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

int FileSize(char* filename);
int* instances(char* page, char* str);

int main(int argc, char** argv){
   
   if(argc != 4)
   {
      printf("Invalid Syntax\nTry ./fnr filename old_string new_string\n");
      return -1;
   }

   int size = FileSize(argv[1]);
   
   if(size == -1)
   {
      printf("Error could not find file with the name [%s]\n", argv[1]);
      return -1;
   }

   char file_buffer[size];
   int old_str_size = 0;
   int new_str_size = 0;
   
   while(argv[2][old_str_size] != 0)
   {
      old_str_size++;
   }

   while(argv[3][new_str_size] != 0)
   {
      new_str_size++;
   }

   FILE* fptr = fopen(argv[1], "r");
   fread(file_buffer, sizeof(char), size, fptr);
   fclose(fptr);

   int* all_instance = instances(file_buffer, argv[2]);
   int num_instances = 0;
   while(all_instance[num_instances] != -1)
   {
      num_instances++;
   }
   all_instance[num_instances] = 1000000;
   all_instance[num_instances+1] = -1;

   int new_buffer_size = size - (num_instances * old_str_size) + (num_instances * new_str_size);

   char new_buffer[new_buffer_size];

   int i = 0;
   int j = 0;
   int k = 0;
   int n = 0;

   while(i < new_buffer_size)
   {
      if(j < all_instance[n])
      {
         new_buffer[i] = file_buffer[j];
         i++;
         j++;
      }
      else
      {
         for(int k=0;k<new_str_size;k++)
         {
            new_buffer[i] = argv[3][k];
            i++;
         }
         j = j + old_str_size;
         n++;
      }
   }
   
   new_buffer[new_buffer_size] = 0;

   fptr = fopen(argv[1], "w");
   fwrite(new_buffer, sizeof(char), new_buffer_size, fptr);
   fclose(fptr);

   return 0;
}

int FileSize(char* filename){

   int result;
   FILE* fptr = fopen(filename, "r");
   if(fptr == NULL)
   {
      return -1;
   }
   fseek(fptr, 0, SEEK_END);
   result = ftell(fptr);
   fclose(fptr);
   return result;

}

int* instances(char* page, char* str){
   
   int* result = (int*)malloc(sizeof(int) * 1024);
   int n = 0;
   int page_size = 0;
   int str_size = 0;
   int match = 0;

   while(page[page_size] != 0 && page[page_size] != EOF && page_size < 1024)
   {
      page_size++;
   }

   while(str[str_size] != 0 && str_size < 1024)
   {
      str_size++;
   }


   for(int i=0;i<page_size;i++)
   {
      if(page[i] == str[match])
      {
         match++;
         if(match == str_size)
         {
            result[n] = i-str_size+1;
            n++;
            match = 0;
         }
      }
      else if(match > 0)
      {
         match = 0;
      }
   }
   
   result[n] = -1;
   return result;

}