Save an episode without its dead air when DeadAir is on (#186)

The save button fetched the original while the player played it shortened.
/media/{id}/deadair sends the original's ID3 tag and then the pieces of the
person's plan one after another, read as it goes: MP3 and ADTS frames each
stand alone, so that is a file any player takes, and nothing is stored, as
with the playlist. The tag stops at the first kept frame, since what can
follow it, the Xing frame, gives the original's length.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/deadair.rs

@@ -60,6 +60,49 @@ impl Plan {
     }
 }
 
+/// The length of the ID3v2 tag a file starts with, 0 if it has none.
+pub fn id3_len(head: &[u8]) -> u64 {
+    if head.len() < 10 || &head[..3] != b"ID3" {
+        return 0;
+    }
+    // Seven bits a byte, so no byte of the size looks like the start of a frame.
+    let size = head[6..10].iter().fold(0u64, |n, b| n << 7 | u64::from(b & 0x7f));
+    let footer = if head[5] & 0x10 != 0 { 10 } else { 0 };
+    10 + size + footer
+}
+
+/// A file as `plan` plays it, to save (#186): its ID3 tag, so the title and artwork come too, then
+/// the pieces one after another. MP3 and ADTS frames each stand alone, so that is a file any
+/// player plays. Read as it is sent and never stored, as the playlist is: (its length, its bytes).
+pub async fn kept(
+    path: &Path,
+    plan: &Plan,
+) -> std::io::Result<(u64, impl futures_util::Stream<Item = std::io::Result<axum::body::Bytes>> + use<>)> {
+    use tokio::io::{AsyncReadExt, AsyncSeekExt};
+    let mut file = tokio::fs::File::open(path).await?;
+    let mut head = [0u8; 10];
+    let n = file.read(&mut head).await?;
+    // Only up to the first frame kept: past the tag can be the Xing frame, which gives the
+    // original's length.
+    let first = plan.pieces.first().map_or(0, |p| p.0);
+    let tag = id3_len(&head[..n]).min(first);
+    let ranges: std::collections::VecDeque<(u64, u64)> =
+        std::iter::once((0, tag)).chain(plan.pieces.iter().map(|&(o, n, _)| (o, n))).filter(|r| r.1 > 0).collect();
+    let len = ranges.iter().map(|r| r.1).sum();
+    let body = futures_util::stream::try_unfold((file, ranges), |(mut f, mut left)| async move {
+        let Some((off, n)) = left.pop_front() else { return Ok(None) };
+        let mut buf = vec![0; n.min(1 << 16) as usize];
+        f.seek(std::io::SeekFrom::Start(off)).await?;
+        f.read_exact(&mut buf).await?;
+        let got = buf.len() as u64;
+        if got < n {
+            left.push_front((off + got, n - got));
+        }
+        Ok(Some((axum::body::Bytes::from(buf), (f, left))))
+    });
+    Ok((len, body))
+}
+
 /// The silences silencedetect reported, as (start, end) seconds. One still open at the end of
 /// the file has no end and is left alone.
 pub fn parse_silences(log: &str) -> Vec<(f64, f64)> {
@@ -597,6 +640,14 @@ mod tests {
         assert_eq!(w.pause(2, "a"), DEFAULT_PAUSE, "someone else's says nothing of yours");
     }
 
+    #[test]
+    fn id3_len_reads_the_tags_size() {
+        assert_eq!(id3_len(b"ID3\x04\x00\x00\x00\x00\x02\x01xx"), 10 + 257);
+        assert_eq!(id3_len(b"ID3\x04\x00\x10\x00\x00\x02\x01"), 10 + 257 + 10, "with a footer");
+        assert_eq!(id3_len(b"\xff\xfb\x90\x00\x00\x00\x00\x00\x00\x00"), 0);
+        assert_eq!(id3_len(b"ID3"), 0);
+    }
+
     /// The whole of it, on a clip ffmpeg makes: a second of tone, two of silence, and so on,
     /// played back through its playlist by ffmpeg's own HLS reader. Skipped where ffmpeg is not
     /// installed.
@@ -648,6 +699,18 @@ mod tests {
             assert!((short * 1000.0 - cut as f64).abs() < 100.0, "{ext}: {short} s shorter, {cut} ms cut");
             // Decodes without a complaint.
             assert!(ffmpeg(&["-v", "error", "-i", &list.display().to_string(), "-f", "null", "-"]).await.unwrap().is_empty());
+            // Saved as a file of its own (#186): as long as the playlist, and it plays.
+            let (n, body) = kept(&src, p).await.unwrap();
+            let parts: Vec<_> = futures_util::TryStreamExt::try_collect(body).await.unwrap();
+            let bytes = parts.concat();
+            assert_eq!(bytes.len() as u64, n);
+            if ext == "mp3" {
+                assert!(bytes.starts_with(b"ID3"), "the original's tag comes too");
+            }
+            let saved = dir.join(format!("saved.{ext}"));
+            std::fs::write(&saved, &bytes).unwrap();
+            assert!((len(&saved).await - len(&list).await).abs() < 0.05, "{ext}");
+            assert!(ffmpeg(&["-v", "error", "-i", &saved.display().to_string(), "-f", "null", "-"]).await.unwrap().is_empty());
         }
         std::fs::remove_dir_all(&dir).unwrap();
     }