serial.c

/* serial.c: Glulxe code for saving and restoring the VM state.
    Designed by Andrew Plotkin <erkyrath@eblong.com>
    http://eblong.com/zarf/glulx/index.html
*/

#include <string.h>
#include "glk.h"
#include "glulxe.h"

/* This structure allows us to write either to a Glk stream or to
   a dynamically-allocated memory chunk. */
typedef struct dest_struct {
  int ismem;
  
  /* If it's a Glk stream: */
  strid_t str;

  /* If it's a block of memory: */
  unsigned char *ptr;
  glui32 pos;
  glui32 size;
} dest_t;

#define IFFID(c1, c2, c3, c4)  \
  ( (((glui32)c1) << 24)    \
  | (((glui32)c2) << 16)    \
  | (((glui32)c3) << 8)     \
  | (((glui32)c4)) )

/* This can be adjusted before startup by platform-specific startup
   code -- that is, preference code. */
int max_undo_level = 8;

static int undo_chain_size = 0;
static int undo_chain_num = 0;
static unsigned char **undo_chain = NULL;

#ifdef SERIALIZE_CACHE_RAM
/* This will contain a copy of RAM (ramstate to endmem) as it exists
   in the game file. */
static unsigned char *ramcache = NULL;
#endif /* SERIALIZE_CACHE_RAM */

static glui32 write_memstate(dest_t *dest);
static glui32 write_heapstate(dest_t *dest, int portable);
static glui32 write_stackstate(dest_t *dest, int portable);
static glui32 read_memstate(dest_t *dest, glui32 chunklen);
static glui32 read_heapstate(dest_t *dest, glui32 chunklen, int portable,
  glui32 *sumlen, glui32 **summary);
static glui32 read_stackstate(dest_t *dest, glui32 chunklen, int portable);
static glui32 write_heapstate_sub(glui32 sumlen, glui32 *sumarray,
  dest_t *dest, int portable);
static int sort_heap_summary(void *p1, void *p2);
static int write_long(dest_t *dest, glui32 val);
static int read_long(dest_t *dest, glui32 *val);
static int write_byte(dest_t *dest, unsigned char val);
static int read_byte(dest_t *dest, unsigned char *val);
static int reposition_write(dest_t *dest, glui32 pos);

/* init_serial():
   Set up the undo chain and anything else that needs to be set up.
*/
int init_serial()
{
  undo_chain_num = 0;
  undo_chain_size = max_undo_level;
  undo_chain = (unsigned char **)glulx_malloc(sizeof(unsigned char *) * undo_chain_size);
  if (!undo_chain)
    return FALSE;

#ifdef SERIALIZE_CACHE_RAM
  {
    glui32 len = (endmem - ramstart);
    glui32 res;
    ramcache = (unsigned char *)glulx_malloc(sizeof(unsigned char *) * len);
    if (!ramcache)
      return FALSE;
    glk_stream_set_position(gamefile, gamefile_start+ramstart, seekmode_Start);
    res = glk_get_buffer_stream(gamefile, (char *)ramcache, len);
    if (res != len)
      return FALSE;
  }
#endif /* SERIALIZE_CACHE_RAM */

  return TRUE;
}

/* final_serial():
   Clean up memory when the VM shuts down.
*/
void final_serial()
{
  if (undo_chain) {
    int ix;
    for (ix=0; ix<undo_chain_num; ix++) {
      glulx_free(undo_chain[ix]);
    }
    glulx_free(undo_chain);
  }
  undo_chain = NULL;
  undo_chain_size = 0;
  undo_chain_num = 0;

#ifdef SERIALIZE_CACHE_RAM
  if (ramcache) {
    glulx_free(ramcache);
    ramcache = NULL;
  }
#endif /* SERIALIZE_CACHE_RAM */
}

/* perform_saveundo():
   Add a state pointer to the undo chain. This returns 0 on success,
   1 on failure.
*/
glui32 perform_saveundo()
{
  dest_t dest;
  glui32 res;
  glui32 memstart, memlen, heapstart, heaplen, stackstart, stacklen;

