How not to make a mixer (.it player component I've since polished up)

gistfile1.c

void do_channel(struct fatmodule *mod, int idx, SLONG *buf)
{
	SWORD v = 0;
	int pan;
	
	int mch = (mod->chn[idx].master == -1 ? idx : mod->chn[idx].master);
	
	if(!mod->chn[idx].tperiod)
		mod->chn[idx].tperiod = mod->chn[idx].period;
	
	if(mod->chn[idx].tperiod && mod->smpdata[mod->chn[idx].sample].b != NULL)
	{
		v = (
			mod->smplist[mod->chn[idx].sample].flags & 0x02
			? mod->smpdata[mod->chn[idx].sample].w[mod->chn[idx].pos]
			: mod->smpdata[mod->chn[idx].sample].b[mod->chn[idx].pos] << 8
		);
		
		if(mod->chn[idx].tvol == 0xFF)
			mod->chn[idx].tvol = mod->chn[idx].rvol;
		
		if(mod->chn[idx].flags & FC_MIXME)
			if(
				mod->chn[idx].tvol
				&& mod->smplist[mod->chn[idx].sample].gv
				&& !(mod->impm.chanpan[mch] & 0x80)
			)
			{
				v = (v * mod->chn[idx].tvol * mod->smplist[mod->chn[idx].sample].gv) / (64 * 64);
				v = (v * mod->impm.chanvol[mch]) / 64;
				
				pan = mod->chn[idx].pan;
				
				if(mod->impm.flags & 0x0004)
				{
					if(mod->inslist[mod->chn[idx].cinstr-1].env[0].flg & 0x01)
						v = (v * mod->chn[idx].ey[0]) / 64;
					if(mod->inslist[mod->chn[idx].cinstr-1].env[1].flg & 0x01)
						pan = ((pan - 32) * mod->chn[idx].ey[1]) / 64 + 32;
					
					v = (v * mod->inslist[mod->chn[idx].cinstr-1].gv) / 128;
					v = (v * mod->chn[idx].nfc) / 1024;
				}
				
				//if(mod->chn[idx].master != -1)
				//	printf("%02X %04X\n", idx, (UWORD)v);
				
				//printf("%04X\n", (UWORD)v);
				if(v)
				{
					if(mod->impm.chanpan[mch] == 100 || pan == 100)
					{
						buf[0] += v;
						buf[1] -= v;
					} else if(pan < 32) {
						buf[0] += (v * pan) >> 5;
						buf[1] += v;
					} else {
						buf[0] += v;
						buf[1] += (v * (64 - pan)) >> 5;
					}
				}
			}
		
		if(mod->chn[idx].flags & FC_REVERSE)
		{
			mod->chn[idx].subpos -= mod->chn[idx].tperiod;
			while(mod->chn[idx].subpos < 0)
			{
				mod->chn[idx].pos--;
				mod->chn[idx].subpos += mod->bfreq;
			}
		} else {
			mod->chn[idx].subpos += mod->chn[idx].tperiod;
			/*while(mod->chn[idx].subpos >= mod->bfreq)
			{
				mod->chn[idx].pos++;
				mod->chn[idx].subpos -= mod->bfreq;
			}*/
			mod->chn[idx].pos += mod->chn[idx].subpos / mod->bfreq;
			mod->chn[idx].subpos %= mod->bfreq;
		}
		
		//printf("pos %08X/%08X\n", mod->chn[idx].pos, mod->chn[idx].subpos);
		
		if(mod->chn[idx].flags & FC_DOLOOP)
		{
			if(mod->chn[idx].pos >= mod->chn[idx].lpend && !(mod->chn[idx].flags & FC_REVERSE))
			{
				if(mod->chn[idx].flags & FC_BIDI)
				{
					mod->chn[idx].pos = mod->chn[idx].lpend-1;
					mod->chn[idx].flags |= FC_REVERSE;
					//printf("<==\n");
				} else {
					mod->chn[idx].pos = mod->chn[idx].lpbeg;
				}
				mod->chn[idx].subpos = 0;
			} else if(mod->chn[idx].pos <= mod->chn[idx].lpbeg
					&& (mod->chn[idx].flags & FC_REVERSE)) {
				if(mod->chn[idx].flags & FC_BIDI)
				{
					mod->chn[idx].pos = mod->chn[idx].lpbeg+1;
					mod->chn[idx].flags &= ~FC_REVERSE;
					//printf("==>\n");
				} else {
					while(mod->chn[idx].pos > mod->chn[idx].lpend)
						mod->chn[idx].pos -= mod->chn[idx].lpend-mod->chn[idx].lpbeg;
					//mod->chn[idx].pos = mod->chn[idx].lpend-1;
				}
				//mod->chn[idx].subpos = 0;
			}
		}
		
		if(mod->chn[idx].pos >= mod->smplist[mod->chn[idx].sample].len)
		{
			//mod->chn[idx].period = 0;
			//printf("\e[1mbuh %i\e[0m\n",idx);
			mod->chn[idx].flags &= ~(FC_PLAYING|FC_MIXME);
		}
	}
}

// * SNIP *

		int len = (bfreq*5)/(mod->tempo*2);
		SLONG mixbuf[2];
		
		//printf("S/T %03i/%03i\n", mod->speed, mod->tempo);
		
		for(i = 0; i < len; i++)
		{
			wbuf[i*2+1] = wbuf[i*2] = 0;
			mixbuf[0] = mixbuf[1] = 0;
			for(j = 0; j < 256; j++)
			{
				if(mod->chn[j].flags & FC_PLAYING)
					do_channel(mod, j, &mixbuf[0]);
			}
			
			mixbuf[0] = (mixbuf[0] * mod->impm.mv) >> 7;
			mixbuf[1] = (mixbuf[1] * mod->impm.mv) >> 7;
			mixbuf[0] = (mixbuf[0] * mod->impm.gv) >> 7;
			mixbuf[1] = (mixbuf[1] * mod->impm.gv) >> 7;
			
			mixbuf[0] -= mixbuf_offs[0];
			mixbuf[1] -= mixbuf_offs[1];
			
			if(mixbuf[0] > 0x7FFF)
			{
				mixbuf_offs[0] += mixbuf[0]-0x7FFF;
				mixbuf[0] = 0x7FFF;
			}
			if(mixbuf[1] > 0x7FFF)
			{
				mixbuf_offs[1] += mixbuf[1]-0x7FFF;
				mixbuf[1] = 0x7FFF;
			}
			if(mixbuf[0] < -0x7FFF)
			{
				mixbuf_offs[0] += mixbuf[0]+0x7FFF;
				mixbuf[0] = -0x7FFF;
			}
			if(mixbuf[1] < -0x7FFF)
			{
				mixbuf_offs[1] += mixbuf[1]+0x7FFF;
				mixbuf[1] = -0x7FFF;
			}
			
			if(mixbuf_offs[0] < 0)
			{
				mixbuf_offs[0] = -((-mixbuf_offs[0]*7)>>3);
			} else {
				mixbuf_offs[0] = (mixbuf_offs[0]*7)>>3;
			}
			
			if(mixbuf_offs[1] < 0)
			{
				mixbuf_offs[1] = -((-mixbuf_offs[1]*7)>>3);
			} else {
				mixbuf_offs[1] = (mixbuf_offs[1]*7)>>3;
			}
			
			wbuf[i*2] = (SWORD)mixbuf[0];
			wbuf[i*2+1] = (SWORD)mixbuf[1];
		}
		
		mod->tick++;
		if(mod->tick >= mod->speed)
			mod->tick = -1;
		
		write(dsp, &wbuf[0], channels*2*len);