/************************************************

  enumerator.c - provides Enumerator class

  $Author$

  Copyright (C) 2001-2003 Akinori MUSHA

  $Idaemons: /home/cvs/rb/enumerator/enumerator.c,v 1.1.1.1 2001/07/15 10:12:48 knu Exp $
  $RoughId: enumerator.c,v 1.6 2003/07/27 11:03:24 nobu Exp $
  $Id$

************************************************/

#include "internal.h"
#include "id.h"

#ifdef HAVE_FLOAT_H
#include <float.h>
#endif

/*
 * Document-class: Enumerator
 *
 * A class which allows both internal and external iteration.
 *
 * An Enumerator can be created by the following methods.
 * - Kernel#to_enum
 * - Kernel#enum_for
 * - Enumerator.new
 *
 * Most methods have two forms: a block form where the contents
 * are evaluated for each item in the enumeration, and a non-block form
 * which returns a new Enumerator wrapping the iteration.
 *
 *   enumerator = %w(one two three).each
 *   puts enumerator.class # => Enumerator
 *
 *   enumerator.each_with_object("foo") do |item, obj|
 *     puts "#{obj}: #{item}"
 *   end
 *
 *   # foo: one
 *   # foo: two
 *   # foo: three
 *
 *   enum_with_obj = enumerator.each_with_object("foo")
 *   puts enum_with_obj.class # => Enumerator
 *
 *   enum_with_obj.each do |item, obj|
 *     puts "#{obj}: #{item}"
 *   end
 *
 *   # foo: one
 *   # foo: two
 *   # foo: three
 *
 * This allows you to chain Enumerators together.  For example, you
 * can map a list's elements to strings containing the index
 * and the element as a string via:
 *
 *   puts %w[foo bar baz].map.with_index { |w, i| "#{i}:#{w}" }
 *   # => ["0:foo", "1:bar", "2:baz"]
 *
 * An Enumerator can also be used as an external iterator.
 * For example, Enumerator#next returns the next value of the iterator
 * or raises StopIteration if the Enumerator is at the end.
 *
 *   e = [1,2,3].each   # returns an enumerator object.
 *   puts e.next   # => 1
 *   puts e.next   # => 2
 *   puts e.next   # => 3
 *   puts e.next   # raises StopIteration
 *
 * You can use this to implement an internal iterator as follows:
 *
 *   def ext_each(e)
 *     while true
 *       begin
 *         vs = e.next_values
 *       rescue StopIteration
 *         return $!.result
 *       end
 *       y = yield(*vs)
 *       e.feed y
 *     end
 *   end
 *
 *   o = Object.new
 *
 *   def o.each
 *     puts yield
 *     puts yield(1)
 *     puts yield(1, 2)
 *     3
 *   end
 *
 *   # use o.each as an internal iterator directly.
 *   puts o.each {|*x| puts x; [:b, *x] }
 *   # => [], [:b], [1], [:b, 1], [1, 2], [:b, 1, 2], 3
 *
 *   # convert o.each to an external iterator for
 *   # implementing an internal iterator.
 *   puts ext_each(o.to_enum) {|*x| puts x; [:b, *x] }
 *   # => [], [:b], [1], [:b, 1], [1, 2], [:b, 1, 2], 3
 *
 */
VALUE rb_cEnumerator;
static VALUE rb_cLazy;
static ID id_rewind, id_new, id_to_enum;
static ID id_next, id_result, id_receiver, id_arguments, id_memo, id_method, id_force;
static ID id_begin, id_end, id_step, id_exclude_end;
static VALUE sym_each, sym_cycle;

#define id_call idCall
#define id_each idEach
#define id_eqq idEqq
#define id_initialize idInitialize
#define id_size idSize

VALUE rb_eStopIteration;

struct enumerator {
    VALUE obj;
    ID    meth;
    VALUE args;
    VALUE fib;
    VALUE dst;
    VALUE lookahead;
    VALUE feedvalue;
    VALUE stop_exc;
    VALUE size;
    VALUE procs;
    rb_enumerator_size_func *size_fn;
};

static VALUE rb_cGenerator, rb_cYielder;

struct generator {
    VALUE proc;
    VALUE obj;
};

struct yielder {
    VALUE proc;
};

typedef struct MEMO *lazyenum_proc_func(VALUE, struct MEMO *, VALUE, long);
typedef VALUE lazyenum_size_func(VALUE, VALUE);
typedef struct {
    lazyenum_proc_func *proc;
    lazyenum_size_func *size;
} lazyenum_funcs;

struct proc_entry {
    VALUE proc;
    VALUE memo;
    const lazyenum_funcs *fn;
};

static VALUE generator_allocate(VALUE klass);
static VALUE generator_init(VALUE obj, VALUE proc);

static VALUE rb_cArithSeq;

/*
 * Enumerator
 */
static void
enumerator_mark(void *p)
{
    struct enumerator *ptr = p;
    rb_gc_mark(ptr->obj);
    rb_gc_mark(ptr->args);
    rb_gc_mark(ptr->fib);
    rb_gc_mark(ptr->dst);
    rb_gc_mark(ptr->lookahead);
    rb_gc_mark(ptr->feedvalue);
    rb_gc_mark(ptr->stop_exc);
    rb_gc_mark(ptr->size);
    rb_gc_mark(ptr->procs);
}

#define enumerator_free RUBY_TYPED_DEFAULT_FREE

static size_t
enumerator_memsize(const void *p)
{
    return sizeof(struct enumerator);
}

static const rb_data_type_t enumerator_data_type = {
    "enumerator",
    {
	enumerator_mark,
	enumerator_free,
	enumerator_memsize,
    },
    0, 0, RUBY_TYPED_FREE_IMMEDIATELY
};

static struct enumerator *
enumerator_ptr(VALUE obj)
{
    struct enumerator *ptr;

    TypedData_Get_Struct(obj, struct enumerator, &enumerator_data_type, ptr);
    if (!ptr || ptr->obj == Qundef) {
	rb_raise(rb_eArgError, "uninitialized enumerator");
    }
    return ptr;
}

static void
proc_entry_mark(void *p)
{
    struct proc_entry *ptr = p;
    rb_gc_mark(ptr->proc);
    rb_gc_mark(ptr->memo);
}

#define proc_entry_free RUBY_TYPED_DEFAULT_FREE

static size_t
proc_entry_memsize(const void *p)
{
    return p ? sizeof(struct proc_entry) : 0;
}

static const rb_data_type_t proc_entry_data_type = {
    "proc_entry",
    {
	proc_entry_mark,
	proc_entry_free,
	proc_entry_memsize,
    },
};

static struct proc_entry *
proc_entry_ptr(VALUE proc_entry)
{
    struct proc_entry *ptr;

    TypedData_Get_Struct(proc_entry, struct proc_entry, &proc_entry_data_type, ptr);

    return ptr;
}

/*
 * call-seq:
 *   obj.to_enum(method = :each, *args)                 -> enum
 *   obj.enum_for(method = :each, *args)                -> enum
 *   obj.to_enum(method = :each, *args) {|*args| block} -> enum
 *   obj.enum_for(method = :each, *args){|*args| block} -> enum
 *
 * Creates a new Enumerator which will enumerate by calling +method+ on
 * +obj+, passing +args+ if any.
 *
 * If a block is given, it will be used to calculate the size of
 * the enumerator without the need to iterate it (see Enumerator#size).
 *
 * === Examples
 *
 *   str = "xyz"
 *
 *   enum = str.enum_for(:each_byte)
 *   enum.each { |b| puts b }
 *   # => 120
 *   # => 121
 *   # => 122
 *
 *   # protect an array from being modified by some_method
 *   a = [1, 2, 3]
 *   some_method(a.to_enum)
 *
 * It is typical to call to_enum when defining methods for
 * a generic Enumerable, in case no block is passed.
 *
 * Here is such an example, with parameter passing and a sizing block:
 *
 *   module Enumerable
 *     # a generic method to repeat the values of any enumerable
 *     def repeat(n)
 *       raise ArgumentError, "#{n} is negative!" if n < 0
 *       unless block_given?
 *         return to_enum(__method__, n) do # __method__ is :repeat here
 *           sz = size     # Call size and multiply by n...
 *           sz * n if sz  # but return nil if size itself is nil
 *         end
 *       end
 *       each do |*val|
 *         n.times { yield *val }
 *       end
 *     end
 *   end
 *
 *   %i[hello world].repeat(2) { |w| puts w }
 *     # => Prints 'hello', 'hello', 'world', 'world'
 *   enum = (1..14).repeat(3)
 *     # => returns an Enumerator when called without a block
 *   enum.first(4) # => [1, 1, 1, 2]
 *   enum.size # => 42
 */
static VALUE
obj_to_enum(int argc, VALUE *argv, VALUE obj)
{
    VALUE enumerator, meth = sym_each;

    if (argc > 0) {
	--argc;
	meth = *argv++;
    }
    enumerator = rb_enumeratorize_with_size(obj, meth, argc, argv, 0);
    if (rb_block_given_p()) {
	enumerator_ptr(enumerator)->size = rb_block_proc();
    }
    return enumerator;
}

static VALUE
enumerator_allocate(VALUE klass)
{
    struct enumerator *ptr;
    VALUE enum_obj;

    enum_obj = TypedData_Make_Struct(klass, struct enumerator, &enumerator_data_type, ptr);
    ptr->obj = Qundef;

    return enum_obj;
}

static VALUE
enumerator_init(VALUE enum_obj, VALUE obj, VALUE meth, int argc, const VALUE *argv, rb_enumerator_size_func *size_fn, VALUE size)
{
    struct enumerator *ptr;

    rb_check_frozen(enum_obj);
    TypedData_Get_Struct(enum_obj, struct enumerator, &enumerator_data_type, ptr);

    if (!ptr) {
	rb_raise(rb_eArgError, "unallocated enumerator");
    }

    ptr->obj  = obj;
    ptr->meth = rb_to_id(meth);
    if (argc) ptr->args = rb_ary_new4(argc, argv);
    ptr->fib = 0;
    ptr->dst = Qnil;
    ptr->lookahead = Qundef;
    ptr->feedvalue = Qundef;
    ptr->stop_exc = Qfalse;
    ptr->size = size;
    ptr->size_fn = size_fn;

    return enum_obj;
}

/*
 * call-seq:
 *   Enumerator.new(size = nil) { |yielder| ... }
 *   Enumerator.new(obj, method = :each, *args)
 *
 * Creates a new Enumerator object, which can be used as an
 * Enumerable.
 *
 * In the first form, iteration is defined by the given block, in
 * which a "yielder" object, given as block parameter, can be used to
 * yield a value by calling the +yield+ method (aliased as +<<+):
 *
 *   fib = Enumerator.new do |y|
 *     a = b = 1
 *     loop do
 *       y << a
 *       a, b = b, a + b
 *     end
 *   end
 *
 *   p fib.take(10) # => [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]
 *
 * The optional parameter can be used to specify how to calculate the size
 * in a lazy fashion (see Enumerator#size). It can either be a value or
 * a callable object.
 *
 * In the second, deprecated, form, a generated Enumerator iterates over the
 * given object using the given method with the given arguments passed.
 *
 * Use of this form is discouraged.  Use Kernel#enum_for or Kernel#to_enum
 * instead.
 *
 *   e = Enumerator.new(ObjectSpace, :each_object)
 *       #-> ObjectSpace.enum_for(:each_object)
 *
 *   e.select { |obj| obj.is_a?(Class) }  #=> array of all classes
 *
 */
static VALUE
enumerator_initialize(int argc, VALUE *argv, VALUE obj)
{
    VALUE recv, meth = sym_each;
    VALUE size = Qnil;

    if (rb_block_given_p()) {
	rb_check_arity(argc, 0, 1);
	recv = generator_init(generator_allocate(rb_cGenerator), rb_block_proc());
	if (argc) {
            if (NIL_P(argv[0]) || rb_respond_to(argv[0], id_call) ||
                (RB_TYPE_P(argv[0], T_FLOAT) && RFLOAT_VALUE(argv[0]) == HUGE_VAL)) {
                size = argv[0];
            }
            else {
                size = rb_to_int(argv[0]);
            }
            argc = 0;
        }
    }
    else {
	rb_check_arity(argc, 1, UNLIMITED_ARGUMENTS);
	rb_warn("Enumerator.new without a block is deprecated; use Object#to_enum");
	recv = *argv++;
	if (--argc) {
	    meth = *argv++;
	    --argc;
	}
    }

    return enumerator_init(obj, recv, meth, argc, argv, 0, size);
}

/* :nodoc: */
static VALUE
enumerator_init_copy(VALUE obj, VALUE orig)
{
    struct enumerator *ptr0, *ptr1;

    if (!OBJ_INIT_COPY(obj, orig)) return obj;
    ptr0 = enumerator_ptr(orig);
    if (ptr0->fib) {
	/* Fibers cannot be copied */
	rb_raise(rb_eTypeError, "can't copy execution context");
    }

