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67 lines
1.6 KiB
67 lines
1.6 KiB
// Copyright 2014-2021 Ulrich Kunitz. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package hash
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// A is the default constant for Robin-Karp rolling hash. This is a random
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// prime.
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const A = 0x97b548add41d5da1
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// RabinKarp supports the computation of a rolling hash.
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type RabinKarp struct {
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A uint64
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// a^n
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aOldest uint64
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h uint64
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p []byte
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i int
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}
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// NewRabinKarp creates a new RabinKarp value. The argument n defines the
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// length of the byte sequence to be hashed. The default constant will will be
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// used.
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func NewRabinKarp(n int) *RabinKarp {
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return NewRabinKarpConst(n, A)
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}
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// NewRabinKarpConst creates a new RabinKarp value. The argument n defines the
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// length of the byte sequence to be hashed. The argument a provides the
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// constant used to compute the hash.
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func NewRabinKarpConst(n int, a uint64) *RabinKarp {
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if n <= 0 {
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panic("number of bytes n must be positive")
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}
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aOldest := uint64(1)
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// There are faster methods. For the small n required by the LZMA
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// compressor O(n) is sufficient.
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for i := 0; i < n; i++ {
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aOldest *= a
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}
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return &RabinKarp{
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A: a, aOldest: aOldest,
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p: make([]byte, 0, n),
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}
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}
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// Len returns the length of the byte sequence.
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func (r *RabinKarp) Len() int {
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return cap(r.p)
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}
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// RollByte computes the hash after x has been added.
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func (r *RabinKarp) RollByte(x byte) uint64 {
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if len(r.p) < cap(r.p) {
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r.h += uint64(x)
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r.h *= r.A
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r.p = append(r.p, x)
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} else {
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r.h -= uint64(r.p[r.i]) * r.aOldest
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r.h += uint64(x)
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r.h *= r.A
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r.p[r.i] = x
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r.i = (r.i + 1) % cap(r.p)
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}
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return r.h
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}
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