#define _FILE_OFFSET_BITS 64

#include "temp_file.h"

#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>

static off_t ceil_div(off_t dividend, off_t divisor) {
  return (dividend - 1) / divisor + 1;
}

void reverse_file(const char* input_filename, FILE* output_stream) {
  const off_t mmap_chunk_size = 512 << 20;

  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 num_chunks = ceil_div(stats.st_size, mmap_chunk_size);
  for (off_t chunk = num_chunks - 1; chunk >= 0; chunk--) {
    off_t start_offset = chunk * mmap_chunk_size;
    off_t end_offset = (chunk + 1) * mmap_chunk_size;
    if (end_offset > stats.st_size) {
      end_offset = stats.st_size;
    }
    off_t pages = ceil_div(end_offset - start_offset, page_size);
    long map_size = pages * page_size;
    char *m = mmap(NULL, map_size, PROT_READ, MAP_SHARED, fd, start_offset);
    if (m == MAP_FAILED) err(EX_NOINPUT, "Could not mmap chunk %lld of %lld", chunk, num_chunks);

    for (off_t p = (end_offset - start_offset) - 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 chunk %lld of %lld", chunk, num_chunks);
  }

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

/* 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);
}