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

Issue 5203: Transition to pulling open-vcdiff from repository, instead of using snapshot... (Closed) Base URL: svn://chrome-svn/chrome/trunk/src/
Patch Set: Created 12 years, 3 months ago
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Index: sdch/open_vcdiff/depot/opensource/open-vcdiff/src/rolling_hash.h
===================================================================
--- 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_

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