#include <stdint.h>
#include <string.h>

#include "sha-256.h"

#define CHUNK_SIZE 64
#define TOTAL_LEN_LEN 8

/*
 * Comments from pseudo-code at https://en.wikipedia.org/wiki/SHA-2 are reproduced here.
 * When useful for clarification, portions of the pseudo-code are reproduced here too.
 */

/*
 * Initialize array of round constants:
 * (first 32 bits of the fractional parts of the cube roots of the first 64 primes 2..311):
 */
static const uint32_t k[] = {
	0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
	0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,
	0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
	0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7, 0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967,
	0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13, 0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85,
	0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3, 0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
	0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
	0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2
};

struct buffer_state {
	const uint8_t * p;
	size_t len;
	size_t total_len;
	int single_one_delivered;
	int total_len_delivered;
};

static inline uint32_t right_rot(uint32_t value, unsigned int count)
{
	/*
	 * Defined behaviour in standard C for all count where 0 < count < 32,
	 * which is what we need here.
	 */
	return value >> count | value << (32 - count);
}

static void init_buf_state(struct buffer_state * state, const void * input, size_t len)
{
	state->p = input;
	state->len = len;
	state->total_len = len;
	state->single_one_delivered = 0;
	state->total_len_delivered = 0;
}

static int calc_chunk(uint8_t chunk[CHUNK_SIZE], struct buffer_state * state)
{
	size_t space_in_chunk;

	if (state->total_len_delivered) {
		return 0;
	}

	if (state->len >= CHUNK_SIZE) {
		memcpy(chunk, state->p, CHUNK_SIZE);
		state->p += CHUNK_SIZE;
		state->len -= CHUNK_SIZE;
		return 1;

	}

	memcpy(chunk, state->p, state->len);
	chunk += state->len;
	space_in_chunk = CHUNK_SIZE - state->len;
	state->p += state->len;
	state->len = 0;

	/* If we are here, space_in_chunk is one at minimum. */
	if (!state->single_one_delivered) {
		*chunk++ = 0x80;
		space_in_chunk -= 1;
		state->single_one_delivered = 1;
	}

	/*
	 * Now:
	 * - either there is enough space left for the total length, and we can conclude,
	 * - or there is too little space left, and we have to pad the rest of this chunk with zeroes.
	 * In the latter case, we will conclude at the next invokation of this function.
	 */
	if (space_in_chunk >= TOTAL_LEN_LEN) {
		const size_t left = space_in_chunk - TOTAL_LEN_LEN;
		const size_t len = state->total_len * 8;
		memset(chunk, 0x00, left);
		chunk += left;

		if (sizeof len > 4) {
			chunk[0] = (uint8_t) (len >> 56);
			chunk[1] = (uint8_t) (len >> 48);
			chunk[2] = (uint8_t) (len >> 40);
			chunk[3] = (uint8_t) (len >> 32);
		} else {
			chunk[0] = 0;
			chunk[1] = 0;
			chunk[2] = 0;
			chunk[3] = 0;
		}
		if (sizeof len > 2) {
			chunk[4] = (uint8_t) (len >> 24);
			chunk[5] = (uint8_t) (len >> 16);
		} else {
			chunk[4] = 0;
			chunk[5] = 0;
		}
		
		if (sizeof len > 1) {
			chunk[6] = (uint8_t) (len >> 8);
		} else {
			chunk[6] = 0;
		}
		
		chunk[7] = (uint8_t) len;

		state->total_len_delivered = 1;
	} else {
		memset(chunk, 0x00, space_in_chunk);
	}

	return 1;
}

/*
 * Limitations:
 * - len must be small enough for (8 * len) to fit in len. Otherwise, the results are unpredictable.
 * - sizeof size_t is assumed to be either 8, 16, 32 or 64. Otherwise, the results are unpredictable.
 * - Since input is a pointer in RAM, the data to hash should be in RAM, which could be a problem
 *   for large data sizes.
 */
void calc_sha_256(uint8_t hash[32], const void * input, size_t len)
{
	/*
	 * Note 1: All variables are 32 bit unsigned integers and addition is calculated modulo 232
	 * Note 2: For each round, there is one round constant k[i] and one entry in the message schedule array w[i], 0 = i = 63
	 * Note 3: The compression function uses 8 working variables, a through h
	 * Note 4: Big-endian convention is used when expressing the constants in this pseudocode,
	 *     and when parsing message block data from bytes to words, for example,
	 *     the first word of the input message "abc" after padding is 0x61626380
	 */