  /* The format for undo-saves is simpler than for saves on disk. We
     just have a memory chunk, a heap chunk, and a stack chunk, in
     that order. We skip the IFF chunk headers (although the size
     fields are still there.) We also don't bother with IFF's 16-bit
     alignment. */

  if (undo_chain_size == 0)
    return 1;

  dest.ismem = TRUE;
  dest.size = 0;
  dest.pos = 0;
  dest.ptr = NULL;
  dest.str = NULL;

  res = 0;
  if (res == 0) {
    res = write_long(&dest, 0); /* space for chunk length */
  }
  if (res == 0) {
    memstart = dest.pos;
    res = write_memstate(&dest);
    memlen = dest.pos - memstart;
  }
  if (res == 0) {
    res = write_long(&dest, 0); /* space for chunk length */
  }
  if (res == 0) {
    heapstart = dest.pos;
    res = write_heapstate(&dest, FALSE);
    heaplen = dest.pos - heapstart;
  }
  if (res == 0) {
    res = write_long(&dest, 0); /* space for chunk length */
  }
  if (res == 0) {
    stackstart = dest.pos;
    res = write_stackstate(&dest, FALSE);
    stacklen = dest.pos - stackstart;
  }

  if (res == 0) {
    /* Trim it down to the perfect size. */
    dest.ptr = glulx_realloc(dest.ptr, dest.pos);
    if (!dest.ptr)
      res = 1;
  }
  if (res == 0) {
    res = reposition_write(&dest, memstart-4);
  }
  if (res == 0) {
    res = write_long(&dest, memlen);
  }
  if (res == 0) {
    res = reposition_write(&dest, heapstart-4);
  }
  if (res == 0) {
    res = write_long(&dest, heaplen);
  }
  if (res == 0) {
    res = reposition_write(&dest, stackstart-4);
  }
  if (res == 0) {
    res = write_long(&dest, stacklen);
  }

  if (res == 0) {
    /* It worked. */
    if (undo_chain_num >= undo_chain_size) {
      glulx_free(undo_chain[undo_chain_num-1]);
      undo_chain[undo_chain_num-1] = NULL;
    }
    if (undo_chain_size > 1)
      memmove(undo_chain+1, undo_chain, 
        (undo_chain_size-1) * sizeof(unsigned char *));
    undo_chain[0] = dest.ptr;
    if (undo_chain_num < undo_chain_size)
      undo_chain_num += 1;
    dest.ptr = NULL;
  }
  else {
    /* It didn't work. */
    if (dest.ptr) {
      glulx_free(dest.ptr);
      dest.ptr = NULL;
    }
  }
    
  return res;
}

/* perform_restoreundo():
   Pull a state pointer from the undo chain. This returns 0 on success,
   1 on failure. Note that if it succeeds, the frameptr, localsbase,
   and valstackbase registers are invalid; they must be rebuilt from
   the stack.
*/
glui32 perform_restoreundo()
{
  dest_t dest;
  glui32 res, val;
  glui32 heapsumlen = 0;
  glui32 *heapsumarr = NULL;

  /* If profiling is enabled and active then fail. */
  #if VM_PROFILING
  if (profile_profiling_active())
    return 1;
  #endif /* VM_PROFILING */

  if (undo_chain_size == 0 || undo_chain_num == 0)
    return 1;

  dest.ismem = TRUE;
  dest.size = 0;
  dest.pos = 0;
  dest.ptr = undo_chain[0];
  dest.str = NULL;

  res = 0;
  if (res == 0) {
    res = read_long(&dest, &val);
  }
  if (res == 0) {
    res = read_memstate(&dest, val);
  }
  if (res == 0) {
    res = read_long(&dest, &val);
  }
  if (res == 0) {
    res = read_heapstate(&dest, val, FALSE, &heapsumlen, &heapsumarr);
  }
  if (res == 0) {
    res = read_long(&dest, &val);
  }
  if (res == 0) {
    res = read_stackstate(&dest, val, FALSE);
  }
  /* ### really, many of the failure modes of those calls ought to
     cause fatal errors. The stack or main memory may be damaged now. */