    TypedData_Get_Struct(obj, struct enumerator, &enumerator_data_type, ptr1);

    if (!ptr1) {
	rb_raise(rb_eArgError, "unallocated enumerator");
    }

    ptr1->obj  = ptr0->obj;
    ptr1->meth = ptr0->meth;
    ptr1->args = ptr0->args;
    ptr1->fib  = 0;
    ptr1->lookahead  = Qundef;
    ptr1->feedvalue  = Qundef;
    ptr1->size  = ptr0->size;
    ptr1->size_fn  = ptr0->size_fn;

    return obj;
}

/*
 * For backwards compatibility; use rb_enumeratorize_with_size
 */
VALUE
rb_enumeratorize(VALUE obj, VALUE meth, int argc, const VALUE *argv)
{
    return rb_enumeratorize_with_size(obj, meth, argc, argv, 0);
}

static VALUE
lazy_to_enum_i(VALUE self, VALUE meth, int argc, const VALUE *argv, rb_enumerator_size_func *size_fn);

VALUE
rb_enumeratorize_with_size(VALUE obj, VALUE meth, int argc, const VALUE *argv, rb_enumerator_size_func *size_fn)
{
    /* Similar effect as calling obj.to_enum, i.e. dispatching to either
       Kernel#to_enum vs Lazy#to_enum */
    if (RTEST(rb_obj_is_kind_of(obj, rb_cLazy)))
	return lazy_to_enum_i(obj, meth, argc, argv, size_fn);
    else
	return enumerator_init(enumerator_allocate(rb_cEnumerator),
			       obj, meth, argc, argv, size_fn, Qnil);
}

static VALUE
enumerator_block_call(VALUE obj, rb_block_call_func *func, VALUE arg)
{
    int argc = 0;
    const VALUE *argv = 0;
    const struct enumerator *e = enumerator_ptr(obj);
    ID meth = e->meth;

    if (e->args) {
	argc = RARRAY_LENINT(e->args);
	argv = RARRAY_CONST_PTR(e->args);
    }
    return rb_block_call(e->obj, meth, argc, argv, func, arg);
}

/*
 * call-seq:
 *   enum.each { |elm| block }                    -> obj
 *   enum.each                                    -> enum
 *   enum.each(*appending_args) { |elm| block }   -> obj
 *   enum.each(*appending_args)                   -> an_enumerator
 *
 * Iterates over the block according to how this Enumerator was constructed.
 * If no block and no arguments are given, returns self.
 *
 * === Examples
 *
 *   "Hello, world!".scan(/\w+/)                     #=> ["Hello", "world"]
 *   "Hello, world!".to_enum(:scan, /\w+/).to_a      #=> ["Hello", "world"]
 *   "Hello, world!".to_enum(:scan).each(/\w+/).to_a #=> ["Hello", "world"]
 *
 *   obj = Object.new
 *
 *   def obj.each_arg(a, b=:b, *rest)
 *     yield a
 *     yield b
 *     yield rest
 *     :method_returned
 *   end
 *
 *   enum = obj.to_enum :each_arg, :a, :x
 *
 *   enum.each.to_a                  #=> [:a, :x, []]
 *   enum.each.equal?(enum)          #=> true
 *   enum.each { |elm| elm }         #=> :method_returned
 *
 *   enum.each(:y, :z).to_a          #=> [:a, :x, [:y, :z]]
 *   enum.each(:y, :z).equal?(enum)  #=> false
 *   enum.each(:y, :z) { |elm| elm } #=> :method_returned
 *
 */
static VALUE
enumerator_each(int argc, VALUE *argv, VALUE obj)
{
    if (argc > 0) {
	struct enumerator *e = enumerator_ptr(obj = rb_obj_dup(obj));
	VALUE args = e->args;
	if (args) {
#if SIZEOF_INT < SIZEOF_LONG
	    /* check int range overflow */
	    rb_long2int(RARRAY_LEN(args) + argc);
#endif
	    args = rb_ary_dup(args);
	    rb_ary_cat(args, argv, argc);
	}
	else {
	    args = rb_ary_new4(argc, argv);
	}
	e->args = args;
    }
    if (!rb_block_given_p()) return obj;
    return enumerator_block_call(obj, 0, obj);
}

static VALUE
enumerator_with_index_i(RB_BLOCK_CALL_FUNC_ARGLIST(val, m))
{
    struct MEMO *memo = (struct MEMO *)m;
    VALUE idx = memo->v1;
    MEMO_V1_SET(memo, rb_int_succ(idx));

    if (argc <= 1)
	return rb_yield_values(2, val, idx);

    return rb_yield_values(2, rb_ary_new4(argc, argv), idx);
}

static VALUE
enumerator_size(VALUE obj);

static VALUE
enumerator_enum_size(VALUE obj, VALUE args, VALUE eobj)
{
    return enumerator_size(obj);
}

/*
 * call-seq:
 *   e.with_index(offset = 0) {|(*args), idx| ... }
 *   e.with_index(offset = 0)
 *
 * Iterates the given block for each element with an index, which
 * starts from +offset+.  If no block is given, returns a new Enumerator
 * that includes the index, starting from +offset+
 *
 * +offset+:: the starting index to use
 *
 */
static VALUE
enumerator_with_index(int argc, VALUE *argv, VALUE obj)
{
    VALUE memo;

    rb_scan_args(argc, argv, "01", &memo);
    RETURN_SIZED_ENUMERATOR(obj, argc, argv, enumerator_enum_size);
    if (NIL_P(memo))
	memo = INT2FIX(0);
    else
	memo = rb_to_int(memo);
    return enumerator_block_call(obj, enumerator_with_index_i, (VALUE)MEMO_NEW(memo, 0, 0));
}

/*
 * call-seq:
 *   e.each_with_index {|(*args), idx| ... }
 *   e.each_with_index
 *
 * Same as Enumerator#with_index(0), i.e. there is no starting offset.
 *
 * If no block is given, a new Enumerator is returned that includes the index.
 *
 */
static VALUE
enumerator_each_with_index(VALUE obj)
{
    return enumerator_with_index(0, NULL, obj);
}

static VALUE
enumerator_with_object_i(RB_BLOCK_CALL_FUNC_ARGLIST(val, memo))
{
    if (argc <= 1)
	return rb_yield_values(2, val, memo);

    return rb_yield_values(2, rb_ary_new4(argc, argv), memo);
}

/*
 * call-seq:
 *   e.each_with_object(obj) {|(*args), obj| ... }
 *   e.each_with_object(obj)
 *   e.with_object(obj) {|(*args), obj| ... }
 *   e.with_object(obj)
 *
 * Iterates the given block for each element with an arbitrary object, +obj+,
 * and returns +obj+
 *
 * If no block is given, returns a new Enumerator.
 *
 * === Example
 *
 *   to_three = Enumerator.new do |y|
 *     3.times do |x|
 *       y << x
 *     end
 *   end
 *
 *   to_three_with_string = to_three.with_object("foo")
 *   to_three_with_string.each do |x,string|
 *     puts "#{string}: #{x}"
 *   end
 *
 *   # => foo:0
 *   # => foo:1
 *   # => foo:2
 */
static VALUE
enumerator_with_object(VALUE obj, VALUE memo)
{
    RETURN_SIZED_ENUMERATOR(obj, 1, &memo, enumerator_enum_size);
    enumerator_block_call(obj, enumerator_with_object_i, memo);

    return memo;
}

static VALUE
next_ii(RB_BLOCK_CALL_FUNC_ARGLIST(i, obj))
{
    struct enumerator *e = enumerator_ptr(obj);
    VALUE feedvalue = Qnil;
    VALUE args = rb_ary_new4(argc, argv);
    rb_fiber_yield(1, &args);
    if (e->feedvalue != Qundef) {
        feedvalue = e->feedvalue;
        e->feedvalue = Qundef;
    }
    return feedvalue;
}

static VALUE
next_i(VALUE curr, VALUE obj)
{
    struct enumerator *e = enumerator_ptr(obj);
    VALUE nil = Qnil;
    VALUE result;

    result = rb_block_call(obj, id_each, 0, 0, next_ii, obj);
    e->stop_exc = rb_exc_new2(rb_eStopIteration, "iteration reached an end");
    rb_ivar_set(e->stop_exc, id_result, result);
    return rb_fiber_yield(1, &nil);
}

static void
next_init(VALUE obj, struct enumerator *e)
{
    VALUE curr = rb_fiber_current();
    e->dst = curr;
    e->fib = rb_fiber_new(next_i, obj);
    e->lookahead = Qundef;
}

static VALUE
get_next_values(VALUE obj, struct enumerator *e)
{
    VALUE curr, vs;

    if (e->stop_exc)
	rb_exc_raise(e->stop_exc);

    curr = rb_fiber_current();

    if (!e->fib || !rb_fiber_alive_p(e->fib)) {
	next_init(obj, e);
    }

    vs = rb_fiber_resume(e->fib, 1, &curr);
    if (e->stop_exc) {
	e->fib = 0;
	e->dst = Qnil;
	e->lookahead = Qundef;
	e->feedvalue = Qundef;
	rb_exc_raise(e->stop_exc);
    }
    return vs;
}

