compute_checksum.cpp

#include <cstdio>
#include <algorithm>
#include <sys/time.h>

// Current time in seconds.
long double wallclock() {
  timeval tim;
  gettimeofday(&tim, NULL);
  return tim.tv_sec + (tim.tv_usec / 1000000.0L);
}

// Compute checksum.
long compute_checksum(const char *filename) {
  std::FILE * const f = std::fopen(filename, "r");
  std::fseek(f, 0, SEEK_END);
  const long size = std::ftell(f);
  std::rewind(f);
  char *buf = new char[1 << 20];
  long ans = 0, left = size;
  while (left > 0) {
    long toread = std::min(1L << 20, left);
    std::fread(buf, 1, toread, f);
    for (long i = 0; i < toread; ++i)
      ans += buf[i];
    left -= toread;
  }
  delete[] buf; 
  std::fclose(f);
  return ans;
}

int main(int, char **argv) {
  for (int i = 0; i < 3; ++i) {
    long double start = wallclock();
    long sum = compute_checksum(argv[1]);
    fprintf(stderr, "checksum = %ld, time = %.2Lfs\n",
      sum, wallclock() - start);
  }
}