xref: /dports/games/libretro-nxengine/nxengine-libretro-10c4381/nxengine/sound/org.cpp

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

#include "../nx.h"
#include "../common/basics.h"
#include "org.h"
#include "pxt.h"			// for loading drums
#include "sslib.h"			// SAMPLE_RATE
#include "org.fdh"

#include "libretro_shared.h"

#define DRUM_PXT

#define drumK		22050

static bool org_inited = false;

static stNoteChannel note_channel[16];

static stSong song;

static int cache_ahead_time = 17;		// approximate number of ms to cache ahead (is rounded to a # of beats)

static int buffer_beats;				// # of beats to cache ahead in each buffer
static int buffer_samples;				// how many samples are in each outbuffer
static int outbuffer_size_bytes;		// length of each outbuffer, and of final_buffer, in bytes

static struct
{
	signed short *samples;		// pointer to the raw PCM sound data
	int firstbeat;				// beat # of the first beat contained in this chunk
} final_buffer[2];

static uint8_t current_buffer;
static bool buffers_full;

static int OrgVolume;

signed short wavetable[100][256];

// sound effect numbers which correspond to the drums
static const unsigned char drum_pxt[] =
{
   0x96, 0, 0x97, 0, 0x9a, 0x98,
   0x99, 0, 0x9b, 0, 0, 0
};

static struct
{
	signed short *samples;
	int nsamples;
} drumtable[NUM_DRUMS];

static int pitch[NUM_NOTES];


static void init_pitch(void)
{
	NX_LOG("Calculating pitch scale...\n");

	for(int i=0;i<NUM_NOTES;i++)
	{
		pitch[i] = (int)(441.0*(pow(2.0,((i-19.0)/12.0))));
	}
}


// given an instrument pitch and a note, returns the sampling rate that the
// wavetable sample should be played at in order to seem as if it is a recording of that note.
static float_type GetNoteSampleRate(int note, int instrument_pitch)
{
	return ((instrument_pitch - 1000.0)/100.0 + pitch[note])*44100/1550;
}

// converts a time in milliseconds to that same time length in samples
static int MSToSamples(int ms)
{
   return (int)(((float_type)SAMPLE_RATE / (float_type)1000) * (float_type)ms);
}

// converts a sample length to milliseconds
static int SamplesToMS(int samples)
{
   return (int)(((float_type)samples * 1000) / SAMPLE_RATE);
}

static bool load_drum_pxt(FILE *fd, int s, int d)
{
   int i;
   signed short sample;
   stPXSound snd;

   if (pxt_load(fd, &snd, s)) return 1;
   pxt_Render(&snd);

   drumtable[d].nsamples = snd.final_size;
   drumtable[d].samples = (signed short *)malloc(snd.final_size * 2);		// *2 - it is 16-bit

   // read data out of pxt's render result and put it into our drum sample table
   for(i=0;i<drumtable[d].nsamples;i++)
   {
      sample = snd.final_buffer[i];
      //i'm upscaling the 8-bit value to 16-bit;
      //but this also sets volume of drums relative to music
      sample *= 200;

      drumtable[d].samples[i] = sample;
   }

   FreePXTBuf(&snd);
   return 0;
}

static bool load_drumtable(FILE *fp)		// pxt_path = the path where drum pxt files can be found
{
   NX_LOG("load_drumtable: cache gone; rebuilding drums...\n");

   pxt_initsynth();

   for (int d = 0; d < NUM_DRUMS; d++)
   {
      if (drum_pxt[d])
         if (load_drum_pxt(fp, drum_pxt[d], d)) return 1;
   }

   return 0;
}


/*
void c------------------------------() {}
*/

extern bool extract_org(FILE *exefp);

int org_init(FILE *fp, int org_volume)
{
   int i;

	SSReserveChannel(ORG_CHANNEL);
	OrgVolume = org_volume;