/*
 * call-seq:
 *   e.next_values   -> array
 *
 * Returns the next object as an array in the enumerator, and move the
 * internal position forward.  When the position reached at the end,
 * StopIteration is raised.
 *
 * This method can be used to distinguish <code>yield</code> and <code>yield
 * nil</code>.
 *
 * === Example
 *
 *   o = Object.new
 *   def o.each
 *     yield
 *     yield 1
 *     yield 1, 2
 *     yield nil
 *     yield [1, 2]
 *   end
 *   e = o.to_enum
 *   p e.next_values
 *   p e.next_values
 *   p e.next_values
 *   p e.next_values
 *   p e.next_values
 *   e = o.to_enum
 *   p e.next
 *   p e.next
 *   p e.next
 *   p e.next
 *   p e.next
 *
 *   ## yield args       next_values      next
 *   #  yield            []               nil
 *   #  yield 1          [1]              1
 *   #  yield 1, 2       [1, 2]           [1, 2]
 *   #  yield nil        [nil]            nil
 *   #  yield [1, 2]     [[1, 2]]         [1, 2]
 *
 * Note that +next_values+ does not affect other non-external enumeration
 * methods unless underlying iteration method itself has side-effect, e.g.
 * IO#each_line.
 *
 */

static VALUE
enumerator_next_values(VALUE obj)
{
    struct enumerator *e = enumerator_ptr(obj);
    VALUE vs;

    if (e->lookahead != Qundef) {
        vs = e->lookahead;
        e->lookahead = Qundef;
        return vs;
    }

    return get_next_values(obj, e);
}

static VALUE
ary2sv(VALUE args, int dup)
{
    if (!RB_TYPE_P(args, T_ARRAY))
        return args;

    switch (RARRAY_LEN(args)) {
      case 0:
        return Qnil;

      case 1:
        return RARRAY_AREF(args, 0);

      default:
        if (dup)
            return rb_ary_dup(args);
        return args;
    }
}

/*
 * call-seq:
 *   e.next   -> object
 *
 * Returns the next object in the enumerator, and move the internal position
 * forward.  When the position reached at the end, StopIteration is raised.
 *
 * === Example
 *
 *   a = [1,2,3]
 *   e = a.to_enum
 *   p e.next   #=> 1
 *   p e.next   #=> 2
 *   p e.next   #=> 3
 *   p e.next   #raises StopIteration
 *
 * Note that enumeration sequence by +next+ does not affect other non-external
 * enumeration methods, unless the underlying iteration methods itself has
 * side-effect, e.g. IO#each_line.
 *
 */

static VALUE
enumerator_next(VALUE obj)
{
    VALUE vs = enumerator_next_values(obj);
    return ary2sv(vs, 0);
}

static VALUE
enumerator_peek_values(VALUE obj)
{
    struct enumerator *e = enumerator_ptr(obj);

    if (e->lookahead == Qundef) {
        e->lookahead = get_next_values(obj, e);
    }
    return e->lookahead;
}

/*
 * call-seq:
 *   e.peek_values   -> array
 *
 * Returns the next object as an array, similar to Enumerator#next_values, but
 * doesn't move the internal position forward.  If the position is already at
 * the end, StopIteration is raised.
 *
 * === Example
 *
 *   o = Object.new
 *   def o.each
 *     yield
 *     yield 1
 *     yield 1, 2
 *   end
 *   e = o.to_enum
 *   p e.peek_values    #=> []
 *   e.next
 *   p e.peek_values    #=> [1]
 *   p e.peek_values    #=> [1]
 *   e.next
 *   p e.peek_values    #=> [1, 2]
 *   e.next
 *   p e.peek_values    # raises StopIteration
 *
 */

static VALUE
enumerator_peek_values_m(VALUE obj)
{
    return rb_ary_dup(enumerator_peek_values(obj));
}

/*
 * call-seq:
 *   e.peek   -> object
 *
 * Returns the next object in the enumerator, but doesn't move the internal
 * position forward.  If the position is already at the end, StopIteration
 * is raised.
 *
 * === Example
 *
 *   a = [1,2,3]
 *   e = a.to_enum
 *   p e.next   #=> 1
 *   p e.peek   #=> 2
 *   p e.peek   #=> 2
 *   p e.peek   #=> 2
 *   p e.next   #=> 2
 *   p e.next   #=> 3
 *   p e.peek   #raises StopIteration
 *
 */

static VALUE
enumerator_peek(VALUE obj)
{
    VALUE vs = enumerator_peek_values(obj);
    return ary2sv(vs, 1);
}

/*
 * call-seq:
 *   e.feed obj   -> nil
 *
 * Sets the value to be returned by the next yield inside +e+.
 *
 * If the value is not set, the yield returns nil.
 *
 * This value is cleared after being yielded.
 *
 *   # Array#map passes the array's elements to "yield" and collects the
 *   # results of "yield" as an array.
 *   # Following example shows that "next" returns the passed elements and
 *   # values passed to "feed" are collected as an array which can be
 *   # obtained by StopIteration#result.
 *   e = [1,2,3].map
 *   p e.next           #=> 1
 *   e.feed "a"
 *   p e.next           #=> 2
 *   e.feed "b"
 *   p e.next           #=> 3
 *   e.feed "c"
 *   begin
 *     e.next
 *   rescue StopIteration
 *     p $!.result      #=> ["a", "b", "c"]
 *   end
 *
 *   o = Object.new
 *   def o.each
 *     x = yield         # (2) blocks
 *     p x               # (5) => "foo"
 *     x = yield         # (6) blocks
 *     p x               # (8) => nil
 *     x = yield         # (9) blocks
 *     p x               # not reached w/o another e.next
 *   end
 *
 *   e = o.to_enum
 *   e.next              # (1)
 *   e.feed "foo"        # (3)
 *   e.next              # (4)
 *   e.next              # (7)
 *                       # (10)
 */

static VALUE
enumerator_feed(VALUE obj, VALUE v)
{
    struct enumerator *e = enumerator_ptr(obj);

    if (e->feedvalue != Qundef) {
	rb_raise(rb_eTypeError, "feed value already set");
    }
    e->feedvalue = v;

    return Qnil;
}

/*
 * call-seq:
 *   e.rewind   -> e
 *
 * Rewinds the enumeration sequence to the beginning.
 *
 * If the enclosed object responds to a "rewind" method, it is called.
 */

static VALUE
enumerator_rewind(VALUE obj)
{
    struct enumerator *e = enumerator_ptr(obj);

    rb_check_funcall(e->obj, id_rewind, 0, 0);

    e->fib = 0;
    e->dst = Qnil;
    e->lookahead = Qundef;
    e->feedvalue = Qundef;
    e->stop_exc = Qfalse;
    return obj;
}

static struct generator *generator_ptr(VALUE obj);
static VALUE append_method(VALUE obj, VALUE str, ID default_method, VALUE default_args);

static VALUE
inspect_enumerator(VALUE obj, VALUE dummy, int recur)
{
    struct enumerator *e;
    VALUE eobj, str, cname;

    TypedData_Get_Struct(obj, struct enumerator, &enumerator_data_type, e);

    cname = rb_obj_class(obj);

    if (!e || e->obj == Qundef) {
	return rb_sprintf("#<%"PRIsVALUE": uninitialized>", rb_class_path(cname));
    }

    if (recur) {
	str = rb_sprintf("#<%"PRIsVALUE": ...>", rb_class_path(cname));
	OBJ_TAINT(str);
	return str;
    }

    if (e->procs) {
	long i;

	eobj = generator_ptr(e->obj)->obj;
	/* In case procs chained enumerator traversing all proc entries manually */
	if (rb_obj_class(eobj) == cname) {
	    str = rb_inspect(eobj);
	}
	else {
	    str = rb_sprintf("#<%"PRIsVALUE": %+"PRIsVALUE">", rb_class_path(cname), eobj);
	}
	for (i = 0; i < RARRAY_LEN(e->procs); i++) {
	    str = rb_sprintf("#<%"PRIsVALUE": %"PRIsVALUE, cname, str);
	    append_method(RARRAY_AREF(e->procs, i), str, e->meth, e->args);
	    rb_str_buf_cat2(str, ">");
	}
	return str;
    }

    eobj = rb_attr_get(obj, id_receiver);
    if (NIL_P(eobj)) {
	eobj = e->obj;
    }

    /* (1..100).each_cons(2) => "#<Enumerator: 1..100:each_cons(2)>" */
    str = rb_sprintf("#<%"PRIsVALUE": %+"PRIsVALUE, rb_class_path(cname), eobj);
    append_method(obj, str, e->meth, e->args);

    rb_str_buf_cat2(str, ">");

    return str;
}

static int
key_symbol_p(VALUE key, VALUE val, VALUE arg)
{
    if (SYMBOL_P(key)) return ST_CONTINUE;
    *(int *)arg = FALSE;
    return ST_STOP;
}

static int
kwd_append(VALUE key, VALUE val, VALUE str)
{
    if (!SYMBOL_P(key)) rb_raise(rb_eRuntimeError, "non-symbol key inserted");
    rb_str_catf(str, "% "PRIsVALUE": %"PRIsVALUE", ", key, val);
    return ST_CONTINUE;
}

static VALUE
append_method(VALUE obj, VALUE str, ID default_method, VALUE default_args)
{
    VALUE method, eargs;

    method = rb_attr_get(obj, id_method);
    if (method != Qfalse) {
	if (!NIL_P(method)) {
	    Check_Type(method, T_SYMBOL);
	    method = rb_sym2str(method);
	}
	else {
	    method = rb_id2str(default_method);
	}
	rb_str_buf_cat2(str, ":");
	rb_str_buf_append(str, method);
    }

    eargs = rb_attr_get(obj, id_arguments);
    if (NIL_P(eargs)) {
	eargs = default_args;
    }
    if (eargs != Qfalse) {
	long   argc = RARRAY_LEN(eargs);
	const VALUE *argv = RARRAY_CONST_PTR(eargs); /* WB: no new reference */

	if (argc > 0) {
	    VALUE kwds = Qnil;

	    rb_str_buf_cat2(str, "(");

	    if (RB_TYPE_P(argv[argc-1], T_HASH)) {
		int all_key = TRUE;
		rb_hash_foreach(argv[argc-1], key_symbol_p, (VALUE)&all_key);
		if (all_key) kwds = argv[--argc];
	    }

	    while (argc--) {
		VALUE arg = *argv++;

		rb_str_append(str, rb_inspect(arg));
		rb_str_buf_cat2(str, ", ");
		OBJ_INFECT(str, arg);
	    }
	    if (!NIL_P(kwds)) {
		rb_hash_foreach(kwds, kwd_append, str);
	    }
	    rb_str_set_len(str, RSTRING_LEN(str)-2);
	    rb_str_buf_cat2(str, ")");
	}
    }

    return str;
}

/*
 * call-seq:
 *   e.inspect  -> string
 *
 * Creates a printable version of <i>e</i>.
 */

static VALUE
enumerator_inspect(VALUE obj)
{
    return rb_exec_recursive(inspect_enumerator, obj, 0);
}

/*
 * call-seq:
 *   e.size          -> int, Float::INFINITY or nil
 *
 * Returns the size of the enumerator, or +nil+ if it can't be calculated lazily.
 *
 *   (1..100).to_a.permutation(4).size # => 94109400
 *   loop.size # => Float::INFINITY
 *   (1..100).drop_while.size # => nil
 */

static VALUE
enumerator_size(VALUE obj)
{
    struct enumerator *e = enumerator_ptr(obj);
    int argc = 0;
    const VALUE *argv = NULL;
    VALUE size;

    if (e->procs) {
	struct generator *g = generator_ptr(e->obj);
	VALUE receiver = rb_check_funcall(g->obj, id_size, 0, 0);
	long i = 0;

	for (i = 0; i < RARRAY_LEN(e->procs); i++) {
	    VALUE proc = RARRAY_AREF(e->procs, i);
	    struct proc_entry *entry = proc_entry_ptr(proc);
	    lazyenum_size_func *size_fn = entry->fn->size;
	    if (!size_fn) {
		return Qnil;
	    }
	    receiver = (*size_fn)(proc, receiver);
	}
	return receiver;
    }

    if (e->size_fn) {
	return (*e->size_fn)(e->obj, e->args, obj);
    }
    if (e->args) {
	argc = (int)RARRAY_LEN(e->args);
	argv = RARRAY_CONST_PTR(e->args);
    }
    size = rb_check_funcall(e->size, id_call, argc, argv);
    if (size != Qundef) return size;
    return e->size;
}

