uxn/src/apu.c

/*
Copyright (c) 2021 Devine Lu Linvega
Copyright (c) 2021 Andrew Alderwick

Permission to use, copy, modify, and distribute this software for any
purpose with or without fee is hereby granted, provided that the above
copyright notice and this permission notice appear in all copies.

THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
WITH REGARD TO THIS SOFTWARE.
*/

#include "uxn.h"
#include "apu.h"

#define NOTE_PERIOD 0x10000
#define ADSR_STEP (SAMPLE_FREQUENCY / 0xf)

/* clang-format off */

static Uint32 advances[12] = {
	0x80000, 0x879c8, 0x8facd, 0x9837f, 0xa1451, 0xaadc1,
	0xb504f, 0xbfc88, 0xcb2ff, 0xd7450, 0xe411f, 0xf1a1c
};

/* clang-format on */

static Sint32
envelope(Apu *c, Uint32 age)
{
	if(!c->r) return 0x0888;
	if(age < c->a) return 0x0888 * age / c->a;
	if(age < c->d) return 0x0444 * (2 * c->d - c->a - age) / (c->d - c->a);
	if(age < c->s) return 0x0444;
	if(age < c->r) return 0x0444 * (c->r - age) / (c->r - c->s);
	c->advance = 0;
	return 0x0000;
}

void
apu_render(Apu *c, Sint16 *sample, Sint16 *end)
{
	Sint32 s;
	if(!c->advance || !c->period) return;
	while(sample < end) {
		c->count += c->advance;
		c->i += c->count / c->period;
		c->count %= c->period;
		if(c->i >= c->len) {
			if(!c->repeat) {
				c->advance = 0;
				return;
			}
			c->i %= c->len;
		}
		s = (Sint8)(c->addr[c->i]) * envelope(c, c->age++);
		*sample++ += s * c->volume_l / 0x80;
		*sample++ += s * c->volume_r / 0x80;
	}
}

void
apu_start(Apu *c, Uint16 adsr, Uint8 pitch)
{
	if(pitch < 108 && c->len)
		c->advance = advances[pitch % 12] >> (8 - pitch / 12);
	else {
		c->advance = 0;
		return;
	}
	c->a = ADSR_STEP * (adsr >> 12);
	c->d = ADSR_STEP * (adsr >> 8 & 0xf) + c->a;
	c->s = ADSR_STEP * (adsr >> 4 & 0xf) + c->d;
	c->r = ADSR_STEP * (adsr >> 0 & 0xf) + c->s;
	c->age = 0;
	c->i = 0;
	if(c->len <= 0x100) /* single cycle mode */
		c->period = NOTE_PERIOD * 337 / 2 / c->len;
	else /* sample repeat mode */
		c->period = NOTE_PERIOD;
}