/*
 * utility fn for MD5 style padding as follows:
 *   ret = strtopb([in] in, [in] src_len, [out ptr] out_size
 *
 * in: src is source buf, src_len is len in bits
 * out: out_size is filled with size of padded buffer, return value is a pointer
 * to beginning of padded buffer.
 *
 * cross platform & leakless per valgrind:
 *
 * == HEAP SUMMARY:
 * ==     in use at exit: 0 bytes in 0 blocks
 * ==   total heap usage: 2 allocs, 2 frees, 525 bytes allocated
 * ==
 * == All heap blocks were freed -- no leaks are possible
 *
 * TODO: add [in] block_size, require congruent mod { 2**n }
 *
 * James Skarzinskas <james@jskarzin.org>
 */
#include <stdio.h>      /* stderr, stdout */
#include <stdlib.h>     /* malloc(), free(), EXIT_SUCCESS */
#include <string.h>     /* memset(), strlen() */
#include <inttypes.h>   /* uint64_t */
#include <assert.h>     /* assert() */

#define _LINUX          /* uncomment on windows */
#define _TEST_H         /* include entry point main to do sanity tests */

/* if on Linux, macro stdint's _byteswap_uint64 -> __builtin_bswap64 */
#ifdef _LINUX
#define _byteswap_uint64(X) __builtin_bswap64((X))
#endif // _LINUX

/* given an input, return a buffer of 512-bit blocks tailed by input bit
 * size as a big endian uint64:
 *
 * 1. set n to next multiple of m congrent mod 448 (why? 512 - 64 bits)
 * 2. allocate buffer of size n, prepending input data
 * 3. append bit '1' and then zero until n
 * 4. set last 8 bytes to big endian 64-bit length
 *
 * next multiple: (x + n) & ~(n-1) <-- modulo of 2^n := AND, could also & 0x200
 * returns padded buffer or NULL on failure.
 */
char*
strtopb(const char *src, size_t src_len, size_t *out_size) {
	/* sanity check inputs */
	assert(src_len > 0);
	assert(src != NULL);

	unsigned int ml = src_len * 8;   /* msg length in bits */
								     /* number of bits required to store in 512-bit chunks */
	unsigned int     nb = ((ml + sizeof(uint64_t)) + 512) & ~511;
	unsigned int     rb = nb - sizeof(uint64_t);
	uint64_t  u64_be_len = _byteswap_uint64(ml);  /* cross platform endianness swap */

	char *buf = (char *)malloc(nb);  /* allocate the padded message block */
	memset(buf, 0, nb);              /* zero the buffer */
	memcpy(buf, (char *)src, ml);    /* prepend the source */
                                     /* cheat: append a big endian 1 */
	*(buf + ml) = 0x80;              /* 0b1000000*/
	if (NULL != out_size) {          /* avoid exception if no out_size */
		*out_size = nb;              /* set output size */
	}
	/* copy the big endian length to last 64 bits */
	memcpy((unsigned char *)buf + rb, (unsigned char *)&u64_be_len, sizeof(uint64_t));
	return buf;
}

#ifdef _TEST_H
int main()
{
	/* test case variables */
	const char* input_string = "Hello there!";
	size_t out, len = strlen(input_string);
	uint64_t result_len;
	char* output_string;
	int nb, i;

	/* sanity on control data */
	assert(input_string == "Hello there!");
	assert(len == 12);
	char* result = strtopb(input_string, len, &out);
	assert(out == 512);

	memcpy((unsigned char *)&result_len, &result[504], sizeof(uint64_t));
	result_len = _byteswap_uint64(result_len);
	assert(result_len / 8 == 12);

	/* test assumes 32-bit integers */
	nb = sizeof(unsigned char) * ~~(result_len / 8);
	output_string = (char *)malloc(nb + 1);

	for (i = 0; i <= result_len / 8; i++) {
		output_string[i] = (unsigned char)result[i];
	}

	/* test we haven't mangled our data */
	assert((0 == memcmp(input_string, output_string, nb)));

	/* dispose resources & exit */
	free(output_string);
	free(result);

	fprintf(stdout, "Done successfully.\r\n");
	return EXIT_SUCCESS;
}
#endif // _TEST_H