	// set all buffer pointers and things to NULL, so if something fails to load,
	// we won't crash on org_close.
	memset(drumtable, 0, sizeof(drumtable));
	for(i=0;i<16;i++)
      note_channel[i].outbuffer = NULL;
	for(i=0;i<2;i++)
      final_buffer[i].samples = NULL;

   extract_org(fp);

	init_pitch();
	if (load_drumtable(fp))
   {
      return 1;
   }

	song.playing = false;
	org_inited = true;
	return 0;
}


void org_close(void)
{
int d;

	org_stop();
	free_buffers();

	for(d=0;d<NUM_DRUMS;d++)
		if (drumtable[d].samples) free(drumtable[d].samples);
}

int mgetc(char **fp)
{
   unsigned char volatile *f = *((unsigned char volatile **)fp);
   unsigned char volatile c = *f;
   (*fp)++;
   return c;
}

uint16_t mgeti(char **fp)
{
uint16_t a, b;
	a = mgetc(fp);
	b = mgetc(fp);
	return (b << 8) | a;
}

uint32_t mgetl(char **fp)
{
uint32_t a, b, c, d;
	a = mgetc(fp);
	b = mgetc(fp);
	c = mgetc(fp);
	d = mgetc(fp);
	return (d<<24)|(c<<16)|(b<<8)|(a);
}

extern char *org_data[42];

char org_load(int songno)
{
static const char *magic = "Org-02";
char buf[8];
char *f;
char **fp;
int i, j;

	f = org_data[songno];
   fp = &f;
	if (!fp) {
      NX_WARN("org_load: no such file: '%d'\n", songno);
      return 1;
   }

	for(i=0;i<6;i++) { buf[i] = mgetc(fp); } buf[i] = 0;
	if (strcmp(buf, magic)) {
      NX_WARN("org-load: not an org file (got '%s')\n", buf);
      //fclose(fp);
      return 1;
   }
	NX_LOG("%d: %s detected\n", songno, magic);

	//fseek(fp, 0x06, SEEK_SET);

	song.ms_per_beat = mgeti(fp);
	song.steps_per_bar = mgetc(fp);
	song.beats_per_step = mgetc(fp);
	song.loop_start = mgetl(fp);
	song.loop_end = mgetl(fp);

	//song.ms_per_beat = 500;
	//song.loop_start = 64;

	if (song.loop_end < song.loop_start)
	{
		visible_warning("org_load: loop end is before loop start");
		//fclose(fp);
		return 1;
	}

	// compute how long the last beat of a note should be (it should not use up the whole beat)
   song.ms_of_last_beat_of_note = song.ms_per_beat - (int)((float_type)song.ms_per_beat * 0.1);

	// not actually used in this module, but the larger program might want to know this
	song.beats_per_bar = (song.beats_per_step * song.steps_per_bar);

	/*lprintf("tempo: %d ms/beat\n", song.ms_per_beat);
	lprintf("beats_per_step: %d\n", song.beats_per_step);
	lprintf("steps_per_bar: %d\n", song.steps_per_bar);
	lprintf("loop begins on beat %d\n", song.loop_start);
	lprintf("loop ends on beat %d\n", song.loop_end);*/

	for(i=0;i<16;i++)
	{
		song.instrument[i].pitch = mgeti(fp);
		song.instrument[i].wave = mgetc(fp);
		song.instrument[i].pi = mgetc(fp);
		song.instrument[i].nnotes = mgeti(fp);

		if (song.instrument[i].nnotes >= MAX_SONG_LENGTH)
		{
			visible_warning(" * org_load: instrument %d has too many notes! (has %d, max %d)", i, song.instrument[i].nnotes, MAX_SONG_LENGTH);
			//fclose(fp);
			return 1;
		}

		/*if (song.instrument[i].nnotes)
		{
			lprintf("Instrument %d: ", i);
			lprintf(" Pitch: %d, ", song.instrument[i].pitch);
			lprintf(" Wave: %d, ", song.instrument[i].wave);
			lprintf(" Pi: %d, ", song.instrument[i].pi);
			lprintf(" Nnotes: %d\n", song.instrument[i].nnotes);
		}*/