  if (res == 0) {
    if (heapsumarr)
      res = heap_apply_summary(heapsumlen, heapsumarr);
  }

  if (res == 0) {
    /* It worked. */
    if (undo_chain_size > 1)
      memmove(undo_chain, undo_chain+1,
        (undo_chain_size-1) * sizeof(unsigned char *));
    undo_chain_num -= 1;
    glulx_free(dest.ptr);
    dest.ptr = NULL;
  }
  else {
    /* It didn't work. */
    dest.ptr = NULL;
  }

  return res;
}

/* perform_save():
   Write the state to the output stream. This returns 0 on success,
   1 on failure.
*/
glui32 perform_save(strid_t str)
{
  dest_t dest;
  int ix;
  glui32 res, lx, val;
  glui32 memstart, memlen, stackstart, stacklen, heapstart, heaplen;
  glui32 filestart=0, filelen;

  stream_get_iosys(&val, &lx);
  if (val != 2) {
    /* Not using the Glk I/O system, so bail. This function only
       knows how to write to a Glk stream. */
    fatal_error("Streams are only available in Glk I/O system.");
  }

  if (str == 0)
    return 1;

  dest.ismem = FALSE;
  dest.size = 0;
  dest.pos = 0;
  dest.ptr = NULL;
  dest.str = str;

  res = 0;

  /* Quetzal header. */
  if (res == 0) {
    res = write_long(&dest, IFFID('F', 'O', 'R', 'M'));
  }
  if (res == 0) {
    res = write_long(&dest, 0); /* space for file length */
    filestart = dest.pos;
  }

  if (res == 0) {
    res = write_long(&dest, IFFID('I', 'F', 'Z', 'S')); /* ### ? */
  }

  /* Header chunk. This is the first 128 bytes of memory. */
  if (res == 0) {
    res = write_long(&dest, IFFID('I', 'F', 'h', 'd'));
  }
  if (res == 0) {
    res = write_long(&dest, 128);
  }
  for (ix=0; res==0 && ix<128; ix++) {
    res = write_byte(&dest, Mem1(ix));
  }
  /* Always even, so no padding necessary. */
  
  /* Memory chunk. */
  if (res == 0) {
    res = write_long(&dest, IFFID('C', 'M', 'e', 'm'));
  }
  if (res == 0) {
    res = write_long(&dest, 0); /* space for chunk length */
  }
  if (res == 0) {
    memstart = dest.pos;
    res = write_memstate(&dest);
    memlen = dest.pos - memstart;
  }
  if (res == 0 && (memlen & 1) != 0) {
    res = write_byte(&dest, 0);
  }

  /* Heap chunk. */
  if (res == 0) {
    res = write_long(&dest, IFFID('M', 'A', 'l', 'l'));
  }
  if (res == 0) {
    res = write_long(&dest, 0); /* space for chunk length */
  }
  if (res == 0) {
    heapstart = dest.pos;
    res = write_heapstate(&dest, TRUE);
    heaplen = dest.pos - heapstart;
  }
  /* Always even, so no padding necessary. */

  /* Stack chunk. */
  if (res == 0) {
    res = write_long(&dest, IFFID('S', 't', 'k', 's'));
  }
  if (res == 0) {
    res = write_long(&dest, 0); /* space for chunk length */
  }
  if (res == 0) {
    stackstart = dest.pos;
    res = write_stackstate(&dest, TRUE);
    stacklen = dest.pos - stackstart;
  }
  if (res == 0 && (stacklen & 1) != 0) {
    res = write_byte(&dest, 0);
  }

  filelen = dest.pos - filestart;

  /* Okay, fill in all the lengths. */
  if (res == 0) {
    res = reposition_write(&dest, memstart-4);
  }
  if (res == 0) {
    res = write_long(&dest, memlen);
  }
  if (res == 0) {
    res = reposition_write(&dest, heapstart-4);
  }
  if (res == 0) {
    res = write_long(&dest, heaplen);
  }
  if (res == 0) {
    res = reposition_write(&dest, stackstart-4);
  }
  if (res == 0) {
    res = write_long(&dest, stacklen);
  }
  if (res == 0) {
    res = reposition_write(&dest, filestart-4);
  }
  if (res == 0) {
    res = write_long(&dest, filelen);
  }

  /* All done. */
    
  return res;
}

/* perform_restore():
   Pull a state pointer from a stream. This returns 0 on success,
   1 on failure. Note that if it succeeds, the frameptr, localsbase,
   and valstackbase registers are invalid; they must be rebuilt from
   the stack.
 
