Дан текстовый файл. решить на С!!

#include <stdio.h>
#include <conio.h>
#include <stdlib.h>
#include <string.h>
 
#define INPUT_FILENAME "input.txt"
#define MAX_SIZE 64
 
#define SUB_STR_ONE "AB"
#define SUB_STR_TWO "abcdef"
#define SUB_STR_THREE "abcd"
 
int main()
{
    FILE* input;
    char buffer[MAX_SIZE];
    int count = 0;
    int find_state = 0;
    char* pos;
    int count_array[4] = { 0 };
 
    if ((input = fopen(INPUT_FILENAME, "r")) == NULL)
    {
        fprintf(stderr, "Can't open file  \"%s\"", INPUT_FILENAME);
        _getch();
        _exit(EXIT_FAILURE);
    }
 
    while (fgets(buffer, MAX_SIZE, input))
    {
        pos = buffer;
        while ((pos = strstr(pos + 1, SUB_STR_ONE)))
            ++count;
 
        if (!find_state && strstr(buffer, SUB_STR_TWO))
            find_state = !find_state;
 
        for (int i = 0; i < 4; ++i)
        {
            for (pos = buffer; pos = strchr(pos + 1, SUB_STR_THREE[i]); (*(count_array + i))++);
        }
    }
    
    fclose(input);
 
    if ((input = fopen(INPUT_FILENAME, "a")) == NULL)
    {
        fprintf(stderr, "Can't open file  \"%s\"", INPUT_FILENAME);
        _getch();
        _exit(EXIT_FAILURE);
    }
 
    fprintf(input, "\nCount of \"%s\": %d", SUB_STR_ONE, count);
    fprintf(input, "\n\"%s\": %d", SUB_STR_TWO, find_state);
 
    for (int i = 0; i < 4; ++i)
        fprintf(input, "\n\"%c\": %d", SUB_STR_THREE[i], count_array[i]);
 
    _getch();
    return 0;
}