		// substitute unavailable drums
		// credits track for one, has Per02 set which CS didn't actually have, I don't think
		if (i >= 8)
		{
			switch(song.instrument[i].wave)
			{
				case 9: song.instrument[i].wave = 8; break;
			}
		}
	}

	for(i=0;i<16;i++)
	{
		for(j=0;j<song.instrument[i].nnotes;j++) song.instrument[i].note[j].beat = mgetl(fp);
		for(j=0;j<song.instrument[i].nnotes;j++) song.instrument[i].note[j].note = mgetc(fp);
		for(j=0;j<song.instrument[i].nnotes;j++) song.instrument[i].note[j].length = mgetc(fp);
		for(j=0;j<song.instrument[i].nnotes;j++) song.instrument[i].note[j].volume = mgetc(fp);
		for(j=0;j<song.instrument[i].nnotes;j++) song.instrument[i].note[j].panning = mgetc(fp);
	}

	//fclose(fp);
	return init_buffers();
}

/*
void c------------------------------() {}
*/


static bool init_buffers(void)
{
int i;

	// free the old buffers, as we're probably going to change their size here in a sec
	free_buffers();

	/* figure some stuff out real quick about buffer lengths --- */

	// convert the ms-per-beat stuff into samples
	song.samples_per_beat = MSToSamples(song.ms_per_beat);
	song.note_closing_samples = MSToSamples(song.ms_of_last_beat_of_note);
	// take the suggestion on cache ahead time (which is in ms) and figure out how many beats that is
	buffer_beats = (cache_ahead_time / song.ms_per_beat) + 1;

#ifndef MIN_AUDIO_PROCESSING_PER_FRAME
   if (buffer_beats < 3) buffer_beats = 3;
#endif

	// now figure out how many samples that is.
	buffer_samples = (buffer_beats * song.samples_per_beat);
	// now figure out how many bytes THAT is.
	outbuffer_size_bytes = buffer_samples * 2 * 2;		// @ 16-bits, and stereo sound


	// initialize the per-channel output buffers
	for(i=0;i<16;i++)
	{
		note_channel[i].outbuffer = (signed short *)malloc(outbuffer_size_bytes);
		note_channel[i].number = i;
		//memset(note_channel[i].outbuffer, 0, outbuffer_size_bytes);
	}

	// initialize the final (mixed) output buffers
	for(i=0;i<2;i++)
	{
		final_buffer[i].samples = (signed short *)malloc(outbuffer_size_bytes);
		//memset(final_buffer[i].samples, 0, outbuffer_size_bytes);
	}

	return 0;
}

static void free_buffers(void)
{
int i;

	for(i=0;i<16;i++)
		if (note_channel[i].outbuffer) free(note_channel[i].outbuffer);

	for(i=0;i<2;i++)
		if (final_buffer[i].samples) free(final_buffer[i].samples);
}


/*
void c------------------------------() {}
*/


// start the currently-loaded track playing at beat startbeat.
bool org_start(int startbeat)
{
	org_stop();		// stop any old music

	// set all the note-tracking stuff to starting values
	song.beat = startbeat;
	song.haslooped = false;

	for(int i=0;i<16;i++)
	{
		song.instrument[i].curnote = 0;
		note_channel[i].volume = ORG_MAX_VOLUME;
		note_channel[i].panning = ORG_PAN_CENTERED;
		note_channel[i].length = 0;
	}

	// fill the first buffer and play it to jumpstart the playback cycle
	//lprintf(" ** org_start: Jumpstarting buffer cycle\n");

	song.playing = true;
	song.fading = false;

	song.volume = OrgVolume;
	SSSetVolume(ORG_CHANNEL, song.volume);

	// kickstart the first buffer
	current_buffer = 0;
	generate_music();
	queue_final_buffer();
	buffers_full = 0;				// tell org_run to generate the other buffer right away

	return 0;
}


// pause/stop playback of the current song
void org_stop(void)
{
	if (song.playing)
	{
		song.playing = false;
		// cancel whichever buffer is playing
		SSAbortChannel(ORG_CHANNEL);
	}
}

bool org_is_playing(void)
{
	return song.playing;
}

void org_fade(void)
{
	NX_LOG("org_fade\n");
	song.fading = true;
	song.last_fade_time = 0;
}

void org_set_volume(int newvolume)
{
	if (newvolume != song.volume)
	{
		song.volume = newvolume;
		SSSetVolume(ORG_CHANNEL, newvolume);
	}
}

unsigned retro_get_tick(void);

static void runfade()
{
	uint32_t curtime = retro_get_tick();
	if ((curtime - song.last_fade_time) >= 25)
	{
		int newvol = (song.volume - 1);
		if (newvol <= 0)
		{
			song.fading = false;
			org_stop();
		}
		else
		{
			org_set_volume(newvol);
		}

		song.last_fade_time = curtime;
	}
}

/*
void c------------------------------() {}
*/

// combines all of the individual channel output buffers into a single, final, buffer.
static void mix_buffers(void)
{
	int i, cursample, len;
	int mixed_sample;
	signed short *final;