   If fromshell is true, the restore is being invoked by the library
   shell (an autorestore of some kind). This currently happens only in
   iosglk.
*/
glui32 perform_restore(strid_t str, int fromshell)
{
  dest_t dest;
  int ix;
  glui32 lx, res, val;
  glui32 filestart, filelen;
  glui32 heapsumlen = 0;
  glui32 *heapsumarr = NULL;

  /* If profiling is enabled and active then fail. */
  #if VM_PROFILING
  if (profile_profiling_active())
    return 1;
  #endif /* VM_PROFILING */

  stream_get_iosys(&val, &lx);
  if (val != 2 && !fromshell) {
    /* Not using the Glk I/O system, so bail. This function only
       knows how to read from a Glk stream. (But in the autorestore
       case, iosys hasn't been set yet, so ignore this test.) */
    fatal_error("Streams are only available in Glk I/O system.");
  }

  if (str == 0)
    return 1;

  dest.ismem = FALSE;
  dest.size = 0;
  dest.pos = 0;
  dest.ptr = NULL;
  dest.str = str;

  res = 0;

  /* ### the format errors checked below should send error messages to
     the current stream. */

  if (res == 0) {
    res = read_long(&dest, &val);
  }
  if (res == 0 && val != IFFID('F', 'O', 'R', 'M')) {
    /* ### bad header */
    return 1;
  }
  if (res == 0) {
    res = read_long(&dest, &filelen);
  }
  filestart = dest.pos;

  if (res == 0) {
    res = read_long(&dest, &val);
  }
  if (res == 0 && val != IFFID('I', 'F', 'Z', 'S')) { /* ### ? */
    /* ### bad header */
    return 1;
  }

  while (res == 0 && dest.pos < filestart+filelen) {
    /* Read a chunk and deal with it. */
    glui32 chunktype=0, chunkstart=0, chunklen=0;
    unsigned char dummy;

    if (res == 0) {
      res = read_long(&dest, &chunktype);
    }
    if (res == 0) {
      res = read_long(&dest, &chunklen);
    }
    chunkstart = dest.pos;

    if (chunktype == IFFID('I', 'F', 'h', 'd')) {
      for (ix=0; res==0 && ix<128; ix++) {
        res = read_byte(&dest, &dummy);
        if (res == 0 && Mem1(ix) != dummy) {
          /* ### non-matching header */
          return 1;
        }
      }
    }
    else if (chunktype == IFFID('C', 'M', 'e', 'm')) {
      res = read_memstate(&dest, chunklen);
    }
    else if (chunktype == IFFID('M', 'A', 'l', 'l')) {
      res = read_heapstate(&dest, chunklen, TRUE, &heapsumlen, &heapsumarr);
    }
    else if (chunktype == IFFID('S', 't', 'k', 's')) {
      res = read_stackstate(&dest, chunklen, TRUE);
    }
    else {
      /* Unknown chunk type. Skip it. */
      for (lx=0; res==0 && lx<chunklen; lx++) {
        res = read_byte(&dest, &dummy);
      }
    }

    if (chunkstart+chunklen != dest.pos) {
      /* ### funny chunk length */
      return 1;
    }

    if ((chunklen & 1) != 0) {
      if (res == 0) {
        res = read_byte(&dest, &dummy);
      }
    }
  }

  if (res == 0) {
    if (heapsumarr) {
      /* The summary might have come from any interpreter, so it could
         be out of order. We'll sort it. */
      glulx_sort(heapsumarr+2, (heapsumlen-2)/2, 2*sizeof(glui32),
        &sort_heap_summary);
      res = heap_apply_summary(heapsumlen, heapsumarr);
    }
  }

