Sylpheed/crates/sylpheed-export/src/audio.rs

//! Menu audio: disc XMA → Ogg Vorbis, because Godot 4 plays Vorbis natively and
//! will never be taught to read XMA or to open `sound.pak`.
//!
//! Two kinds of thing come out of here and they are not symmetric:
//!
//! * **SE cues** — three short mono waves out of `Static.slb`, one per menu
//!   event. Where each one lives was *measured off the running game* (HANDOFF
//!   Q8) and is **not on the disc in any findable form**.
//! * **BGM** — one bank of `sound.pak`, which is **two stems of one
//!   performance** (HANDOFF Q10). They are summed here into a single file.
//!
//! ## Neither table lives in this file
//!
//! Both come from `authored/audio.json`, and that is the point of the module
//! rather than an accident of configuration. MISSION §3: a value that is
//! *measured* rather than *decoded* lives in `authored/`, carries a `why`, and
//! is deleted the day the disc states it. A measured offset compiled into a Rust
//! `const` is a measurement wearing the costume of a decoded field — it reads
//! like the exporter derived it, and nobody deletes it, because nobody can see
//! it. Contrast [`crate::video::MOVIES`], which *is* a `const` here: Q9 decoded
//! that mapping from the movie manifest on the disc.
//!
//! ## The assembly is not reimplemented here
//!
//! `sylpheed_formats::media` owns every question of the form *"which bytes
//! belong together"* — segment-spanning reads, multi-sub-wave banks, and the
//! delimiter-less `Static.slb` where a wave is only `(offset, packet_count)`.
//! This module asks it for `RIFF`s and converts them. The seam is deliberate:
//! everything before it is disc knowledge, everything after it is a codec
//! choice, and re-deriving the first half here is exactly the mistake the
//! mission names.

use anyhow::{bail, Context, Result};
use serde::Deserialize;
use std::collections::BTreeMap;
use std::path::{Path, PathBuf};
use std::process::Command;
use sylpheed_formats::media::{self, DiscSource};

/// One menu sound effect, and where its wave sits in a delimiter-less bank.
///
/// `offset` is a string so the file can carry `"0x1ec0"` — the form the RE
/// finding is written in. A reader comparing the two should not have to convert
/// 125 632 in their head to believe they match.
#[derive(Deserialize)]
pub struct CueSpec {
    pub bank: String,
    pub offset: String,
    pub packets: usize,
    pub channels: u8,
    pub rate: u32,
    /// The game's own cue identifier, where one has been *guessed by name*.
    /// Absent means nobody claimed one — never that the binding is unknown.
    #[serde(default)]
    pub name_match: Option<String>,
    pub why: String,
}

impl CueSpec {
    fn offset(&self, event: &str) -> Result<usize> {
        let s = self.offset.trim();
        let parsed = match s.strip_prefix("0x").or_else(|| s.strip_prefix("0X")) {
            Some(hex) => usize::from_str_radix(&hex.replace('_', ""), 16),
            None => s.replace('_', "").parse(),
        };
        parsed.with_context(|| format!("cue `{event}`: `{s}` is not an offset"))
    }
}

/// One music bank and what to do with it.
#[derive(Deserialize)]
pub struct BgmSpec {
    pub bank: String,
    /// What happens at the end of the file. Carried through to the manifest so
    /// the runtime does not have to reach into `authored/` to find out, and so
    /// that the *why* travels with the decision.
    #[serde(default)]
    pub r#loop: Option<String>,
    pub why: String,
    #[serde(default)]
    pub loop_why: Option<String>,
    #[serde(default)]
    pub stems_why: Option<String>,
}

/// Which of a voice region's full-length presentations to export.
///
/// A region carries three presentations of one take and **nothing on the disc
/// ranks them** — `wEncodeOptions`, channel count and channel mask are
/// byte-identical across them. So this is a CHOICE, it lives in
/// `authored/audio.json` with its `why`, and it is deleted the day a capture
/// says which one the game plays.
#[derive(Deserialize, Default, Clone, Copy, PartialEq)]
#[serde(rename_all = "snake_case")]
pub enum Presentation {
    /// Peak nearest full scale.
    #[default]
    Loudest,
    /// Most bytes per second.
    HighestRate,
}

/// `authored/audio.json`, with the documentation keys dropped.
pub struct Config {
    pub se: Vec<(String, CueSpec)>,
    pub bgm: Vec<(String, BgmSpec)>,
    pub voice: Presentation,
}

