midi.c - audio/dosmid - Rivoreo Source Code Repositories

 /*
  * A simple MIDI parsing library
  *
  * Copyright (C) 2014-2022 Mateusz Viste
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
  *
  * 1. Redistributions of source code must retain the above copyright notice,
  *    this list of conditions and the following disclaimer.
  *
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
  * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  * POSSIBILITY OF SUCH DAMAGE.
  */

 #include <string.h>  /* memcpy() */

 #include "bitfield.h"
 #include "fio.h"
 #include "mem.h"
 #include "midi.h"   /* include self for control */

 #define BSWAPL(x) ((((unsigned long)(x) & 0x000000FFul) << 24) | (((unsigned long)(x) & 0x0000FF00ul) << 8) | (((unsigned long)(x) & 0x00FF0000ul) >> 8) | (((unsigned long)(x) & 0xFF000000ul) >> 24))

 extern unsigned char wbuff[];

 /* PRIVATE ROUTINES USED FOR INTERNAL PROCESSING ONLY */

 /* fetch a variable length quantity value from a given offset. returns number of bytes read */
 static int midi_fetch_variablelen_fromfile(struct fiofile_t *f, unsigned long *result) {
   unsigned char bytebuff;
   int offset = 0;
   *result = 0;
   for (;;) {
     fio_read(f, &bytebuff, 1);
     *result <<= 7;
     *result |= (bytebuff & 127);
     if ((bytebuff & 128) == 0) break;
   }
   return(offset);
 }


 /* reads a MIDI file and computes a map of chunks (ie a list of offsets) */
 static int midi_gettrackmap(struct fiofile_t *f, unsigned long *tracklist, int maxchunks) {
   short i;
   unsigned long ulvar;
   for (i = 0; i < maxchunks; i++) {
     /* read and validate chunk's id */
     if (fio_read(f, &ulvar, 4) != 4) break;
     if (ulvar != 0x6b72544dL) return(-1); /* if header != "MTrk" */
     /* compute the track's byte length */
     if (fio_read(f, &ulvar, 4) != 4) break;
     ulvar = BSWAPL(ulvar);
     /* remember chunk data offset */
     tracklist[i] = fio_seek(f, FIO_SEEK_CUR, 0);
     /* skip to next chunk */
     fio_seek(f, FIO_SEEK_CUR, ulvar);
   }
   return(i);
 }


 /* PUBLIC INTERFACE */


 int midi_readhdr(struct fiofile_t *f, int *format, unsigned short *timeunitdiv, unsigned long *tracklist, int maxtracks) {
   unsigned short tracks;
   /*
    * Here's an example of a complete MThd chunk:
    *  4D 54 68 64     MThd ID
    *  00 00 00 06     Length of the MThd chunk is always 6
    *  00 01           The Format type is 1
    *  00 02           There are 2 MTrk chunks in this file
    *  E7 28           Each increment of delta-time represents a millisecond
    */

   /* test for RMID header */
   /* a RMID file starts with RIFFxxxxRMID (xxxx being the data size) followed
    * by the word 'data' followed by a 32-bit data size. */
   /* read first 14 bytes - if unable, return an error */
   if (fio_read(f, wbuff, 14) != 14) return(-8);
   /* if no RMID header, then assume it's normal MIDI */
   if ((wbuff[0] == 'R') && (wbuff[1] == 'I')  && (wbuff[2] == 'F') && (wbuff[3] == 'F')
    && (wbuff[8] == 'R') && (wbuff[9] == 'M') && (wbuff[10] == 'I') && (wbuff[11] == 'D')) {
     /* skip 6 bytes and there we should have our MThd MIDI header */
     fio_seek(f, FIO_SEEK_CUR, 6);
     if (fio_read(f, wbuff, 14) != 14) return(-7);
   }

   /* check id (MThd) and len (must be exactly 6 bytes) */
   if ((((unsigned long *)wbuff)[0] != 0x6468544dL) || (wbuff[4] != 0) || (wbuff[5] != 0) || (wbuff[6] != 0) || (wbuff[7] != 6)) { /* 0x6468544dL == "MThd" */
     return(-6);
   }

   if (wbuff[8] != 0) return(-5); /* format is 1 or 2 so 1st digit must be 0 */
   *format = wbuff[9];

   tracks = wbuff[10];
   tracks <<= 8;
   tracks |= wbuff[11];
   /* midi_gettrackmap() should not try reading more tracks than declared in
    * the header - some MIDI/RMI files may be trailed with some extra stuff */
   if (tracks < maxtracks) maxtracks = tracks;

   *timeunitdiv = wbuff[12];
   *timeunitdiv <<= 8;
   *timeunitdiv |= wbuff[13];

   /* timeunitdiv must be a positive number */
   if (*timeunitdiv < 1) return(-3);

   /* default tempo -> quarter note (1 beat) == 500'000 microseconds (0.5s), ie 120 bpm.
      a delta time unit is therefore (0.5s / DIV) long. */

   if (*format > 2) return(-2);

   /* read the tracks map and return number of tracks */
   return(midi_gettrackmap(f, tracklist, maxtracks));
 }


 /* returns a negative value on error, 0 on success, 1 on end of track */
 #ifdef DBGFILE
 static int ld_meta(struct midi_event_t *event, struct fiofile_t *f, FILE *logfd, unsigned long *tracklen, char *title, int titlemaxlen, char *copyright, int copyrightmaxlen, char *text, int textmaxlen) {
 #else
 static int ld_meta(struct midi_event_t *event, struct fiofile_t *f, unsigned long *tracklen, char *title, int titlemaxlen, char *copyright, int copyrightmaxlen, char *text, int textmaxlen) {
 #endif