  if (res)
    return 1;

  return 0;
}

static int reposition_write(dest_t *dest, glui32 pos)
{
  if (dest->ismem) {
    dest->pos = pos;
  }
  else {
    glk_stream_set_position(dest->str, pos, seekmode_Start);
    dest->pos = pos;
  }

  return 0;
}

static int write_buffer(dest_t *dest, unsigned char *ptr, glui32 len)
{
  if (dest->ismem) {
    if (dest->pos+len > dest->size) {
      dest->size = dest->pos+len+1024;
      if (!dest->ptr) {
        dest->ptr = glulx_malloc(dest->size);
      }
      else {
        dest->ptr = glulx_realloc(dest->ptr, dest->size);
      }
      if (!dest->ptr)
        return 1;
    }
    memcpy(dest->ptr+dest->pos, ptr, len);
  }
  else {
    glk_put_buffer_stream(dest->str, (char *)ptr, len);
  }

  dest->pos += len;

  return 0;
}

static int read_buffer(dest_t *dest, unsigned char *ptr, glui32 len)
{
  glui32 newlen;

  if (dest->ismem) {
    memcpy(ptr, dest->ptr+dest->pos, len);
  }
  else {
    newlen = glk_get_buffer_stream(dest->str, (char *)ptr, len);
    if (newlen != len)
      return 1;
  }

  dest->pos += len;

  return 0;
}

static int write_long(dest_t *dest, glui32 val)
{
  unsigned char buf[4];
  Write4(buf, val);
  return write_buffer(dest, buf, 4);
}

static int write_short(dest_t *dest, glui16 val)
{
  unsigned char buf[2];
  Write2(buf, val);
  return write_buffer(dest, buf, 2);
}

static int write_byte(dest_t *dest, unsigned char val)
{
  return write_buffer(dest, &val, 1);
}

static int read_long(dest_t *dest, glui32 *val)
{
  unsigned char buf[4];
  int res = read_buffer(dest, buf, 4);
  if (res)
    return res;
  *val = Read4(buf);
  return 0;
}

static int read_short(dest_t *dest, glui16 *val)
{
  unsigned char buf[2];
  int res = read_buffer(dest, buf, 2);
  if (res)
    return res;
  *val = Read2(buf);
  return 0;
}

static int read_byte(dest_t *dest, unsigned char *val)
{
  return read_buffer(dest, val, 1);
}

static glui32 write_memstate(dest_t *dest)
{
  glui32 res, pos;
  int val;
  int runlen;
  unsigned char ch;
#ifdef SERIALIZE_CACHE_RAM
  glui32 cachepos;
#endif /* SERIALIZE_CACHE_RAM */

  res = write_long(dest, endmem);
  if (res)
    return res;

  runlen = 0;

#ifdef SERIALIZE_CACHE_RAM
  cachepos = 0;
#else /* SERIALIZE_CACHE_RAM */
  glk_stream_set_position(gamefile, gamefile_start+ramstart, seekmode_Start);
#endif /* SERIALIZE_CACHE_RAM */

  for (pos=ramstart; pos<endmem; pos++) {
    ch = Mem1(pos);
    if (pos < endgamefile) {
#ifdef SERIALIZE_CACHE_RAM
      val = ramcache[cachepos];
      cachepos++;
#else /* SERIALIZE_CACHE_RAM */
      val = glk_get_char_stream(gamefile);
      if (val == -1) {
        fatal_error("The game file ended unexpectedly while saving.");
      }
#endif /* SERIALIZE_CACHE_RAM */
      ch ^= (unsigned char)val;
    }
    if (ch == 0) {
      runlen++;
    }
    else {
      /* Write any run we've got. */
      while (runlen) {
        if (runlen >= 0x100)
          val = 0x100;
        else
          val = runlen;
        res = write_byte(dest, 0);
        if (res)
          return res;
        res = write_byte(dest, (val-1));
        if (res)
          return res;
        runlen -= val;
      }
      /* Write the byte we got. */
      res = write_byte(dest, ch);
      if (res)
        return res;
    }
  }
  /* It's possible we've got a run left over, but we don't write it. */