/// Read `authored/audio.json`, or `None` when there is no such file.
///
/// Absent is not an error: an export with no audio is what every milestone
/// before P6 produced, and it should stay possible to take one.
pub fn load(authored: &Path) -> Result<Option<Config>> {
    let path = authored.join("audio.json");
    if !path.exists() {
        return Ok(None);
    }
    #[derive(Deserialize)]
    struct File {
        #[serde(default)]
        se: BTreeMap<String, serde_json::Value>,
        #[serde(default)]
        bgm: BTreeMap<String, serde_json::Value>,
        #[serde(default)]
        voice: BTreeMap<String, serde_json::Value>,
    }
    let raw = std::fs::read_to_string(&path)
        .with_context(|| format!("read {}", path.display()))?;
    let file: File = serde_json::from_str(&raw)
        .with_context(|| format!("parse {}", path.display()))?;

    // `_` is the house convention for a prose block explaining the section it
    // sits in -- see `authored/flow.json` and `authored/timing.json`. It is
    // documentation, not an entry, and the schema must skip it rather than
    // force the reasoning out of the file that holds the decision.
    fn entries<T: serde::de::DeserializeOwned>(
        m: BTreeMap<String, serde_json::Value>,
        what: &str,
    ) -> Result<Vec<(String, T)>> {
        m.into_iter()
            .filter(|(k, _)| k != "_")
            .map(|(k, v)| {
                let parsed = serde_json::from_value(v)
                    .with_context(|| format!("authored/audio.json: {what}.{k}"))?;
                Ok((k, parsed))
            })
            .collect()
    }

    // Absent means `loudest`, which is what the file says today. A default here
    // is safe in a way a default matrix is not: the manifest records which
    // presentation was taken and why, on every entry.
    let voice = match file.voice.get("presentation") {
        Some(v) => serde_json::from_value(v.clone())
            .with_context(|| format!("authored/audio.json: voice.presentation {v}"))?,
        None => Presentation::default(),
    };
    Ok(Some(Config {
        se: entries(file.se, "se")?,
        bgm: entries(file.bgm, "bgm")?,
        voice,
    }))
}

/// Vorbis quality. `-q:a 5` is ffmpeg's usual transparent-ish setting and is
/// what [`crate::video`] already uses for the movies' audio; using one value
/// across the export means a level difference between a cue and a movie cannot
/// be a codec artefact.
const VORBIS_Q: &str = "5";

pub struct Exported {
    pub name: String,
    pub file: String,
    pub command: String,
    pub why: String,
    /// dBFS peak of the decoded result. Recorded because the BGM sum can clip
    /// and a clipped file is not visibly different from a correct one.
    pub peak_dbfs: Option<f32>,
    /// Seconds, as ffprobe reads them back off the finished file. Recorded
    /// because the cue durations are the one thing about the SE export that an
    /// outside finding predicts, so they are the one thing that can be checked.
    pub duration_s: Option<f32>,
    pub kind: &'static str,
    /// The game's own identifier where it is a name match, never a measurement.
    pub name_match: Option<String>,
    /// What the runtime should do at the end of the file, where that was
    /// authored. `None` on a cue: a cue ends.
    pub loop_mode: Option<String>,
    /// How many sub-waves `media` returned for a bank. Carried out of here so
    /// the caller can warn when it contradicts HANDOFF's census -- this module
    /// does not get to decide that one of them is not a stem.
    pub sub_waves: usize,
}

/// Run ffmpeg, writing to a temp name and renaming on success.
///
/// The rename is not tidiness: the filesystem is shared with another agent, and
/// a reader that catches a half-written Ogg gets a confident wrong duration
/// rather than an error. `docs/port/AUDIO-VERIFICATION.md` records that this
/// already happened once on a video.
fn run_ffmpeg(argv: &[String], out: &Path) -> Result<()> {
    // The extension goes LAST, not the `.partial`. ffmpeg picks its muxer from
    // the output filename, so `.back.ogg.partial` is not a slightly uglier
    // temp name -- it is a hard failure before a byte is written: "Unable to
    // choose an output format". `video.rs` already had this shape; this
    // function was written from scratch and did not.
    let stem = out.file_stem().unwrap_or_default().to_string_lossy().into_owned();
    let ext = out.extension().unwrap_or_default().to_string_lossy().into_owned();
    let partial = out.with_file_name(format!(".{stem}.partial.{ext}"));
    let mut argv = argv.to_vec();
    let last = argv.len() - 1;
    argv[last] = partial.display().to_string();
    let res = Command::new("ffmpeg")
        .args(&argv)
        .output()
        .context("run ffmpeg -- is it on PATH?")?;
    if !res.status.success() {
        let _ = std::fs::remove_file(&partial);
        bail!(
            "ffmpeg failed writing {}:\n{}",
            out.display(),
            String::from_utf8_lossy(&res.stderr)
        );
    }
    std::fs::rename(&partial, out)?;
    Ok(())
}

