fix(seek/delay) seek < delay duration ate up the delay

src/queue.rs

@@ -170,6 +170,15 @@ where
         None
     }
 
+    /// Only seeks within the current source.
+    // We can not go back to previous sources. We could implement seek such
+    // that it advances the queue if the position is beyond the current song.
+    //
+    // We would then however need to enable seeking backwards across sources too.
+    // That no longer seems in line with the queue behaviour.
+    //
+    // A final pain point is that we would need the total duration for the 
+    // next few songs.
     #[inline]
     fn try_seek(&mut self, pos: Duration) -> Result<(), SeekError> {
         self.current.try_seek(pos)

src/sink.rs

@@ -205,7 +205,7 @@ impl Sink {
     ///
     /// This blocks between 0 and ~5 milliseconds.
     ///
-    /// As long as the duration of the source is known seek is guaranteed to saturate
+    /// As long as the duration of the source is known, seek is guaranteed to saturate
     /// at the end of the source. For example given a source that reports a total duration
     /// of 42 seconds calling `try_seek()` with 60 seconds as argument will seek to
     /// 42 seconds.

src/source/buffered.rs

@@ -242,6 +242,8 @@ where
         self.total_duration
     }
 
+    /// Can not support seek, in the end state we lose the underlying source
+    /// which makes seeking back impossible.
     #[inline]
     fn try_seek(&mut self, _: Duration) -> Result<(), SeekError> {
         Err(SeekError::NotSupported {

src/source/delay.rs

@@ -4,19 +4,22 @@ use crate::{Sample, Source};
 
 use super::SeekError;
 
+fn remaining_samples(until_playback: Duration, sample_rate: u32, channels: u16) -> usize {
+    let ns = until_playback.as_secs() * 1_000_000_000 + until_playback.subsec_nanos() as u64;
+    let samples = ns * channels as u64 * sample_rate as u64 / 1_000_000_000;
+    samples as usize
+}
+
 /// Internal function that builds a `Delay` object.
 pub fn delay<I>(input: I, duration: Duration) -> Delay<I>
 where
     I: Source,
     I::Item: Sample,
 {
-    let duration_ns = duration.as_secs() * 1000000000 + duration.subsec_nanos() as u64;
-    let samples = duration_ns * input.sample_rate() as u64 / 1000000000 * input.channels() as u64;
-
     Delay {
-        input,
-        remaining_samples: samples as usize,
+        remaining_samples: remaining_samples(duration, input.sample_rate(), input.channels()),
         requested_duration: duration,
+        input,
     }
 }
 
@@ -108,9 +111,28 @@ where
             .map(|val| val + self.requested_duration)
     }
 
+    /// Pos is seen from the perspective of the api user.
+    ///
+    /// # Example
+    ///
+    /// ```ignore
+    /// use std::time::Duration;
+    ///
+    /// let mut source = inner_source.delay(Duration::from_secs(10));
+    /// source.try_seek(Duration::from_secs(15));
+    ///
+    /// // inner_source is now at pos: Duration::from_secs(5);
+    /// ```
+    ///
     #[inline]
     fn try_seek(&mut self, pos: Duration) -> Result<(), SeekError> {
-        let pos_without_delay = pos.saturating_sub(self.requested_duration);
-        self.input.try_seek(pos_without_delay)
+        if pos < self.requested_duration {
+            self.input.try_seek(Duration::ZERO)?;
+            let until_playback = self.requested_duration - pos;
+            self.remaining_samples =
+                remaining_samples(until_playback, self.sample_rate(), self.channels());
+        }
+        let compensated_for_delay = pos.saturating_sub(self.requested_duration);
+        self.input.try_seek(compensated_for_delay)
     }
 }