gecko-dev/xpcom/ds/tools/perfecthash.py

# perfecthash.py - Helper for generating perfect hash functions
#
# This Source Code Form is subject to the terms of the Mozilla Public
# License, v. 2.0. If a copy of the MPL was not distributed with this
# file, You can obtain one at http://mozilla.org/MPL/2.0/.

"""
A perfect hash function (PHF) is a function which maps distinct elements from a
source set to a sequence of integers with no collisions.

PHFs generated by this module use a 32-bit FNV hash to index an intermediate
table. The value from that table is then used to run a second 32-bit FNV hash to
get a final index.

The algorithm works by starting with the largest set of conflicts, and testing
intermediate table values until no conflicts are generated, allowing efficient
index lookup at runtime. (see also: http://stevehanov.ca/blog/index.php?id=119)

NOTE: This perfect hash generator was designed to be simple, easy to follow, and
maintainable, rather than being as optimized as possible, due to the relatively
small dataset it was designed for. In the future we may want to optimize further.
"""

from collections import namedtuple
import textwrap


class PerfectHash(object):
    """PerfectHash objects represent a computed perfect hash function, which
    can be generated at compile time to provide highly efficient and compact
    static HashTables.

    Consumers must provide an intermediate table size to store the generated
    hash function. Larger tables will generate more quickly, while smaller
    tables will consume less space on disk."""

    # 32-bit FNV offset basis and prime value.
    # NOTE: Must match values in |PerfectHash.h|
    FNV_OFFSET_BASIS = 0x811C9DC5
    FNV_PRIME = 16777619
    U32_MAX = 0xffffffff

    # Bucket of entries which map into a given intermediate index.
    Bucket = namedtuple('Bucket', 'index entries')

    def __init__(self, entries, size, validate=True, key=lambda e: e[0]):
        """Create a new PerfectHash

        @param entries  set of entries to generate a PHF for
        @param size     size of the PHF intermediate table
        @param validate test the generated hash function after generation
        @param key      function to get 'memoryview'-compatible key for an
                        entry. defaults to extracting the first element of an
                        entry 2-tuple.
        """

        assert 0 < len(entries) < self.U32_MAX, "bad # of entries!"
        self._key = key

        # Allocate the intermediate table and keys.
        self.table = [0] * size
        self.entries = [None] * len(entries)

        # A set of Bucket values. Each bucket contains the index into the
        # intermediate table, and a list of Entries which map into that bucket.
        buckets = [self.Bucket(idx, []) for idx in range(size)]

        # Determine which input strings map to which buckets in the table.
        for entry in entries:
            assert entry is not None, "None entry in entries"
            assert self.key(entry).itemsize == 1, "non-byte key elements"
            buckets[self._hash(self.key(entry)) % size].entries.append(entry)

        # Sort the buckets such that the largest one comes first.
        buckets.sort(key=lambda b: len(b.entries), reverse=True)

        for bucket in buckets:
            # Once we've reached an empty bucket, we're done.
            if len(bucket.entries) == 0:
                break

            # Try values for the basis until we find one with no conflicts.
            idx = 0
            basis = 1
            slots = []
            while idx < len(bucket.entries):
                slot = self._hash(self.key(bucket.entries[idx]), basis) % len(self.entries)
                if self.entries[slot] is not None or slot in slots:
                    # There was a conflict, try the next basis.
                    basis += 1
                    idx = 0
                    del slots[:]
                    assert basis < self.U32_MAX, "table too small"
                else:
                    slots.append(slot)
                    idx += 1

            assert basis < self.U32_MAX

            # We've found a basis which doesn't conflict
            self.table[bucket.index] = basis
            for slot, entry in zip(slots, bucket.entries):
                self.entries[slot] = entry

        # Validate that looking up each key succeeds
        if validate:
            for entry in entries:
                assert self.get_entry(self.key(entry)), "get_entry(%s)" % repr(entry)

    @classmethod
    def _hash(cls, key, basis=FNV_OFFSET_BASIS):
        """A basic FNV-based hash function. key is the memoryview to hash.
        32-bit FNV is used for indexing into the first table, and the value
        stored in that table is used as the offset basis for indexing into the
        values table."""
        for byte in memoryview(key):
            basis ^= ord(byte)      # xor-in the byte
            basis *= cls.FNV_PRIME  # Multiply by the FNV prime
            basis &= cls.U32_MAX    # clamp to 32-bits
        return basis

    def key(self, entry):
        return memoryview(self._key(entry))

    def get_raw_index(self, key):
        """Determine the index in self.entries without validating"""
        mid = self.table[self._hash(key) % len(self.table)]
        return self._hash(key, mid) % len(self.entries)

    def get_index(self, key):
        """Given a key, determine the index in self.entries"""
        idx = self.get_raw_index(key)
        if memoryview(key) != self.key(self.entries[idx]):
            return None
        return idx

    def get_entry(self, key):
        """Given a key, get the corresponding entry"""
        idx = self.get_index(key)
        if idx is None:
            return None
        return self.entries[idx]

    @staticmethod
    def _indent(text, amount=1):
        return text.replace('\n', '\n' + ('  ' * amount))

    def codegen(self, name, entry_type):
        """Create a helper for codegening PHF logic"""
        return CGHelper(self, name, entry_type)

    def cxx_codegen(self, name, entry_type, lower_entry, entries_name=None,
                    return_type=None, return_entry='return entry;',
                    key_type='const char*', key_bytes='aKey',
                    key_length='strlen(aKey)'):
        """Generate complete C++ code for a get_entry-style method.

