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

#define DELTA 0x9E3779B9
#define ROUNDS 32

void decrypt(uint32_t v[2], uint32_t const key[4]) {
    uint32_t v0 = v[0], v1 = v[1], sum = DELTA * ROUNDS;
    for (int i = 0; i < ROUNDS; i++) {
        v1 -= (((v0 << 4) ^ (v0 >> 5)) + v0) ^ (sum + key[(sum >> 11) & 3]);
        sum -= DELTA;
        v0 -= (((v1 << 4) ^ (v1 >> 5)) + v1) ^ (sum + key[sum & 3]);
    }
    v[0] = v0;
    v[1] = v1;
}

void encrypt(uint32_t v[2], uint32_t const key[4]) {
    uint32_t v0 = v[0], v1 = v[1], sum = 0;
    for (int i = 0; i < ROUNDS; i++) {
        v0 += (((v1 << 4) ^ (v1 >> 5)) + v1) ^ (sum + key[sum & 3]);
        sum += DELTA;
        v1 += (((v0 << 4) ^ (v0 >> 5)) + v0) ^ (sum + key[(sum >> 11) & 3]);
    }
    v[0] = v0;
    v[1] = v1;
}

void process_file(const char *mode, const char *keyfile, const char *inputfile, const char *outputfile) {
    uint32_t key[4];
    uint32_t block[2];
    int fd_key = open(keyfile, O_RDONLY);
    int fd_in = open(inputfile, O_RDONLY);
    int fd_out = open(outputfile, O_WRONLY | O_CREAT | O_TRUNC, 0600);
    
    if (fd_key < 0 || fd_in < 0 || fd_out < 0) {
        perror("Error opening file");
        exit(1);
    }
    read(fd_key, key, sizeof(key));
    close(fd_key);
    
    while (read(fd_in, block, sizeof(block)) == sizeof(block)) {
        if (strcmp(mode, "-d") == 0) {
            decrypt(block, key);
        } else {
            encrypt(block, key);
        }
        write(fd_out, block, sizeof(block));
    }
    close(fd_in);
    close(fd_out);
}

int main(int argc, char *argv[]) {
    if (argc != 5) {
        fprintf(stderr, "Usage: %s -e|-d filekey file1 file2\n", argv[0]);
        return 1;
    }
    process_file(argv[1], argv[2], argv[3], argv[4]);
    return 0;
}