#define _FILE_OFFSET_BITS 64

#define BUFFER_SIZE 4096

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <err.h>
#include <sysexits.h>

static void write_all(int fd, const void *buf, size_t count) {
  const char* cbuf = buf;
  size_t written = 0;
  while (written < count) {
    int ret = write(fd, &cbuf[written], count - written);
    if (ret == -1) err(EX_IOERR, "Could not write");
    written += ret;
  }
}

void reverse_file(const char* input_filename, int output_fd) {
  int fd = open(input_filename, O_RDONLY);
  if (fd == -1) err(EX_NOINPUT, "Could not open specified file");

  struct stat stats;
  if (fstat(fd, &stats) == -1) err(EX_NOINPUT, "Could not stat input");

  long page_size = sysconf(_SC_PAGE_SIZE);
  off_t pages = (stats.st_size - 1) / page_size + 1;
  long map_size = pages * page_size;
  char *m = mmap(NULL, map_size, PROT_READ, MAP_SHARED, fd, 0);
  if (m == MAP_FAILED) err(EX_NOINPUT, "Could not mmap input");

  char buf[BUFFER_SIZE];
  off_t buf_offset = 0;
  for (off_t p = stats.st_size - 1; p >= 0; p--) {
    buf[buf_offset++] = m[p];
    if (buf_offset >= BUFFER_SIZE) {
      write_all(output_fd, buf, buf_offset);
      buf_offset = 0;
    }
  }
  if (buf_offset) {
    write_all(output_fd, buf, buf_offset);
  }

  if (munmap(m, map_size) == -1) err(EX_IOERR, "Could not unmap input");
  if (close(fd) == -1) err(EX_IOERR, "Could not close input");
}