/* -*- Mode: C; -*-
 * ----------------------------------------------------------------------------
 *     Title: Dynamic Bind Syntactic Sugar
 *   Created: 2020-05-23
 *    Author: Gilbert Baumann <gilbert@bauhh.de> 
 * ----------------------------------------------------------------------------
 *  (c) copyright 2020 by Gilbert Baumann
 */

#include <stdio.h>
#include <string.h>

struct dynbind_frame {
  void *addr;                   /* pointer to lvalue bound */
  const void *old;              /* pointer to saved value */
  size_t sz;                    /* size of value */
};

static void dynbind_cleanup (const struct dynbind_frame *frame)
{
  memcpy (frame->addr, frame->old, frame->sz);
}

#define dynbind(lvalue, val)                                            \
  for (int run##__LINE__ = 1; run##__LINE__; )                          \
  for ( __typeof__(lvalue) * const addr##__LINE__ = &(lvalue);          \
       run##__LINE__; )                                                 \
  for (const __typeof__(lvalue) old##__LINE__ = *addr##__LINE__;        \
       run##__LINE__;)                                                  \
  for (const struct dynbind_frame save##__LINE__                        \
       __attribute__ ((cleanup (dynbind_cleanup)))                      \
         = { addr##__LINE__,                                            \
             &old##__LINE__,                                            \
             sizeof (lvalue) };                                         \
       run##__LINE__;)                                                  \
  for (*addr##__LINE__ = (val);                                         \
       run##__LINE__;                                                   \
       run##__LINE__ = 0)

/* -- Demo ----------------------------------------------------------------- */

int xyz = 0;
struct point { int x, y; } pt;
extern struct point *quux (void);

extern int bar (void);

void foo (void)
{
  dynbind (xyz, 46)
    {
      bar ();
    }
}

void foo_pt (void)
{
  struct point q = { 100, 200 };
  
  dynbind (pt, q)
    {
      bar ();
    }
}

void foo_eval (void)
{
  dynbind (quux()->x, 100)
    bar ();
}

/* -- Changes -------------------------------------------------------------- */

/*
 * - renamed a few things
 *
 * - what was a struct for k, addr, and old value is now three
 *   separate for()s
 *
 * - spend a few 'const' in the hope to help the compiler.
 *
 * - renamed 'k' to 'run' and made it a flag insted of a counter.
 *
 * - fixed it, so that lvalue is evaluated just once.
 *
 */