	// go up to samples*2 because we're mixing the stereo audio output from calls to WAV_Synth
	len = buffer_samples * 2;
	final = final_buffer[current_buffer].samples;

	//NX_LOG("mixing %d samples\n", len);
	for(cursample=0;cursample<len;cursample++)
	{
		// first mix instruments
		mixed_sample = note_channel[0].outbuffer[cursample];
		for(i=1;i<16;i++)
         mixed_sample += note_channel[i].outbuffer[cursample];

      CLAMP16(mixed_sample);

		final[cursample] = mixed_sample;
	}
}

// start whichever buffer is queued to play next, and flag the other one as needing
// to be filled
static void queue_final_buffer(void)
{
	SSEnqueueChunk(ORG_CHANNEL, final_buffer[current_buffer].samples, buffer_samples,
						current_buffer, OrgBufferFinished);

	current_buffer ^= 1;
}


// callback from sslib when a buffer is finished playing.
static void OrgBufferFinished(int channel, int buffer_no)
{
	buffers_full = false;
}

/*
void c------------------------------() {}
*/


/* given a volume and a panning value, it returns three values
 between 0 and 1.00 which are how much to scale:
the whole sound (volume_ratio)
just the left channel (volume_left_ratio)
just the right channel (volume_right_ratio) */
static void ComputeVolumeRatios(int volume, int panning, float_type *volume_ratio, \
                        float_type *volume_left_ratio, float_type *volume_right_ratio)
{
   *volume_ratio = ((float_type)volume / ORG_MAX_VOLUME);

	// get volume ratios for left and right channels (panning)
	if (panning < ORG_PAN_CENTERED)
	{	// panning left (make right channel quieter)
      *volume_right_ratio = ((float_type)panning / ORG_PAN_CENTERED);
		*volume_left_ratio = 1.00f;
	}
	else if (panning > ORG_PAN_CENTERED)
	{	// panning right (make left channel quieter)
      *volume_left_ratio = ((float_type)(ORG_PAN_FULL_RIGHT - panning) / ORG_PAN_CENTERED);
		*volume_right_ratio = 1.00f;
	}
	else
	{	// perfectly centered (both channels get the full volume)
		*volume_left_ratio = 1.00f;
		*volume_right_ratio = 1.00f;
	}
}


// Interpolates a new sample from two samples which will be "in-between" the two samples.
// if ratio is 0.00, it will return exactly sample1.
// if ratio is 1.00, it will return exactly sample2.
// and if ratio is something like 0.5, it will mix the samples together.
static float_type Interpolate(int sample1, int sample2, float_type ratio)
{
float_type s1, s2;
   s1 = ((float_type)sample1 * (1.00f - ratio));
   s2 = ((float_type)sample2 * ratio);
	return (s1 + s2);
}


// ensures that there are exactly desired_samples contained in the output buffer of instrument m.
// if there are fewer samples than desired, the gap is filled with silence.
// if there are more, the extra audio is truncated.
static void ForceSamplePos(int m, int desired_samples)
{
	if (note_channel[m].samples_so_far != desired_samples)
	{
		if (desired_samples > note_channel[m].samples_so_far)
		{
			silence_gen(&note_channel[m], (desired_samples - note_channel[m].samples_so_far));
		}
		else
		{	// this should NEVER actually happen!!
			NX_WARN("ForceSamplePos: WARNING: !!! truncated channel %d from %d to %d samples !!!\n", m, note_channel[m].samples_so_far, desired_samples);
			note_channel[m].samples_so_far = desired_samples;
			note_channel[m].outpos = desired_samples * 2;
		}
	}
}