   unsigned long metalen;
   unsigned long i;
   int result = 0;
   unsigned char subtype;
   fio_read(f, &subtype, 1); /* fetch the META subtype fields */
   midi_fetch_variablelen_fromfile(f, &metalen);
   /* printf("got META[0x%02X] of %ld bytes\n", subtype, metalen); */
   switch (subtype) {
     case 1: /* text or marker - often used to describe the file... */
     case 6: /* marker */
       i = 0;
       if ((text != NULL) && (text[0] == 0) && (textmaxlen > 3)) { /* title might be NULL */
         for (; i < metalen; i++) {
           if (i+1 >= textmaxlen) break; /* avoid overflow */
           fio_read(f, text + i, 1);
         }
         text[i] = 0;
         /* recompute the available maxlen */
         text += i;
         textmaxlen -= i;
         /* add a LF trailer, just in case we'd like to append more data */
         if (textmaxlen > 2) {
           *text = '\n';
           text++;
           *text = 0;
           textmaxlen--;
         }
       }
       /* skip the rest, if we had to truncate the string */
       fio_seek(f, FIO_SEEK_CUR, metalen - i);
 #ifdef DBGFILE
       if (logfd != NULL) fprintf(logfd, "%lu: TEXT OR MARKER EVENT\n", *tracklen);
 #endif
       break;
     case 2: /* copyright notice */
       i = 0;
       if ((copyright != NULL) && (copyright[0] == 0)) { /* take care, copyright might be NULL */
         for (; i < metalen; i++) {
           if (i+1 >= copyrightmaxlen) break; /* avoid overflow */
           fio_read(f, copyright + i, 1);
         }
         copyright[i] = 0;
       }
       fio_seek(f, FIO_SEEK_CUR, metalen - i); /* skip the rest, if we had to truncate the string */
       break;
     case 3: /* track name */
       i = 0;
       if (title != NULL) { /* title might be NULL */
         for (; i < metalen; i++) {
           if (i+1 >= titlemaxlen) break; /* avoid overflow */
           fio_read(f, title + i, 1);
         }
         title[i] = 0;
       }
       fio_seek(f, FIO_SEEK_CUR, metalen - i); /* skip the rest, if we had to truncate the string */
       break;
     case 4: /* instrument name */
       fio_seek(f, FIO_SEEK_CUR, metalen);
 #ifdef DBGFILE
       if (logfd != NULL) fprintf(logfd, "%lu: INSTRUMENT EVENT (ignored)\n", *tracklen);
 #endif
       break;
     case 5: /* lyric */
       fio_seek(f, FIO_SEEK_CUR, metalen);
 #ifdef DBGFILE
       if (logfd != NULL) fprintf(logfd, "%lu: LYRIC EVENT (ignored)\n", *tracklen);
 #endif
       break;
     case 0x21:  /* MIDI port -- no support for multi-MIDI files, I just ignore it */
       fio_seek(f, FIO_SEEK_CUR, metalen);
 #ifdef DBGFILE
       if (logfd != NULL) fprintf(logfd, "%lu: MIDI PORT EVENT (ignored)\n", *tracklen);
 #endif
       break;
     case 0x2F: /* end of track */
 #ifdef DBGFILE
       if (logfd != NULL) fprintf(logfd, "%lu: END OF TRACK\n", *tracklen);
 #endif
       result = 1;
       break;
     case 0x51:  /* set tempo */
       if (metalen != 3) {
 #ifdef DBGFILE
         if (logfd != NULL) fprintf(logfd, "%lu: TEMPO ERROR\n", *tracklen);
 #endif
         return(-1);
       } else {
         unsigned char b[3];
         event->type = EVENT_TEMPO;
         fio_read(f, b, 3);
         event->data.tempoval = b[0];
         event->data.tempoval <<= 8;
         event->data.tempoval |= b[1];
         event->data.tempoval <<= 8;
         event->data.tempoval |= b[2];
 #ifdef DBGFILE
         if (logfd != NULL) fprintf(logfd, "%lu: TEMPO -> %lu\n", *tracklen, event->data.tempoval);
 #endif
       }
       break;
     case 0x54:  /* SMPTE offset -> since I expect only format 0/1 files, I ignore this because I want to start playing asap anyway */
       fio_seek(f, FIO_SEEK_CUR, metalen);
 #ifdef DBGFILE
       if (logfd != NULL) fprintf(logfd, "%lu: SMPTE OFFSET (ignored)\n", *tracklen);
 #endif
       break;
     case 0x58:  /* Time signature */
       if (metalen != 4) {
 #ifdef DBGFILE
         if (logfd != NULL) fprintf(logfd, "%lu: INVALID TIME SIGNATURE!\n", *tracklen);
 #endif
         return(-1);
       } else {
         fio_seek(f, FIO_SEEK_CUR, metalen);
 #ifdef DBGFILE
         if (logfd != NULL) fprintf(logfd, "%lu: TIME SIGNATURE (ignored)\n", *tracklen);
 #endif
       }
       break;
     case 0x59:  /* key signature */
       if (metalen != 2) {
 #ifdef DBGFILE
         if (logfd != NULL) fprintf(logfd, "%lu: INVALID KEY SIGNATURE!\n", *tracklen);
 #endif
         return(-1);
       } else {
         fio_seek(f, FIO_SEEK_CUR, metalen);
 #ifdef DBGFILE
         if (logfd != NULL) fprintf(logfd, "%lu: KEY SIGNATURE (ignored)\n", *tracklen);
 #endif
       }
       break;
     case 0x7F:  /* proprietary event -> this is non-standard stuff, I ignore it */
       fio_seek(f, FIO_SEEK_CUR, metalen);
 #ifdef DBGFILE
       if (logfd != NULL) fprintf(logfd, "%lu: PROPRIETARY EVENT (ignored)\n", *tracklen);
 #endif
       break;
     default:
       fio_seek(f, FIO_SEEK_CUR, metalen); /* skip the meta data */
 #ifdef DBGFILE
       if (logfd != NULL) fprintf(logfd, "%lu: UNHANDLED META EVENT [0x%02Xh] (ignored)\n", *tracklen, subtype);
 #endif
       break;
   }
   return(result);
 }


