little-things/urldecode.c

/*
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/

// OrIdow6, late November 2021


// stdin - a urlencoded string
// stdout - the decoded string

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

#define BUFFER_SIZE 1024 * 300


int main() {
	uint8_t* inbuf = malloc(sizeof(uint8_t) * BUFFER_SIZE);
	uint8_t* outbuf = malloc(sizeof(uint8_t) * BUFFER_SIZE + 2);
	size_t outp = 0;
	int state = -1;
	int8_t digita = 0x0;
	int8_t digitb = 0x0;
	uint8_t digita_real;
	uint8_t digitb_real;

	// https://stackoverflow.com/questions/10324/convert-a-hexadecimal-string-to-an-integer-efficiently-in-c
	// Because I can't be bothered to generate this manually
	// Could be faster if you had a table for 16-bit integers, depending on caching
	static const int8_t hextable[] = {
		-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
		-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
		-1,-1, 0,1,2,3,4,5,6,7,8,9,-1,-1,-1,-1,-1,-1,-1,10,11,12,13,14,15,-1,
		-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
		-1,-1,10,11,12,13,14,15,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
		-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
		-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
		-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
		-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
		-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
		-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1
	};

	while (1) {
		size_t readS = fread(inbuf, sizeof(char), BUFFER_SIZE, stdin);
		if (readS == 0) {
			break;
		}
		for (size_t p = 0; p < readS; p++) {
			if (state == -1) {
				if (inbuf[p] != '%') {
					outbuf[outp++] = inbuf[p];
				} else {
					state = 0;
				}
			} else if (state == 0) {
				digita = hextable[inbuf[p]];
				if (digita == (int8_t)-1 ) {
					outbuf[outp++] = '%';
					if (inbuf[p] != '%') {
						outbuf[outp++] = inbuf[p];
						state = -1;
					} // else state remains 0
					continue;
				}
				digita_real = inbuf[p];
				state = 1;
			} else {
				digitb = hextable[inbuf[p]];
				if (digitb == (int8_t)-1 ) {
					digitb_real = inbuf[p];
					outbuf[outp++] = '%';
					outbuf[outp++] = digita_real;
					if (inbuf[p] != '%') {
						outbuf[outp++] = digitb_real;
						state = -1;
					} else {
						state = 0;
					}
					continue;
				}
				outbuf[outp++] = digita << 4 | digitb;
				state = -1;
			}
		}

		fwrite(outbuf, outp, 1, stdout);
		outp = 0;

		if (readS < BUFFER_SIZE) {
			break;
		}
	}

	if (state == 0 || state == 1) {
		fwrite("%", 1, 1, stdout);
	}
	if (state == 1) {
		fwrite(&digita_real, 1, 1, stdout);
	}
}