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

/*
file structure:
    ID3v2/file identifier   "ID3"
    ID3v2 version           $03 00
    ID3v2 flags             %abc00000
    ID3v2 size              4 * %0xxxxxxx
    Frame ID       $xx xx xx xx (four characters)
    Size           $xx xx xx xx
    Flags          $xx xx
    <Frame contents>
    etc...
 */

unsigned int bswap_32(unsigned int num){
    return ((num >> 24) & 0xff) | ((num << 8) & 0xff0000) | ((num >> 8) & 0xff00) | ((num << 24) & 0xff000000);
}

void filecopy(FILE *fp, FILE *ofp){
    int c;
    while ((c = getc(fp)) != EOF) putc(c, ofp);
}

union tag_header{
    char buffer[12];
    struct{
        unsigned short empty;
        unsigned char version[3];
        unsigned char rev1;
        unsigned char rev2;
        unsigned char flags;
        unsigned int size;
    } data;
};

union frame_header{
    char buffer[10];
    struct{
        unsigned char name[4];
        unsigned int size;
        unsigned short flags;
    } data;
};

void show(char* media_file_name){
    FILE* media_file = fopen(media_file_name, "rb");
    fseek(media_file, 0, SEEK_SET);
    union tag_header tag_header;
    fread(tag_header.buffer + 2, sizeof(char), 10, media_file);
    unsigned int tag_size = bswap_32(tag_header.data.size);
    printf("%sv%d.%d\n", tag_header.data.version, tag_header.data.rev1, tag_header.data.rev2);
    while (ftell(media_file) < tag_size + 10){
        union frame_header frame_header;
        fread(frame_header.buffer, sizeof(char), 10, media_file);
        printf("%s: ", frame_header.data.name);
        unsigned int frame_size = bswap_32(frame_header.data.size);
        unsigned char* frame_contents = malloc(sizeof(char) * frame_size);
        fread(frame_contents, sizeof(char), frame_size, media_file);
        for (unsigned int i = 0; i < frame_size; i++) printf("%c", frame_contents[i]);
        printf("\n");
        memset(frame_contents, 0, frame_size);
        free(frame_contents);
    }
    fclose(media_file);
}

void get(char* media_file_name, char sought_frame_name[4]){
    FILE* media_file = fopen(media_file_name, "rb");
    union tag_header tag_header;
    fread(tag_header.buffer + 2, sizeof(char), 10, media_file);
    unsigned int tag_size = bswap_32(tag_header.data.size);
    while (ftell(media_file) < tag_size + 10){
        union frame_header frame_header;
        fread(frame_header.buffer, sizeof(char), 10, media_file);
        long long current_frame_size = bswap_32(frame_header.data.size);
        if (!strcmp(frame_header.data.name, sought_frame_name)){
            printf("%s: ", frame_header.data.name);
            unsigned int frame_size = bswap_32(frame_header.data.size);
            unsigned char* frame_contents = malloc(sizeof(char) * frame_size);
            fread(frame_contents, sizeof(char), frame_size, media_file);
            for (unsigned int i = 0; i < frame_size; i++) printf("%c", frame_contents[i]);
            printf("\n");
            free(frame_contents);
            fclose(media_file);
            return;
        }
        fseek(media_file, current_frame_size, SEEK_CUR);
    }
    fclose(media_file);
    printf("No value found for %s!\n", sought_frame_name);
}

void set(char* media_file_name, char new_frame_name[4], char* new_frame_value){
    FILE* media_file = fopen(media_file_name, "rb");
    union tag_header tag_header;
    fread(tag_header.buffer + 2, sizeof(char), 10, media_file);
    unsigned int tag_size = bswap_32(tag_header.data.size);
    long long frame_to_change_pos = 0, frame_to_change_size = 0;
    while (ftell(media_file) < tag_size + 10){
        union frame_header frame_header;
        fread(frame_header.buffer, sizeof(char), 10, media_file);
        long long current_frame_size = bswap_32(frame_header.data.size);
        if (!strcmp(frame_header.data.name, new_frame_name)){
            frame_to_change_pos = ftell(media_file) - 10;
            frame_to_change_size = current_frame_size;
            break;
        }
        fseek(media_file, current_frame_size, SEEK_CUR);
    }
    if (frame_to_change_pos == 0) frame_to_change_pos = ftell(media_file);
    unsigned int new_value_size = strlen(new_frame_value);
    unsigned int new_tag_size = tag_size - frame_to_change_size + new_value_size;
    if (new_value_size == 0) new_tag_size -= 10;
    char* media_file_copy_name;
    media_file_copy_name = "temp.mp3";
    FILE* media_file_copy = fopen(media_file_copy_name, "wb");
    fseek(media_file, 0, SEEK_SET);
    fseek(media_file_copy, 0, SEEK_SET);
    filecopy(media_file, media_file_copy);
    fclose(media_file_copy);
    fclose(media_file);
    media_file_copy = fopen(media_file_copy_name, "rb");
    media_file = fopen(media_file_name, "wb");
    tag_header.data.size = bswap_32(new_tag_size);
    fwrite(tag_header.buffer + 2, sizeof(char), 10, media_file);
    fseek(media_file_copy, 10, SEEK_SET);
    for(unsigned long long i = 0; i < frame_to_change_pos - 10; i++){
        int c;
        c = getc(media_file_copy);
        putc(c, media_file);
    }
    if (new_value_size){
        union frame_header frame_header;
        frame_header.data.name[0] = new_frame_name[0];
        frame_header.data.name[1] = new_frame_name[1];
        frame_header.data.name[2] = new_frame_name[2];
        frame_header.data.name[3] = new_frame_name[3];
        frame_header.data.size = bswap_32(new_value_size);
        frame_header.data.flags = 0;
        fwrite(frame_header.buffer, sizeof(char), 10, media_file);
    }
    fwrite(new_frame_value, sizeof(char), new_value_size, media_file);
    fseek(media_file_copy, frame_to_change_pos + 10 + frame_to_change_size , SEEK_SET);
    for(unsigned long long i = ftell(media_file); i < new_tag_size; i++){
        unsigned short int c;
        c = getc(media_file_copy);
        putc(c, media_file);
    }
    printf("Frame set: %s -> \"%s\"\n", new_frame_name, new_frame_value);
    filecopy(media_file_copy, media_file);
    fclose(media_file);
    fclose(media_file_copy);
    remove(media_file_copy_name);
}

int main(int argc, char* argv[]){
    char* filepath;
    filepath = strpbrk(argv[1],"=") + 1;
    for (int i = 2;i < argc; i++){
        if (!strcmp(argv[i], "--show")){
            show(filepath);
            continue;
        }
        if (argv[i][2]=='g'){
            get(filepath, strpbrk(argv[i],"=") + 1);
            continue;
        }
        if (argv[i][2]=='s'){
            set(filepath, strpbrk(argv[i],"=") + 1, strpbrk(argv[i + 1], "=") + 1);
            continue;
        }
    }
    return 0;
}