Weak handles for audio

gistfile1.cpp

// ---- INTERFACE

// Sound handle. This is a weak reference; they automatically go invalid as voices
// stop playing and get reused. You don't need to worry about leaks.
struct SoundHandle
{
	uint32_t opaque; // Mixer decides what this means (but 0=null)

	explicit SoundHandle(uint32_t handle=0) : opaque(handle) {}

	bool is_null() const { return opaque == 0; }
	bool operator ==(const SoundHandle& x) const { return opaque == x.opaque; }
	bool operator !=(const SoundHandle& x) const { return opaque != x.opaque; }
};

// ---- IMPLEMENTATION

// Handle is concat(valid,voice,counter)
//   "valid" is a single bit (0 or 1)
//   "voice" is the voice ID (as many bits as required for kMaxVoices)
//   "counter" is the rest and gets incremented each time a voice is used (with wrap-around).

Mixer::Voice* Mixer::voice_from_handle(SoundHandle h)
{
	// is valid?
	if ((h.opaque & 1) == 0)
		return nullptr;

	Voice& v = new_app_state->voice[(h.opaque >> 1) & (kMaxVoices - 1)];
	if (v.handle == h)
		return &v;
	else
		return nullptr;
}

void Mixer::assign_new_handle(Voice& v, int voice_id)
{
	assert(voice_id >= 0 && voice_id < kMaxVoices);

	// counter bit mask
	static const uint32_t counter_bit_mask = ~((kMaxVoices << 1) - 1);

	// increment counter by 1 (using unsigned wraparound) while setting "valid"
	// and "voice" bits.
	v.handle.opaque = ((v.handle.opaque - counter_bit_mask) & counter_bit_mask)
		| ((voice_id << 1) | 1); // voice/vald bits.
}