 /* returns a negative value on error, 0 on success, 1 on end of track */
 #ifdef DBGFILE
 static int ld_sysex(struct midi_event_t *event, struct fiofile_t *f, FILE *logfd, unsigned char statusbyte, unsigned long *tracklen) {
 #else
 static int ld_sysex(struct midi_event_t *event, struct fiofile_t *f, unsigned char statusbyte, unsigned long *tracklen) {
 #endif
   unsigned long sysexlen;
   int sysexleneven; /* can be int, guaranteed to be less than 4K */
   unsigned char *sysexbuff;
   midi_fetch_variablelen_fromfile(f, &sysexlen); /* get length */
   sysexlen += 1; /* add one byte for the status byte that is not counted, but that we will add to the top of the buffer later */
 #ifdef DBGFILE
   if (logfd != NULL) fprintf(logfd, "%lu: SYSEX EVENT OF %ld BYTES ON CHAN #%d\n", *tracklen, sysexlen, statusbyte & 0x0F);
 #endif
   if (sysexlen > 4096) { /* skip SYSEX events that are more than 4K big */
     fio_seek(f, FIO_SEEK_CUR, sysexlen);
     return(0);
   }
   /* read the sysex string */
   sysexleneven = sysexlen + 2; /* add two bytes for the sysex length that I will add in front of the actual sysex string */
   if ((sysexleneven & 1) != 0) sysexleneven++; /* make sysexleneven an even number (XMS moves MUST occur on even numbers of bytes) */
   sysexbuff = wbuff;
   event->type = EVENT_SYSEX;

   ((unsigned short *)sysexbuff)[0] = sysexlen;
   sysexbuff[2] = statusbyte; /* I store the entire sysex string in memory */
   fio_read(f, sysexbuff + 3, sysexlen - 1); /* read sysexlen-1 because I have already read the status byte */
   event->data.sysex.sysexptr = mem_alloc(sysexleneven);
   if (event->data.sysex.sysexptr >= 0) {
     mem_push(sysexbuff, event->data.sysex.sysexptr, sysexleneven);
   } else {
     event->type = EVENT_NONE;
 #ifdef DBGFILE
     if (logfd != NULL) fprintf(logfd, "%lu: SYSEX MEM_ALLOC FAILED FOR %ld BYTES\n", *tracklen, sysexlen);
 #endif
     return(MIDI_OUTOFMEM);
   }
   return(0);
 }


 #ifdef DBGFILE
 static int ld_note(struct midi_event_t *event, struct fiofile_t *f, FILE *logfd, unsigned char statusbyte, unsigned long *tracklen, unsigned short *channelsusage, void *reqpatches) {
 #else
 static int ld_note(struct midi_event_t *event, struct fiofile_t *f, unsigned char statusbyte, unsigned long *tracklen, unsigned short *channelsusage, void *reqpatches) {
 #endif
   unsigned char ubuff[2]; /* micro buffer for loading data */
   switch (statusbyte & 0xF0) { /* I care only about NoteOn/NoteOff events */
     case 0x80:  /* Note OFF */
       fio_read(f, ubuff, 2);
       event->type = EVENT_NOTEOFF;
       event->data.note.chan = statusbyte & 0x0F;
       event->data.note.note = ubuff[0] & 127; /* a note must be in range 0..127 */
       event->data.note.velocity = ubuff[1];
       break;
     case 0x90:  /* Note ON */
       fio_read(f, ubuff, 2);
       event->type = EVENT_NOTEON;
       event->data.note.chan = statusbyte & 0x0F;
       event->data.note.note = ubuff[0] & 127;
       event->data.note.velocity = ubuff[1];
       if (event->data.note.velocity == 0) {
         event->type = EVENT_NOTEOFF; /* if no velocity, it's in fact a note OFF */
       } else {
         *channelsusage |= (1 << event->data.note.chan); /* update the channel usage flags */
       }
       /* if it's percussion, mark the required patch */
       if (event->data.note.chan == 9) BIT_SET(reqpatches, event->data.note.note | 128);
       break;
     case 0xA0:  /* key after-touch */
       fio_read(f, ubuff, 2);
       event->type = EVENT_KEYPRESSURE;
       event->data.keypressure.chan = statusbyte & 0x0F;
       event->data.keypressure.note = ubuff[0];
       event->data.keypressure.pressure = ubuff[1];
       break;
     case 0xB0:  /* control change */
       fio_read(f, ubuff, 2);
       event->type = EVENT_CONTROL;
       event->data.control.chan = statusbyte & 0x0F;
       event->data.control.id = ubuff[0];
       event->data.control.val = ubuff[1];
       break;
     case 0xC0:  /* program (patch) change */
       fio_read(f, ubuff, 1);
       event->type = EVENT_PROGCHAN;
       event->data.prog.chan = statusbyte & 0x0F;
       event->data.prog.prog = ubuff[0] & 127;
       BIT_SET(reqpatches, event->data.prog.prog);
       break;
     case 0xD0:  /* channel after-touch (aka "channel pressure") */
       fio_read(f, ubuff, 1);
       event->type = EVENT_CHANPRESSURE;
       event->data.chanpressure.chan = statusbyte & 0x0F;
       event->data.chanpressure.pressure = ubuff[0];
       break;
     case 0xE0:  /* pitch wheel change */
       fio_read(f, ubuff, 2);
       event->type = EVENT_PITCH;
       event->data.pitch.chan = statusbyte & 0x0F;
       event->data.pitch.wheel = ubuff[1];
       event->data.pitch.wheel <<= 7;
       event->data.pitch.wheel |= ubuff[0];
       break;
     default:
 #ifdef DBGFILE
       if (logfd != NULL) fprintf(logfd, "%lu: Unknown note data\n", *tracklen);
 #endif
       return(-1);
       break;
   }
   return(0);
 }


 /* parse a track object and returns the id of the first events in the linked
  * list. channelsusage contains 16 flags indicating what channels are used.
  * titlemaxlen and copyrightmaxlen are the maximum lengths of the strings,
  * including the NULL terminator.
  * returns MIDI_EMPTYTRACK if no event found in the track
  * returns MIDI_TRACKERROR if the track is corrupted
  * returns MIDI_OUTOFMEM if failed to store events in memory */
 #ifdef DBGFILE
 long midi_track2events(struct fiofile_t *f, char *title, int titlemaxlen, char *copyright, int copyrightmaxlen, char *text, int textmaxlen, unsigned short *channelsusage, FILE *logfd, unsigned long *tracklen, void *reqpatches) {
 #else
 long midi_track2events(struct fiofile_t *f, char *title, int titlemaxlen, char *copyright, int copyrightmaxlen, char *text, int textmaxlen, unsigned short *channelsusage, unsigned long *tracklen, void *reqpatches) {
 #endif
   unsigned long deltatime;
   unsigned char statusbyte = 0;
   struct midi_event_t event;
   long result = MIDI_EMPTYTRACK;
   unsigned long ignoreddeltas = 0;

