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

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

    // take program arguments
    if (argc != 3) {
        printf("Bad call. Syntax: 'filtertouppercase source destination'\n");
        return 1;
    }
    else {
        source = argv[1];
        destination = argv[2];
    }

    // open source
    int source_fd = open(source, O_RDONLY); // source file descriptor
    if (source_fd == -1) {
        printf("Error opening source file. Aborting...\n");
        return 2;
    }

    // check for destination and confirm overwrite
    if (access(destination, R_OK) != -1) {
        printf("The output file already exists. Confirm overwrite? Y/N\n");
        char answer;
        scanf("%c", &answer);
        if (answer != 'Y') {
            printf("Could not write to destination file. Aborting...\n");
            return 4;
        }
    }

    // open destination (create if it doesn't exist, wipe it if it does)
    int destination_fd = open(destination, O_WRONLY | O_CREAT | O_TRUNC);
    if (destination_fd == -1) {
        printf("Error opening destination file. Aborting...\n");
        return 5;
    }

    // read text in 1KB chunks, convert them, and write them
    char chunk[1024];
    // first read
    int chunk_size = read(source_fd, chunk, 1024);
    if (chunk_size == -1) {
        printf("Error reading from source file. Aborting...\n");
        return 3;
    }

    while (chunk_size > 0) {
        // conversion
        for (int i = 0; i < chunk_size; i++) {
            if (chunk[i] >= 'a' && chunk[i] <= 'z') {
                chunk[i] += 'A' - 'a';
            }
        }
        // writing
        write(destination_fd, chunk, chunk_size);
        // reading
        chunk_size = read(source_fd, chunk, 1024);
    }

    // close files
    close(source_fd);
    close(destination_fd);
    printf("Program finished execution succesfully.\n");

    return 0;
}