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

bool es_digito(char c) {
    return (c >= '0' && c <= '9');
}

bool es_letra(char c) {
    return ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z'));
}

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

bool validar_siguientes(FILE* f, int* seek_lectura) {
    char c;
    for (size_t i = 0; i < 2; i++) {
        if ( (c = leer_byte(f, seek_lectura)) == EOF || !es_digito(c))
            return false;
    }
    for (size_t i = 0; i < 4; i++) {
        if ( (c = leer_byte(f, seek_lectura)) == EOF || !es_letra(c))
            return false;
    }
    return true;
}

int main() {
    FILE* f = fopen("a.bin", "r+");
    if (!f) return -1;
    int seek_lectura = 0;
    int seek_escritura = 0;
    char c;
    while ( (c = leer_byte(f, &seek_lectura)) != EOF ) {
        int seek_actual = seek_lectura;
        --seek_lectura;
        fseek(f, seek_lectura, SEEK_SET);
        if (!validar_siguientes(f, &seek_lectura)) {
            fseek(f, seek_actual, SEEK_SET);
            printf("%c", c);
            escribir_byte(f, &seek_escritura, c);
        }
    }
    fclose(f);
    truncate("a.bin", seek_escritura);
    return 0;
}