#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include "images.h"

// write an int to a byte array
void bmp__write_int(FILE *fp, int num) {
    for (int i = 0; i < 4; i++) {
        putc(num >> (i * 8), fp);
    }
}

void bmp__write_16bit(FILE *fp, int num) {
    for (int i = 0; i < 2; i++) {
        putc(num >> (i * 8), fp);
    }
}

unsigned long int bmp__align_to_four_bytes(unsigned long int row_length) {
    unsigned long int os =row_length % 4;
    return os == 0 ?row_length : row_length + (4 - os);
}

unsigned long int bmp__image_byte_size(Image image) {
    // calculate size:
    // bitmap file header = 14
    // BITMAPCOREHEADER = 12 bytes
    unsigned long int size = 14 + 12;
    // width must be padded to align to 4 bytes
    unsigned long int row_bytes = bmp__align_to_four_bytes(image.width * 3); // 3 bytes per pixel
    // add row_bytes * height to total size
    size += row_bytes * image.height;

    return size;
}

// header format: 
// BMP header (14 bytes)
// 0x42 0x4D            "BM"
// 0xAA 0xBB 0xCC 0xDD  bmp file size (int)
// 0x00 0x00 0x00 0x00  nulls
// 0x1A 0x00 0x00 0x00  pixel array offset (int) (= 14 + 12);
// DIB  Header (BITMAPCOREHEADER) (12 bytes)
// 0x0C 0x00 0x00 0x00  header size (int) (= 12)
// 0xAA 0xBB            width (unsigned 16 bit int)
// 0xCC 0xDD            height (unsigned 16 bit int)
// 0x01 0x00            number of color planes (must be 1)
// 0x18 0x00            number of bits per pixel (3 * 8 = 24)


// char* bytes;

// bytes + 2    => zeigt auf offset 2   (ist char*)
// bytes[2]     => wert an offset 2     (ist char)

int image_save_bmp(Image image, const char* path) {
    unsigned long int size = bmp__image_byte_size(image);
    if ((unsigned long int) size > (unsigned long int) UINT_MAX) {
        printf("bitmap is too big!\n");
        return 0;
    } else {
        printf("predicted size is %lu out of %u possible", size, UINT_MAX);
    }
    FILE *fp = fopen(path, "w");
    //unsigned char* bytes = calloc(sizeof(char), size);
    // magic bytes, signature "BM"
    // bmp header:
    fputs("BM", fp);              // offset  0
    // write size to header
    bmp__write_int(fp, size);     // offset  2
    bmp__write_int(fp, 0);        // offset  6 - the next 4 byte are zeros
    // pixel array offset
    bmp__write_int(fp, 14 + 12);  // offset 10
    // DIB header: 
    bmp__write_int(fp, 12);             // header size
    bmp__write_16bit(fp, image.width);  // width
    bmp__write_16bit(fp, image.height); // height
    bmp__write_16bit(fp, 1);            // color planes
    bmp__write_16bit(fp, 3 * 8);        // bits per pixel

    int line, px;
    for (line = 0; line < image.height; line++) {
        for (px = 0; px < image.width * 3; px += 3) {
            putc(image.bitmap[line][px + 2], fp);
            putc(image.bitmap[line][px + 1], fp);
            putc(image.bitmap[line][px + 0], fp);
        }
        int mod = px % 4;
        if (mod != 0) {
            for (int i = 0; i < 4-mod; i++) {
                putc(0, fp);
            }
        }
    }

    printf("wrote %li bytes...\n", size);
    fclose(fp);
    return 1;
}