/* FIXME */
#include <assert.h>

/* FIXME: this is small for testing */
#define BUFFER_SIZE (8)

#define LEN(X, I) ((X)->p[(I)].one_past_end - (X)->p[(I)].start)
#define EMPTY(X) ((X)->n_bufs == 1 && LEN (X, 0) == 0)

#define ONE_PAST_END(X, I) ((X)->p[(I)].one_past_end)

struct Line_ptr
{
  size_t i;
  char *ptr;
};
typedef struct Line_ptr Line_ptr;

struct B_pair
{
  char *start;
  char *one_past_end;
};

struct Buf
{
  size_t n_bufs;
  struct obstack obs;
  struct B_pair *p;
};
typedef struct Buf Buf;

static int
buf_init_from_stdin (Buf *x, int eol_byte)
{
  int last_byte_is_eol_byte = 1;
  int fail = 0;

#define OBS (&(x->obs))
  obstack_init (OBS);

  while (1)
    {
      char *buf = (char *) malloc (BUFFER_SIZE);
      int bytes_read;

      if (buf == NULL)
	{
	  /* Fall back on the code that relies on a temporary file.
	     Write all buffers to that file and free them.  */
	  /* FIXME */
	  fail = 1;
	  break;
	}
      bytes_read = full_read (STDIN_FILENO, buf, BUFFER_SIZE);
      if (bytes_read < 0)
	error (EXIT_FAILURE, errno, _("read error"));

      {
	struct B_pair bp;
	bp.start = buf;
	bp.one_past_end = buf + bytes_read;
	obstack_grow (OBS, &bp, sizeof (bp));
      }

      if (bytes_read > 0)
	last_byte_is_eol_byte = (buf[bytes_read - 1] == eol_byte);

      if (bytes_read < BUFFER_SIZE)
	break;
    }

  if (!fail)
    {
      /* If the file was non-empty and lacked an EOL_BYTE at its end,
	 then add a buffer containing just that one byte.  */
      if (!last_byte_is_eol_byte)
	{
	  char *buf = malloc (1);
	  if (buf == NULL)
	    {
	      /* FIXME: just like above */
	      fail = 1;
	    }
	  else
	    {
	      struct B_pair bp;
	      *buf = eol_byte;
	      bp.start = buf;
	      bp.one_past_end = buf + 1;
	      obstack_grow (OBS, &bp, sizeof (bp));
	    }
	}
    }

  x->n_bufs = obstack_object_size (OBS) / sizeof (x->p[0]);
  x->p = (struct B_pair *) obstack_finish (OBS);

  /* If there are two or more buffers and the last buffer is empty,
     then free the last one and decrement the buffer count.  */
  if (x->n_bufs >= 2
      && x->p[x->n_bufs - 1].start == x->p[x->n_bufs - 1].one_past_end)
    free (x->p[--(x->n_bufs)].start);

  return fail;
}

static void
buf_free (Buf *x)
{
  int i;
  for (i = 0; i < x->n_bufs; i++)
    free (x->p[i].start);
  obstack_free (OBS, NULL);
}

Line_ptr
line_ptr_decrement (const Buf *x, const Line_ptr *lp)
{
  Line_ptr lp_new;

  if (lp->ptr > x->p[lp->i].start)
    {
      lp_new.i = lp->i;
      lp_new.ptr = lp->ptr - 1;
    }
  else
    {
      assert (lp->i > 0);
      lp_new.i = lp->i - 1;
      lp_new.ptr = ONE_PAST_END (x, lp->i - 1) - 1;
    }
  return lp_new;
}

Line_ptr
line_ptr_increment (const Buf *x, const Line_ptr *lp)
{
  Line_ptr lp_new;

  assert (lp->ptr <= ONE_PAST_END (x, lp->i) - 1);
  if (lp->ptr < ONE_PAST_END (x, lp->i) - 1)
    {
      lp_new.i = lp->i;
      lp_new.ptr = lp->ptr + 1;
    }
  else
    {
      assert (lp->i < x->n_bufs - 1);
      lp_new.i = lp->i + 1;
      lp_new.ptr = x->p[lp->i + 1].start;
    }
  return lp_new;
}

static int
find_bol (const Buf *x,
	  const Line_ptr *last_bol, Line_ptr *new_bol, char eol_byte)
{
  int i;
  Line_ptr tmp;
  char *last_bol_ptr;

  if (last_bol->ptr == x->p[0].start)
    return 0;

  tmp = line_ptr_decrement (x, last_bol);
  last_bol_ptr = tmp.ptr;
  i = tmp.i;
  while (1)
    {
      char *nl = memrchr (x->p[i].start, last_bol_ptr, eol_byte);
      if (nl)
	{
	  Line_ptr nl_pos;
	  nl_pos.i = i;
	  nl_pos.ptr = nl;
	  *new_bol = line_ptr_increment (x, &nl_pos);
	  return 1;
	}

      if (i <= 0)
	break;

      --i;
      last_bol_ptr = ONE_PAST_END (x, i);
    }

  /* If last_bol->ptr didn't point at the first byte of X, then reaching
     this point means that we're about to return the line that is at the
     beginning of X.  */
  if (last_bol->ptr != x->p[0].start)
    {
      new_bol->i = 0;
      new_bol->ptr = x->p[0].start;
      return 1;
    }

  return 0;
}

static void
print_line (FILE *out_stream, const Buf *x,
	    const Line_ptr *bol, const Line_ptr *bol_next)
{
  int i;
  for (i = bol->i; i <= bol_next->i; i++)
    {
      char *a = (i == bol->i ? bol->ptr : x->p[i].start);
      char *b = (i == bol_next->i ? bol_next->ptr : ONE_PAST_END (x, i));
      fwrite (a, 1, b - a, out_stream);
    }
}

static int
tac_mem ()
{
  Buf x;
  Line_ptr bol;
  char eol_byte = '\n';
  int fail;

  fail = buf_init_from_stdin (&x, eol_byte);
  if (fail)
    {
      buf_free (&x);
      return 1;
    }

  /* Special case the empty file.  */
  if (EMPTY (&x))
    return 0;

  /* Initially, point at one past the last byte of the file.  */
  bol.i = x.n_bufs - 1;
  bol.ptr = ONE_PAST_END (&x, bol.i);

  while (1)
    {
      Line_ptr new_bol;
      int found = find_bol (&x, &bol, &new_bol, eol_byte);
      if (!found)
	break;
      print_line (stdout, &x, &new_bol, &bol);
      bol = new_bol;
    }
  return 0;
}
