open-vault/vendor/github.com/dsnet/compress/bzip2/bwt.go

// Copyright 2015, Joe Tsai. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE.md file.

package bzip2

import "github.com/dsnet/compress/bzip2/internal/sais"

// The Burrows-Wheeler Transform implementation used here is based on the
// Suffix Array by Induced Sorting (SA-IS) methodology by Nong, Zhang, and Chan.
// This implementation uses the sais algorithm originally written by Yuta Mori.
//
// The SA-IS algorithm runs in O(n) and outputs a Suffix Array. There is a
// mathematical relationship between Suffix Arrays and the Burrows-Wheeler
// Transform, such that a SA can be converted to a BWT in O(n) time.
//
// References:
//	http://www.hpl.hp.com/techreports/Compaq-DEC/SRC-RR-124.pdf
//	https://github.com/cscott/compressjs/blob/master/lib/BWT.js
//	https://www.quora.com/How-can-I-optimize-burrows-wheeler-transform-and-inverse-transform-to-work-in-O-n-time-O-n-space
type burrowsWheelerTransform struct {
	buf  []byte
	sa   []int
	perm []uint32
}

func (bwt *burrowsWheelerTransform) Encode(buf []byte) (ptr int) {
	if len(buf) == 0 {
		return -1
	}

	// TODO(dsnet): Find a way to avoid the duplicate input string method.
	// We only need to do this because suffix arrays (by definition) only
	// operate non-wrapped suffixes of a string. On the other hand,
	// the BWT specifically used in bzip2 operate on a strings that wrap-around
	// when being sorted.

	// Step 1: Concatenate the input string to itself so that we can use the
	// suffix array algorithm for bzip2's variant of BWT.
	n := len(buf)
	bwt.buf = append(append(bwt.buf[:0], buf...), buf...)
	if cap(bwt.sa) < 2*n {
		bwt.sa = make([]int, 2*n)
	}
	t := bwt.buf[:2*n]
	sa := bwt.sa[:2*n]

	// Step 2: Compute the suffix array (SA). The input string, t, will not be
	// modified, while the results will be written to the output, sa.
	sais.ComputeSA(t, sa)

	// Step 3: Convert the SA to a BWT. Since ComputeSA does not mutate the
	// input, we have two copies of the input; in buf and buf2. Thus, we write
	// the transformation to buf, while using buf2.
	var j int
	buf2 := t[n:]
	for _, i := range sa {
		if i < n {
			if i == 0 {
				ptr = j
				i = n
			}
			buf[j] = buf2[i-1]
			j++
		}
	}
	return ptr
}

func (bwt *burrowsWheelerTransform) Decode(buf []byte, ptr int) {
	if len(buf) == 0 {
		return
	}

	// Step 1: Compute cumm, where cumm[ch] reports the total number of
	// characters that precede the character ch in the alphabet.
	var cumm [256]int
	for _, v := range buf {
		cumm[v]++
	}
	var sum int
	for i, v := range cumm {
		cumm[i] = sum
		sum += v
	}

	// Step 2: Compute perm, where perm[ptr] contains a pointer to the next
	// byte in buf and the next pointer in perm itself.
	if cap(bwt.perm) < len(buf) {
		bwt.perm = make([]uint32, len(buf))
	}
	perm := bwt.perm[:len(buf)]
	for i, b := range buf {
		perm[cumm[b]] = uint32(i)
		cumm[b]++
	}

	// Step 3: Follow each pointer in perm to the next byte, starting with the
	// origin pointer.
	if cap(bwt.buf) < len(buf) {
		bwt.buf = make([]byte, len(buf))
	}
	buf2 := bwt.buf[:len(buf)]
	i := perm[ptr]
	for j := range buf2 {
		buf2[j] = buf[i]
		i = perm[i]
	}
	copy(buf, buf2)
}