/// Peak level and duration of a finished file.
///
/// Measured rather than assumed because the BGM is a **sum of two stems** and a
/// sum can clip, and because silence is the audio failure that looks like
/// success: a file of exactly the right duration, full of zeroes. Both numbers
/// pass every check that is not looking for them, so the export looks for them.
fn measure(path: &Path) -> (Option<f32>, Option<f32>) {
    let out = Command::new("ffmpeg")
        .args(["-hide_banner", "-v", "info", "-i"])
        .arg(path)
        .args(["-af", "astats=measure_perchannel=none", "-f", "null", "-"])
        .output();
    let Ok(out) = out else { return (None, None) };
    let text = String::from_utf8_lossy(&out.stderr).into_owned();
    // `astats` writes through the filter log, so every line carries a
    // `[Parsed_astats_0 @ 0x…] ` prefix. Matching on the line START silently
    // finds nothing and reports "peak unmeasured", which is the failure this
    // measurement exists to catch -- so it is found as a SUBSTRING.
    const KEY: &str = "Peak level dB:";
    let peak = text
        .lines()
        .find_map(|l| l.split_once(KEY)?.1.trim().parse().ok());
    let dur = Command::new("ffprobe")
        .args([
            "-v", "error", "-show_entries", "format=duration",
            "-of", "csv=p=0",
        ])
        .arg(path)
        .output()
        .ok()
        .and_then(|o| String::from_utf8_lossy(&o.stdout).trim().parse().ok());
    (peak, dur)
}

/// Write one XMA `RIFF` beside the output so ffmpeg has something to open.
///
/// Kept next to the result rather than in `/tmp` so a failed export leaves the
/// intermediate where the person debugging it will look, and removed on success
/// so the tree holds only formats a modder can open (MODDING rule 3).
fn stage_riff(dir: &Path, stem: &str, riff: &[u8]) -> Result<PathBuf> {
    let path = dir.join(format!(".{stem}.xma.wav"));
    std::fs::write(&path, riff).with_context(|| format!("write {}", path.display()))?;
    Ok(path)
}

/// The menu cues, as `audio/se/<event>.ogg`.
pub fn export_cues<S: DiscSource + ?Sized>(
    source: &S,
    out: &Path,
    cues: &[(String, CueSpec)],
) -> Result<Vec<Exported>> {
    if cues.is_empty() {
        return Ok(Vec::new());
    }
    let dir = out.join("audio/se");
    std::fs::create_dir_all(&dir)?;
    let mut done = Vec::new();
    for (event, cue) in cues {
        let offset = cue.offset(event)?;
        // `media` owns the assembly. Asking it for the RIFF rather than reading
        // `Static.slb` here is the whole point of the seam -- and it REFUSES a
        // short read rather than returning a truncated stream, because a
        // truncated XMA decodes to plausible-sounding garbage.
        let riff = media::se_wave_riff(
            source, &cue.bank, offset, cue.packets, cue.channels, cue.rate,
        )
        .map_err(anyhow::Error::msg)
        .with_context(|| format!("assemble the {event} cue"))?;

        let staged = stage_riff(&dir, event, &riff)?;
        let ogg = dir.join(format!("{event}.ogg"));
        let argv: Vec<String> = [
            "-hide_banner", "-loglevel", "error", "-y",
            "-i", &staged.display().to_string(),
            "-c:a", "libvorbis", "-q:a", VORBIS_Q,
            &ogg.display().to_string(),
        ]
        .iter()
        .map(|s| s.to_string())
        .collect();
        let command = format!("ffmpeg {}", argv.join(" "));
        run_ffmpeg(&argv, &ogg)?;
        let (peak, dur) = measure(&ogg);
        std::fs::remove_file(&staged).ok();

        done.push(Exported {
            name: event.clone(),
            file: format!("audio/se/{event}.ogg"),
            command,
            why: format!(
                "{} AUTHORED because it is measured, not decoded: authored/audio.json \
                 se.{event}. Located in {} at {:#x} for {} packet(s); the ASSEMBLY is \
                 sylpheed_formats::media::se_wave_riff, which refuses a short read.",
                cue.why, cue.bank, offset, cue.packets
            ),
            peak_dbfs: peak,
            duration_s: dur,
            kind: "se",
            name_match: cue.name_match.clone(),
            loop_mode: None,
            sub_waves: 1,
        });
    }
    Ok(done)
}

