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64 lines
2.7 KiB
C
64 lines
2.7 KiB
C
#include "proto.h"
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/*
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Repetitions compressor: compresses the data only using a subset of the available repetition commands.
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Methods defined: 6
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Flags values: the value plus one is taken as a bitfield indicating which kinds of repetition commands are used
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(lowest bit to highest: repeat single byte (1), repeat two bytes (2), repeat zeros (3)).
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*/
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struct command * try_compress_repetitions (const unsigned char * data, __attribute__((unused)) const unsigned char * bitflipped, unsigned short * size, unsigned flags) {
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unsigned short pos = 0, skipped = 0;
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struct command * result = malloc(*size * sizeof(struct command));
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struct command * current = result;
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struct command candidate;
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flags = (flags + 1) << 1;
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while (pos < *size) {
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candidate = find_repetition_at_position(data, pos, *size);
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if ((candidate.command == 3) && !(flags & 8)) {
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candidate.command = 1;
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candidate.value = 0;
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}
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if ((candidate.command == 1) && !(flags & 2)) {
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candidate.command = 2;
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candidate.value |= candidate.value << 8;
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}
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if ((flags & (1 << candidate.command)) && (command_size(candidate) <= candidate.count)) {
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if (skipped) *(current ++) = (struct command) {.command = 0, .count = skipped, .value = pos - skipped};
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skipped = 0;
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*(current ++) = candidate;
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pos += candidate.count;
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} else {
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pos ++;
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if ((++ skipped) == MAX_COMMAND_COUNT) {
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*(current ++) = (struct command) {.command = 0, .count = MAX_COMMAND_COUNT, .value = pos - MAX_COMMAND_COUNT};
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skipped = 0;
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}
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}
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}
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if (skipped) *(current ++) = (struct command) {.command = 0, .count = skipped, .value = pos - skipped};
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*size = current - result;
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result = realloc(result, *size * sizeof(struct command));
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return result;
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}
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struct command find_repetition_at_position (const unsigned char * data, unsigned short position, unsigned short length) {
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if ((position + 1) >= length) return data[position] ? ((struct command) {.command = 7}) : ((struct command) {.command = 3, .count = 1});
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unsigned char value[2] = {data[position], data[position + 1]};
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unsigned repcount, limit = length - position;
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if (limit > MAX_COMMAND_COUNT) limit = MAX_COMMAND_COUNT;
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for (repcount = 2; (repcount < limit) && (data[position + repcount] == value[repcount & 1]); repcount ++);
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struct command result;
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result.count = repcount;
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if (*value != value[1]) {
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if (!*value && (repcount < 3)) return (struct command) {.command = 3, .count = 1};
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result.command = 2;
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result.value = ((unsigned) (*value)) | (((unsigned) (value[1])) << 8);
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} else if (*value) {
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result.command = 1;
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result.value = *value;
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} else
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result.command = 3;
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return result;
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}
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