   /* zero out title and copyright strings, if provided */
   if (titlemaxlen > 0) title[0] = 0;
   if (copyrightmaxlen > 0) copyright[0] = 0;
   if (textmaxlen > 0) text[0] = 0;

   *tracklen = 0;

   for (;;) {
     int r;
     unsigned char bytebuff;
     /* read the delta time first - variable length */
     midi_fetch_variablelen_fromfile(f, &deltatime);
     *tracklen += deltatime;
     /* check the type of the event */
     /* if it's a byte with MSB set, we are dealing with running status (so it's same status as last time */
     if (fio_read(f, &bytebuff, 1) == 0) return(MIDI_TRACKERROR);
     if ((bytebuff & 128) != 0) {
       statusbyte = bytebuff;
     } else { /* get back one byte */
       fio_seek(f, FIO_SEEK_CUR, -1);
     }
     event.type = EVENT_NONE;
     event.deltatime = deltatime;
     event.next = -1;
     if (statusbyte == 0xFF) { /* META event */
 #ifdef DBGFILE
       r = ld_meta(&event, f, logfd, tracklen, title, titlemaxlen, copyright, copyrightmaxlen, text, textmaxlen);
 #else
       r = ld_meta(&event, f, tracklen, title, titlemaxlen, copyright, copyrightmaxlen, text, textmaxlen);
 #endif
       if (r < 0) return(MIDI_TRACKERROR);
       if (r == 1) break; /* end of track */
     } else if ((statusbyte >= 0xF0) && (statusbyte <= 0xF7)) { /* SYSEX event */
 #ifdef DBGFILE
       r = ld_sysex(&event, f, logfd, statusbyte, tracklen);
 #else
       r = ld_sysex(&event, f, statusbyte, tracklen);
 #endif
       if (r != 0) return(MIDI_TRACKERROR);
     } else if ((statusbyte >= 0x80) && (statusbyte <= 0xEF)) { /* else it's a note-related command */
 #ifdef DBGFILE
       r = ld_note(&event, f, logfd, statusbyte, tracklen, channelsusage, reqpatches);
 #else
       r = ld_note(&event, f, statusbyte, tracklen, channelsusage, reqpatches);
 #endif
       if (r != 0) return(MIDI_TRACKERROR);
     } else { /* else it's an error - free memory we allocated and return NULL */
 #ifdef DBGFILE
       if (logfd != NULL) fprintf(logfd, "Err. at offset %04lX (bytebuff = 0x%02X)\n", fio_seek(f, FIO_SEEK_CUR, 0), statusbyte);
 #endif
       return(MIDI_TRACKERROR);
     }
     /* add the event to the queue */
     if (event.type == EVENT_NONE) {
       ignoreddeltas += event.deltatime;
     } else {
       int pusheventres;
       event.deltatime += ignoreddeltas; /* add any previously ignored delta times */
       ignoreddeltas = 0;
       /* add the event to the queue */
       if (result == MIDI_EMPTYTRACK) { /* this is the first event in the queue */
         pusheventres = pusheventqueue(&event, &result);
       } else {
         pusheventres = pusheventqueue(&event, NULL);
       }
       if (pusheventres != 0) {
         return(MIDI_OUTOFMEM);
       }
     }
   }
   if (result >= 0) {
     if (pusheventqueue(NULL, NULL) != 0) return(MIDI_OUTOFMEM); /* flush last event in buffer to memory */
   }
   return(result);
 }


 /* merge two MIDI tracks into a single (serialized) one. returns a "pointer"
  * to the unique track. I take care not to allocate/free memory here.
  * All notes are already in RAM after all. totlen is filled with the total
  * time of the merged tracks (in seconds). */
 long midi_mergetrack(long t0, long t1, unsigned long *totlen, unsigned short timeunitdiv) {
   long res = -1, lasteventid = -1, selectedid;
   int selected;
   unsigned long curtempo = 500000l, utotlen = 0;
   struct midi_event_t event[2], lastevent;

   if (totlen != NULL) *totlen = 0;
   /* fetch first events for both tracks */
   if (t0 >= 0) mem_pull(t0, &event[0], sizeof(struct midi_event_t));
   if (t1 >= 0) mem_pull(t1, &event[1], sizeof(struct midi_event_t));
   /* start looping */
   while ((t0 >= 0) || (t1 >= 0)) {
     /* compare both tracks, and select the soonest one */
     if (t0 >= 0) {
       if ((t1 >= 0) && (event[1].deltatime < event[0].deltatime)) {
         selected = 1;
         selectedid = t1;
       } else {
         selected = 0;
         selectedid = t0;
       }
     } else {
       selected = 1;
       selectedid = t1;
     }
     /* on first iteration, make sure to assign a result */
     if (lasteventid < 0) {
       res = selectedid;
     } else if (lastevent.next != selectedid) {                        /* otherwise attach selected */
       lastevent.next = selectedid;                                    /* track to last note, and   */
       mem_push(&lastevent, lasteventid, sizeof(struct midi_event_t)); /* update last pointer (if   */
     }                                                                 /* not good already)         */
     /* save the last event into buffer for later, and remember its id */
     lasteventid = selectedid;
     memcpy(&lastevent, &(event[selected]), sizeof(struct midi_event_t));
     /* increment timer */
     if ((totlen != NULL) && (event[selected].deltatime != 0)) {
       utotlen += DELTATIME2US(event[selected].deltatime, curtempo, timeunitdiv);
       while (utotlen >= 1000000lu) {
         utotlen -= 1000000lu;
         *totlen += 1;
       }
     }
     if (event[selected].type == EVENT_TEMPO) curtempo = event[selected].data.tempoval;
     /* decrement timer on the non-selected track, and synch it to xms if needed */
     /* then move along on the selected track, and fetch it from xms */
     if (selected == 0) {
       if ((t1 >= 0) && (event[0].deltatime != 0)) {
         event[1].deltatime -= event[0].deltatime;
         mem_push(&event[1], t1, sizeof(struct midi_event_t));
       }
       t0 = event[0].next;
       if (t0 >= 0) mem_pull(t0, &event[0], sizeof(struct midi_event_t));
     } else { /* selected == 1 */
       if ((t0 >= 0) && (event[1].deltatime != 0)) {
         event[0].deltatime -= event[1].deltatime;
         mem_push(&event[0], t0, sizeof(struct midi_event_t));
       }
       t1 = event[1].next;
       if (t1 >= 0) mem_pull(t1, &event[1], sizeof(struct midi_event_t));
     }
   }
   return(res);
 }
	/*
	* A simple MIDI parsing library
	*
	* Copyright (C) 2014-2022 Mateusz Viste
	* All rights reserved.
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions are met:
	*
	* 1. Redistributions of source code must retain the above copyright notice,
	* this list of conditions and the following disclaimer.
	*
	* 2. Redistributions in binary form must reproduce the above copyright
	* notice, this list of conditions and the following disclaimer in the
	* documentation and/or other materials provided with the distribution.
	*
	* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
	* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
	* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
	* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
	* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
	* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
	* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
	* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
	* POSSIBILITY OF SUCH DAMAGE.
	*/

