/*-
 * Copyright (c) 2017 Taylor R. Campbell
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * Base32 encoding of octet strings, safe for URLs, domain labels, and
 * human eyeball distinction to avoid confusing 0/O and 1/l/I.
 * Specified in
 *
 *	S. Josefsson, `The Base16, Base32, and Base64 Data Encodings',
 *	IETF RFC 4648, October 2006.
 */

#define	_POSIX_C_SOURCE	200809L

#include "base32enc.h"

#include <assert.h>
#include <stddef.h>
#include <stdint.h>

/*
 * base32enc_size(s)
 *
 *	Return the size in bytes of the base32 encoding of a string of
 *	s octets.
 */
size_t
base32enc_size(size_t s)
{

	return 8*((s + 4)/5);
}

/*
 * base32enc(target, tlen, source, slen)
 *
 *	Encode slen bytes of data at source as NUL-terminated base32 in
 *	the buffer target which has tlen bytes.  tlen must be at least
 *	base32enc_size(slen) + 1.
 */
void
base32enc(char *target, size_t tlen, const void *source, size_t slen)
{
	static const char b32[] = "abcdefghijklmnopqrstuvwxyz234567";
	char *t = target;
	const uint8_t *s = source;

	/* Require enough room in target buffer.  */
	assert(tlen >= base32enc_size(slen) + 1);

	/* Map 5-octet chunks to 8-character chunks.  */
	while (5 <= slen) {
		assert(8 <= tlen);
		t[0] = b32[                     (s[0] & 0xf8) >> 3];
		t[1] = b32[(s[0] & 0x07) << 2 | (s[1] & 0xc0) >> 6];
		t[2] = b32[                     (s[1] & 0x3e) >> 1];
		t[3] = b32[(s[1] & 0x01) << 4 | (s[2] & 0xf0) >> 4];
		t[4] = b32[(s[2] & 0x0f) << 1 | (s[3] & 0x80) >> 7];
		t[5] = b32[                     (s[3] & 0x7c) >> 2];
		t[6] = b32[(s[3] & 0x03) << 3 | (s[4] & 0xe0) >> 5];
		t[7] = b32[(s[4] & 0x1f) << 0                     ];

		s += 5, slen -= 5;
		t += 8, tlen -= 8;
	}

	/* Map trailing chunk with padding if need be.  */
	if (0 < slen) {
		uint8_t s0, s1, s2, s3, s4;

		assert(8 <= tlen);
		assert(slen == 1 || slen == 2 || slen == 3 || slen == 4);
		s0 = s[0];
		s1 = (slen <= 1? 0 : s[1]);
		s2 = (slen <= 2? 0 : s[2]);
		s3 = (slen <= 3? 0 : s[3]);
		s4 = 0;

		t[0] = b32[		      (s0 & 0xf8) >> 3];
		t[1] = b32[(s0 & 0x07) << 2 | (s1 & 0xc0) >> 6];
		t[2] = (slen <= 1? '='
		     : b32[                   (s1 & 0x3e) >> 1]);
		t[3] = (slen <= 1? '='
		     : b32[(s1 & 0x01) << 4 | (s2 & 0xf0) >> 4]);
		t[4] = (slen <= 2? '='
		     : b32[(s2 & 0x0f) << 1 | (s3 & 0x80) >> 7]);
		t[5] = (slen <= 3? '='
		     : b32[                   (s3 & 0x7c) >> 2]);
		t[6] = (slen <= 3? '='
		     : b32[(s3 & 0x03) << 3 | (s4 & 0xe0) >> 5]);
		t[7] = (slen <= 4? '='
		     : b32[(s4 & 0x1f) << 0                   ]);

		s += slen, slen -= slen;
		t += 8, tlen -= 8;
	}

	/* NUL-terminate.  */
	*t++ = '\0', tlen--;

	/* We had better have encoded the size we promised.  */
	assert((size_t)(t - target) ==
	    base32enc_size((size_t)(s - (const uint8_t *)source)) + 1);
}