* array.c (rb_ary_permutation): Support for Array#permutation.size

[Feature #6636]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37501 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

array.c

@@ -4323,6 +4323,29 @@ permute0(long n, long r, long *p, long index, char *used, VALUE values)
     }
 }
 
+/*
+ * Returns the product of from, from-1, ..., from - how_many + 1.
+ * http://en.wikipedia.org/wiki/Pochhammer_symbol
+ */
+static VALUE
+descending_factorial(long from, long how_many)
+{
+    VALUE cnt = LONG2FIX(how_many >= 0);
+    while(how_many-- > 0) {
+        cnt = rb_funcall(cnt, '*', 1, LONG2FIX(from--));
+    }
+    return cnt;
+}
+
+static VALUE
+rb_ary_permutation_size(VALUE ary, VALUE args)
+{
+    long n = RARRAY_LEN(ary);
+    long k = (args && (RARRAY_LEN(args) > 0)) ? NUM2LONG(RARRAY_PTR(args)[0]) : n;
+
+    return descending_factorial(n, k);
+}
+
 /*
  *  call-seq:
  *     ary.permutation { |p| block }          -> ary
@@ -4358,7 +4381,7 @@ rb_ary_permutation(int argc, VALUE *argv, VALUE ary)
     long r, n, i;
 
     n = RARRAY_LEN(ary);                  /* Array length */
-    RETURN_ENUMERATOR(ary, argc, argv);   /* Return Enumerator if no block */
+    RETURN_SIZED_ENUMERATOR(ary, argc, argv, rb_ary_permutation_size);   /* Return enumerator if no block */
     rb_scan_args(argc, argv, "01", &num);
     r = NIL_P(num) ? n : NUM2LONG(num);   /* Permutation size from argument */
 

test/ruby/test_enumerator.rb

@@ -429,5 +429,18 @@ class TestEnumerator < Test::Unit::TestCase
     end
   end
 
+  def check_consistency_for_combinatorics(method)
+    [ [], [:a, :b, :c, :d, :e] ].product([-2, 0, 2, 5, 6]) do |array, arg|
+      assert_equal array.send(method, arg).to_a.size, array.send(method, arg).size,
+        "inconsistent size for #{array}.#{method}(#{arg})"
+    end
+  end
+
+  def test_size_for_array_combinatorics
+    check_consistency_for_combinatorics(:permutation)
+    assert_equal 24, [0, 1, 2, 4].permutation.size
+    assert_equal 2933197128679486453788761052665610240000000,
+      (1..42).to_a.permutation(30).size # 1.upto(42).inject(:*) / 1.upto(12).inject(:*)
+  end
 end