	#include <string.h> /* memcpy() */

	#include "bitfield.h"
	#include "fio.h"
	#include "mem.h"
	#include "midi.h" /* include self for control */

	#define BSWAPL(x) ((((unsigned long)(x) & 0x000000FFul) << 24) \| (((unsigned long)(x) & 0x0000FF00ul) << 8) \| (((unsigned long)(x) & 0x00FF0000ul) >> 8) \| (((unsigned long)(x) & 0xFF000000ul) >> 24))

	extern unsigned char wbuff[];

	/* PRIVATE ROUTINES USED FOR INTERNAL PROCESSING ONLY */

	/* fetch a variable length quantity value from a given offset. returns number of bytes read */
	static int midi_fetch_variablelen_fromfile(struct fiofile_t f, unsigned long result) {
	unsigned char bytebuff;
	int offset = 0;
	*result = 0;
	for (;;) {
	fio_read(f, &bytebuff, 1);
	*result <<= 7;
	*result \|= (bytebuff & 127);
	if ((bytebuff & 128) == 0) break;
	}
	return(offset);
	}


	/* reads a MIDI file and computes a map of chunks (ie a list of offsets) */
	static int midi_gettrackmap(struct fiofile_t f, unsigned long tracklist, int maxchunks) {
	short i;
	unsigned long ulvar;
	for (i = 0; i < maxchunks; i++) {
	/* read and validate chunk's id */
	if (fio_read(f, &ulvar, 4) != 4) break;
	if (ulvar != 0x6b72544dL) return(-1); /* if header != "MTrk" */
	/* compute the track's byte length */
	if (fio_read(f, &ulvar, 4) != 4) break;
	ulvar = BSWAPL(ulvar);
	/* remember chunk data offset */
	tracklist[i] = fio_seek(f, FIO_SEEK_CUR, 0);
	/* skip to next chunk */
	fio_seek(f, FIO_SEEK_CUR, ulvar);
	}
	return(i);
	}


	/* PUBLIC INTERFACE */


	int midi_readhdr(struct fiofile_t f, int format, unsigned short timeunitdiv, unsigned long tracklist, int maxtracks) {
	unsigned short tracks;
	/*
	* Here's an example of a complete MThd chunk:
	* 4D 54 68 64 MThd ID
	* 00 00 00 06 Length of the MThd chunk is always 6
	* 00 01 The Format type is 1
	* 00 02 There are 2 MTrk chunks in this file
	* E7 28 Each increment of delta-time represents a millisecond
	*/

	/* test for RMID header */
	/* a RMID file starts with RIFFxxxxRMID (xxxx being the data size) followed
	* by the word 'data' followed by a 32-bit data size. */
	/* read first 14 bytes - if unable, return an error */
	if (fio_read(f, wbuff, 14) != 14) return(-8);
	/* if no RMID header, then assume it's normal MIDI */
	if ((wbuff[0] == 'R') && (wbuff[1] == 'I') && (wbuff[2] == 'F') && (wbuff[3] == 'F')
	&& (wbuff[8] == 'R') && (wbuff[9] == 'M') && (wbuff[10] == 'I') && (wbuff[11] == 'D')) {
	/* skip 6 bytes and there we should have our MThd MIDI header */
	fio_seek(f, FIO_SEEK_CUR, 6);
	if (fio_read(f, wbuff, 14) != 14) return(-7);
	}

	/* check id (MThd) and len (must be exactly 6 bytes) */
	if ((((unsigned long )wbuff)[0] != 0x6468544dL) \|\| (wbuff[4] != 0) \|\| (wbuff[5] != 0) \|\| (wbuff[6] != 0) \|\| (wbuff[7] != 6)) { / 0x6468544dL == "MThd" */
	return(-6);
	}

	if (wbuff[8] != 0) return(-5); /* format is 1 or 2 so 1st digit must be 0 */
	*format = wbuff[9];

	tracks = wbuff[10];
	tracks <<= 8;
	tracks \|= wbuff[11];
	/* midi_gettrackmap() should not try reading more tracks than declared in
	* the header - some MIDI/RMI files may be trailed with some extra stuff */
	if (tracks < maxtracks) maxtracks = tracks;

	*timeunitdiv = wbuff[12];
	*timeunitdiv <<= 8;
	*timeunitdiv \|= wbuff[13];

	/* timeunitdiv must be a positive number */
	if (*timeunitdiv < 1) return(-3);

	/* default tempo -> quarter note (1 beat) == 500'000 microseconds (0.5s), ie 120 bpm.
	a delta time unit is therefore (0.5s / DIV) long. */

	if (*format > 2) return(-2);

	/* read the tracks map and return number of tracks */
	return(midi_gettrackmap(f, tracklist, maxtracks));
	}


