AmigaExamples/tools/external/shrinkler/MatchFinder.h

// Copyright 1999-2015 Aske Simon Christensen. See LICENSE.txt for usage terms.

/*

Find repeated strings in a data block.

Matches are reported from longest to shortest. A match is only reported
if it is closer (smaller offset, higher position) than all longer matches.

Two parameters control the speed/precision tradeoff of the matcher:

The match_patience parameter controls how many matches outside the current
reporting range (between last longer match and current position) are skipped
before the matcher gives up finding more matches.

The max_same_length parameter controls how many matches of the same length
are reported. The matches reported will be the closest ones of that length.

*/

#pragma once

#include <vector>
#include <algorithm>
#include <queue>
#include <functional>

using std::vector;

class MatchFinder {
	// Inputs
	unsigned char *data;
	int length;
	int min_length;
	int match_patience;
	int max_same_length;

	// Suffix array
	vector<int> suffix_array;
	vector<int> rev_suffix_array;
	vector<int> longest_common_prefix;

	// Matcher parameters
	int current_pos;
	int min_pos;

	// Matcher state
	int left_index;
	int left_length;
	int right_index;
	int right_length;
	int current_length;

	// Best matches seen with current length
	std::priority_queue<int, vector<int>, std::greater<int> > match_buffer;

	struct suffix_compare {
		MatchFinder* finder;

		suffix_compare(MatchFinder* finder) : finder(finder) {}

		// Compare suffixes starting at positions a and b
		bool operator()(int a, int b) {
			vector<int>& same = finder->rev_suffix_array;
			unsigned char *data = finder->data;
			if (a == b) return false;
			if (data[a] == data[b]) {
				// Skip stretch of equal bytes
				int skip = std::min(same[a], same[b]);
				a += skip;
				b += skip;
			}
			int until_end = finder->length - std::max(a,b);
			for (int i = 0 ; i < until_end ; i++) {
				if (data[a + i] != data[b + i]) {
					return data[a + i] < data[b + i];
				}
			}
			return a < b;
		}
	};

	// Quick'n'dirty suffix array construction:
	// plain sorting with accelleration of same-value blocks
	void make_suffix_array() {
		// Temporary same-value block accelleration array
		rev_suffix_array.resize(length + 1);
		int count = 0;
		char c = 0;
		rev_suffix_array[length] = 0;
		for (int i = length - 1 ; i >= 0 ; i--) {
			if (data[i] != c) count = 0;
			rev_suffix_array[i] = ++count;
			c = data[i];
		}

		// Compute suffix array
		suffix_array.resize(length + 1);
		for (int i = 0 ; i <= length ; i++) {
			suffix_array[i] = i;
		}
		std::sort(&suffix_array[0], &suffix_array[length], suffix_compare(this));

		// Compute reverse suffix array
		for (int i = 0 ; i <= length ; i++) {
			rev_suffix_array[suffix_array[i]] = i;
		}

		// Compute LCP array
		longest_common_prefix.resize(length + 1);
		longest_common_prefix[0] = 0;
		longest_common_prefix[length] = 0;
		int h = 0;
		for (int i = 0 ; i < length ; i++) {
			int r = rev_suffix_array[i];
			if (r > 0) {
				int j = suffix_array[r - 1];
				while (data[i + h] == data[j + h]) {
					h = h + 1;
				}
				longest_common_prefix[r] = h;
				if (h > 0) h = h - 1;
			}
		}
	}

	void extend_left() {
		int iter = 0;
		while (left_length >= min_length) {
			left_length = std::min(left_length, longest_common_prefix[left_index]);
			int pos = suffix_array[--left_index];
			if (pos < current_pos && pos >= min_pos) break;
			if (++iter > match_patience) left_length = 0;
		}
	}

	void extend_right() {
		int iter = 0;
		while (right_length >= min_length) {
			right_length = std::min(right_length, longest_common_prefix[++right_index]);
			int pos = suffix_array[right_index];
			if (pos < current_pos && pos >= min_pos) break;
			if (++iter > match_patience) right_length = 0;
		}
	}

	int next_length() {
		return std::max(left_length, right_length);
	}

public:
	MatchFinder(unsigned char *data, int length, int min_length, int match_patience, int max_same_length) :
		data(data), length(length), min_length(min_length), match_patience(match_patience), max_same_length(max_same_length) {
		make_suffix_array();
		reset();
	}

	void reset() {
	}

	// Start finding matches between strings starting at pos and earlier strings.
	void beginMatching(int pos) {
		current_pos = pos;
		min_pos = 0;

		left_index = rev_suffix_array[pos];
		left_length = length;
		extend_left();
		right_index = rev_suffix_array[pos];
		right_length = length;
		extend_right();
	}

	// Report next match. Returns whether a match was found.
	bool nextMatch(int *match_pos_out, int *match_length_out) {
		if (match_buffer.empty()) {
			// Fill match buffer
			current_length = next_length();
			if (current_length < min_length) return false;
			int new_min_pos = min_pos;
			do {
				int match_pos;
				if (left_length > right_length) {
					match_pos = suffix_array[left_index];
					extend_left();
				} else {
					match_pos = suffix_array[right_index];
					extend_right();
				}
				new_min_pos = std::max(new_min_pos, match_pos);
				if (match_buffer.size() < max_same_length) {
					match_buffer.push(match_pos);
				} else {
					if (match_pos > match_buffer.top()) {
						match_buffer.pop();
						match_buffer.push(match_pos);
					}
					min_pos = match_buffer.top();
				}
			} while (next_length() == current_length);
			assert(!match_buffer.empty());
			min_pos = new_min_pos;
		}

		*match_length_out = current_length;
		*match_pos_out = match_buffer.top();
		match_buffer.pop();
		assert(*match_pos_out < current_pos);
		return true;
	}
};