/*
 * File:	base64.c
 * Purpose:	Implementation of MIME's Base64 encoding and decoding.
 * Author:	Lars Wirzenius
 * Version:	$Id: base64.c,v 1.2 2003/11/15 18:24:10 liw Exp $
 */

#include <limits.h>
#include "publib/base64.h"

static const unsigned char sixtet_to_base64[] = 
	"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";


/*
 * Function:	base64_length
 * Purpose:	Compute minimum length of encoded output.
 */
size_t base64_length(size_t n) {
	return 4*(n+3)/3 + 1;
}


/*
 * Function:	base64_encode
 * Purpose:	Convert octets to base64
 * Note:	Output buffer must be at least base64_length(n) chars.
 */
size_t base64_encode(char *to, const char *from, size_t n) {
    	unsigned w;
	char *to_start;
	unsigned char *fromp;
	
    	to_start = to;

    	fromp = (unsigned char *) from;
    	for (; n >= 3; n -= 3, fromp += 3) {
	    w = (fromp[0] << 16) | (fromp[1] << 8) | fromp[2];
	    *to++ = sixtet_to_base64[(w >> 18) & 0x3f];
	    *to++ = sixtet_to_base64[(w >> 12) & 0x3f];
	    *to++ = sixtet_to_base64[(w >> 6) & 0x3f];
	    *to++ = sixtet_to_base64[w & 0x3f];
	}

    	switch (n) {
	case 0:
	    /* Nothing to do */
	    break;
    	
	case 1:
	    w = fromp[0];
	    *to++ = sixtet_to_base64[(w >> 2) & 0x3f];
	    *to++ = sixtet_to_base64[(w << 4) & 0x3f];
	    *to++ = '=';
	    *to++ = '=';
	    break;

	case 2:
	    w = (fromp[0] << 8) | fromp[1];
	    *to++ = sixtet_to_base64[(w >> 10) & 0x3f];
	    *to++ = sixtet_to_base64[(w >> 4) & 0x3f];
	    *to++ = sixtet_to_base64[(w << 2) & 0x3f];
	    *to++ = '=';
	    break;
	}
	
	return to - to_start;
}


/*
 * Function:	base64_decode
 * Purpose:	Convert base64 to octets.
 * Note:	Output buffer must be at least 3*n/4 chars.
 *		The output buffer does not get a '\0' appended.
 */
size_t base64_decode(char *to, const char *from, size_t len) {
    	static int base64_to_sixtet[UCHAR_MAX + 1];
	static int tab_init = 0;
	int i;
	unsigned bitbuf;
	int nbits;
	unsigned char *fromp;
	char *to_start;

    	if (!tab_init) {
	    tab_init = 1;
	    for (i = 0; i <= UCHAR_MAX + 1; ++i)
	    	base64_to_sixtet[i] = -1;
	    for (i = 0; sixtet_to_base64[i] != '\0'; ++i)
	    	base64_to_sixtet[sixtet_to_base64[i]] = i;
	}

    	to_start = to;

    	bitbuf = 0;
	nbits = 0;
	fromp = (unsigned char *) from;
    	for (i = 0; i < len && fromp[i] != '='; ++i) {
	    if (base64_to_sixtet[fromp[i]] != -1) {
		bitbuf = (bitbuf << 6) | base64_to_sixtet[fromp[i]];
		nbits += 6;
		if (nbits >= 8) {
		    *to++ = (bitbuf >> (nbits - 8)) & 0xff;
		    bitbuf >>= 8;
		    nbits -= 8;
		}
	    }
	}
	
	return to - to_start;
}