Transition to pulling open-vcdiff from repository, instead of using snapshot...

sdch/open_vcdiff/depot/opensource/open-vcdiff/src/rolling_hash.h

Index: sdch/open_vcdiff/depot/opensource/open-vcdiff/src/rolling_hash.h
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--- sdch/open_vcdiff/depot/opensource/open-vcdiff/src/rolling_hash.h	(revision 2678)
+++ sdch/open_vcdiff/depot/opensource/open-vcdiff/src/rolling_hash.h	(working copy)
@@ -1,237 +0,0 @@
-// Copyright 2007, 2008 Google Inc.
-// Authors: Jeff Dean, Sanjay Ghemawat, Lincoln Smith
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#ifndef OPEN_VCDIFF_ROLLING_HASH_H_
-#define OPEN_VCDIFF_ROLLING_HASH_H_
-
-#include <config.h>
-#include <stdint.h>  // uint32_t
-#include "logging.h"
-
-namespace open_vcdiff {
-
-// Rabin-Karp hasher module -- this is a faster version with different
-// constants, so it's not quite Rabin-Karp fingerprinting, but its behavior is
-// close enough for most applications.
-
-// Definitions common to all hash window sizes.
-class RollingHashUtil {
- public:
-  // Multiplier for incremental hashing.  The compiler should be smart enough to
-  // convert (val * kMult) into ((val << 8) + val).
-  static const uint32_t kMult = 257;
-
-  // All hashes are returned modulo "kBase".  Current implementation requires
-  // kBase <= 2^32/kMult to avoid overflow.  Also, kBase must be a power of two
-  // so that we can compute modulus efficiently.
-  static const uint32_t kBase = (1 << 23);
-
-  // Returns operand % kBase, assuming that kBase is a power of two.
-  static inline uint32_t ModBase(uint32_t operand) {
-    return operand & (kBase - 1);
-  }
-
-  // Given an unsigned integer "operand", returns an unsigned integer "result"
-  // such that
-  //     result < kBase
-  // and
-  //     ModBase(operand + result) == 0
-  static inline uint32_t FindModBaseInverse(uint32_t operand) {
-    // The subtraction (0 - operand) produces an unsigned underflow for any
-    // operand except 0.  The underflow results in a (very large) unsigned
-    // number.  Binary subtraction is used instead of unary negation because
-    // some compilers (e.g. Visual Studio 7+) produce a warning if an unsigned
-    // value is negated.
-    //
-    // The C++ mod operation (operand % kBase) may produce different results for
-    // different compilers if operand is negative.  That is not a problem in
-    // this case, since all numbers used are unsigned, and ModBase does its work
-    // using bitwise arithmetic rather than the % operator.
-    return ModBase(uint32_t(0) - operand);
-  }
-
-  // Here's the heart of the hash algorithm.  Start with a partial_hash value of
-  // 0, and run this HashStep once against each byte in the data window to be
-  // hashed.  The result will be the hash value for the entire data window.  The
-  // Hash() function, below, does exactly this, albeit with some refinements.
-  static inline uint32_t HashStep(uint32_t partial_hash,
-                                  unsigned char next_byte) {
-    return ModBase((partial_hash * kMult) + next_byte);
-  }
-
-  // Use this function to start computing a new hash value based on the first
-  // two bytes in the window.  It is equivalent to calling
-  //     HashStep(HashStep(0, ptr[0]), ptr[1])
-  // but takes advantage of the fact that the maximum value of
-  // (ptr[0] * kMult) + ptr[1] is not large enough to exceed kBase, thus
-  // avoiding an unnecessary ModBase operation.
-  static inline uint32_t HashFirstTwoBytes(const char* ptr) {
-    return (static_cast<unsigned char>(ptr[0]) * kMult)
-        + static_cast<unsigned char>(ptr[1]);
-  }
- private:
-  // Making these private avoids copy constructor and assignment operator.
-  // No objects of this type should be constructed.
-  RollingHashUtil();
-  RollingHashUtil(const RollingHashUtil&);  // NOLINT
-  void operator=(const RollingHashUtil&);
-};
-
-// window_size must be >= 2.
-template<int window_size>
-class RollingHash {
- public:
-  // Perform global initialization that is required in order to instantiate a
-  // RollingHash.  This function *must* be called (preferably on startup) by any
-  // program that uses a RollingHash.  It is harmless to call this function more
-  // than once.  It is not thread-safe, but calling it from two different
-  // threads at the same time can only cause a memory leak, not incorrect
-  // behavior.  Make sure to call it before spawning any threads that could use
-  // RollingHash.  The function returns true if initialization succeeds, or
-  // false if initialization fails, in which case the caller should not proceed
-  // to construct any objects of type RollingHash.
-  static bool Init();
-
-  // Initialize hasher to maintain a window of the specified size.  You need an
-  // instance of this type to use UpdateHash(), but Hash() does not depend on
-  // remove_table_, so it is static.
-  RollingHash() {
-    if (!remove_table_) {
-      LOG(DFATAL) << "RollingHash object instantiated"
-                     " before calling RollingHash::Init()" << LOG_ENDL;
-    }
-  }
-
-  // Compute a hash of the window "ptr[0, window_size - 1]".
