text: #include <stdint.h>
  #include <string.h>
  
  #include "sha-256.h"
  
  #define CHUNK_SIZE 64
  #define TOTAL_LEN_LEN 8
  
  /*
   * ABOUT bool: this file does not use bool in order to be as pre-C99 compatible as possible.
   */
  
  /*
   * 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; /* bool */
  	int total_len_delivered; /* bool */
  };
  
  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;
  }
  
  /* Return value: bool */
  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;
  		size_t len = state->total_len;
  		int i;
  		memset(chunk, 0x00, left);
  		chunk += left;
  
  		/* Storing of len * 8 as a big endian 64-bit without overflow. */
  		chunk[7] = (uint8_t) (len << 3);
  		len >>= 5;
  		for (i = 6; i >= 0; i--) {
  			chunk[i] = (uint8_t) len;
  			len >>= 8;
  		}
  		state->total_len_delivered = 1;
  	} else {
  		memset(chunk, 0x00, space_in_chunk);
  	}
  
  	return 1;
  }
  
  /*
   * Limitations:
   * - Since input is a pointer in RAM, the data to hash should be in RAM, which could be a problem
   *   for large data sizes.
   * - SHA algorithms theoretically operate on bit strings. However, this implementation has no support
   *   for bit string lengths that are not multiples of eight, and it really operates on arrays of bytes.
   *   In particular, the len parameter is a number of bytes.
   */
  void calc_sha_256(uint8_t hash[32], const void * input, size_t len)
  {
  	/*
  	 * Note 1: All integers (expect indexes) are 32-bit unsigned integers and addition is calculated modulo 2^32.
  	 * 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 h[] = { 0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19 };
  	unsigned i, j;
  
  	/* 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 ah[8];
  
  		const uint8_t *p = chunk;
  
  		/* Initialize working variables to current hash value: */
  		for (i = 0; i < 8; i++)
  			ah[i] = h[i];
  
  		/* Compression function main loop: */
  		for (i = 0; i < 4; i++) {
  			/*
  			 * The w-array is really w[64], but since we only need
  			 * 16 of them at a time, we save stack by calculating
  			 * 16 at a time.
  			 *
  			 * This optimization was not there initially and the
  			 * rest of the comments about w[64] are kept in their
  			 * initial state.
  			 */
  
  			/*
  			 * 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[16];
  
  			for (j = 0; j < 16; j++) {
  				if (i == 0) {
  					w[j] = (uint32_t) p[0] << 24 | (uint32_t) p[1] << 16 |
  						(uint32_t) p[2] << 8 | (uint32_t) p[3];
  					p += 4;
  				} else {
  					/* Extend the first 16 words into the remaining 48 words w[16..63] of the message schedule array: */
  					const uint32_t s0 = right_rot(w[(j + 1) & 0xf], 7) ^ right_rot(w[(j + 1) & 0xf], 18) ^ (w[(j + 1) & 0xf] >> 3);
  					const uint32_t s1 = right_rot(w[(j + 14) & 0xf], 17) ^ right_rot(w[(j + 14) & 0xf], 19) ^ (w[(j + 14) & 0xf] >> 10);
  					w[j] = w[j] + s0 + w[(j + 9) & 0xf] + s1;
  				}
  				const uint32_t s1 = right_rot(ah[4], 6) ^ right_rot(ah[4], 11) ^ right_rot(ah[4], 25);
  				const uint32_t ch = (ah[4] & ah[5]) ^ (~ah[4] & ah[6]);
  				const uint32_t temp1 = ah[7] + s1 + ch + k[i << 4 | j] + w[j];
  				const uint32_t s0 = right_rot(ah[0], 2) ^ right_rot(ah[0], 13) ^ right_rot(ah[0], 22);
  				const uint32_t maj = (ah[0] & ah[1]) ^ (ah[0] & ah[2]) ^ (ah[1] & ah[2]);
  				const uint32_t temp2 = s0 + maj;
  
  				ah[7] = ah[6];
  				ah[6] = ah[5];
  				ah[5] = ah[4];
  				ah[4] = ah[3] + temp1;
  				ah[3] = ah[2];
  				ah[2] = ah[1];
  				ah[1] = ah[0];
  				ah[0] = temp1 + temp2;
  			}
  		}
  
  		/* Add the compressed chunk to the current hash value: */
  		for (i = 0; i < 8; i++)
  			h[i] += ah[i];
  	}
  
  	/* Produce the final hash value (big-endian): */
  	for (i = 0, j = 0; i < 8; i++)
  	{
  		hash[j++] = (uint8_t) (h[i] >> 24);
  		hash[j++] = (uint8_t) (h[i] >> 16);
  		hash[j++] = (uint8_t) (h[i] >> 8);
  		hash[j++] = (uint8_t) h[i];
  	}
  }
  