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/*
Challenge 022
Task #2
Write a script to implement Lempel-Ziv-Welch (LZW) compression algorithm.
The script should have method to encode/decode algorithm. The wiki page
explains the compression algorithm very nicely.
*/
#include <assert.h>
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "uthash.h"
#include "utstring.h"
#include "utarray.h"
#define EOM '#'
#define SYMBOLS "#ABCDEFGHIJKLMNOPQRSTUVWXYZ"
typedef struct dict {
char* key;
int id;
UT_hash_handle hh;
} Dict;
typedef struct lzw {
Dict* dict;
UT_array* symbols;
} LZW;
int bits_width(int n) {
int width = 0;
while (n > 0) {
n /= 2;
width++;
}
return width;
}
void output_bits(UT_string* result, int n, int width) {
UT_string* reverse_bits;
utstring_new(reverse_bits);
while (width > 0 || n > 0) {
utstring_printf(reverse_bits, "%c", '0' + (n & 1));
n /= 2;
width--;
}
for (const char* p = utstring_body(reverse_bits) + utstring_len(reverse_bits) - 1;
p >= utstring_body(reverse_bits); p--)
utstring_printf(result, "%c", *p);
utstring_free(reverse_bits);
}
int input_bits(const char** p, int width) {
int result = 0;
while (**p != '\0' && width-- > 0) {
result *= 2;
result += **p == '1' ? 1 : 0;
(*p)++;
}
return result;
}
void* check_mem(void* p) {
if (!p) {
fputs("out of memory", stderr);
exit(EXIT_FAILURE);
}
return p;
}
int lzw_last(LZW* lzw) {
return utarray_len(lzw->symbols) - 1;
}
int lzw_width(LZW* lzw) {
return bits_width(lzw_last(lzw));
}
int lzw_next_width(LZW* lzw) {
return bits_width(lzw_last(lzw) + 1);
}
void lzw_add(LZW* lzw, const char* seq) {
int id = lzw_last(lzw) + 1;
Dict* elt = check_mem(calloc(1, sizeof(Dict)));
elt->key = check_mem(strdup(seq));
elt->id = id;
HASH_ADD_KEYPTR(hh, lzw->dict, elt->key, strlen(elt->key), elt);
utarray_push_back(lzw->symbols, &seq);
}
LZW* lzw_new() {
LZW* lzw = check_mem(calloc(1, sizeof(LZW)));
utarray_new(lzw->symbols, &ut_str_icd);
for (const char* p = SYMBOLS; *p; p++) {
char seq[2];
seq[0] = *p;
seq[1] = '\0';
lzw_add(lzw, seq);
}
return lzw;
}
void lzw_free(LZW* lzw) {
Dict* elt, * tmp;
HASH_ITER(hh, lzw->dict, elt , tmp) {
HASH_DEL(lzw->dict, elt);
free(elt->key);
free(elt);
}
utarray_free(lzw->symbols);
}
void lzw_longest_match(LZW* lzw, UT_string* encoded, const char** p) {
UT_string* prefix;
utstring_new(prefix);
// find longest match
int len = 0;
while (len < strlen(*p)) {
utstring_clear(prefix);
utstring_printf(prefix, "%-.*s", len + 1, *p);
Dict* elt;
HASH_FIND_STR(lzw->dict, utstring_body(prefix), elt);
if (elt == NULL)
break;
len++;
}
utstring_clear(prefix);
utstring_printf(prefix, "%-.*s", len, *p);
(*p) += len;
Dict* elt;
HASH_FIND_STR(lzw->dict, utstring_body(prefix), elt);
assert(elt != NULL);
int code = elt->id;
int code_width = lzw_width(lzw);
// store new prefix in dictionary
if (**p != '\0') {
utstring_printf(prefix, "%c", **p);
lzw_add(lzw, utstring_body(prefix));
}
output_bits(encoded, code, code_width);
utstring_free(prefix);
}
void lzw_encode(LZW* lzw, UT_string* encoded, const char* text_) {
UT_string* text;
utstring_new(text);
for (const char* p = text_; *p; p++)
if (isalpha(*p))
utstring_printf(text, "%c", toupper(*p));
utstring_printf(text, "%c", EOM);
utstring_clear(encoded);
const char* p = utstring_body(text);
while (*p) {
lzw_longest_match(lzw, encoded, &p);
}
utstring_free(text);
}
void lzw_decode(LZW* lzw, UT_string* decoded, const char* text) {
const char* p = text;
utstring_clear(decoded);
while (*p) {
int code = input_bits(&p, lzw_width(lzw));
const char* seq = *(const char**)utarray_eltptr(lzw->symbols, code);
utstring_printf(decoded, "%s", seq);
if (*p) {
int next_width = lzw_next_width(lzw);
const char* p1 = p;
int next_code = input_bits(&p1, next_width);
const char* next_seq= *(const char**)utarray_eltptr(lzw->symbols, next_code);
UT_string* next_prefix;
utstring_new(next_prefix);
utstring_printf(next_prefix, "%s%c", seq, *next_seq);
lzw_add(lzw, utstring_body(next_prefix));
utstring_free(next_prefix);
}
}
// remove EOM
utstring_body(decoded)[utstring_len(decoded) - 1] = '\0';
utstring_len(decoded) = strlen(utstring_body(decoded));
}
void usage() {
fputs("usage: ch-2 encode|decode text", stderr);
}
int main(int argc, char* argv[]) {
if (argc != 3) {
usage();
return EXIT_FAILURE;
}
UT_string* result;
utstring_new(result);
LZW* lzw = lzw_new();
if (strcmp(argv[1], "encode") == 0) {
lzw_encode(lzw, result, argv[2]);
}
else if (strcmp(argv[1], "decode") == 0) {
lzw_decode(lzw, result, argv[2]);
}
else {
usage();
return EXIT_FAILURE;
}
printf("%s\n", utstring_body(result));
utstring_free(result);
lzw_free(lzw);
}
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