/*
 * Yielder
 */
static void
yielder_mark(void *p)
{
    struct yielder *ptr = p;
    rb_gc_mark(ptr->proc);
}

#define yielder_free RUBY_TYPED_DEFAULT_FREE

static size_t
yielder_memsize(const void *p)
{
    return sizeof(struct yielder);
}

static const rb_data_type_t yielder_data_type = {
    "yielder",
    {
	yielder_mark,
	yielder_free,
	yielder_memsize,
    },
    0, 0, RUBY_TYPED_FREE_IMMEDIATELY
};

static struct yielder *
yielder_ptr(VALUE obj)
{
    struct yielder *ptr;

    TypedData_Get_Struct(obj, struct yielder, &yielder_data_type, ptr);
    if (!ptr || ptr->proc == Qundef) {
	rb_raise(rb_eArgError, "uninitialized yielder");
    }
    return ptr;
}

/* :nodoc: */
static VALUE
yielder_allocate(VALUE klass)
{
    struct yielder *ptr;
    VALUE obj;

    obj = TypedData_Make_Struct(klass, struct yielder, &yielder_data_type, ptr);
    ptr->proc = Qundef;

    return obj;
}

static VALUE
yielder_init(VALUE obj, VALUE proc)
{
    struct yielder *ptr;

    TypedData_Get_Struct(obj, struct yielder, &yielder_data_type, ptr);

    if (!ptr) {
	rb_raise(rb_eArgError, "unallocated yielder");
    }

    ptr->proc = proc;

    return obj;
}

/* :nodoc: */
static VALUE
yielder_initialize(VALUE obj)
{
    rb_need_block();

    return yielder_init(obj, rb_block_proc());
}

/* :nodoc: */
static VALUE
yielder_yield(VALUE obj, VALUE args)
{
    struct yielder *ptr = yielder_ptr(obj);

    return rb_proc_call(ptr->proc, args);
}

/* :nodoc: */
static VALUE
yielder_yield_push(VALUE obj, VALUE arg)
{
    struct yielder *ptr = yielder_ptr(obj);

    rb_proc_call_with_block(ptr->proc, 1, &arg, Qnil);

    return obj;
}

static VALUE
yielder_yield_i(RB_BLOCK_CALL_FUNC_ARGLIST(obj, memo))
{
    return rb_yield_values2(argc, argv);
}

static VALUE
yielder_new(void)
{
    return yielder_init(yielder_allocate(rb_cYielder), rb_proc_new(yielder_yield_i, 0));
}

/*
 * Generator
 */
static void
generator_mark(void *p)
{
    struct generator *ptr = p;
    rb_gc_mark(ptr->proc);
    rb_gc_mark(ptr->obj);
}

#define generator_free RUBY_TYPED_DEFAULT_FREE

static size_t
generator_memsize(const void *p)
{
    return sizeof(struct generator);
}

static const rb_data_type_t generator_data_type = {
    "generator",
    {
	generator_mark,
	generator_free,
	generator_memsize,
    },
    0, 0, RUBY_TYPED_FREE_IMMEDIATELY
};

static struct generator *
generator_ptr(VALUE obj)
{
    struct generator *ptr;

    TypedData_Get_Struct(obj, struct generator, &generator_data_type, ptr);
    if (!ptr || ptr->proc == Qundef) {
	rb_raise(rb_eArgError, "uninitialized generator");
    }
    return ptr;
}

/* :nodoc: */
static VALUE
generator_allocate(VALUE klass)
{
    struct generator *ptr;
    VALUE obj;

    obj = TypedData_Make_Struct(klass, struct generator, &generator_data_type, ptr);
    ptr->proc = Qundef;

    return obj;
}

static VALUE
generator_init(VALUE obj, VALUE proc)
{
    struct generator *ptr;

    rb_check_frozen(obj);
    TypedData_Get_Struct(obj, struct generator, &generator_data_type, ptr);

    if (!ptr) {
	rb_raise(rb_eArgError, "unallocated generator");
    }

    ptr->proc = proc;

    return obj;
}

/* :nodoc: */
static VALUE
generator_initialize(int argc, VALUE *argv, VALUE obj)
{
    VALUE proc;

    if (argc == 0) {
	rb_need_block();

	proc = rb_block_proc();
    }
    else {
	rb_scan_args(argc, argv, "1", &proc);

	if (!rb_obj_is_proc(proc))
	    rb_raise(rb_eTypeError,
		     "wrong argument type %"PRIsVALUE" (expected Proc)",
		     rb_obj_class(proc));

	if (rb_block_given_p()) {
	    rb_warn("given block not used");
	}
    }

    return generator_init(obj, proc);
}

/* :nodoc: */
static VALUE
generator_init_copy(VALUE obj, VALUE orig)
{
    struct generator *ptr0, *ptr1;

    if (!OBJ_INIT_COPY(obj, orig)) return obj;

    ptr0 = generator_ptr(orig);

    TypedData_Get_Struct(obj, struct generator, &generator_data_type, ptr1);

    if (!ptr1) {
	rb_raise(rb_eArgError, "unallocated generator");
    }

    ptr1->proc = ptr0->proc;

    return obj;
}

/* :nodoc: */
static VALUE
generator_each(int argc, VALUE *argv, VALUE obj)
{
    struct generator *ptr = generator_ptr(obj);
    VALUE args = rb_ary_new2(argc + 1);

    rb_ary_push(args, yielder_new());
    if (argc > 0) {
	rb_ary_cat(args, argv, argc);
    }

    return rb_proc_call(ptr->proc, args);
}

/* Lazy Enumerator methods */
static VALUE
enum_size(VALUE self)
{
    VALUE r = rb_check_funcall(self, id_size, 0, 0);
    return (r == Qundef) ? Qnil : r;
}

static VALUE
lazyenum_size(VALUE self, VALUE args, VALUE eobj)
{
    return enum_size(self);
}

static VALUE
lazy_size(VALUE self)
{
    return enum_size(rb_ivar_get(self, id_receiver));
}

static VALUE
lazy_receiver_size(VALUE generator, VALUE args, VALUE lazy)
{
    return lazy_size(lazy);
}

static VALUE
lazy_init_iterator(RB_BLOCK_CALL_FUNC_ARGLIST(val, m))
{
    VALUE result;
    if (argc == 1) {
	VALUE args[2];
	args[0] = m;
	args[1] = val;
	result = rb_yield_values2(2, args);
    }
    else {
	VALUE args;
	int len = rb_long2int((long)argc + 1);
	VALUE *nargv = ALLOCV_N(VALUE, args, len);

	nargv[0] = m;
	if (argc > 0) {
	    MEMCPY(nargv + 1, argv, VALUE, argc);
	}
	result = rb_yield_values2(len, nargv);
	ALLOCV_END(args);
    }
    if (result == Qundef) rb_iter_break();
    return Qnil;
}

static VALUE
lazy_init_block_i(RB_BLOCK_CALL_FUNC_ARGLIST(val, m))
{
    rb_block_call(m, id_each, argc-1, argv+1, lazy_init_iterator, val);
    return Qnil;
}

#define memo_value v2
#define memo_flags u3.state
#define LAZY_MEMO_BREAK 1
#define LAZY_MEMO_PACKED 2
#define LAZY_MEMO_BREAK_P(memo) ((memo)->memo_flags & LAZY_MEMO_BREAK)
#define LAZY_MEMO_PACKED_P(memo) ((memo)->memo_flags & LAZY_MEMO_PACKED)
#define LAZY_MEMO_SET_BREAK(memo) ((memo)->memo_flags |= LAZY_MEMO_BREAK)
#define LAZY_MEMO_SET_VALUE(memo, value) MEMO_V2_SET(memo, value)
#define LAZY_MEMO_SET_PACKED(memo) ((memo)->memo_flags |= LAZY_MEMO_PACKED)
#define LAZY_MEMO_RESET_PACKED(memo) ((memo)->memo_flags &= ~LAZY_MEMO_PACKED)

static VALUE
lazy_init_yielder(VALUE val, VALUE m, int argc, VALUE *argv)
{
    VALUE yielder = RARRAY_AREF(m, 0);
    VALUE procs_array = RARRAY_AREF(m, 1);
    VALUE memos = rb_attr_get(yielder, id_memo);
    long i = 0;
    struct MEMO *result;
    int cont = 1;

    result = MEMO_NEW(Qnil, rb_enum_values_pack(argc, argv),
		      argc > 1 ? LAZY_MEMO_PACKED : 0);

    for (i = 0; i < RARRAY_LEN(procs_array); i++) {
	VALUE proc = RARRAY_AREF(procs_array, i);
	struct proc_entry *entry = proc_entry_ptr(proc);
	if (!(*entry->fn->proc)(proc, result, memos, i)) {
	    cont = 0;
	    break;
	}
    }

    if (cont) {
	rb_funcall2(yielder, idLTLT, 1, &(result->memo_value));
    }
    if (LAZY_MEMO_BREAK_P(result)) {
	rb_iter_break();
    }
    return result->memo_value;
}

static VALUE
lazy_init_block(VALUE val, VALUE m, int argc, VALUE *argv)
{
    VALUE procs = RARRAY_AREF(m, 1);

    rb_ivar_set(val, id_memo, rb_ary_new2(RARRAY_LEN(procs)));
    rb_block_call(RARRAY_AREF(m, 0), id_each, 0, 0,
		  lazy_init_yielder, rb_ary_new3(2, val, procs));
    return Qnil;
}

static VALUE
lazy_generator_init(VALUE enumerator, VALUE procs)
{
    VALUE generator;
    VALUE obj;
    struct generator *gen_ptr;
    struct enumerator *e = enumerator_ptr(enumerator);

    if (RARRAY_LEN(procs) > 0) {
	struct generator *old_gen_ptr = generator_ptr(e->obj);
	obj = old_gen_ptr->obj;
    }
    else {
	obj = enumerator;
    }

    generator = generator_allocate(rb_cGenerator);

    rb_block_call(generator, id_initialize, 0, 0,
		  lazy_init_block, rb_ary_new3(2, obj, procs));

    gen_ptr = generator_ptr(generator);
    gen_ptr->obj = obj;

    return generator;
}

/*
 * call-seq:
 *   Lazy.new(obj, size=nil) { |yielder, *values| ... }
 *
 * Creates a new Lazy enumerator. When the enumerator is actually enumerated
 * (e.g. by calling #force), +obj+ will be enumerated and each value passed
 * to the given block. The block can yield values back using +yielder+.
 * For example, to create a method +filter_map+ in both lazy and
 * non-lazy fashions:
 *
 *   module Enumerable
 *     def filter_map(&block)
 *       map(&block).compact
 *     end
 *   end
 *
 *   class Enumerator::Lazy
 *     def filter_map
 *       Lazy.new(self) do |yielder, *values|
 *         result = yield *values
 *         yielder << result if result
 *       end
 *     end
 *   end
 *
 *   (1..Float::INFINITY).lazy.filter_map{|i| i*i if i.even?}.first(5)
 *       # => [4, 16, 36, 64, 100]
 */
static VALUE
lazy_initialize(int argc, VALUE *argv, VALUE self)
{
    VALUE obj, size = Qnil;
    VALUE generator;

