Voice Rework -- Events, Track Queues (#806)

This implements a proof-of-concept for an improved audio frontend. The largest change is the introduction of events and event handling: both by time elapsed and by track events, such as ending or looping. Following on from this, the library now includes a basic, event-driven track queue system (which people seem to ask for unusually often). A new sample, `examples/13_voice_events`, demonstrates both the `TrackQueue` system and some basic events via the `~queue` and `~play_fade` commands.

Locks are removed from around the control of `Audio` objects, which should allow the backend to be moved to a more granular futures-based backend solution in a cleaner way.

src/input/utils.rs

@@ -0,0 +1,41 @@
+//! Utility methods for seeking or decoding.
+
+use crate::constants::*;
+use audiopus::{coder::Decoder, Channels, Result as OpusResult, SampleRate};
+use std::{mem, time::Duration};
+
+/// Calculates the sample position in a FloatPCM stream from a timestamp.
+pub fn timestamp_to_sample_count(timestamp: Duration, stereo: bool) -> usize {
+    ((timestamp.as_millis() as usize) * (MONO_FRAME_SIZE / FRAME_LEN_MS)) << stereo as usize
+}
+
+/// Calculates the time position in a FloatPCM stream from a sample index.
+pub fn sample_count_to_timestamp(amt: usize, stereo: bool) -> Duration {
+    Duration::from_millis((((amt * FRAME_LEN_MS) / MONO_FRAME_SIZE) as u64) >> stereo as u64)
+}
+
+/// Calculates the byte position in a FloatPCM stream from a timestamp.
+///
+/// Each sample is sized by `mem::size_of::<f32>() == 4usize`.
+pub fn timestamp_to_byte_count(timestamp: Duration, stereo: bool) -> usize {
+    timestamp_to_sample_count(timestamp, stereo) * mem::size_of::<f32>()
+}
+
+/// Calculates the time position in a FloatPCM stream from a byte index.
+///
+/// Each sample is sized by `mem::size_of::<f32>() == 4usize`.
+pub fn byte_count_to_timestamp(amt: usize, stereo: bool) -> Duration {
+    sample_count_to_timestamp(amt / mem::size_of::<f32>(), stereo)
+}
+
+/// Create an Opus decoder outputting at a sample rate of 48kHz.
+pub fn decoder(stereo: bool) -> OpusResult<Decoder> {
+    Decoder::new(
+        SampleRate::Hz48000,
+        if stereo {
+            Channels::Stereo
+        } else {
+            Channels::Mono
+        },
+    )
+}