        @param name         Name for the entry getter function.
        @param entry_type   C++ type of each entry in static memory.
        @param lower_entry  Function called with each entry to convert it to a
                            C++ literal of type 'entry_type'.

        # Optional Keyword Parameters:
        @param entries_name Name for the generated entry table.

        @param return_type  C++ return type, default: 'const entry_type&'
        @param return_entry Override the default behaviour for returning the
                            found entry. 'entry' is a reference to the found
                            entry, and 'aKey' is the lookup key. 'return_entry'
                            can be used for additional checks, e.g. for keys
                            not in the table.

        @param key_type     C++ key type, default: 'const char*'
        @param key_bytes    'const char*' expression to get bytes for 'aKey'
        @param key_length   'size_t' expression to get length of 'aKey'"""

        if entries_name is None:
            entries_name = 's%sEntries' % name

        cg = self.codegen(entries_name, entry_type)
        entries = cg.gen_entries(lower_entry)
        getter = cg.gen_getter(name, return_type, return_entry,
                               key_type, key_bytes, key_length)

        return "%s\n\n%s" % (entries, getter)


class CGHelper(object):
    """Helper object for generating C++ code for a PerfectHash.
    Created using PerfectHash.codegen()."""

    def __init__(self, phf, entries_name, entry_type):
        self.phf = phf
        self.entries_name = entries_name
        self.entry_type = entry_type

    @staticmethod
    def _indent(text, amount=1):
        return text.replace('\n', '\n' + ('  ' * amount))

    def basis_ty(self):
        """Determine how big of an integer is needed for bases table."""
        max_base = max(self.phf.table)
        if max_base <= 0xff:
            return 'uint8_t'
        elif max_base <= 0xffff:
            return 'uint16_t'
        return 'uint32_t'

    def basis_table(self, name='BASES'):
        """Generate code for a static basis table"""
        bases = ''
        for idx, base in enumerate(self.phf.table):
            if idx and idx % 16 == 0:  # 16 bases per line
                bases += '\n  '
            bases += '%4d,' % base
        return textwrap.dedent("""\
            static const %s %s[] = {
              %s
            };
            """) % (self.basis_ty(), name, bases)

    def gen_entries(self, lower_entry):
        """Generate code for an entries table"""
        entries = self._indent(',\n'.join(lower_entry(entry).rstrip()
                                          for entry in self.phf.entries))
        return textwrap.dedent("""\
            const %s %s[] = {
              %s
            };
            """) % (self.entry_type, self.entries_name, entries)

    def gen_getter(self, name, return_type=None, return_entry='return entry;',
                   key_type='const char*', key_bytes='aKey',
                   key_length='strlen(aKey)'):
        """Generate the code for a C++ getter.

        @param name         Name for the entry getter function.
        @param return_type  C++ return type, default: 'const entry_type&'
        @param return_entry Override the default behaviour for returning the
                            found entry. 'entry' is a reference to the found
                            entry, and 'aKey' is the lookup key. 'return_entry'
                            can be used for additional checks, e.g. for keys
                            not in the table.

        @param key_type     C++ key type, default: 'const char*'
        @param key_bytes    'const char*' expression to get bytes for 'aKey'
        @param key_length   'size_t' expression to get length of 'aKey'"""

        if return_type is None:
            return_type = 'const %s&' % self.entry_type

        return textwrap.dedent("""
            %(return_type)s
            %(name)s(%(key_type)s aKey)
            {
              %(basis_table)s

              const char* bytes = %(key_bytes)s;
              size_t length = %(key_length)s;
              auto& entry = mozilla::perfecthash::Lookup(bytes, length, BASES,
                                                         %(entries_name)s);
              %(return_entry)s
            }
            """) % {
                'name': name,
                'basis_table': self._indent(self.basis_table()),
                'entries_name': self.entries_name,

                'return_type': return_type,
                'return_entry': self._indent(return_entry),

                'key_type': key_type,
                'key_bytes': key_bytes,
                'key_length': key_length,
            }

    def gen_jslinearstr_getter(self, name, return_type=None,
                               return_entry='return entry;'):
        """Generate code for a specialized getter taking JSLinearStrings.
        This getter avoids copying the JS string, but only supports ASCII keys.

        @param name         Name for the entry getter function.
        @param return_type  C++ return type, default: 'const entry_type&'
        @param return_entry Override the default behaviour for returning the
                            found entry. 'entry' is a reference to the found
                            entry, and 'aKey' is the lookup key. 'return_entry'
                            can be used for additional checks, e.g. for keys
                            not in the table."""

        assert all(ord(b) <= 0x7f
                   for e in self.phf.entries
                   for b in self.phf.key(e)), "non-ASCII key"

        if return_type is None:
            return_type = 'const %s&' % self.entry_type

        return textwrap.dedent("""
            %(return_type)s
            %(name)s(JSLinearString* aKey)
            {
              %(basis_table)s

              size_t length = js::GetLinearStringLength(aKey);

              JS::AutoCheckCannotGC nogc;
              if (js::LinearStringHasLatin1Chars(aKey)) {
                auto& entry = mozilla::perfecthash::Lookup(
                  js::GetLatin1LinearStringChars(nogc, aKey),
                  length, BASES, %(entries_name)s);

                %(return_entry)s
              } else {
                auto& entry = mozilla::perfecthash::Lookup(
                  js::GetTwoByteLinearStringChars(nogc, aKey),
                  length, BASES, %(entries_name)s);

                %(return_entry)s
              }
            }
            """) % {
                'name': name,
                'basis_table': self._indent(self.basis_table()),
                'entries_name': self.entries_name,

                'return_type': return_type,
                'return_entry': self._indent(return_entry, 2),
            }