/// One music bank, as a single `audio/bgm/<name>.ogg`.
///
/// **The two stems are summed, not concatenated and not split into two files.**
///
/// HANDOFF Q10: a bank's sub-waves are two stems of one performance, played
/// together — sample-synchronous, equal duration, on all 32 banks.
/// Concatenating them is explicitly wrong. Emitting two files would be wrong
/// here for a different reason: MODDING rule 1 is *one logical asset, one
/// file*, and a modder who had to line two stems up by hand would be
/// reassembling exactly what the exporter exists to resolve.
///
/// **The sum is scaled by 1/n, and an earlier version of this comment argued the
/// opposite.** It said `normalize=0` sums at unity "because halving is a mix
/// decision nobody made". That was wrong twice over. Unity summing IS a decision
/// — and it is the one that can clip, which it duly did: `BGM_103` came out at
/// **+1.8 dBFS**. And 1/n is not a taste call but the smallest constant that
/// makes an n-input sum of unity-scale signals provably clip-free, which is the
/// same reasoning `video.rs` already uses for its 0.4142-normalised 5.1 downmix.
/// It is written out as an explicit `volume=` rather than left to `amix`'s
/// `normalize=1` default so the coefficient appears in the manifest's command
/// line: a default is a decision nobody made, and it can move under an ffmpeg
/// upgrade.
///
/// It preserves the stems' relative balance exactly, which is the only thing
/// about the sum that HANDOFF Q10 actually settles. The peak is still measured
/// and reported.
///
/// The file is named for the **role** (`main_menu`), not for the bank
/// (`BGM_001`). Which bank plays here is authored and expected to change; the
/// role is what the runtime asks for, and a rename of the disc asset should not
/// be a change to the Godot project.
pub fn export_bgm<S: DiscSource + ?Sized>(
    source: &S,
    out: &Path,
    role: &str,
    spec: &BgmSpec,
) -> Result<Option<Exported>> {
    let riffs = match media::sound_bank_riffs(source, &spec.bank) {
        Ok(r) if !r.is_empty() => r,
        Ok(_) => return Ok(None),
        // "This disc does not have that bank" is a MISSING ASSET, not a broken
        // exporter: the manifest carries a warning and everything else still
        // exports. Any other failure -- a short read, a malformed bank -- is a
        // real error and stops the run, because a partly-read bank produces a
        // file that plays.
        Err(e) if e.contains("not present in sound.pak") => return Ok(None),
        Err(e) => bail!("{}: {e}", spec.bank),
    };
    let dir = out.join("audio/bgm");
    std::fs::create_dir_all(&dir)?;

    let mut staged = Vec::new();
    for (i, riff) in riffs.iter().enumerate() {
        staged.push(stage_riff(&dir, &format!("{role}.{i}"), riff)?);
    }

    let ogg = dir.join(format!("{role}.ogg"));
    let mut argv: Vec<String> = ["-hide_banner", "-loglevel", "error", "-y"]
        .iter()
        .map(|s| s.to_string())
        .collect();
    for s in &staged {
        argv.push("-i".into());
        argv.push(s.display().to_string());
    }
    if staged.len() > 1 {
        argv.push("-filter_complex".into());
        argv.push(format!(
            "amix=inputs={n}:normalize=0,volume={:.6}",
            1.0 / staged.len() as f64,
            n = staged.len()
        ));
    }
    argv.extend(
        ["-c:a", "libvorbis", "-q:a", VORBIS_Q, &ogg.display().to_string()]
            .iter()
            .map(|s| s.to_string()),
    );
    let command = format!("ffmpeg {}", argv.join(" "));
    run_ffmpeg(&argv, &ogg)?;
    let (peak, dur) = measure(&ogg);
    for s in &staged {
        std::fs::remove_file(s).ok();
    }

    let mut why = format!(
        "{} authored/audio.json bgm.{role} names bank {}. Its {} \
         sub-wave(s) are SUMMED into one file and the sum is scaled by 1/{}, \
         which is the smallest constant that cannot clip.",
        spec.why,
        spec.bank,
        riffs.len(),
        riffs.len()
    );
    if let Some(s) = &spec.stems_why {
        why.push(' ');
        why.push_str(s);
    }
    if let Some(s) = &spec.loop_why {
        why.push(' ');
        why.push_str(s);
    }