  return 0;
}

static glui32 read_memstate(dest_t *dest, glui32 chunklen)
{
  glui32 chunkend = dest->pos + chunklen;
  glui32 newlen;
  glui32 res, pos;
  int val;
  int runlen;
  unsigned char ch, ch2;
#ifdef SERIALIZE_CACHE_RAM
  glui32 cachepos;
#endif /* SERIALIZE_CACHE_RAM */

  heap_clear();

  res = read_long(dest, &newlen);
  if (res)
    return res;

  res = change_memsize(newlen, FALSE);
  if (res)
    return res;

  runlen = 0;

#ifdef SERIALIZE_CACHE_RAM
  cachepos = 0;
#else /* SERIALIZE_CACHE_RAM */
  glk_stream_set_position(gamefile, gamefile_start+ramstart, seekmode_Start);
#endif /* SERIALIZE_CACHE_RAM */

  for (pos=ramstart; pos<endmem; pos++) {
    if (pos < endgamefile) {
#ifdef SERIALIZE_CACHE_RAM
      val = ramcache[cachepos];
      cachepos++;
#else /* SERIALIZE_CACHE_RAM */
      val = glk_get_char_stream(gamefile);
      if (val == -1) {
        fatal_error("The game file ended unexpectedly while restoring.");
      }
#endif /* SERIALIZE_CACHE_RAM */
      ch = (unsigned char)val;
    }
    else {
      ch = 0;
    }

    if (dest->pos >= chunkend) {
      /* we're into the final, unstored run. */
    }
    else if (runlen) {
      runlen--;
    }
    else {
      res = read_byte(dest, &ch2);
      if (res)
        return res;
      if (ch2 == 0) {
        res = read_byte(dest, &ch2);
        if (res)
          return res;
        runlen = (glui32)ch2;
      }
      else {
        ch ^= ch2;
      }
    }

    if (pos >= protectstart && pos < protectend)
      continue;

    MemW1(pos, ch);
  }

  return 0;
}

static glui32 write_heapstate(dest_t *dest, int portable)
{
  glui32 res;
  glui32 sumlen;
  glui32 *sumarray;

  res = heap_get_summary(&sumlen, &sumarray);
  if (res)
    return res;

  if (!sumarray)
    return 0; /* no heap */

  res = write_heapstate_sub(sumlen, sumarray, dest, portable);

  glulx_free(sumarray);
  return res;
}

static glui32 write_heapstate_sub(glui32 sumlen, glui32 *sumarray,
  dest_t *dest, int portable) 
{
  glui32 res, lx;

  /* If we're storing for the purpose of undo, we don't need to do any
     byte-swapping, because the result will only be used by this session. */
  if (!portable) {
    res = write_buffer(dest, (void *)sumarray, sumlen*sizeof(glui32));
    if (res)
      return res;
    return 0;
  }

  for (lx=0; lx<sumlen; lx++) {
    res = write_long(dest, sumarray[lx]);
    if (res)
      return res;
  }

  return 0;
}

static int sort_heap_summary(void *p1, void *p2)
{
  glui32 v1 = *(glui32 *)p1;
  glui32 v2 = *(glui32 *)p2;

  if (v1 < v2)
    return -1;
  if (v1 > v2)
    return 1;
  return 0;
}

static glui32 read_heapstate(dest_t *dest, glui32 chunklen, int portable,
  glui32 *sumlen, glui32 **summary)
{
  glui32 res, count, lx;
  glui32 *arr;

  *sumlen = 0;
  *summary = NULL;

  if (chunklen == 0)
    return 0; /* no heap */

  if (!portable) {
    count = chunklen / sizeof(glui32);

    arr = glulx_malloc(chunklen);
    if (!arr)
      return 1;

    res = read_buffer(dest, (void *)arr, chunklen);
    if (res)
      return res;

    *sumlen = count;
    *summary = arr;

    return 0;
  }

  count = chunklen / 4;

  arr = glulx_malloc(count * sizeof(glui32));
  if (!arr)
    return 1;
  
  for (lx=0; lx<count; lx++) {
    res = read_long(dest, arr+lx);
    if (res)
      return res;
  }