	/* returns a negative value on error, 0 on success, 1 on end of track */
	#ifdef DBGFILE
	static int ld_meta(struct midi_event_t event, struct fiofile_t f, FILE logfd, unsigned long tracklen, char title, int titlemaxlen, char copyright, int copyrightmaxlen, char *text, int textmaxlen) {
	#else
	static int ld_meta(struct midi_event_t event, struct fiofile_t f, unsigned long tracklen, char title, int titlemaxlen, char copyright, int copyrightmaxlen, char text, int textmaxlen) {
	#endif

	unsigned long metalen;
	unsigned long i;
	int result = 0;
	unsigned char subtype;
	fio_read(f, &subtype, 1); /* fetch the META subtype fields */
	midi_fetch_variablelen_fromfile(f, &metalen);
	/* printf("got META[0x%02X] of %ld bytes\n", subtype, metalen); */
	switch (subtype) {
	case 1: /* text or marker - often used to describe the file... */
	case 6: /* marker */
	i = 0;
	if ((text != NULL) && (text[0] == 0) && (textmaxlen > 3)) { /* title might be NULL */
	for (; i < metalen; i++) {
	if (i+1 >= textmaxlen) break; /* avoid overflow */
	fio_read(f, text + i, 1);
	}
	text[i] = 0;
	/* recompute the available maxlen */
	text += i;
	textmaxlen -= i;
	/* add a LF trailer, just in case we'd like to append more data */
	if (textmaxlen > 2) {
	*text = '\n';
	text++;
	*text = 0;
	textmaxlen--;
	}
	}
	/* skip the rest, if we had to truncate the string */
	fio_seek(f, FIO_SEEK_CUR, metalen - i);
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: TEXT OR MARKER EVENT\n", *tracklen);
	#endif
	break;
	case 2: /* copyright notice */
	i = 0;
	if ((copyright != NULL) && (copyright[0] == 0)) { /* take care, copyright might be NULL */
	for (; i < metalen; i++) {
	if (i+1 >= copyrightmaxlen) break; /* avoid overflow */
	fio_read(f, copyright + i, 1);
	}
	copyright[i] = 0;
	}
	fio_seek(f, FIO_SEEK_CUR, metalen - i); /* skip the rest, if we had to truncate the string */
	break;
	case 3: /* track name */
	i = 0;
	if (title != NULL) { /* title might be NULL */
	for (; i < metalen; i++) {
	if (i+1 >= titlemaxlen) break; /* avoid overflow */
	fio_read(f, title + i, 1);
	}
	title[i] = 0;
	}
	fio_seek(f, FIO_SEEK_CUR, metalen - i); /* skip the rest, if we had to truncate the string */
	break;
	case 4: /* instrument name */
	fio_seek(f, FIO_SEEK_CUR, metalen);
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: INSTRUMENT EVENT (ignored)\n", *tracklen);
	#endif
	break;
	case 5: /* lyric */
	fio_seek(f, FIO_SEEK_CUR, metalen);
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: LYRIC EVENT (ignored)\n", *tracklen);
	#endif
	break;
	case 0x21: /* MIDI port -- no support for multi-MIDI files, I just ignore it */
	fio_seek(f, FIO_SEEK_CUR, metalen);
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: MIDI PORT EVENT (ignored)\n", *tracklen);
	#endif
	break;
	case 0x2F: /* end of track */
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: END OF TRACK\n", *tracklen);
	#endif
	result = 1;
	break;
	case 0x51: /* set tempo */
	if (metalen != 3) {
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: TEMPO ERROR\n", *tracklen);
	#endif
	return(-1);
	} else {
	unsigned char b[3];
	event->type = EVENT_TEMPO;
	fio_read(f, b, 3);
	event->data.tempoval = b[0];
	event->data.tempoval <<= 8;
	event->data.tempoval \|= b[1];
	event->data.tempoval <<= 8;
	event->data.tempoval \|= b[2];
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: TEMPO -> %lu\n", *tracklen, event->data.tempoval);
	#endif
	}
	break;
	case 0x54: /* SMPTE offset -> since I expect only format 0/1 files, I ignore this because I want to start playing asap anyway */
	fio_seek(f, FIO_SEEK_CUR, metalen);
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: SMPTE OFFSET (ignored)\n", *tracklen);
	#endif
	break;
	case 0x58: /* Time signature */
	if (metalen != 4) {
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: INVALID TIME SIGNATURE!\n", *tracklen);
	#endif
	return(-1);
	} else {
	fio_seek(f, FIO_SEEK_CUR, metalen);
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: TIME SIGNATURE (ignored)\n", *tracklen);
	#endif
	}
	break;
	case 0x59: /* key signature */
	if (metalen != 2) {
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: INVALID KEY SIGNATURE!\n", *tracklen);
	#endif
	return(-1);
	} else {
	fio_seek(f, FIO_SEEK_CUR, metalen);
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: KEY SIGNATURE (ignored)\n", *tracklen);
	#endif
	}
	break;
	case 0x7F: /* proprietary event -> this is non-standard stuff, I ignore it */
	fio_seek(f, FIO_SEEK_CUR, metalen);
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: PROPRIETARY EVENT (ignored)\n", *tracklen);
	#endif
	break;
	default:
	fio_seek(f, FIO_SEEK_CUR, metalen); /* skip the meta data */
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: UNHANDLED META EVENT [0x%02Xh] (ignored)\n", *tracklen, subtype);
	#endif
	break;
	}
	return(result);
	}


	/* returns a negative value on error, 0 on success, 1 on end of track */
	#ifdef DBGFILE
	static int ld_sysex(struct midi_event_t event, struct fiofile_t f, FILE logfd, unsigned char statusbyte, unsigned long tracklen) {
	#else
	static int ld_sysex(struct midi_event_t event, struct fiofile_t f, unsigned char statusbyte, unsigned long *tracklen) {
	#endif
	unsigned long sysexlen;
	int sysexleneven; /* can be int, guaranteed to be less than 4K */
	unsigned char *sysexbuff;
	midi_fetch_variablelen_fromfile(f, &sysexlen); /* get length */
	sysexlen += 1; /* add one byte for the status byte that is not counted, but that we will add to the top of the buffer later */
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: SYSEX EVENT OF %ld BYTES ON CHAN #%d\n", *tracklen, sysexlen, statusbyte & 0x0F);
	#endif
	if (sysexlen > 4096) { /* skip SYSEX events that are more than 4K big */
	fio_seek(f, FIO_SEEK_CUR, sysexlen);
	return(0);
	}
	/* read the sysex string */
	sysexleneven = sysexlen + 2; /* add two bytes for the sysex length that I will add in front of the actual sysex string */
	if ((sysexleneven & 1) != 0) sysexleneven++; /* make sysexleneven an even number (XMS moves MUST occur on even numbers of bytes) */
	sysexbuff = wbuff;
	event->type = EVENT_SYSEX;