    rb_check_arity(argc, 1, 2);
    if (!rb_block_given_p()) {
	rb_raise(rb_eArgError, "tried to call lazy new without a block");
    }
    obj = argv[0];
    if (argc > 1) {
	size = argv[1];
    }
    generator = generator_allocate(rb_cGenerator);
    rb_block_call(generator, id_initialize, 0, 0, lazy_init_block_i, obj);
    enumerator_init(self, generator, sym_each, 0, 0, 0, size);
    rb_ivar_set(self, id_receiver, obj);

    return self;
}

#if 0 /* for RDoc */
/*
 * call-seq:
 *   lazy.to_a  -> array
 *   lazy.force -> array
 *
 * Expands +lazy+ enumerator to an array.
 * See Enumerable#to_a.
 */
static VALUE lazy_to_a(VALUE self)
{
}
#endif

static void
lazy_set_args(VALUE lazy, VALUE args)
{
    ID id = rb_frame_this_func();
    rb_ivar_set(lazy, id_method, ID2SYM(id));
    if (NIL_P(args)) {
	/* Qfalse indicates that the arguments are empty */
	rb_ivar_set(lazy, id_arguments, Qfalse);
    }
    else {
	rb_ivar_set(lazy, id_arguments, args);
    }
}

static VALUE
lazy_set_method(VALUE lazy, VALUE args, rb_enumerator_size_func *size_fn)
{
    struct enumerator *e = enumerator_ptr(lazy);
    lazy_set_args(lazy, args);
    e->size_fn = size_fn;
    return lazy;
}

static VALUE
lazy_add_method(VALUE obj, int argc, VALUE *argv, VALUE args, VALUE memo,
		const lazyenum_funcs *fn)
{
    struct enumerator *new_e;
    VALUE new_obj;
    VALUE new_generator;
    VALUE new_procs;
    struct enumerator *e = enumerator_ptr(obj);
    struct proc_entry *entry;
    VALUE entry_obj = TypedData_Make_Struct(rb_cObject, struct proc_entry,
					    &proc_entry_data_type, entry);
    if (rb_block_given_p()) {
	entry->proc = rb_block_proc();
    }
    entry->fn = fn;
    entry->memo = args;

    lazy_set_args(entry_obj, memo);

    new_procs = RTEST(e->procs) ? rb_ary_dup(e->procs) : rb_ary_new();
    new_generator = lazy_generator_init(obj, new_procs);
    rb_ary_push(new_procs, entry_obj);

    new_obj = enumerator_init_copy(enumerator_allocate(rb_cLazy), obj);
    new_e = DATA_PTR(new_obj);
    new_e->obj = new_generator;
    new_e->procs = new_procs;

    if (argc > 0) {
	new_e->meth = rb_to_id(*argv++);
	--argc;
    }
    else {
	new_e->meth = id_each;
    }
    new_e->args = rb_ary_new4(argc, argv);
    return new_obj;
}

/*
 * call-seq:
 *   e.lazy -> lazy_enumerator
 *
 * Returns a lazy enumerator, whose methods map/collect,
 * flat_map/collect_concat, select/find_all, reject, grep, grep_v, zip, take,
 * take_while, drop, and drop_while enumerate values only on an
 * as-needed basis.  However, if a block is given to zip, values
 * are enumerated immediately.
 *
 * === Example
 *
 * The following program finds pythagorean triples:
 *
 *   def pythagorean_triples
 *     (1..Float::INFINITY).lazy.flat_map {|z|
 *       (1..z).flat_map {|x|
 *         (x..z).select {|y|
 *           x**2 + y**2 == z**2
 *         }.map {|y|
 *           [x, y, z]
 *         }
 *       }
 *     }
 *   end
 *   # show first ten pythagorean triples
 *   p pythagorean_triples.take(10).force # take is lazy, so force is needed
 *   p pythagorean_triples.first(10)      # first is eager
 *   # show pythagorean triples less than 100
 *   p pythagorean_triples.take_while { |*, z| z < 100 }.force
 */
static VALUE
enumerable_lazy(VALUE obj)
{
    VALUE result = lazy_to_enum_i(obj, sym_each, 0, 0, lazyenum_size);
    /* Qfalse indicates that the Enumerator::Lazy has no method name */
    rb_ivar_set(result, id_method, Qfalse);
    return result;
}

static VALUE
lazy_to_enum_i(VALUE obj, VALUE meth, int argc, const VALUE *argv, rb_enumerator_size_func *size_fn)
{
    return enumerator_init(enumerator_allocate(rb_cLazy),
			   obj, meth, argc, argv, size_fn, Qnil);
}

/*
 * call-seq:
 *   lzy.to_enum(method = :each, *args)                 -> lazy_enum
 *   lzy.enum_for(method = :each, *args)                -> lazy_enum
 *   lzy.to_enum(method = :each, *args) {|*args| block} -> lazy_enum
 *   lzy.enum_for(method = :each, *args){|*args| block} -> lazy_enum
 *
 * Similar to Kernel#to_enum, except it returns a lazy enumerator.
 * This makes it easy to define Enumerable methods that will
 * naturally remain lazy if called from a lazy enumerator.
 *
 * For example, continuing from the example in Kernel#to_enum:
 *
 *   # See Kernel#to_enum for the definition of repeat
 *   r = 1..Float::INFINITY
 *   r.repeat(2).first(5) # => [1, 1, 2, 2, 3]
 *   r.repeat(2).class # => Enumerator
 *   r.repeat(2).map{|n| n ** 2}.first(5) # => endless loop!
 *   # works naturally on lazy enumerator:
 *   r.lazy.repeat(2).class # => Enumerator::Lazy
 *   r.lazy.repeat(2).map{|n| n ** 2}.first(5) # => [1, 1, 4, 4, 9]
 */

static VALUE
lazy_to_enum(int argc, VALUE *argv, VALUE self)
{
    VALUE lazy, meth = sym_each;

    if (argc > 0) {
	--argc;
	meth = *argv++;
    }
    lazy = lazy_to_enum_i(self, meth, argc, argv, 0);
    if (rb_block_given_p()) {
	enumerator_ptr(lazy)->size = rb_block_proc();
    }
    return lazy;
}

static VALUE
lazyenum_yield(VALUE proc_entry, struct MEMO *result)
{
    struct proc_entry *entry = proc_entry_ptr(proc_entry);
    return rb_proc_call_with_block(entry->proc, 1, &result->memo_value, Qnil);
}

static VALUE
lazyenum_yield_values(VALUE proc_entry, struct MEMO *result)
{
    struct proc_entry *entry = proc_entry_ptr(proc_entry);
    int argc = 1;
    const VALUE *argv = &result->memo_value;
    if (LAZY_MEMO_PACKED_P(result)) {
	const VALUE args = *argv;
	argc = RARRAY_LENINT(args);
	argv = RARRAY_CONST_PTR(args);
    }
    return rb_proc_call_with_block(entry->proc, argc, argv, Qnil);
}

static struct MEMO *
lazy_map_proc(VALUE proc_entry, struct MEMO *result, VALUE memos, long memo_index)
{
    VALUE value = lazyenum_yield_values(proc_entry, result);
    LAZY_MEMO_SET_VALUE(result, value);
    LAZY_MEMO_RESET_PACKED(result);
    return result;
}

static VALUE
lazy_map_size(VALUE entry, VALUE receiver)
{
    return receiver;
}

static const lazyenum_funcs lazy_map_funcs = {
    lazy_map_proc, lazy_map_size,
};

static VALUE
lazy_map(VALUE obj)
{
    if (!rb_block_given_p()) {
	rb_raise(rb_eArgError, "tried to call lazy map without a block");
    }

    return lazy_add_method(obj, 0, 0, Qnil, Qnil, &lazy_map_funcs);
}

static VALUE
lazy_flat_map_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, yielder))
{
    VALUE arg = rb_enum_values_pack(argc, argv);

    return rb_funcallv(yielder, idLTLT, 1, &arg);
}

static VALUE
lazy_flat_map_each(VALUE obj, VALUE yielder)
{
    rb_block_call(obj, id_each, 0, 0, lazy_flat_map_i, yielder);
    return Qnil;
}

static VALUE
lazy_flat_map_to_ary(VALUE obj, VALUE yielder)
{
    VALUE ary = rb_check_array_type(obj);
    if (NIL_P(ary)) {
	rb_funcall(yielder, idLTLT, 1, obj);
    }
    else {
	long i;
	for (i = 0; i < RARRAY_LEN(ary); i++) {
	    rb_funcall(yielder, idLTLT, 1, RARRAY_AREF(ary, i));
	}
    }
    return Qnil;
}

static VALUE
lazy_flat_map_proc(RB_BLOCK_CALL_FUNC_ARGLIST(val, m))
{
    VALUE result = rb_yield_values2(argc - 1, &argv[1]);
    if (RB_TYPE_P(result, T_ARRAY)) {
	long i;
	for (i = 0; i < RARRAY_LEN(result); i++) {
	    rb_funcall(argv[0], idLTLT, 1, RARRAY_AREF(result, i));
	}
    }
    else {
	if (rb_respond_to(result, id_force) && rb_respond_to(result, id_each)) {
	    lazy_flat_map_each(result, argv[0]);
	}
	else {
	    lazy_flat_map_to_ary(result, argv[0]);
	}
    }
    return Qnil;
}

/*
 *  call-seq:
 *     lazy.collect_concat { |obj| block } -> a_lazy_enumerator
 *     lazy.flat_map       { |obj| block } -> a_lazy_enumerator
 *
 *  Returns a new lazy enumerator with the concatenated results of running
 *  <i>block</i> once for every element in <i>lazy</i>.
 *
 *    ["foo", "bar"].lazy.flat_map {|i| i.each_char.lazy}.force
 *    #=> ["f", "o", "o", "b", "a", "r"]
 *
 *  A value <i>x</i> returned by <i>block</i> is decomposed if either of
 *  the following conditions is true:
 *
 *    a) <i>x</i> responds to both each and force, which means that
 *       <i>x</i> is a lazy enumerator.
 *    b) <i>x</i> is an array or responds to to_ary.
 *
 *  Otherwise, <i>x</i> is contained as-is in the return value.
 *
 *    [{a:1}, {b:2}].lazy.flat_map {|i| i}.force
 *    #=> [{:a=>1}, {:b=>2}]
 */
static VALUE
lazy_flat_map(VALUE obj)
{
    if (!rb_block_given_p()) {
	rb_raise(rb_eArgError, "tried to call lazy flat_map without a block");
    }

    return lazy_set_method(rb_block_call(rb_cLazy, id_new, 1, &obj,
					 lazy_flat_map_proc, 0),
			   Qnil, 0);
}

static struct MEMO *
lazy_select_proc(VALUE proc_entry, struct MEMO *result, VALUE memos, long memo_index)
{
    VALUE chain = lazyenum_yield(proc_entry, result);
    if (!RTEST(chain)) return 0;
    return result;
}

static const lazyenum_funcs lazy_select_funcs = {
    lazy_select_proc, 0,
};

static VALUE
lazy_select(VALUE obj)
{
    if (!rb_block_given_p()) {
	rb_raise(rb_eArgError, "tried to call lazy select without a block");
    }

    return lazy_add_method(obj, 0, 0, Qnil, Qnil, &lazy_select_funcs);
}

static struct MEMO *
lazy_reject_proc(VALUE proc_entry, struct MEMO *result, VALUE memos, long memo_index)
{
    VALUE chain = lazyenum_yield(proc_entry, result);
    if (RTEST(chain)) return 0;
    return result;
}

