#include <stdio.h>
#include <stdlib.h>

unsigned char *readBMP(char *filename, int *size) {
    int width, height;
    unsigned char *data;
    unsigned char info[54];

    FILE *file = fopen(filename, "rb");
    if (file == NULL)
        return 0;

    fread(info, sizeof(unsigned char), 54, file); // read the 54-byte header

    // extract image height and width from header
    width = *(int *) &info[18];
    height = *(int *) &info[22];

    *size = 3 * width * height;
    data = (unsigned char *) malloc(*size * sizeof(unsigned char)); // allocate 3 bytes per pixel
    fread(data, sizeof(unsigned char), (size_t) *size, file); // read the rest of the data at once

    for (int i = 0; i < *size; i += 3) {
        unsigned char tmp = data[i];
        data[i] = data[i + 2];
        data[i + 2] = tmp;
    }

    fclose(file);
    return data;
}

int main() {
    int size = 0;
    char filename[] = "output.bmp";
    unsigned char *data = readBMP(filename, &size);

    for (int i = 0; i < size; i++) {
        printf("%d. %d\n", i + 1, (int) data[i]);
        if ((i + 1) % 3 == 0)
            printf("\n");
    }

    free(data);
    return 0;
}