#define _GNU_SOURCE
#define _FILE_OFFSET_BITS 64

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

void reverse_file(const char* input_filename, FILE* output_stream) {
  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");

  for (off_t p = stats.st_size - 1; p >= 0; p--) {
    if (fputc(m[p], output_stream) == EOF) errx(EX_IOERR, "Could not write");
  }

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

/* Create a temporary file in $TMPDIR, or /tmp if TMPDIR is not set.
 * Caller must free temp_filename and fclose temp_file.  Succeeds or terminates
 * the process. */
static void make_temporary_file(char** temp_filename, FILE** temp_file) {
  char* TMPDIR = getenv("TMPDIR");
  if (TMPDIR == NULL) {
    TMPDIR = "/tmp";
  }
  if (asprintf(temp_filename, "%s/reverse.XXXXXX", TMPDIR) == -1) {
    errx(EX_OSERR, "Could not assemble temporary filename");
  }
  int fd = mkstemp(*temp_filename);
  if (fd == -1) err(EX_IOERR, "Could not make a temporary file");
  *temp_file = fdopen(fd, "w");
  if (*temp_file == NULL) err(EX_IOERR, "Could not open temporary file");
}

/* Copy data from input to output until EOF is reached. */
static void copy(FILE* input, FILE* output) {
  for (;;) {
    int c = fgetc(input);
    if (c == EOF) {
      if (ferror(input)) errx(EX_IOERR, "Could not read");
      if (!feof(input)) errx(EX_IOERR, "Unexpected end of file");
      break;
    }
    if (fputc(c, output) == EOF) errx(EX_IOERR, "Could not write");
  }
}

void reverse_stream(FILE* input_stream, FILE* output_stream) {
  char* temp_filename;
  FILE* temp_file;
  make_temporary_file(&temp_filename, &temp_file);

  copy(input_stream, temp_file);
  if (fclose(temp_file) != 0) err(EX_IOERR, "Could not close temporary file");

  reverse_file(temp_filename, output_stream);

  if (unlink(temp_filename) == -1) err(EX_IOERR, "Could not remove temporary file");
  free(temp_filename);
}