// Copyright (c) 2011 The LevelDB Authors. All rights reserved.

// Use of this source code is governed by a BSD-style license that can be

// found in the LICENSE file. See the AUTHORS file for names of contributors.

#include "util/hash.h"

#include <string.h>

#include "util/coding.h"

// The FALLTHROUGH_INTENDED macro can be used to annotate implicit fall-through

// between switch labels. The real definition should be provided externally.

// This one is a fallback version for unsupported compilers.

#ifndef FALLTHROUGH_INTENDED

#define FALLTHROUGH_INTENDED \

  do {                       \

  } while (0)

#endif

namespace leveldb {

uint32_t Hash(const char* data, size_t n, uint32_t seed) {

  // Similar to murmur hash

  const uint32_t m = 0xc6a4a793;

  const uint32_t r = 24;

  const char* limit = data + n;

  uint32_t h = seed ^ (n * m);

  // Pick up four bytes at a time

  while (data + 4 <= limit) {

    uint32_t w = DecodeFixed32(data);

    data += 4;

    h += w;

    h *= m;

    h ^= (h >> 16);

  }

  // Pick up remaining bytes

  switch (limit - data) {

    case 3:

      h += static_cast<uint8_t>(data[2]) << 16;

      FALLTHROUGH_INTENDED;

    case 2:

      h += static_cast<uint8_t>(data[1]) << 8;

      FALLTHROUGH_INTENDED;

    case 1:

      h += static_cast<uint8_t>(data[0]);

      h *= m;

      h ^= (h >> r);

      break;

  }

  return h;

}

}  // namespace leveldb