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/metadata.rs

@@ -0,0 +1,166 @@
+use crate::constants::*;
+use serde_json::Value;
+use std::time::Duration;
+
+/// Information about an [`Input`] source.
+///
+/// [`Input`]: struct.Input.html
+#[derive(Clone, Debug, Default, Eq, PartialEq)]
+pub struct Metadata {
+    /// The title of this stream.
+    pub title: Option<String>,
+    /// The main artist of this stream.
+    pub artist: Option<String>,
+    /// The date of creation of this stream.
+    pub date: Option<String>,
+
+    /// The number of audio channels in this stream.
+    ///
+    /// Any number `>= 2` is treated as stereo.
+    pub channels: Option<u8>,
+    /// The time at which the first true sample is played back.
+    ///
+    /// This occurs as an artefact of coder delay.
+    pub start_time: Option<Duration>,
+    /// The reported duration of this stream.
+    pub duration: Option<Duration>,
+    /// The sample rate of this stream.
+    pub sample_rate: Option<u32>,
+}
+
+impl Metadata {
+    /// Extract metadata and details from the output of
+    /// `ffprobe`.
+    pub fn from_ffprobe_json(value: &Value) -> Self {
+        let format = value.as_object().and_then(|m| m.get("format"));
+
+        let duration = format
+            .and_then(|m| m.get("duration"))
+            .and_then(Value::as_str)
+            .and_then(|v| v.parse::<f64>().ok())
+            .map(Duration::from_secs_f64);
+
+        let start_time = format
+            .and_then(|m| m.get("start_time"))
+            .and_then(Value::as_str)
+            .and_then(|v| v.parse::<f64>().ok())
+            .map(Duration::from_secs_f64);
+
+        let tags = format.and_then(|m| m.get("tags"));
+
+        let title = tags
+            .and_then(|m| m.get("title"))
+            .and_then(Value::as_str)
+            .map(str::to_string);
+
+        let artist = tags
+            .and_then(|m| m.get("artist"))
+            .and_then(Value::as_str)
+            .map(str::to_string);
+
+        let date = tags
+            .and_then(|m| m.get("date"))
+            .and_then(Value::as_str)
+            .map(str::to_string);
+
+        let stream = value
+            .as_object()
+            .and_then(|m| m.get("streams"))
+            .and_then(|v| v.as_array())
+            .and_then(|v| {
+                v.iter()
+                    .find(|line| line.get("codec_type").and_then(Value::as_str) == Some("audio"))
+            });
+
+        let channels = stream
+            .and_then(|m| m.get("channels"))
+            .and_then(Value::as_u64)
+            .map(|v| v as u8);
+
+        let sample_rate = stream
+            .and_then(|m| m.get("sample_rate"))
+            .and_then(Value::as_str)
+            .and_then(|v| v.parse::<u64>().ok())
+            .map(|v| v as u32);
+
+        Self {
+            title,
+            artist,
+            date,
+
+            channels,
+            start_time,
+            duration,
+            sample_rate,
+        }
+    }
+
+    /// Use `youtube-dl` to extract metadata for an online resource.
+    pub fn from_ytdl_output(value: Value) -> Self {
+        let obj = value.as_object();
+
+        let track = obj
+            .and_then(|m| m.get("track"))
+            .and_then(Value::as_str)
+            .map(str::to_string);
+
+        let title = track.or_else(|| {
+            obj.and_then(|m| m.get("title"))
+                .and_then(Value::as_str)
+                .map(str::to_string)
+        });
+
+        let true_artist = obj
+            .and_then(|m| m.get("artist"))
+            .and_then(Value::as_str)
+            .map(str::to_string);
+
+        let artist = true_artist.or_else(|| {
+            obj.and_then(|m| m.get("uploader"))
+                .and_then(Value::as_str)
+                .map(str::to_string)
+        });
+
+        let r_date = obj
+            .and_then(|m| m.get("release_date"))
+            .and_then(Value::as_str)
+            .map(str::to_string);
+
+        let date = r_date.or_else(|| {
+            obj.and_then(|m| m.get("upload_date"))
+                .and_then(Value::as_str)
+                .map(str::to_string)
+        });
+
+        let duration = obj
+            .and_then(|m| m.get("duration"))
+            .and_then(Value::as_f64)
+            .map(Duration::from_secs_f64);
+
+        Self {
+            title,
+            artist,
+            date,
+
+            channels: Some(2),
+            duration,
+            sample_rate: Some(SAMPLE_RATE_RAW as u32),
+
+            ..Default::default()
+        }
+    }
+
+    /// Move all fields from a `Metadata` object into a new one.
+    pub fn take(&mut self) -> Self {
+        Self {
+            title: self.title.take(),
+            artist: self.artist.take(),
+            date: self.date.take(),
+
+            channels: self.channels.take(),
+            start_time: self.start_time.take(),
+            duration: self.duration.take(),
+            sample_rate: self.sample_rate.take(),
+        }
+    }
+}