// adds num_samples samples of silence to the output buffer of channel "m".
static void silence_gen(stNoteChannel *chan, int num_samples)
{
int clear_bytes;

	//NX_LOG("silence_gen: making %d samples of silence\n", num_samples);

	clear_bytes = (num_samples * 2 * 2);		// clear twice as many shorts as = num_samples
	memset(&chan->outbuffer[chan->outpos], 0, clear_bytes);

	chan->samples_so_far += num_samples;
	chan->outpos += (num_samples * 2);
}

// -------------------
// note_open
// -------------------
// initializes the synthesis of a new note.
// chan: the instrument channel the note will play on
// wave: the instrument no to play the note with
// pitch: the pitch variation of the instrument as set in the org
// note: the note # we'll be playing
// total_ms: the maximum length the note will play for (controls buffer allocation length)
static void note_open(stNoteChannel *chan, int wave, int pitch, int note)
{
float_type new_sample_rate;
#define	samplK	 	 11025		// constant is original sampling rate of the samples in the wavetable

	// compute how quickly, or slowly, to play back the wavetable sample
	new_sample_rate = GetNoteSampleRate(note, pitch);
   chan->sample_inc = (new_sample_rate / (float_type)samplK);

	chan->wave = wave;
	chan->phaseacc = 0;

	//lprintf("note_open: new note opened for channel %08x at sample_inc %.2f, using wave %d\n", chan, chan->sample_inc, chan->wave);
}

// -------------------
// note_gen
// -------------------
// Adds num_samples worth of audio data to the channel at the current frequency, volume,
// panning, and pitch settings, and at the note & wave spec'd in note_open.
static void note_gen(stNoteChannel *chan, int num_samples)
{
int i;
float_type audioval;
float_type master_volume_ratio, volume_left_ratio, volume_right_ratio;
int wave;
unsigned char pos1, pos2;
float_type iratio;

	wave = chan->wave;

	// compute volume ratios; unlike drums we have to do this every time
	// since they can change in the middle of the note.
	ComputeVolumeRatios(chan->volume, chan->panning,
				&master_volume_ratio, &volume_left_ratio, &volume_right_ratio);

	//statbuild("Entering note_gen with phaseacc=%.2f and sample_inc=%.2f", chan->phaseacc, chan->sample_inc);
	//statbuild(", using buffer %08x\n", chan->outbuffer);

	//NX_LOG("note_gen(%d, %d)\n", chan->number, num_samples);

	// generate however many output samples we were asked for
	for(i=0;i<num_samples;i++)
	{
		// interpolate a sample that's at the fractional "phaseacc" sample position in the wavetable form
		pos1 = (int)chan->phaseacc;
		pos2 = pos1 + 1;		// since pos1&2 are chars, this wraps at 255
		iratio = chan->phaseacc - (int)chan->phaseacc;

		audioval = Interpolate(wavetable[wave][pos1], wavetable[wave][pos2], iratio);
		audioval *= master_volume_ratio;

		chan->outbuffer[chan->outpos++] = (int)(audioval * volume_left_ratio);
		chan->outbuffer[chan->outpos++] = (int)(audioval * volume_right_ratio);
		chan->samples_so_far++;

		chan->phaseacc += chan->sample_inc;
		if ((int)chan->phaseacc >= 256) chan->phaseacc -= 256;
	}
}


// -------------------
// note_close
// -------------------
// ends a note smoothly by ensuring that it's wave stops near the 0-crossing point.
// this avoids a slight popping noise which can be caused by abruptly moving from
// a high sample value to a close-to-zero sample value.
// returns the # of extra samples generated.
static int note_close(stNoteChannel *chan)
{
	if (chan->outpos == 0)
		return 0;

	int samples_made = 0;
	while(chan->samples_so_far < buffer_samples)	// avoid potential buffer overflow
	{
		// get the value of the last sample in the buffer and check
		// if it's close enough to silence yet. if not, let the note
		// run on a teeny bit longer than it's supposed to until it's wave
		// hits the zero-crossing point, to avoid a click.
		int last_sample = chan->outbuffer[chan->outpos - 1];
		if (abs(last_sample) < 1000) break;

		note_gen(chan, 1);
		samples_made++;
	}

	return samples_made;
}


// set up to make a drum noise using drum "wave" at note "note" on channel "m_channel".
// the total number of samples the drum will last is returned.
static int drum_open(int m_channel, int wave, int note)
{
stNoteChannel *chan = &note_channel[m_channel];
float_type new_sample_rate;
int gen_samples;

