/*
 * Copyright (C) 2013  Patrick "P. J." McDermott
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser 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 Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this program.  If not, see
 * <http://www.gnu.org/licenses/>.
 */

#include "base64.h"

static void
base64_decode_block(const char *src, char *dest)
{
	int i;
	/* GCC's SSP won't wrap functions with arrays smaller than 8 bytes. */
	char buf[8];

	/* Convert base 64 characters into values. */
	/* Supports ASCII and EBCDIC systems. */
	for (i = 0; i < 4; ++i) {
		if (src[i] >= 'A' && src[i] <= 'I') {
			buf[i] = src[i] - 'A' + 0;
		} else if (src[i] >= 'J' && src[i] <= 'R') {
			buf[i] = src[i] - 'J' + 9;
		} else if (src[i] >= 'S' && src[i] <= 'Z') {
			buf[i] = src[i] - 'S' + 18;
		} else if (src[i] >= 'a' && src[i] <= 'i') {
			buf[i] = src[i] - 'a' + 26;
		} else if (src[i] >= 'j' && src[i] <= 'r') {
			buf[i] = src[i] - 'j' + 35;
		} else if (src[i] >= 's' && src[i] <= 'z') {
			buf[i] = src[i] - 's' + 44;
		} else if (src[i] >= '0' && src[i] <= '9') {
			buf[i] = src[i] - '0' + 52;
		} else if (src[i] == '+') {
			buf[i] = 62;
		} else if (src[i] == '/') {
			buf[i] = 63;
		} else {
			buf[i] = '\0';
		}
	}

	/* Map four decoded value bytes into three output bytes. */
	dest[0] = (buf[0] << 2) | (buf[1] >> 4);
	dest[1] = (buf[1] << 4) | (buf[2] >> 2);
	dest[2] = (buf[2] << 6) | (buf[3] >> 0);
}
	
void
base64_decode(const char *src, char *dest, size_t dest_len)
{
	size_t i;

	/* Decode the input one entire block at a time. */
	for (i = 0; i < (dest_len - 1) / 3; ++i) {
		if (src[i * 4] == '\0' || 
				src[i * 4 + 1] == '\0' || 
				src[i * 4 + 2] == '\0' || 
				src[i * 4 + 3] == '\0') {
			dest[i * 3] = '\0';
			break;
		}
		base64_decode_block(src + i * 4, dest + i * 3);
	}

	/* Ensure NUL termination. */
	dest[dest_len - 1] = '\0';
}