	((unsigned short *)sysexbuff)[0] = sysexlen;
	sysexbuff[2] = statusbyte; /* I store the entire sysex string in memory */
	fio_read(f, sysexbuff + 3, sysexlen - 1); /* read sysexlen-1 because I have already read the status byte */
	event->data.sysex.sysexptr = mem_alloc(sysexleneven);
	if (event->data.sysex.sysexptr >= 0) {
	mem_push(sysexbuff, event->data.sysex.sysexptr, sysexleneven);
	} else {
	event->type = EVENT_NONE;
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: SYSEX MEM_ALLOC FAILED FOR %ld BYTES\n", *tracklen, sysexlen);
	#endif
	return(MIDI_OUTOFMEM);
	}
	return(0);
	}


	#ifdef DBGFILE
	static int ld_note(struct midi_event_t event, struct fiofile_t f, FILE logfd, unsigned char statusbyte, unsigned long tracklen, unsigned short channelsusage, void reqpatches) {
	#else
	static int ld_note(struct midi_event_t event, struct fiofile_t f, unsigned char statusbyte, unsigned long tracklen, unsigned short channelsusage, void *reqpatches) {
	#endif
	unsigned char ubuff[2]; /* micro buffer for loading data */
	switch (statusbyte & 0xF0) { /* I care only about NoteOn/NoteOff events */
	case 0x80: /* Note OFF */
	fio_read(f, ubuff, 2);
	event->type = EVENT_NOTEOFF;
	event->data.note.chan = statusbyte & 0x0F;
	event->data.note.note = ubuff[0] & 127; /* a note must be in range 0..127 */
	event->data.note.velocity = ubuff[1];
	break;
	case 0x90: /* Note ON */
	fio_read(f, ubuff, 2);
	event->type = EVENT_NOTEON;
	event->data.note.chan = statusbyte & 0x0F;
	event->data.note.note = ubuff[0] & 127;
	event->data.note.velocity = ubuff[1];
	if (event->data.note.velocity == 0) {
	event->type = EVENT_NOTEOFF; /* if no velocity, it's in fact a note OFF */
	} else {
	channelsusage \|= (1 << event->data.note.chan); / update the channel usage flags */
	}
	/* if it's percussion, mark the required patch */
	if (event->data.note.chan == 9) BIT_SET(reqpatches, event->data.note.note \| 128);
	break;
	case 0xA0: /* key after-touch */
	fio_read(f, ubuff, 2);
	event->type = EVENT_KEYPRESSURE;
	event->data.keypressure.chan = statusbyte & 0x0F;
	event->data.keypressure.note = ubuff[0];
	event->data.keypressure.pressure = ubuff[1];
	break;
	case 0xB0: /* control change */
	fio_read(f, ubuff, 2);
	event->type = EVENT_CONTROL;
	event->data.control.chan = statusbyte & 0x0F;
	event->data.control.id = ubuff[0];
	event->data.control.val = ubuff[1];
	break;
	case 0xC0: /* program (patch) change */
	fio_read(f, ubuff, 1);
	event->type = EVENT_PROGCHAN;
	event->data.prog.chan = statusbyte & 0x0F;
	event->data.prog.prog = ubuff[0] & 127;
	BIT_SET(reqpatches, event->data.prog.prog);
	break;
	case 0xD0: /* channel after-touch (aka "channel pressure") */
	fio_read(f, ubuff, 1);
	event->type = EVENT_CHANPRESSURE;
	event->data.chanpressure.chan = statusbyte & 0x0F;
	event->data.chanpressure.pressure = ubuff[0];
	break;
	case 0xE0: /* pitch wheel change */
	fio_read(f, ubuff, 2);
	event->type = EVENT_PITCH;
	event->data.pitch.chan = statusbyte & 0x0F;
	event->data.pitch.wheel = ubuff[1];
	event->data.pitch.wheel <<= 7;
	event->data.pitch.wheel \|= ubuff[0];
	break;
	default:
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "%lu: Unknown note data\n", *tracklen);
	#endif
	return(-1);
	break;
	}
	return(0);
	}


	/* parse a track object and returns the id of the first events in the linked
	* list. channelsusage contains 16 flags indicating what channels are used.
	* titlemaxlen and copyrightmaxlen are the maximum lengths of the strings,
	* including the NULL terminator.
	* returns MIDI_EMPTYTRACK if no event found in the track
	* returns MIDI_TRACKERROR if the track is corrupted
	* returns MIDI_OUTOFMEM if failed to store events in memory */
	#ifdef DBGFILE
	long midi_track2events(struct fiofile_t f, char title, int titlemaxlen, char copyright, int copyrightmaxlen, char text, int textmaxlen, unsigned short channelsusage, FILE logfd, unsigned long tracklen, void reqpatches) {
	#else
	long midi_track2events(struct fiofile_t f, char title, int titlemaxlen, char copyright, int copyrightmaxlen, char text, int textmaxlen, unsigned short channelsusage, unsigned long tracklen, void *reqpatches) {
	#endif
	unsigned long deltatime;
	unsigned char statusbyte = 0;
	struct midi_event_t event;
	long result = MIDI_EMPTYTRACK;
	unsigned long ignoreddeltas = 0;

	/* zero out title and copyright strings, if provided */
	if (titlemaxlen > 0) title[0] = 0;
	if (copyrightmaxlen > 0) copyright[0] = 0;
	if (textmaxlen > 0) text[0] = 0;