	//lprintf("drum_hit: playing drum %d[%s] on channel %d, note %02x volume %d panning %d\n", wave, drum_names[wave], m_channel, note, chan->volume, chan->panning);

	new_sample_rate = GetNoteSampleRate(note, song.instrument[m_channel].pitch);
   chan->sample_inc = (new_sample_rate / (float_type)drumK);

	// get the new number of samples for the sound
   gen_samples = (int)((float_type)drumtable[wave].nsamples / chan->sample_inc);

	// precompute volume and panning values since they're the same over the length of the drum
	ComputeVolumeRatios(chan->volume, chan->panning, &chan->master_volume_ratio, &chan->volume_left_ratio, &chan->volume_right_ratio);

	chan->wave = wave;
	chan->phaseacc = 0;
	return gen_samples;
}


// generates "num_samples" of a drum noise previously set up via drum_open into the output
// buffer of the channel.
static void drum_gen(int m_channel, int num_samples)
{
stNoteChannel *chan = &note_channel[m_channel];
float_type volume_ratio, volume_left_ratio, volume_right_ratio;
int wave;
int pos1, pos2;
float_type iratio;
float_type audioval;
int i;

	volume_ratio = chan->master_volume_ratio;
	volume_left_ratio = chan->volume_left_ratio;
	volume_right_ratio = chan->volume_right_ratio;
	wave = chan->wave;

	//NX_LOG("drum_gen(%d, %d)\n", m_channel, num_samples);

	// generate the drum sound
	for(i=0;i<num_samples;i++)
	{
		pos1 = (int)chan->phaseacc;
		pos2 = pos1 + 1;
		if (pos2 >= drumtable[wave].nsamples) pos2 = pos1;

		iratio = chan->phaseacc - (int)chan->phaseacc;

		audioval = Interpolate(drumtable[wave].samples[pos1], drumtable[wave].samples[pos2], iratio);
		audioval *= volume_ratio;

		chan->outbuffer[chan->outpos++] = (signed short)(audioval * volume_left_ratio);
		chan->outbuffer[chan->outpos++] = (signed short)(audioval * volume_right_ratio);
		chan->samples_so_far++;

		chan->phaseacc += chan->sample_inc;
		if ((int)chan->phaseacc > drumtable[wave].nsamples)
		{
			NX_ERR(" **ERROR-phaseacc ran over end of drumsample %.2f %d\n", chan->phaseacc, drumtable[wave].nsamples);
			break;
		}
	}
}


void org_run(void)
{
	if (!song.playing)
		return;

	// keep both buffers queued. if one of them isn't queued, then it's time to
	// generate more music for it and queue it back on.
	if (!buffers_full)
	{
		generate_music();				// generate more music into current_buffer

		queue_final_buffer();			// enqueue current_buffer and switch buffers
		buffers_full = true;			// both buffers full again until OrgBufferFinished called
	}

	if (song.fading) runfade();
}


// generate a buffer's worth of music and place it in the current final buffer.
static void generate_music(void)
{
int m;
int beats_left;
int out_position;

	//NX_LOG("generate_music: cb=%d buffer_beats=%d\n", current_buffer, buffer_beats);

	// save beat # of the first beat in buffer for calculating current beat for TrackFuncs
	final_buffer[current_buffer].firstbeat = song.beat;

	// clear all the channel buffers
	for(m=0;m<16;m++)
	{
		note_channel[m].samples_so_far = 0;
		note_channel[m].outpos = 0;
	}

	//NX_LOG("generate_music: generating %d beats of music\n", buffer_beats);
	beats_left = buffer_beats;
	out_position = 0;

	while(beats_left)
	{
		out_position += song.samples_per_beat;

