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

#define HEADER_SIZE 54
#define PIXEL_SIZE  3
#define MAX_PATH_SIZE 4096

int main(int argc, char *argv[]) {

    printf("Starting ...\n");

    int c = 0;
    char input[MAX_PATH_SIZE];
    char output[MAX_PATH_SIZE];
    char * ptr;
    char type[3];
    int keep_going = 1;
    unsigned short int bpp;
    char median = 0;
    char header_tab[HEADER_SIZE];
    char pixel_tab[PIXEL_SIZE];
    char pixel_write_tab[PIXEL_SIZE];

    FILE * fin;
    FILE * fout;
    FILE * temp;

    printf("Enter the file name of a 24 bit bitmap image :\n");
    if(fgets(input, MAX_PATH_SIZE, stdin) == NULL){
        printf("Error - Unable to get the char\n");
        return 1;
    }
    if((ptr = strchr(input, '\n')) != NULL) *ptr = '\0';

    printf("Enter a file name for output file:\n");
    if(fgets(output, MAX_PATH_SIZE, stdin) == NULL){
        printf("Error - Unable to get the char\n");
        return 1;
    }
    if((ptr = strchr(output, '\n')) != NULL) *ptr = '\0';

    if(!(temp = fopen(input, "rb"))) {
        printf("Error - Unable to open the file given (errno:%d)\n", errno);
        return 1;
    } else{
        fseek(temp,0,0);
        fread(type,1,2,temp);
        type[2] = '\0';
        fseek(temp,28,0);
        fread(&bpp,1,2,temp);
        fclose(temp);
    }

    if(strcmp(type,"BM") || (bpp != 24)){
        printf("Error - The file is not BMP format or is not 24 bits\n");
        return 1;
    }

    if(strcmp(input, output) != 0){
        fin = fopen(input, "rb");
        //Read header
        c = fread(header_tab, 1, HEADER_SIZE ,fin);
        if (c<HEADER_SIZE){
            printf("Warning - BMP Header to small\n Exiting properly ...\n");
            fclose(fin);
            return 1;
        }
        if(!(fout = fopen(output, "wb"))) {
            printf("Error - Unable to open/create the output file (errno:%d)\n", errno);
            fclose(fin);
            return 1;
        }
        //Write header
        fwrite(header_tab, 1, HEADER_SIZE, fout);

        //To shade of grey
        do
        {
            c = fread(pixel_tab, 1,PIXEL_SIZE, fin);
            if(c!=3){
                keep_going = 0;
            } else{
                median = (char) ((pixel_tab[0] + pixel_tab[1] + pixel_tab[2]) / 3);
                pixel_write_tab[0] = median;
                pixel_write_tab[1] = median;
                pixel_write_tab[2] = median;
                fwrite(pixel_write_tab, 1, PIXEL_SIZE, fout);
            }
        }
        while(keep_going == 1);
    }
    else{
        printf("Error - Unable to write the output in the same file !\n");
        return 1;
    }

    fclose(fin);
    fclose(fout);

    printf("Ending without errors\n");
    return 0;
}