/* Strings with implicit length.
 * Richard W.M. Jones <rjones@redhat.com>
 *
 * https://rwmj.wordpress.com/2016/01/08/half-baked-ideas-c-strings-with-implicit-length-field/
 *
 * Inspired by this OCaml implementation of strings:
 * https://rwmj.wordpress.com/2009/08/05/ocaml-internals-part-2-strings-and-other-types/
 *
 * NB: It's just for playing with.  Don't actually use this in
 * production code!!  In particular this uses malloc allocations in a
 * way which is not valid, standard C, and may break or corrupt things
 * at any time.
 */

#ifndef ilenstr_h_
#define ilenstr_h_

#include <stdint.h>
#include <string.h>
#include <malloc.h>
#include <assert.h>

/* If your libc doesn't have glibc's malloc_usable_size, then
 * you need to find the equivalent function and define it here.
 */
#define ilenstr_malloc_size malloc_usable_size

/* Although ilenstrs have a separate length field, it's implicit,
 * so they are convertable back to C strings (although C strings
 * are NOT ilenstrs so don't try to cast them the other way).
 */
typedef char *ilenstr;

/* Allocate a new ilenstr from an arbitrary string + length.  The
 * string may contain \0 characters within the string.
 */
static ilenstr
new_ilenstr_with_len (const char *str, size_t len)
{
  char *ilstr;
  size_t msize;
  uint8_t *p;

  ilstr = malloc (len+1);
  if (!ilstr) return NULL;
  memcpy (ilstr, str, len);
  ilstr[len] = '\0';

  msize = ilenstr_malloc_size (ilstr);
  assert (len <= msize);

  /* Because we only have a single byte to use at the end of the
   * malloc allocation, we have this limit.  It could be bypassed with
   * some horrible encoding scheme, but that would make getting the
   * string length slower.  Hopefully this never asserts in practice.
   */
  assert (msize-len <= 255);

  p = (uint8_t *) &ilstr[msize-1];
  *p = msize-len;

  return ilstr;
}

/* Allocate an ilenstr from a C string.  The C string cannot
 * contain \0 characters within the string.
 */
static ilenstr
new_ilenstr (const char *cstr)
{
  return new_ilenstr_with_len (cstr, strlen (cstr));
}

/* Get the implicit length of an ilenstr. */
static size_t
ilstrlen (const ilenstr ilstr)
{
  size_t msize;
  uint8_t *p;

  msize = ilenstr_malloc_size (ilstr);
  p = (uint8_t *) (ilstr + msize - 1);
  return msize - *p;
}

#endif /* ilenstr_h_ */