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

#include "fileReader.h"
#include "fileUtilities.h"

//minimum of two integers
#define min(a,b) \
  ({ __typeof__ (a) _a = (a); \
    __typeof__ (b) _b = (b); \
  _a < _b ? _a : _b; })


//Opens the given file at the given offset
void initFileReader(char path[], unsigned long offset) {
  file.name = path;

  file.filePointer = fopen(file.name, "r");
  if (file.filePointer == NULL) {
    fprintf(stderr, "Could not open file %s for reading: %s\n", file.name, strerror(errno));
    exit(1);
  }

  //get file size
  fseek(file.filePointer, 0L, SEEK_END);
  file.size = ftell(file.filePointer);

  //seek to offset
  if (offset != 0UL) {
    fseek(file.filePointer, offset, SEEK_SET);
  } else {
    rewind(file.filePointer);
  }
  file.offset = offset;
}


//Reads the given amount of bytes from the file and returns them. Automatically opens next file if EOF is reached.
char* getBytes(unsigned long numBytes) {
  char* output = malloc(numBytes);
  if (output == NULL) {
    fprintf(stderr, "Could not allocate %lu bytes.\n", numBytes);
    exit(1);
  }
  char* bufferPosition = output;
  unsigned long remainingBytes = numBytes;

  //As long as we still need to read bytes
  while (remainingBytes > 0) {
    //Read as many bytes as we can from the current file
    const unsigned long availableBytes = min(remainingBytes, file.size - file.offset);
    readBytesIntoBuffer(bufferPosition, availableBytes);
    bufferPosition += availableBytes;
    remainingBytes -= availableBytes;

    //If we've reached end of file, close file and open next file
    if (file.offset == file.size) {
      openNextFile();
    }
  }

  return output;
}