	*tracklen = 0;

	for (;;) {
	int r;
	unsigned char bytebuff;
	/* read the delta time first - variable length */
	midi_fetch_variablelen_fromfile(f, &deltatime);
	*tracklen += deltatime;
	/* check the type of the event */
	/* if it's a byte with MSB set, we are dealing with running status (so it's same status as last time */
	if (fio_read(f, &bytebuff, 1) == 0) return(MIDI_TRACKERROR);
	if ((bytebuff & 128) != 0) {
	statusbyte = bytebuff;
	} else { /* get back one byte */
	fio_seek(f, FIO_SEEK_CUR, -1);
	}
	event.type = EVENT_NONE;
	event.deltatime = deltatime;
	event.next = -1;
	if (statusbyte == 0xFF) { /* META event */
	#ifdef DBGFILE
	r = ld_meta(&event, f, logfd, tracklen, title, titlemaxlen, copyright, copyrightmaxlen, text, textmaxlen);
	#else
	r = ld_meta(&event, f, tracklen, title, titlemaxlen, copyright, copyrightmaxlen, text, textmaxlen);
	#endif
	if (r < 0) return(MIDI_TRACKERROR);
	if (r == 1) break; /* end of track */
	} else if ((statusbyte >= 0xF0) && (statusbyte <= 0xF7)) { /* SYSEX event */
	#ifdef DBGFILE
	r = ld_sysex(&event, f, logfd, statusbyte, tracklen);
	#else
	r = ld_sysex(&event, f, statusbyte, tracklen);
	#endif
	if (r != 0) return(MIDI_TRACKERROR);
	} else if ((statusbyte >= 0x80) && (statusbyte <= 0xEF)) { /* else it's a note-related command */
	#ifdef DBGFILE
	r = ld_note(&event, f, logfd, statusbyte, tracklen, channelsusage, reqpatches);
	#else
	r = ld_note(&event, f, statusbyte, tracklen, channelsusage, reqpatches);
	#endif
	if (r != 0) return(MIDI_TRACKERROR);
	} else { /* else it's an error - free memory we allocated and return NULL */
	#ifdef DBGFILE
	if (logfd != NULL) fprintf(logfd, "Err. at offset %04lX (bytebuff = 0x%02X)\n", fio_seek(f, FIO_SEEK_CUR, 0), statusbyte);
	#endif
	return(MIDI_TRACKERROR);
	}
	/* add the event to the queue */
	if (event.type == EVENT_NONE) {
	ignoreddeltas += event.deltatime;
	} else {
	int pusheventres;
	event.deltatime += ignoreddeltas; /* add any previously ignored delta times */
	ignoreddeltas = 0;
	/* add the event to the queue */
	if (result == MIDI_EMPTYTRACK) { /* this is the first event in the queue */
	pusheventres = pusheventqueue(&event, &result);
	} else {
	pusheventres = pusheventqueue(&event, NULL);
	}
	if (pusheventres != 0) {
	return(MIDI_OUTOFMEM);
	}
	}
	}
	if (result >= 0) {
	if (pusheventqueue(NULL, NULL) != 0) return(MIDI_OUTOFMEM); /* flush last event in buffer to memory */
	}
	return(result);
	}


	/* merge two MIDI tracks into a single (serialized) one. returns a "pointer"
	* to the unique track. I take care not to allocate/free memory here.
	* All notes are already in RAM after all. totlen is filled with the total
	* time of the merged tracks (in seconds). */
	long midi_mergetrack(long t0, long t1, unsigned long *totlen, unsigned short timeunitdiv) {
	long res = -1, lasteventid = -1, selectedid;
	int selected;
	unsigned long curtempo = 500000l, utotlen = 0;
	struct midi_event_t event[2], lastevent;

	if (totlen != NULL) *totlen = 0;
	/* fetch first events for both tracks */
	if (t0 >= 0) mem_pull(t0, &event[0], sizeof(struct midi_event_t));
	if (t1 >= 0) mem_pull(t1, &event[1], sizeof(struct midi_event_t));
	/* start looping */
	while ((t0 >= 0) \|\| (t1 >= 0)) {
	/* compare both tracks, and select the soonest one */
	if (t0 >= 0) {
	if ((t1 >= 0) && (event[1].deltatime < event[0].deltatime)) {
	selected = 1;
	selectedid = t1;
	} else {
	selected = 0;
	selectedid = t0;
	}
	} else {
	selected = 1;
	selectedid = t1;
	}
	/* on first iteration, make sure to assign a result */
	if (lasteventid < 0) {
	res = selectedid;
	} else if (lastevent.next != selectedid) { /* otherwise attach selected */
	lastevent.next = selectedid; /* track to last note, and */
	mem_push(&lastevent, lasteventid, sizeof(struct midi_event_t)); /* update last pointer (if */
	} /* not good already) */
	/* save the last event into buffer for later, and remember its id */
	lasteventid = selectedid;
	memcpy(&lastevent, &(event[selected]), sizeof(struct midi_event_t));
	/* increment timer */
	if ((totlen != NULL) && (event[selected].deltatime != 0)) {
	utotlen += DELTATIME2US(event[selected].deltatime, curtempo, timeunitdiv);
	while (utotlen >= 1000000lu) {
	utotlen -= 1000000lu;
	*totlen += 1;
	}
	}
	if (event[selected].type == EVENT_TEMPO) curtempo = event[selected].data.tempoval;
	/* decrement timer on the non-selected track, and synch it to xms if needed */
	/* then move along on the selected track, and fetch it from xms */
	if (selected == 0) {
	if ((t1 >= 0) && (event[0].deltatime != 0)) {
	event[1].deltatime -= event[0].deltatime;
	mem_push(&event[1], t1, sizeof(struct midi_event_t));
	}
	t0 = event[0].next;
	if (t0 >= 0) mem_pull(t0, &event[0], sizeof(struct midi_event_t));
	} else { /* selected == 1 */
	if ((t0 >= 0) && (event[1].deltatime != 0)) {
	event[0].deltatime -= event[1].deltatime;
	mem_push(&event[0], t0, sizeof(struct midi_event_t));
	}
	t1 = event[1].next;
	if (t1 >= 0) mem_pull(t1, &event[1], sizeof(struct midi_event_t));
	}
	}
	return(res);
	}