static const lazyenum_funcs lazy_reject_funcs = {
    lazy_reject_proc, 0,
};

static VALUE
lazy_reject(VALUE obj)
{
    if (!rb_block_given_p()) {
	rb_raise(rb_eArgError, "tried to call lazy reject without a block");
    }

    return lazy_add_method(obj, 0, 0, Qnil, Qnil, &lazy_reject_funcs);
}

static struct MEMO *
lazy_grep_proc(VALUE proc_entry, struct MEMO *result, VALUE memos, long memo_index)
{
    struct proc_entry *entry = proc_entry_ptr(proc_entry);
    VALUE chain = rb_funcall(entry->memo, id_eqq, 1, result->memo_value);
    if (!RTEST(chain)) return 0;
    return result;
}

static struct MEMO *
lazy_grep_iter_proc(VALUE proc_entry, struct MEMO *result, VALUE memos, long memo_index)
{
    struct proc_entry *entry = proc_entry_ptr(proc_entry);
    VALUE value, chain = rb_funcall(entry->memo, id_eqq, 1, result->memo_value);

    if (!RTEST(chain)) return 0;
    value = rb_proc_call_with_block(entry->proc, 1, &(result->memo_value), Qnil);
    LAZY_MEMO_SET_VALUE(result, value);
    LAZY_MEMO_RESET_PACKED(result);

    return result;
}

static const lazyenum_funcs lazy_grep_iter_funcs = {
    lazy_grep_iter_proc, 0,
};

static const lazyenum_funcs lazy_grep_funcs = {
    lazy_grep_proc, 0,
};

static VALUE
lazy_grep(VALUE obj, VALUE pattern)
{
    const lazyenum_funcs *const funcs = rb_block_given_p() ?
	&lazy_grep_iter_funcs : &lazy_grep_funcs;
    return lazy_add_method(obj, 0, 0, pattern, rb_ary_new3(1, pattern), funcs);
}

static struct MEMO *
lazy_grep_v_proc(VALUE proc_entry, struct MEMO *result, VALUE memos, long memo_index)
{
    struct proc_entry *entry = proc_entry_ptr(proc_entry);
    VALUE chain = rb_funcall(entry->memo, id_eqq, 1, result->memo_value);
    if (RTEST(chain)) return 0;
    return result;
}

static struct MEMO *
lazy_grep_v_iter_proc(VALUE proc_entry, struct MEMO *result, VALUE memos, long memo_index)
{
    struct proc_entry *entry = proc_entry_ptr(proc_entry);
    VALUE value, chain = rb_funcall(entry->memo, id_eqq, 1, result->memo_value);

    if (RTEST(chain)) return 0;
    value = rb_proc_call_with_block(entry->proc, 1, &(result->memo_value), Qnil);
    LAZY_MEMO_SET_VALUE(result, value);
    LAZY_MEMO_RESET_PACKED(result);

    return result;
}

static const lazyenum_funcs lazy_grep_v_iter_funcs = {
    lazy_grep_v_iter_proc, 0,
};

static const lazyenum_funcs lazy_grep_v_funcs = {
    lazy_grep_v_proc, 0,
};

static VALUE
lazy_grep_v(VALUE obj, VALUE pattern)
{
    const lazyenum_funcs *const funcs = rb_block_given_p() ?
        &lazy_grep_v_iter_funcs : &lazy_grep_v_funcs;
    return lazy_add_method(obj, 0, 0, pattern, rb_ary_new3(1, pattern), funcs);
}

static VALUE
call_next(VALUE obj)
{
    return rb_funcall(obj, id_next, 0);
}

static VALUE
next_stopped(VALUE obj)
{
    return Qnil;
}

static VALUE
lazy_zip_arrays_func(RB_BLOCK_CALL_FUNC_ARGLIST(val, arrays))
{
    VALUE yielder, ary, memo;
    long i, count;

    yielder = argv[0];
    memo = rb_attr_get(yielder, id_memo);
    count = NIL_P(memo) ? 0 : NUM2LONG(memo);

    ary = rb_ary_new2(RARRAY_LEN(arrays) + 1);
    rb_ary_push(ary, argv[1]);
    for (i = 0; i < RARRAY_LEN(arrays); i++) {
	rb_ary_push(ary, rb_ary_entry(RARRAY_AREF(arrays, i), count));
    }
    rb_funcall(yielder, idLTLT, 1, ary);
    rb_ivar_set(yielder, id_memo, LONG2NUM(++count));
    return Qnil;
}

static VALUE
lazy_zip_func(RB_BLOCK_CALL_FUNC_ARGLIST(val, zip_args))
{
    VALUE yielder, ary, arg, v;
    long i;

    yielder = argv[0];
    arg = rb_attr_get(yielder, id_memo);
    if (NIL_P(arg)) {
	arg = rb_ary_new2(RARRAY_LEN(zip_args));
	for (i = 0; i < RARRAY_LEN(zip_args); i++) {
	    rb_ary_push(arg, rb_funcall(RARRAY_AREF(zip_args, i), id_to_enum, 0));
	}
	rb_ivar_set(yielder, id_memo, arg);
    }

    ary = rb_ary_new2(RARRAY_LEN(arg) + 1);
    v = Qnil;
    if (--argc > 0) {
	++argv;
	v = argc > 1 ? rb_ary_new_from_values(argc, argv) : *argv;
    }
    rb_ary_push(ary, v);
    for (i = 0; i < RARRAY_LEN(arg); i++) {
	v = rb_rescue2(call_next, RARRAY_AREF(arg, i), next_stopped, 0,
		       rb_eStopIteration, (VALUE)0);
	rb_ary_push(ary, v);
    }
    rb_funcall(yielder, idLTLT, 1, ary);
    return Qnil;
}

static VALUE
lazy_zip(int argc, VALUE *argv, VALUE obj)
{
    VALUE ary, v;
    long i;
    rb_block_call_func *func = lazy_zip_arrays_func;

    if (rb_block_given_p()) {
	return rb_call_super(argc, argv);
    }

    ary = rb_ary_new2(argc);
    for (i = 0; i < argc; i++) {
	v = rb_check_array_type(argv[i]);
	if (NIL_P(v)) {
	    for (; i < argc; i++) {
		if (!rb_respond_to(argv[i], id_each)) {
		    rb_raise(rb_eTypeError, "wrong argument type %"PRIsVALUE" (must respond to :each)",
			     rb_obj_class(argv[i]));
		}
	    }
	    ary = rb_ary_new4(argc, argv);
	    func = lazy_zip_func;
	    break;
	}
	rb_ary_push(ary, v);
    }

    return lazy_set_method(rb_block_call(rb_cLazy, id_new, 1, &obj,
					 func, ary),
			   ary, lazy_receiver_size);
}

static struct MEMO *
lazy_take_proc(VALUE proc_entry, struct MEMO *result, VALUE memos, long memo_index)
{
    long remain;
    struct proc_entry *entry = proc_entry_ptr(proc_entry);
    VALUE memo = rb_ary_entry(memos, memo_index);

    if (NIL_P(memo)) {
	memo = entry->memo;
    }

    remain = NUM2LONG(memo);
    if (remain == 0) {
	LAZY_MEMO_SET_BREAK(result);
    }
    else {
	if (--remain == 0) LAZY_MEMO_SET_BREAK(result);
	rb_ary_store(memos, memo_index, LONG2NUM(remain));
    }
    return result;
}

static VALUE
lazy_take_size(VALUE entry, VALUE receiver)
{
    long len = NUM2LONG(RARRAY_AREF(rb_ivar_get(entry, id_arguments), 0));
    if (NIL_P(receiver) || (FIXNUM_P(receiver) && FIX2LONG(receiver) < len))
	return receiver;
    return LONG2NUM(len);
}

static const lazyenum_funcs lazy_take_funcs = {
    lazy_take_proc, lazy_take_size,
};

static VALUE
lazy_take(VALUE obj, VALUE n)
{
    long len = NUM2LONG(n);
    int argc = 0;
    VALUE argv[2];

    if (len < 0) {
	rb_raise(rb_eArgError, "attempt to take negative size");
    }

    if (len == 0) {
       argv[0] = sym_cycle;
       argv[1] = INT2NUM(0);
       argc = 2;
    }

    return lazy_add_method(obj, argc, argv, n, rb_ary_new3(1, n), &lazy_take_funcs);
}

static struct MEMO *
lazy_take_while_proc(VALUE proc_entry, struct MEMO *result, VALUE memos, long memo_index)
{
    VALUE take = lazyenum_yield_values(proc_entry, result);
    if (!RTEST(take)) {
	LAZY_MEMO_SET_BREAK(result);
	return 0;
    }
    return result;
}

static const lazyenum_funcs lazy_take_while_funcs = {
    lazy_take_while_proc, 0,
};

static VALUE
lazy_take_while(VALUE obj)
{
    if (!rb_block_given_p()) {
	rb_raise(rb_eArgError, "tried to call lazy take_while without a block");
    }

    return lazy_add_method(obj, 0, 0, Qnil, Qnil, &lazy_take_while_funcs);
}

static VALUE
lazy_drop_size(VALUE proc_entry, VALUE receiver)
{
    long len = NUM2LONG(RARRAY_AREF(rb_ivar_get(proc_entry, id_arguments), 0));
    if (NIL_P(receiver))
	return receiver;
    if (FIXNUM_P(receiver)) {
	len = FIX2LONG(receiver) - len;
	return LONG2FIX(len < 0 ? 0 : len);
    }
    return rb_funcall(receiver, '-', 1, LONG2NUM(len));
}

static struct MEMO *
lazy_drop_proc(VALUE proc_entry, struct MEMO *result, VALUE memos, long memo_index)
{
    long remain;
    struct proc_entry *entry = proc_entry_ptr(proc_entry);
    VALUE memo = rb_ary_entry(memos, memo_index);

    if (NIL_P(memo)) {
	memo = entry->memo;
    }
    remain = NUM2LONG(memo);
    if (remain > 0) {
	--remain;
	rb_ary_store(memos, memo_index, LONG2NUM(remain));
	return 0;
    }

    return result;
}

static const lazyenum_funcs lazy_drop_funcs = {
    lazy_drop_proc, lazy_drop_size,
};

static VALUE
lazy_drop(VALUE obj, VALUE n)
{
    long len = NUM2LONG(n);
    VALUE argv[2];
    argv[0] = sym_each;
    argv[1] = n;

    if (len < 0) {
	rb_raise(rb_eArgError, "attempt to drop negative size");
    }

    return lazy_add_method(obj, 2, argv, n, rb_ary_new3(1, n), &lazy_drop_funcs);
}

static struct MEMO *
lazy_drop_while_proc(VALUE proc_entry, struct MEMO* result, VALUE memos, long memo_index)
{
    struct proc_entry *entry = proc_entry_ptr(proc_entry);
    VALUE memo = rb_ary_entry(memos, memo_index);

    if (NIL_P(memo)) {
	memo = entry->memo;
    }

    if (!RTEST(memo)) {
	VALUE drop = lazyenum_yield_values(proc_entry, result);
	if (RTEST(drop)) return 0;
	rb_ary_store(memos, memo_index, Qtrue);
    }
    return result;
}

static const lazyenum_funcs lazy_drop_while_funcs = {
    lazy_drop_while_proc, 0,
};

static VALUE
lazy_drop_while(VALUE obj)
{
    if (!rb_block_given_p()) {
	rb_raise(rb_eArgError, "tried to call lazy drop_while without a block");
    }

