/*
 * Scan for and extract gzip from stdin stream to stdout
 *
 * Written by Harry Sintonen <sintonen@iki.fi>.
 * Public Domain.
 *
 * USAGE
 *
 * ./scangzip numgzip < infile > outfile
 * ...where numgzip is the index of the gzip file to extract, starting
 * from 0.
 *
 * NOTES
 *
 * If false positive is found, the program will still report success.
 * If this happens just bump the index to skip the bogus entry.
 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <errno.h>

#ifndef STDIN_FILENO
#define STDIN_FILENO 0
#endif

typedef unsigned char u8;

#define GZF_ASCII        (1 << 0)
#define GZF_CONTINUATION (1 << 1)
#define GZF_EXTRA_FIELD  (1 << 2)
#define GZF_ORIG_NAME    (1 << 3)
#define GZF_COMMENT      (1 << 4)
#define GZF_ENCRYPTED    (1 << 5)
#define GZF_RESERVED0    (1 << 6)
#define GZF_RESERVED1    (1 << 7)
#define INVALIDFLAGS (GZF_CONTINUATION | GZF_ENCRYPTED | GZF_RESERVED0 | GZF_RESERVED1)

#define MSIZE 4
static const u8 gzip_magic[MSIZE * 2] = {
  0x1f, 0x8b, 0x08, 0,
  0xff, 0xff, 0xff, INVALIDFLAGS
};
static const u8 old_gzip_magic[MSIZE * 2] =
{
  0x1f, 0x9e, 0x08, 0,
  0xff, 0xff, 0xff, INVALIDFLAGS
};

static int gunzip(u8 *header, int hsize)
{
  FILE *fh;
  ssize_t actual;
  u8 buf[4096];
  int rc = -1;

  fh = popen("gzip -d", "w");
  if (!fh)
  {
    perror("popen");
    return rc;
  }

  if (hsize && fwrite(header, 1, hsize, fh) != hsize)
  {
    perror("fwrite");
    return rc;
  }

  rc = 0;
  while ((actual = fread(buf, 1, sizeof(buf), stdin)) > 0)
  {
    if (fwrite(buf, 1, actual, fh) != actual)
    {
      if (errno != EPIPE)
      {
        perror("fwrite");
        rc = -1;
      }
      break;
    }
  }
  if (actual < 0)
  {
    perror("fread");
    rc = -1;
  }
  if (pclose(fh) < 0)
  {
    perror("pclose");
    rc = -1;
  }
  return rc;
}

struct data
{
  int c;
  int offs;
  int cnt;
  int num;
};

static inline void chkgzip(struct data *data, const u8 *magic, int *ip)
{
  u8 c = (u8) data->c;
  int i = *ip;

  if ((c & magic[i + MSIZE]) == magic[i])
  {
    if (i == MSIZE - 1)
    {
      fprintf(stderr, "%d: found potential gzip header at offset 0x%x\n",
              data->cnt, data->offs - 3);
      if (data->cnt == data->num)
      {
        u8 buf[4] = {magic[0], magic[1], magic[2], (u8)c};
        int rc;

        rc = gunzip(buf, sizeof(buf));
        if (rc == 0)
        {
          fprintf(stderr, "Ok\n");
          exit(EXIT_SUCCESS);
        }
      }
      data->cnt++;

      *ip = 0;
    }
    else
    {
      *ip = i + 1;
    }
  }
  else
  {
    *ip = 0;
  }
}

int main(int argc, char **argv)
{
  struct data data;
  int gmi, ogmi;

  if (argc != 2)
  {
    fprintf(stderr, "usage: %s numgzip < infile > outfile\n", argv[0]);
    return EXIT_FAILURE;
  }

  data.offs = 0;
  data.cnt = 0;
  data.num = atoi(argv[1]);
  gmi = 0;
  ogmi = 0;

  signal(SIGPIPE, SIG_IGN);

  for (;;)
  {
    data.c = getchar();
    if (data.c == EOF)
    {
      break;
    }

    chkgzip(&data, gzip_magic, &gmi);
    chkgzip(&data, old_gzip_magic, &ogmi);

    data.offs++;
  }

  fprintf(stderr, "could not find gzip number %d\n", data.num);
  return EXIT_FAILURE;
}