-  static uint32_t Hash(const char* ptr) {
-    uint32_t h = RollingHashUtil::HashFirstTwoBytes(ptr);
-    for (int i = 2; i < window_size; ++i) {
-      h = RollingHashUtil::HashStep(h, ptr[i]);
-    }
-    return h;
-  }
-
-  // Update a hash by removing the oldest byte and adding a new byte.
-  //
-  // UpdateHash takes the hash value of buffer[0] ... buffer[window_size -1]
-  // along with the value of buffer[0] (the "old_first_byte" argument)
-  // and the value of buffer[window_size] (the "new_last_byte" argument).
-  // It quickly computes the hash value of buffer[1] ... buffer[window_size]
-  // without having to run Hash() on the entire window.
-  //
-  // The larger the window, the more advantage comes from using UpdateHash()
-  // (which runs in time independent of window_size) instead of Hash().
-  // Each time window_size doubles, the time to execute Hash() also doubles,
-  // while the time to execute UpdateHash() remains constant.  Empirical tests
-  // have borne out this statement.
-  uint32_t UpdateHash(uint32_t old_hash,
-                      const char old_first_byte,
-                      const char new_last_byte) const {
-    uint32_t partial_hash = RemoveFirstByteFromHash(old_hash, old_first_byte);
-    return RollingHashUtil::HashStep(partial_hash, new_last_byte);
-  }
-
- protected:
-  // Given a full hash value for buffer[0] ... buffer[window_size -1], plus the
-  // value of the first byte buffer[0], this function returns a *partial* hash
-  // value for buffer[1] ... buffer[window_size -1].  See the comments in
-  // Init(), below, for a description of how the contents of remove_table_ are
-  // computed.
-  static uint32_t RemoveFirstByteFromHash(uint32_t full_hash,
-                                          unsigned char first_byte) {
-    return RollingHashUtil::ModBase(full_hash + remove_table_[first_byte]);
-  }
-
- private:
-  // We keep a table that maps from any byte "b" to
-  //    (- b * pow(kMult, window_size - 1)) % kBase
-  static const uint32_t* remove_table_;
-};
-
-// For each window_size, fill a 256-entry table such that
-//        the hash value of buffer[0] ... buffer[window_size - 1]
-//      + remove_table_[buffer[0]]
-//     == the hash value of buffer[1] ... buffer[window_size - 1]
-// See the comments in Init(), below, for a description of how the contents of
-// remove_table_ are computed.
-template<int window_size>
-const uint32_t* RollingHash<window_size>::remove_table_ = NULL;
-
-// Init() checks to make sure that the static object remove_table_ has been
-// initialized; if not, it does the considerable work of populating it.  Once
-// it's ready, the table can be used for any number of RollingHash objects of
-// the same window_size.
-//
-template<int window_size>
-bool RollingHash<window_size>::Init() {
-  if (window_size < 2) {
-    LOG(ERROR) << "RollingHash window size " << window_size
-               << " is too small" << LOG_ENDL;
-    return false;
-  }
-  if (remove_table_ == NULL) {
-    // The new object is placed into a local pointer instead of directly into
-    // remove_table_, for two reasons:
-    //   1. remove_table_ is a pointer to const.  The table is populated using
-    //      the non-const local pointer and then assigned to the global const
-    //      pointer once it's ready.
-    //   2. No other thread will ever see remove_table_ pointing to a
-    //      partially-initialized table.  If two threads happen to call Init()
-    //      at the same time, two tables with the same contents may be created
-    //      (causing a memory leak), but the results will be consistent
-    //      no matter which of the two tables is used.
-    uint32_t* new_remove_table = new uint32_t[256];
-    // Compute multiplier.  Concisely, it is:
-    //     pow(kMult, (window_size - 1)) % kBase,
-    // but we compute the power in integer form.
-    uint32_t multiplier = 1;
-    for (int i = 0; i < window_size - 1; ++i) {
-      multiplier =
-          RollingHashUtil::ModBase(multiplier * RollingHashUtil::kMult);
-    }
-    // For each character removed_byte, compute
-    //     remove_table_[removed_byte] ==
-    //         (- (removed_byte * pow(kMult, (window_size - 1)))) % kBase
-    // where the power operator "pow" is taken in integer form.
-    //
-    // If you take a hash value fp representing the hash of
-    //     buffer[0] ... buffer[window_size - 1]
-    // and add the value of remove_table_[buffer[0]] to it, the result will be
-    // a partial hash value for
-    //     buffer[1] ... buffer[window_size - 1]
-    // that is to say, it no longer includes buffer[0].
-    //
-    // The following byte at buffer[window_size] can then be merged with this
-    // partial hash value to arrive quickly at the hash value for a window that
-    // has advanced by one byte, to
-    //     buffer[1] ... buffer[window_size]
-    // In fact, that is precisely what happens in UpdateHash, above.
-    uint32_t byte_times_multiplier = 0;
-    for (int removed_byte = 0; removed_byte < 256; ++removed_byte) {
-      new_remove_table[removed_byte] =
-          RollingHashUtil::FindModBaseInverse(byte_times_multiplier);
-      // Iteratively adding the multiplier in this loop is equivalent to
-      // computing (removed_byte * multiplier), and is faster
-      byte_times_multiplier =
-          RollingHashUtil::ModBase(byte_times_multiplier + multiplier);
-    }
-    remove_table_ = new_remove_table;
-  }
-  return true;
-}
-
-}  // namespace open_vcdiff
-
-#endif  // OPEN_VCDIFF_ROLLING_HASH_H_