    return lazy_add_method(obj, 0, 0, Qfalse, Qnil, &lazy_drop_while_funcs);
}

static int
lazy_uniq_check(VALUE chain, VALUE memos, long memo_index)
{
    VALUE hash = rb_ary_entry(memos, memo_index);

    if (NIL_P(hash)) {
        hash = rb_obj_hide(rb_hash_new());
        rb_ary_store(memos, memo_index, hash);
    }

    return rb_hash_add_new_element(hash, chain, Qfalse);
}

static struct MEMO *
lazy_uniq_proc(VALUE proc_entry, struct MEMO *result, VALUE memos, long memo_index)
{
    if (lazy_uniq_check(result->memo_value, memos, memo_index)) return 0;
    return result;
}

static struct MEMO *
lazy_uniq_iter_proc(VALUE proc_entry, struct MEMO *result, VALUE memos, long memo_index)
{
    VALUE chain = lazyenum_yield(proc_entry, result);

    if (lazy_uniq_check(chain, memos, memo_index)) return 0;
    return result;
}

static const lazyenum_funcs lazy_uniq_iter_funcs = {
    lazy_uniq_iter_proc, 0,
};

static const lazyenum_funcs lazy_uniq_funcs = {
    lazy_uniq_proc, 0,
};

static VALUE
lazy_uniq(VALUE obj)
{
    const lazyenum_funcs *const funcs =
        rb_block_given_p() ? &lazy_uniq_iter_funcs : &lazy_uniq_funcs;
    return lazy_add_method(obj, 0, 0, Qnil, Qnil, funcs);
}

static VALUE
lazy_super(int argc, VALUE *argv, VALUE lazy)
{
    return enumerable_lazy(rb_call_super(argc, argv));
}

static VALUE
lazy_lazy(VALUE obj)
{
    return obj;
}

/*
 * Document-class: StopIteration
 *
 * Raised to stop the iteration, in particular by Enumerator#next. It is
 * rescued by Kernel#loop.
 *
 *   loop do
 *     puts "Hello"
 *     raise StopIteration
 *     puts "World"
 *   end
 *   puts "Done!"
 *
 * <em>produces:</em>
 *
 *   Hello
 *   Done!
 */

/*
 * call-seq:
 *   result       -> value
 *
 * Returns the return value of the iterator.
 *
 *   o = Object.new
 *   def o.each
 *     yield 1
 *     yield 2
 *     yield 3
 *     100
 *   end
 *
 *   e = o.to_enum
 *
 *   puts e.next                   #=> 1
 *   puts e.next                   #=> 2
 *   puts e.next                   #=> 3
 *
 *   begin
 *     e.next
 *   rescue StopIteration => ex
 *     puts ex.result              #=> 100
 *   end
 *
 */

static VALUE
stop_result(VALUE self)
{
    return rb_attr_get(self, id_result);
}

/*
 * Document-class: Enumerator::ArithmeticSequence
 *
 * Enumerator::ArithmeticSequence is a subclass of Enumerator,
 * that is a representation of sequences of numbers with common difference.
 * The instances of this class can be generated by Range#step and Numeric#step
 * methods.
 */

VALUE
rb_arith_seq_new(VALUE obj, VALUE meth, int argc, VALUE const *argv,
                 rb_enumerator_size_func *size_fn,
                 VALUE beg, VALUE end, VALUE step, int excl)
{
    VALUE aseq = enumerator_init(enumerator_allocate(rb_cArithSeq),
                                 obj, meth, argc, argv, size_fn, Qnil);
    rb_ivar_set(aseq, id_begin, beg);
    rb_ivar_set(aseq, id_end, end);
    rb_ivar_set(aseq, id_step, step);
    rb_ivar_set(aseq, id_exclude_end, excl ? Qtrue : Qfalse);
    return aseq;
}

/*
 * call-seq: aseq.begin -> num
 *
 * Returns the number that defines the first element of this arithmetic
 * sequence.
 */
static inline VALUE
arith_seq_begin(VALUE self)
{
    return rb_ivar_get(self, id_begin);
}

/*
 * call-seq: aseq.end -> num or nil
 *
 * Returns the number that defines the end of this arithmetic sequence.
 */
static inline VALUE
arith_seq_end(VALUE self)
{
    return rb_ivar_get(self, id_end);
}

/*
 * call-seq: aseq.step -> num
 *
 * Returns the number that defines the common difference between
 * two adjacent elements in this arithmetic sequence.
 */
static inline VALUE
arith_seq_step(VALUE self)
{
    return rb_ivar_get(self, id_step);
}

/*
 * call-seq: aseq.exclude_end? -> true or false
 *
 * Returns <code>true</code> if this arithmetic sequence excludes its end value.
 */
static inline VALUE
arith_seq_exclude_end(VALUE self)
{
    return rb_ivar_get(self, id_exclude_end);
}

static inline int
arith_seq_exclude_end_p(VALUE self)
{
    return RTEST(arith_seq_exclude_end(self));
}

/*
 * call-seq:
 *   aseq.first -> num or nil
 *   aseq.first(n) -> an_array
 *
 * Returns the first number in this arithmetic sequence,
 * or an array of the first +n+ elements.
 */
static VALUE
arith_seq_first(int argc, VALUE *argv, VALUE self)
{
    VALUE b, e, s, len_1;

    b = arith_seq_begin(self);
    e = arith_seq_end(self);
    s = arith_seq_step(self);

    if (!NIL_P(e)) {
        len_1 = rb_int_idiv(rb_int_minus(e, b), s);
        if (rb_num_negative_int_p(len_1)) {
            if (argc == 0) {
                return Qnil;
            }
            return rb_ary_new_capa(0);
        }
    }

    if (argc == 0) {
        return b;
    }

    /* TODO: optimization */

    return rb_call_super(argc, argv);
}

/*
 * call-seq:
 *   aseq.last    -> num or nil
 *   aseq.last(n) -> an_array
 *
 * Returns the last number in this arithmetic sequence,
 * or an array of the last +n+ elements.
 */
static VALUE
arith_seq_last(int argc, VALUE *argv, VALUE self)
{
    VALUE b, e, s, len_1, len, last, nv, ary;
    int last_is_adjusted;
    long n;

    e = arith_seq_end(self);
    if (NIL_P(e)) {
        rb_raise(rb_eRangeError,
                 "cannot get the last element of endless arithmetic sequence");
    }

    b = arith_seq_begin(self);
    s = arith_seq_step(self);

    len_1 = rb_int_idiv(rb_int_minus(e, b), s);
    if (rb_num_negative_int_p(len_1)) {
        if (argc == 0) {
            return Qnil;
        }
        return rb_ary_new_capa(0);
    }

    last = rb_int_plus(b, rb_int_mul(s, len_1));
    if ((last_is_adjusted = arith_seq_exclude_end_p(self) && rb_equal(last, e))) {
        last = rb_int_minus(last, s);
    }

    if (argc == 0) {
        return last;
    }

    if (last_is_adjusted) {
        len = len_1;
    }
    else {
        len = rb_int_plus(len_1, INT2FIX(1));
    }

    rb_scan_args(argc, argv, "1", &nv);
    if (!RB_INTEGER_TYPE_P(nv)) {
        nv = rb_to_int(nv);
    }
    if (RTEST(rb_int_gt(nv, len))) {
        nv = len;
    }
    n = NUM2LONG(nv);
    if (n < 0) {
        rb_raise(rb_eArgError, "negative array size");
    }

    ary = rb_ary_new_capa(n);
    b = rb_int_minus(last, rb_int_mul(s, nv));
    while (n) {
        b = rb_int_plus(b, s);
        rb_ary_push(ary, b);
        --n;
    }

    return ary;
}

/*
 * call-seq:
 *   aseq.inspect -> string
 *
 * Convert this arithmetic sequence to a printable form.
 */
static VALUE
arith_seq_inspect(VALUE self)
{
    struct enumerator *e;
    VALUE eobj, str, eargs;
    int range_p;

    TypedData_Get_Struct(self, struct enumerator, &enumerator_data_type, e);

    eobj = rb_attr_get(self, id_receiver);
    if (NIL_P(eobj)) {
        eobj = e->obj;
    }

    range_p = RTEST(rb_obj_is_kind_of(eobj, rb_cRange));
    str = rb_sprintf("(%s%"PRIsVALUE"%s.", range_p ? "(" : "", eobj, range_p ? ")" : "");

    rb_str_buf_append(str, rb_id2str(e->meth));

    eargs = rb_attr_get(eobj, id_arguments);
    if (NIL_P(eargs)) {
        eargs = e->args;
    }
    if (eargs != Qfalse) {
        long argc = RARRAY_LEN(eargs);
        const VALUE *argv = RARRAY_CONST_PTR(eargs); /* WB: no new reference */

        if (argc > 0) {
            VALUE kwds = Qnil;

            rb_str_buf_cat2(str, "(");

            if (RB_TYPE_P(argv[argc-1], T_HASH)) {
                int all_key = TRUE;
                rb_hash_foreach(argv[argc-1], key_symbol_p, (VALUE)&all_key);
                if (all_key) kwds = argv[--argc];
            }

            while (argc--) {
                VALUE arg = *argv++;

                rb_str_append(str, rb_inspect(arg));
                rb_str_buf_cat2(str, ", ");
                OBJ_INFECT(str, arg);
            }
            if (!NIL_P(kwds)) {
                rb_hash_foreach(kwds, kwd_append, str);
            }
            rb_str_set_len(str, RSTRING_LEN(str)-2); /* drop the last ", " */
            rb_str_buf_cat2(str, ")");
        }
    }

    rb_str_buf_cat2(str, ")");

    return str;
}

/*
 * call-seq:
 *   aseq == obj  -> true or false
 *
 * Returns <code>true</code> only if +obj+ is an Enumerator::ArithmeticSequence,
 * has equivalent begin, end, step, and exclude_end? settings.
 */
static VALUE
arith_seq_eq(VALUE self, VALUE other)
{
    if (!RTEST(rb_obj_is_kind_of(other, rb_cArithSeq))) {
        return Qfalse;
    }

    if (!rb_equal(arith_seq_begin(self), arith_seq_begin(other))) {
        return Qfalse;
    }

    if (!rb_equal(arith_seq_end(self), arith_seq_end(other))) {
        return Qfalse;
    }

    if (!rb_equal(arith_seq_step(self), arith_seq_step(other))) {
        return Qfalse;
    }

    if (arith_seq_exclude_end_p(self) != arith_seq_exclude_end_p(other)) {
        return Qfalse;
    }

    return Qtrue;
}

/*
 * call-seq:
 *   aseq.hash  -> integer
 *
 * Compute a hash-value for this arithmetic sequence.
 * Two arithmetic sequences with same begin, end, step, and exclude_end?
 * values will generate the same hash-value.
 *
 * See also Object#hash.
 */
static VALUE
arith_seq_hash(VALUE self)
{
    st_index_t hash;
    VALUE v;

    hash = rb_hash_start(arith_seq_exclude_end_p(self));
    v = rb_hash(arith_seq_begin(self));
    hash = rb_hash_uint(hash, NUM2LONG(v));
    v = rb_hash(arith_seq_end(self));
    hash = rb_hash_uint(hash, NUM2LONG(v));
    v = rb_hash(arith_seq_step(self));
    hash = rb_hash_uint(hash, NUM2LONG(v));
    hash = rb_hash_end(hash);

    return LONG2FIX(hash);
}

struct arith_seq_gen {
    VALUE current;
    VALUE end;
    VALUE step;
    int excl;
};

