A method to compress strings of digit pairs to strings of predefined symbols.

upisc.c

/* upisc (Unlimiter's Pair-based Integer String Compression)
 *   A method to compress strings of digit pairs into strings of predefined symbols.
 *   The symbol dictionary doesn't contain every possible combination of pairs, but pairs that can be reversed to produce other pairs.
 *   
 *   In the command line, the program takes 1 argument, the integer string you want compress. It must have an even length.
 *   If one of the pairs in the argument is not in the dictionary, the program reverses it then translates it, after that, it puts a dot after it to denote it has been reversed.
 * 
 * — Unlimiter
 */

/* 
 * Dictionary of symbols (pairs get translated into symbols at compression):
 *   Symbol  Pair
 *   0       00
 *   1       01
 *   2       02
 *   3       03
 *   4       04
 *   5       05
 *   6       06
 *   7       07
 *   8       08
 *   9       09
 *   a       11
 *   b       12
 *   c       13
 *   d       14
 *   e       15
 *   f       16
 *   g       17
 *   h       18
 *   i       19
 *   j       22
 *   k       23
 *   l       24
 *   m       25
 *   n       26
 *   o       27
 *   p       28
 *   q       29
 *   r       33
 *   s       34
 *   t       35
 *   u       36
 *   v       37
 *   w       38
 *   x       39
 *   y       44
 *   z       45
 *   A       46
 *   B       47
 *   C       48
 *   D       49
 *   E       55
 *   F       56
 *   G       57
 *   H       58
 *   I       59
 *   J       66
 *   K       67
 *   L       68
 *   M       69
 *   N       77
 *   O       78
 *   P       79
 *   Q       88
 *   R       89
 *   S       99
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char* pairs[55] = {
	"00",
	"01",
	"02",
	"03",
	"04",
	"05",
	"06",
	"07",
	"08",
	"09",
	"11",
	"12",
	"13",
	"14",
	"15",
	"16",
	"17",
	"18",
	"19",
	"22",
	"23",
	"24",
	"25",
	"26",
	"27",
	"28",
	"29",
	"33",
	"34",
	"35",
	"36",
	"37",
	"38",
	"39",
	"44",
	"45",
	"46",
	"47",
	"48",
	"49",
	"55",
	"56",
	"57",
	"58",
	"59",
	"66",
	"67",
	"68",
	"69",
	"77",
	"78",
	"79",
	"88",
	"89",
	"99"
};

char symbols[55] = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRS";

char* swap(char* pair) {
	char first[2] = {pair[0], '\0'};
	char second[2] = {pair[1], '\0'};
	return strcat(second, first);
}

int findIndex(char* pair) {
	int i = 0;
	while(i < 55) {
		if (!strcmp(pair, pairs[i])) {
			break;
		}
		else if (!strcmp(swap(pair), pairs[i])) {
			break;
		}
		i++;
	}
	return i;
}

_Bool exists(char* pair) {
	for (int i = 0; i < 55; i++) {
		if (!strcmp(pair, pairs[i])) return 1;
	}
	return 0;
}

char* encode(char* pair) {
	char* str = (char[3]) {(char) symbols[findIndex(pair)], '\0'};
	if (!exists(pair)) {
		strcpy(str, (char[4]) {(char) symbols[findIndex(pair)], '.', '\0'});
	}
	return str;
}

char result[256];
char* translate(char* target) {
	if (!strlen(target)) return "";
	else if ((strlen(target) % 2) != 0) {
		fprintf(
			stderr,
			"%s",
			"\033[1;31mError:\033[0m length of number must be even"
		);
		return "";
	}
	
	for (int i = 0; target[i] != '\0'; i += 2) {
		char cur[3] = {target[i], target[i + 1], '\0'};
		if (strlen(cur) > 1) {
			strcat(result, encode(cur));
			memset(cur, 0, 3);
		}
	}
	return result;
}

int main(int argc, char** argv) {
	printf(
		(argc > 1) ?
			"%s\n"
			: "",
		(argc > 1) ?
			translate(argv[1])
			: translate("")
	);
}