#include <stdio.h>
#include <unistd.h>

char leer_byte(FILE* f, int* seek_lectura) {
    fseek(f, *seek_lectura, SEEK_SET);
    char c = fgetc(f);
    *seek_lectura = ftell(f);
    return c;
}

char leer_byte_reversa(FILE* f, int* seek_lectura) {
    fseek(f, *seek_lectura, SEEK_SET);
    char c = fgetc(f);
    if (ftell(f) == 1) {
        fseek(f, 0, SEEK_SET);
        *seek_lectura = -1;
    } else {
        fseek(f, -2, SEEK_CUR);
        *seek_lectura = ftell(f);
    }
    return c;
}

void escribir_byte(FILE* f, int* seek_escritura, char c) {
    fseek(f, *seek_escritura, SEEK_SET);
    fputc(c, f);
    *seek_escritura = ftell(f);
}

void escribir_byte_reversa(FILE* f, int* seek_escritura, char c) {
    fseek(f, *seek_escritura, SEEK_SET);
    fputc(c, f);
    if (ftell(f) == 1) {
        fseek(f, 0, SEEK_SET);
        *seek_escritura = -1;
    } else {
        fseek(f, -2, SEEK_CUR);
        *seek_escritura = ftell(f);
    }
}


void desplazar_derecha(const char * file_name, long int offset) {
    FILE* f = fopen(file_name, "r");
    fseek(f, 0, SEEK_END);
    int seek_lectura = ftell(f) - 1; // Me paro en la ultima letra
    int seek_escritura = seek_lectura + offset;
    fclose(f);
    truncate(file_name, seek_escritura);
    f = fopen(file_name, "r+");

    while (seek_lectura >= 0) {
        char c = leer_byte_reversa(f, &seek_lectura);
        escribir_byte_reversa(f, &seek_escritura, c);
    }
    for (int i = 0; i < offset; i++){
        escribir_byte_reversa(f, &seek_escritura, '_');
    }
    fclose(f);
}

void desplazar_izquierda(const char * file_name, long int offset) {
    FILE* f = fopen(file_name, "r+");
    int seek_escritura = 0;
    int seek_lectura = offset * -1;

    fseek(f, 0, SEEK_END);
    size_t tam = ftell(f);
    fseek(f, 0, SEEK_SET);

    while (seek_lectura < tam) {
        char c = leer_byte(f, &seek_lectura);
        escribir_byte(f, &seek_escritura, c);
    }
    fclose(f);
    truncate(file_name, tam + offset);
}

void desplazar(const char * file_name, long int offset) {
    if (offset > 0)
        desplazar_derecha(file_name, offset);
    else if (offset < 0)
        desplazar_izquierda(file_name, offset);
}

int main(){
    desplazar("a.bin", -4);
    return 0;
}