  *sumlen = count;
  *summary = arr;

  return 0;
}

static glui32 write_stackstate(dest_t *dest, int portable)
{
  glui32 res;
  glui32 lx;
  glui32 lastframe;

  /* If we're storing for the purpose of undo, we don't need to do any
     byte-swapping, because the result will only be used by this session. */
  if (!portable) {
    res = write_buffer(dest, stack, stackptr);
    if (res)
      return res;
    return 0;
  }

  /* Write a portable stack image. To do this, we have to write stack
     frames in order, bottom to top. Remember that the last word of
     every stack frame is a pointer to the beginning of that stack frame.
     (This includes the last frame, because the save opcode pushes on
     a call stub before it calls perform_save().) */

  lastframe = (glui32)(-1);
  while (1) {
    glui32 frameend, frm, frm2, frm3;
    unsigned char loctype, loccount;
    glui32 numlocals, frlen, locpos;

    /* Find the next stack frame (after the one in lastframe). Sadly,
       this requires searching the stack from the top down. We have to
       do this for *every* frame, which takes N^2 time overall. But
       save routines usually aren't nested very deep. 
       If it becomes a practical problem, we can build a stack-frame 
       array, which requires dynamic allocation. */
    for (frm = stackptr, frameend = stackptr;
         frm != 0 && (frm2 = Stk4(frm-4)) != lastframe;
         frameend = frm, frm = frm2) { };

    /* Write out the frame. */
    frm2 = frm;

    frlen = Stk4(frm2);
    frm2 += 4;
    res = write_long(dest, frlen);
    if (res)
      return res;
    locpos = Stk4(frm2);
    frm2 += 4;
    res = write_long(dest, locpos);
    if (res)
      return res;

    frm3 = frm2;

    numlocals = 0;
    while (1) {
      loctype = Stk1(frm2);
      frm2 += 1;
      loccount = Stk1(frm2);
      frm2 += 1;

      res = write_byte(dest, loctype);
      if (res)
        return res;
      res = write_byte(dest, loccount);
      if (res)
        return res;

      if (loctype == 0 && loccount == 0)
        break;

      numlocals++;
    }

    if ((numlocals & 1) == 0) {
      res = write_byte(dest, 0);
      if (res)
        return res;
      res = write_byte(dest, 0);
      if (res)
        return res;
      frm2 += 2;
    }

    if (frm2 != frm+locpos)
      fatal_error("Inconsistent stack frame during save.");

    /* Write out the locals. */
    for (lx=0; lx<numlocals; lx++) {
      loctype = Stk1(frm3);
      frm3 += 1;
      loccount = Stk1(frm3);
      frm3 += 1;
      
      if (loctype == 0 && loccount == 0)
        break;

      /* Put in up to 0, 1, or 3 bytes of padding, depending on loctype. */
      while (frm2 & (loctype-1)) {
        res = write_byte(dest, 0);
        if (res)
          return res;
        frm2 += 1;
      }

      /* Put in this set of locals. */
      switch (loctype) {

      case 1:
        do {
          res = write_byte(dest, Stk1(frm2));
          if (res)
            return res;
          frm2 += 1;
          loccount--;
        } while (loccount);
        break;

      case 2:
        do {
          res = write_short(dest, Stk2(frm2));
          if (res)
            return res;
          frm2 += 2;
          loccount--;
        } while (loccount);
        break;

      case 4:
        do {
          res = write_long(dest, Stk4(frm2));
          if (res)
            return res;
          frm2 += 4;
          loccount--;
        } while (loccount);
        break;

      }
    }

    if (frm2 != frm+frlen)
      fatal_error("Inconsistent stack frame during save.");

    while (frm2 < frameend) {
      res = write_long(dest, Stk4(frm2));
      if (res)
        return res;
      frm2 += 4;
    }