	/*
	 * Initialize hash values:
	 * (first 32 bits of the fractional parts of the square roots of the first 8 primes 2..19):
	 */

	uint32_t h0 = 0x6a09e667;
	uint32_t h1 = 0xbb67ae85;
	uint32_t h2 = 0x3c6ef372;
	uint32_t h3 = 0xa54ff53a;
	uint32_t h4 = 0x510e527f;
	uint32_t h5 = 0x9b05688c;
	uint32_t h6 = 0x1f83d9ab;
	uint32_t h7 = 0x5be0cd19;

	/* 512-bit chunks is what we will operate on. */

	uint8_t chunk[64];

	struct buffer_state state;

	init_buf_state(&state, input, len);

	while (calc_chunk(chunk, &state)) {
		uint32_t a, b, c, d, e, f, g, h;
		
		/*
		 * create a 64-entry message schedule array w[0..63] of 32-bit words
		 * (The initial values in w[0..63] don't matter, so many implementations zero them here)
		 * copy chunk into first 16 words w[0..15] of the message schedule array
		 */
		uint32_t w[64];
		const uint8_t *p = chunk;

		memset(w, 0x00, sizeof w);
		for (int i = 0; i < 16; i++) {
			w[i] = (uint32_t) p[0] << 24 | (uint32_t) p[1] << 16 |
				(uint32_t) p[2] << 8 | (uint32_t) p[3];
			p += 4;
		}

		/* Extend the first 16 words into the remaining 48 words w[16..63] of the message schedule array: */
		for (int i = 16; i < 64; i++) {
			const uint32_t s0 = right_rot(w[i - 15], 7) ^ right_rot(w[i - 15], 18) ^ (w[i - 15] >> 3);
			const uint32_t s1 = right_rot(w[i - 2], 17) ^ right_rot(w[i - 2], 19) ^ (w[i - 2] >> 10);
			w[i] = w[i - 16] + s0 + w[i - 7] + s1;
		}
		
		/* Initialize working variables to current hash value: */
		a = h0;
		b = h1;
		c = h2;
		d = h3;
		e = h4;
		f = h5;
		g = h6;
		h = h7;

		/* Compression function main loop: */
		for (int i = 0; i < 64; i++) {
			const uint32_t s1 = right_rot(e, 6) ^ right_rot(e, 11) ^ right_rot(e, 25);
			const uint32_t ch = (e & f) ^ (~e & g);
			const uint32_t temp1 = h + s1 + ch + k[i] + w[i];
			const uint32_t s0 = right_rot(a, 2) ^ right_rot(a, 13) ^ right_rot(a, 22);
			const uint32_t maj = (a & b) ^ (a & c) ^ (b & c);
			const uint32_t temp2 = s0 + maj;

			h = g;
			g = f;
			f = e;
			e = d + temp1;
			d = c;
			c = b;
			b = a;
			a = temp1 + temp2;
		}

		/* Add the compressed chunk to the current hash value: */
		h0 = h0 + a;
		h1 = h1 + b;
		h2 = h2 + c;
		h3 = h3 + d;
		h4 = h4 + e;
		h5 = h5 + f;
		h6 = h6 + g;
		h7 = h7 + h;
	}

	hash[0] = (uint8_t) (h0 >> 24);
	hash[1] = (uint8_t) (h0 >> 16);
	hash[2] = (uint8_t) (h0 >> 8);
	hash[3] = (uint8_t) h0;
	hash[4] = (uint8_t) (h1 >> 24);
	hash[5] = (uint8_t) (h1 >> 16);
	hash[6] = (uint8_t) (h1 >> 8);
	hash[7] = (uint8_t) h1;
	hash[8] = (uint8_t) (h2 >> 24);
	hash[9] = (uint8_t) (h2 >> 16);
	hash[10] = (uint8_t) (h2 >> 8);
	hash[11] = (uint8_t) h2;
	hash[12] = (uint8_t) (h3 >> 24);
	hash[13] = (uint8_t) (h3 >> 16);
	hash[14] = (uint8_t) (h3 >> 8);
	hash[15] = (uint8_t) h3;
	hash[16] = (uint8_t) (h4 >> 24);
	hash[17] = (uint8_t) (h4 >> 16);
	hash[18] = (uint8_t) (h4 >> 8);
	hash[19] = (uint8_t) h4;
	hash[20] = (uint8_t) (h5 >> 24);
	hash[21] = (uint8_t) (h5 >> 16);
	hash[22] = (uint8_t) (h5 >> 8);
	hash[23] = (uint8_t) h5;
	hash[24] = (uint8_t) (h6 >> 24);
	hash[25] = (uint8_t) (h6 >> 16);
	hash[26] = (uint8_t) (h6 >> 8);
	hash[27] = (uint8_t) h6;
	hash[28] = (uint8_t) (h7 >> 24);
	hash[29] = (uint8_t) (h7 >> 16);
	hash[30] = (uint8_t) (h7 >> 8);
	hash[31] = (uint8_t) h7;
}