		// for each channel...
		for(m=0;m<16;m++)
		{
			// generate any music that's supposed to go into the current beat
			NextBeat(m);
			// ensure that exactly one beat of samples was added to the channel by inserting silence
			// if needed. sometimes NextBeat may not actually generate a full beats worth, for
			// example if there was no note playing on the track, of if it was the last beat of a note.
			ForceSamplePos(m, out_position);
		}

		if (++song.beat >= song.loop_end)
		{
			song.beat = song.loop_start;
			song.haslooped = true;

			for(m=0;m<16;m++)
			{
				song.instrument[m].curnote = song.instrument[m].loop_note;
				note_channel[m].length = 0;
			}
		}

		beats_left--;
	}

	mix_buffers();
}


// generate up to a 1 beat worth of music from channel "m" at the song.beat cursor point.
// it may generate less.
static void NextBeat(int m)
{
stNoteChannel *chan = &note_channel[m];
stInstrument *track = &song.instrument[m];
//int volume, panning;
stNote *note;
int len;

	// add notes as long as instrument has notes left to add
	if (track->curnote < track->nnotes)
	{
		for(;;)
		{
			// when we hit the loop start point, record the note we were on for later
			if (song.beat == song.loop_start)
			{
				track->loop_note = track->curnote;
			}

			// get a pointer to the note at current position in the song
			note = &track->note[track->curnote];

			// skip ahead if the song got ahead of us somehow
			if (song.beat > note->beat)
			{
				if (++track->curnote >= track->nnotes)
					return;
			}
			else break;
		}

		// 1st- start notes as we arrive at their beat
		if (song.beat == note->beat)
		{
			//NX_LOG(" Beat/Note: %d/%d   Chan: %d   Note: %d  length=%d vol=%d pan=%d wave=%d\n", song.beat, curnote, m, note->note, note->length, note->volume, note->panning, song.instrument[m].wave);

			if (note->volume != 0xff) chan->volume = note->volume;
			if (note->panning != 0xff) chan->panning = note->panning;

			if (note->note != 0xff)
			{
				if (m < 8)
				{
					note_open(chan, track->wave, track->pitch, note->note);
					chan->length = note->length;
				}
				else
				{	// on percussion tracks the length works differently---drum_open returns the
					// number of samples the drum will take to finish playing.
					chan->length = drum_open(m, track->wave, note->note);
				}
			}

			track->curnote++;
		}
	}

	// 2nd- generate any notes which are running
	if (chan->length)
	{
		if (m < 8)
		{	// melody tracks
			if (track->pi)
			{	// pi tracks always generate only 1024 samples for ANY note
				note_gen(chan, 1024);
				chan->length = 0;
			}
			else
			{
				if (chan->length > 1)
				{	// generate a full beat of music
					note_gen(chan, song.samples_per_beat);
				}
				else	// generate only most of the beat--if there's a note immediately after
				{		// this one they should not run together
					note_gen(chan, song.note_closing_samples);
				}

				if (!--chan->length)
				{
					note_close(chan);
				}
			}
		}
		else
		{	// percussion tracks
			// if less than a whole beats worth of samples is left to play on the drum, finish
			// whatever's left. Else generate only one beats worth right now.
			if (chan->length > song.samples_per_beat)
				len = song.samples_per_beat;
			else
				len = chan->length;

			drum_gen(m, len);

			chan->length -= len;
		}
	}

}

/*
void c------------------------------() {}
*/

int org_GetCurrentBeat(void)
{
	if (SSChannelPlaying(ORG_CHANNEL))
	{
		int curbuffer;
		int elapsed;
		int sample_pos;
		int beat;

      curbuffer = SSGetCurUserData(ORG_CHANNEL);
      sample_pos = SSGetSamplePos(ORG_CHANNEL);

		elapsed = SamplesToMS(sample_pos);

		beat = elapsed / song.ms_per_beat;
		beat += final_buffer[curbuffer].firstbeat;
		// wrap at end of song
		while(beat >= song.loop_end)
			beat -= (song.loop_end - song.loop_start);

		return beat;
	}

	return -1;
}

// returns which org buffer is currently playing.
int org_GetCurrentBuffer(void)
{
	if (!SSChannelPlaying(ORG_CHANNEL)) return -1;
	return SSGetCurUserData(ORG_CHANNEL);
}