110 lines
2.0 KiB
C++
110 lines
2.0 KiB
C++
#ifndef BASE_ROLLING_HASH_H
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#define BASE_ROLLING_HASH_H
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#include <boost/circular_buffer.hpp>
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#include <stdint.h>
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#include <boost/optional.hpp>
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//----------------------------------------------------------------
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namespace base {
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namespace hash_detail {
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uint32_t const MULTIPLIER = 4294967291UL;
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uint32_t const SEED = 123;
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}
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class rolling_hash {
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public:
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rolling_hash(unsigned window_size);
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void reset();
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// Returns the current hash
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uint32_t step(uint8_t byte) {
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update_hash(byte);
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return hash_;
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}
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uint32_t get_hash() const {
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return hash_;
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}
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private:
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void update_hash(uint8_t byte) {
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hash_ -= a_to_k_minus_1_ * (buffer_.front() + hash_detail::SEED);
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buffer_.push_back(byte);
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hash_ = (hash_ * a_) + byte + hash_detail::SEED;
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}
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uint32_t a_;
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uint32_t a_to_k_minus_1_;
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uint32_t hash_;
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uint32_t window_size_;
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boost::circular_buffer<uint8_t> buffer_;
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};
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class content_based_hash {
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public:
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content_based_hash(unsigned window_size);
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void reset();
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// Returns a break point relative to the last reset/break.
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boost::optional<unsigned> step(uint8_t byte) {
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boost::optional<unsigned> r;
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rhash_.step(byte);
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len_++;
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if (len_ < min_len_)
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return r;
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if (hit_break(backup_div_))
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backup_break_ = len_;
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if (hit_break(div_)) {
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// found a break
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r = len_;
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len_ = 0;
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backup_break_.reset();
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} else if (len_ >= max_len_) {
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// too big, is there a backup?
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if (backup_break_) {
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len_ -= *backup_break_;
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r = backup_break_;
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backup_break_.reset();
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} else {
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r = len_;
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len_ = 0;
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}
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}
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return r;
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}
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private:
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bool hit_break(uint32_t mask) const {
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uint32_t h = rhash_.get_hash() >> 8;
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return !(h & mask);
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}
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rolling_hash rhash_;
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uint32_t backup_div_;
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uint32_t div_;
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unsigned min_len_;
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unsigned max_len_;
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unsigned len_;
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boost::optional<unsigned> backup_break_;
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};
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}
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//----------------------------------------------------------------
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#endif
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