/*
 * call-seq:
 *   aseq.each {|i| block } -> aseq
 *   aseq.each              -> aseq
 */
static VALUE
arith_seq_each(VALUE self)
{
    VALUE c, e, s, len_1, last;
    int x;

    if (!rb_block_given_p()) return self;

    c = arith_seq_begin(self);
    e = arith_seq_end(self);
    s = arith_seq_step(self);
    x = arith_seq_exclude_end_p(self);

    if (!RB_TYPE_P(s, T_COMPLEX) && ruby_float_step(c, e, s, x, TRUE)) {
        return self;
    }

    if (NIL_P(e)) {
        while (1) {
            rb_yield(c);
            c = rb_int_plus(c, s);
        }

        return self;
    }

    if (rb_equal(s, INT2FIX(0))) {
        while (1) {
            rb_yield(c);
        }

        return self;
    }

    len_1 = rb_int_idiv(rb_int_minus(e, c), s);
    last = rb_int_plus(c, rb_int_mul(s, len_1));
    if (x && rb_equal(last, e)) {
        last = rb_int_minus(last, s);
    }

    if (rb_num_negative_int_p(s)) {
        while (RTEST(rb_int_ge(c, last))) {
            rb_yield(c);
            c = rb_int_plus(c, s);
        }
    }
    else {
        while (RTEST(rb_int_ge(last, c))) {
            rb_yield(c);
            c = rb_int_plus(c, s);
        }
    }

    return self;
}

static double
arith_seq_float_step_size(double beg, double end, double step, int excl)
{
    double const epsilon = DBL_EPSILON;
    double n = (end - beg) / step;
    double err = (fabs(beg) + fabs(end) + fabs(end - beg)) / fabs(step) * epsilon;

    if (isinf(step)) {
        return step > 0 ? beg <= end : beg >= end;
    }
    if (step == 0) {
        return HUGE_VAL;
    }
    if (err > 0.5) err = 0.5;
    if (excl) {
        if (n <= 0) return 0;
        if (n < 1)
            n = 0;
        else
            n = floor(n - err);
    }
    else {
        if (n < 0) return 0;
        n = floor(n + err);
    }
    return n + 1;
}

/*
 * call-seq:
 *   aseq.size -> num or nil
 *
 * Returns the number of elements in this arithmetic sequence if it is a finite
 * sequence.  Otherwise, returns <code>nil</code>.
 */
static VALUE
arith_seq_size(VALUE self)
{
    VALUE b, e, s, len_1, len, last;
    int x;

    b = arith_seq_begin(self);
    e = arith_seq_end(self);
    s = arith_seq_step(self);
    x = arith_seq_exclude_end_p(self);

    if (RB_FLOAT_TYPE_P(b) || RB_FLOAT_TYPE_P(e) || RB_FLOAT_TYPE_P(s)) {
        double ee, n;

        if (NIL_P(e)) {
            if (rb_num_negative_int_p(s)) {
                ee = -HUGE_VAL;
            }
            else {
                ee = HUGE_VAL;
            }
        }
        else {
            ee = NUM2DBL(e);
        }

        n = arith_seq_float_step_size(NUM2DBL(b), ee, NUM2DBL(s), x);
        if (isinf(n)) return DBL2NUM(n);
        if (POSFIXABLE(n)) return LONG2FIX(n);
        return rb_dbl2big(n);
    }

    if (NIL_P(e)) {
        return DBL2NUM(HUGE_VAL);
    }

    if (!rb_obj_is_kind_of(s, rb_cNumeric)) {
        s = rb_to_int(s);
    }

    if (rb_equal(s, INT2FIX(0))) {
        return DBL2NUM(HUGE_VAL);
    }

    len_1 = rb_int_idiv(rb_int_minus(e, b), s);
    if (rb_num_negative_int_p(len_1)) {
        return INT2FIX(0);
    }

    last = rb_int_plus(b, rb_int_mul(s, len_1));
    if (x && rb_equal(last, e)) {
        len = len_1;
    }
    else {
        len = rb_int_plus(len_1, INT2FIX(1));
    }

    return len;
}

void
InitVM_Enumerator(void)
{
    rb_define_method(rb_mKernel, "to_enum", obj_to_enum, -1);
    rb_define_method(rb_mKernel, "enum_for", obj_to_enum, -1);

    rb_cEnumerator = rb_define_class("Enumerator", rb_cObject);
    rb_include_module(rb_cEnumerator, rb_mEnumerable);

    rb_define_alloc_func(rb_cEnumerator, enumerator_allocate);
    rb_define_method(rb_cEnumerator, "initialize", enumerator_initialize, -1);
    rb_define_method(rb_cEnumerator, "initialize_copy", enumerator_init_copy, 1);
    rb_define_method(rb_cEnumerator, "each", enumerator_each, -1);
    rb_define_method(rb_cEnumerator, "each_with_index", enumerator_each_with_index, 0);
    rb_define_method(rb_cEnumerator, "each_with_object", enumerator_with_object, 1);
    rb_define_method(rb_cEnumerator, "with_index", enumerator_with_index, -1);
    rb_define_method(rb_cEnumerator, "with_object", enumerator_with_object, 1);
    rb_define_method(rb_cEnumerator, "next_values", enumerator_next_values, 0);
    rb_define_method(rb_cEnumerator, "peek_values", enumerator_peek_values_m, 0);
    rb_define_method(rb_cEnumerator, "next", enumerator_next, 0);
    rb_define_method(rb_cEnumerator, "peek", enumerator_peek, 0);
    rb_define_method(rb_cEnumerator, "feed", enumerator_feed, 1);
    rb_define_method(rb_cEnumerator, "rewind", enumerator_rewind, 0);
    rb_define_method(rb_cEnumerator, "inspect", enumerator_inspect, 0);
    rb_define_method(rb_cEnumerator, "size", enumerator_size, 0);

    /* Lazy */
    rb_cLazy = rb_define_class_under(rb_cEnumerator, "Lazy", rb_cEnumerator);
    rb_define_method(rb_mEnumerable, "lazy", enumerable_lazy, 0);
    rb_define_method(rb_cLazy, "initialize", lazy_initialize, -1);
    rb_define_method(rb_cLazy, "to_enum", lazy_to_enum, -1);
    rb_define_method(rb_cLazy, "enum_for", lazy_to_enum, -1);
    rb_define_method(rb_cLazy, "map", lazy_map, 0);
    rb_define_method(rb_cLazy, "collect", lazy_map, 0);
    rb_define_method(rb_cLazy, "flat_map", lazy_flat_map, 0);
    rb_define_method(rb_cLazy, "collect_concat", lazy_flat_map, 0);
    rb_define_method(rb_cLazy, "select", lazy_select, 0);
    rb_define_method(rb_cLazy, "find_all", lazy_select, 0);
    rb_define_method(rb_cLazy, "filter", lazy_select, 0);
    rb_define_method(rb_cLazy, "reject", lazy_reject, 0);
    rb_define_method(rb_cLazy, "grep", lazy_grep, 1);
    rb_define_method(rb_cLazy, "grep_v", lazy_grep_v, 1);
    rb_define_method(rb_cLazy, "zip", lazy_zip, -1);
    rb_define_method(rb_cLazy, "take", lazy_take, 1);
    rb_define_method(rb_cLazy, "take_while", lazy_take_while, 0);
    rb_define_method(rb_cLazy, "drop", lazy_drop, 1);
    rb_define_method(rb_cLazy, "drop_while", lazy_drop_while, 0);
    rb_define_method(rb_cLazy, "lazy", lazy_lazy, 0);
    rb_define_method(rb_cLazy, "chunk", lazy_super, -1);
    rb_define_method(rb_cLazy, "slice_before", lazy_super, -1);
    rb_define_method(rb_cLazy, "slice_after", lazy_super, -1);
    rb_define_method(rb_cLazy, "slice_when", lazy_super, -1);
    rb_define_method(rb_cLazy, "chunk_while", lazy_super, -1);
    rb_define_method(rb_cLazy, "uniq", lazy_uniq, 0);

#if 0 /* for RDoc */
    rb_define_method(rb_cLazy, "to_a", lazy_to_a, 0);
#endif
    rb_define_alias(rb_cLazy, "force", "to_a");

    rb_eStopIteration = rb_define_class("StopIteration", rb_eIndexError);
    rb_define_method(rb_eStopIteration, "result", stop_result, 0);

    /* Generator */
    rb_cGenerator = rb_define_class_under(rb_cEnumerator, "Generator", rb_cObject);
    rb_include_module(rb_cGenerator, rb_mEnumerable);
    rb_define_alloc_func(rb_cGenerator, generator_allocate);
    rb_define_method(rb_cGenerator, "initialize", generator_initialize, -1);
    rb_define_method(rb_cGenerator, "initialize_copy", generator_init_copy, 1);
    rb_define_method(rb_cGenerator, "each", generator_each, -1);

    /* Yielder */
    rb_cYielder = rb_define_class_under(rb_cEnumerator, "Yielder", rb_cObject);
    rb_define_alloc_func(rb_cYielder, yielder_allocate);
    rb_define_method(rb_cYielder, "initialize", yielder_initialize, 0);
    rb_define_method(rb_cYielder, "yield", yielder_yield, -2);
    rb_define_method(rb_cYielder, "<<", yielder_yield_push, 1);

    /* ArithmeticSequence */
    rb_cArithSeq = rb_define_class_under(rb_cEnumerator, "ArithmeticSequence", rb_cEnumerator);
    rb_undef_alloc_func(rb_cArithSeq);
    rb_undef_method(CLASS_OF(rb_cArithSeq), "new");
    rb_define_method(rb_cArithSeq, "begin", arith_seq_begin, 0);
    rb_define_method(rb_cArithSeq, "end", arith_seq_end, 0);
    rb_define_method(rb_cArithSeq, "exclude_end?", arith_seq_exclude_end, 0);
    rb_define_method(rb_cArithSeq, "step", arith_seq_step, 0);
    rb_define_method(rb_cArithSeq, "first", arith_seq_first, -1);
    rb_define_method(rb_cArithSeq, "last", arith_seq_last, -1);
    rb_define_method(rb_cArithSeq, "inspect", arith_seq_inspect, 0);
    rb_define_method(rb_cArithSeq, "==", arith_seq_eq, 1);
    rb_define_method(rb_cArithSeq, "===", arith_seq_eq, 1);
    rb_define_method(rb_cArithSeq, "eql?", arith_seq_eq, 1);
    rb_define_method(rb_cArithSeq, "hash", arith_seq_hash, 0);
    rb_define_method(rb_cArithSeq, "each", arith_seq_each, 0);
    rb_define_method(rb_cArithSeq, "size", arith_seq_size, 0);

    rb_provide("enumerator.so");	/* for backward compatibility */
}

#undef rb_intern
void
Init_Enumerator(void)
{
    id_rewind = rb_intern("rewind");
    id_new = rb_intern("new");
    id_next = rb_intern("next");
    id_result = rb_intern("result");
    id_receiver = rb_intern("receiver");
    id_arguments = rb_intern("arguments");
    id_memo = rb_intern("memo");
    id_method = rb_intern("method");
    id_force = rb_intern("force");
    id_to_enum = rb_intern("to_enum");
    id_begin = rb_intern("begin");
    id_end = rb_intern("end");
    id_step = rb_intern("step");
    id_exclude_end = rb_intern("exclude_end");
    sym_each = ID2SYM(id_each);
    sym_cycle = ID2SYM(rb_intern("cycle"));

    InitVM(Enumerator);
}