C-bytebeat-render/src/fwrite_le.c

#include "fwrite_le.h"

int detect_endianness(void) {
	volatile uint32_t i = 0x01234567;
	uint8_t* bytes = (uint8_t*)(&i);
	if (bytes[0] == 0x01 &&
		bytes[1] == 0x23 &&
		bytes[2] == 0x45 &&
		bytes[3] == 0x67)
		return DETECTED_BIG_ENDIAN;
	else if (
		bytes[0] == 0x67 &&
		bytes[1] == 0x45 &&
		bytes[2] == 0x23 &&
		bytes[3] == 0x01)
		return DETECTED_LITTLE_ENDIAN;
	return UNSUPPORTED_ENDIANNESS;
}

size_t fwrite_le(void* ptr, size_t size, size_t count, FILE* stream) {
	/*
	 * warning: this function modifies `void* ptr` by default!
	 *          (if FWRITE_LE_NO_MODIFICATION in the header is 0)
	 */
	if (ptr == NULL)
		return 0;

	int endianness = detect_endianness();
	if (endianness == UNSUPPORTED_ENDIANNESS) {
		fprintf(stderr, "Unsupported endianness\n");
		exit(EXIT_FAILURE);
	} else if (endianness == DETECTED_LITTLE_ENDIAN)
		return fwrite(ptr, size, count, stream);

	// case: big-endian
	size_t bytes_count = size * count;
#if FWRITE_LE_NO_MODIFICATION
	uint8_t* bytes = calloc(bytes_count, sizeof(uint8_t));
	if (bytes == NULL) {
		perror("calloc");
		exit(EXIT_FAILURE);
	}
	memcpy(bytes, ptr, bytes_count);
#else
	uint8_t* bytes = (uint8_t*) ptr;
#endif

	for (size_t i = 0; i < bytes_count; i += size) {
		const size_t div_size = size >> 1; // divide by 2
		for (size_t j = 0; j < div_size; j++) {
			const size_t old_pos = i + j, new_pos = i + size - j - 1;
#if FWRITE_LE_NO_MODIFICATION
			bytes[old_pos] = ptr[new_pos];
			bytes[new_pos] = ptr[old_pos];
#else
			uint8_t temp = bytes[old_pos];
			bytes[old_pos] = bytes[new_pos];
			bytes[new_pos] = temp;
#endif
		}
	}

#if FWRITE_LE_NO_MODIFICATION
	size_t res =
#else
	return
#endif
		fwrite(bytes, size, count, stream);
#if FWRITE_LE_NO_MODIFICATION
	free(bytes);
	return res;
#endif
}