    /* Go on to the next frame. */
    if (frameend == stackptr)
      break; /* All done. */
    lastframe = frm;
  }

  return 0;
}

static glui32 read_stackstate(dest_t *dest, glui32 chunklen, int portable)
{
  glui32 res;
  glui32 frameend, frm, frm2, frm3, locpos, frlen, numlocals;

  if (chunklen > stacksize)
    return 1;

  stackptr = chunklen;
  frameptr = 0;
  valstackbase = 0;
  localsbase = 0;

  if (!portable) {
    res = read_buffer(dest, stack, stackptr);
    if (res)
      return res;
    return 0;
  }

  /* This isn't going to be pleasant; we're going to read the data in
     as a block, and then convert it in-place. */
  res = read_buffer(dest, stack, stackptr);
  if (res)
    return res;

  frameend = stackptr;
  while (frameend != 0) {
    /* Read the beginning-of-frame pointer. Remember, right now, the
       whole frame is stored big-endian. So we have to read with the
       Read*() macros, and then write with the StkW*() macros. */
    frm = Read4(stack+(frameend-4));

    frm2 = frm;

    frlen = Read4(stack+frm2);
    StkW4(frm2, frlen);
    frm2 += 4;
    locpos = Read4(stack+frm2);
    StkW4(frm2, locpos);
    frm2 += 4;

    /* The locals-format list is in bytes, so we don't have to convert it. */
    frm3 = frm2;
    frm2 = frm+locpos;

    numlocals = 0;

    while (1) {
      unsigned char loctype, loccount;
      loctype = Read1(stack+frm3);
      frm3 += 1;
      loccount = Read1(stack+frm3);
      frm3 += 1;

      if (loctype == 0 && loccount == 0)
        break;

      /* Skip up to 0, 1, or 3 bytes of padding, depending on loctype. */
      while (frm2 & (loctype-1)) {
        StkW1(frm2, 0);
        frm2++;
      }
      
      /* Convert this set of locals. */
      switch (loctype) {
        
      case 1:
        do {
          /* Don't need to convert bytes. */
          frm2 += 1;
          loccount--;
        } while (loccount);
        break;

      case 2:
        do {
          glui16 loc = Read2(stack+frm2);
          StkW2(frm2, loc);
          frm2 += 2;
          loccount--;
        } while (loccount);
        break;

      case 4:
        do {
          glui32 loc = Read4(stack+frm2);
          StkW4(frm2, loc);
          frm2 += 4;
          loccount--;
        } while (loccount);
        break;

      }

      numlocals++;
    }

    if ((numlocals & 1) == 0) {
      StkW1(frm3, 0);
      frm3++;
      StkW1(frm3, 0);
      frm3++;
    }

    if (frm3 != frm+locpos) {
      return 1;
    }

    while (frm2 & 3) {
      StkW1(frm2, 0);
      frm2++;
    }

    if (frm2 != frm+frlen) {
      return 1;
    }

    /* Now, the values pushed on the stack after the call frame itself.
       This includes the stub. */
    while (frm2 < frameend) {
      glui32 loc = Read4(stack+frm2);
      StkW4(frm2, loc);
      frm2 += 4;
    }

    frameend = frm;
  }

  return 0;
}

glui32 perform_verify()
{
  glui32 len, checksum, newlen;
  unsigned char buf[4];
  glui32 val, newsum, ix;

  len = gamefile_len;

  if (len < 256 || (len & 0xFF) != 0)
    return 1;

  glk_stream_set_position(gamefile, gamefile_start, seekmode_Start);
  newsum = 0;

  /* Read the header */
  for (ix=0; ix<9; ix++) {
    newlen = glk_get_buffer_stream(gamefile, (char *)buf, 4);
    if (newlen != 4)
      return 1;
    val = Read4(buf);
    if (ix == 3) {
      if (len != val)
        return 1;
    }
    if (ix == 8)
      checksum = val;
    else
      newsum += val;
  }

  /* Read everything else */
  for (; ix < len/4; ix++) {
    newlen = glk_get_buffer_stream(gamefile, (char *)buf, 4);
    if (newlen != 4)
      return 1;
    val = Read4(buf);
    newsum += val;
  }

  if (newsum != checksum)
    return 1;

  return 0;  
}