    Ok(Some(Exported {
        name: role.to_string(),
        file: format!("audio/bgm/{role}.ogg"),
        command,
        why,
        peak_dbfs: peak,
        duration_s: dur,
        kind: "bgm",
        name_match: None,
        loop_mode: spec.r#loop.clone(),
        sub_waves: riffs.len(),
    }))
}

/// One cutscene's voice-over, as `audio/voice/<movie>.ogg`.
///
/// ## Why this is a separate file from the movie at all
///
/// A human play-test heard music under the intro and no dialogue, and the
/// obvious reading — "the transcode dropped a channel" — is wrong. `ADV.wmv`
/// genuinely carries **music and effects only**. On this disc a cutscene's voice
/// is a *different asset*: one continuous XMA stream in `sound.pak`, bound to
/// the movie by the movie manifest in `tables.pak` (`ADV` → `VOICE_ADV`). It was
/// not dropped by [`crate::video`]; it was never exported, because nothing here
/// asked for it.
///
/// ## The binding is resolved, never matched by name
///
/// `sylpheed_formats::media::resolve_movie_voice_region` walks
/// movie → cue token → sound id → byte region. It is the only route taken here,
/// and the reason is that the cheap route looks correct on exactly the movies a
/// person would check first. Measured on the retail disc:
///
/// | movie | region | inside the bank named after it? |
/// |---|---|---|
/// | `ADV` | 433 930 240…437 044 592 | yes |
/// | `S00A` | 452 798 464…455 499 120 | yes |
/// | `RT01A` | 437 044 592…437 345 648 | **no — it is inside `VOICE_ADV.slb`** |
///
/// So `VOICE_<movie>.slb` is a name that happens to hold the right audio twice
/// out of three, and the two it gets right are the two in this port's scope.
/// Reading the bank by name would have shipped, verified clean, and been wrong
/// for the radio cutscenes the moment anybody extended the export.
///
/// ## What a `None` means, and what it must not become
///
/// A movie whose region does not resolve is **genuinely unvoiced** — that is a
/// real answer for most `hokyu_*` resupply cutscenes, and the corpus already
/// paid for the alternative: resolving unbound movies through a shared demo line
/// played the *wrong recording*. Nothing is substituted. A `\Movie\` token that
/// resolved a clip but not a region stays silent for the same reason
/// `sylpheed-viewer` keeps it silent: its raw `.slb` is off by one chunk, so it
/// is not this movie's dialogue.
///
/// ## Three choices made here, and the reason each is not a guess
///
/// * **One file per movie** (MODDING rule 1), and **exactly one region chunk is
///   kept**. Not concatenated, not summed. This function got that wrong twice
///   before it got it right, and the history is kept below because each wrong
///   reading was ended by a measurement, not by an argument.
///
/// ## A region holds THREE PRESENTATIONS OF ONE TAKE — decoded, and not by me
///
/// A resolved region decodes to several chunks. Decoded and timed against the
/// movies' own lengths:
///
/// | movie | movie | chunk 0 | chunk 1 | chunk 2 |
/// |---|---|---|---|---|
/// | `ADV` | 137.437 s | 84.553 | **137.324** | **137.324** |
/// | `S00A` | 93.779 s | 68.072 | **93.694** | **93.694** |
/// | `RT01A` | — | 0.009 | **34.034** | — |
///
/// **Reading 1, concatenate:** 359 s of dialogue for a 137 s movie. Dead.
///
/// **Reading 2, sum them as HANDOFF Q10's two stems** — equal duration, each
/// spanning the movie, which is exactly Q10's *music* shape. ❌ **Refuted here,
/// and the claim had already been adopted into the RE corpus before I tested
/// it**: `S00A`'s second full-length chunk is **digital silence** (peak −inf)
/// and `ADV`'s is **0.60 × the first** with 26.8 dB of residual. Equal duration
/// was a shape match and carrying a music census across on it was my error.
///
/// **Reading 3, one stream. ✅ Decoded disc-wide by the Decoder**, by counting
/// stream starts inside every inter-descriptor span: **258 spans hold one
/// stream, 28 hold three, and nothing holds two or any other number.** The 95
/// movie-voice regions decompose 70 + 8 + 17. So a region is three presentations
/// of one take, and `359 = 84.55 + 137.32 + 137.32`. Summing a take with a
/// scaled copy of itself adds ~4 dB and colours it.
///
/// 🟡 **Which presentation to keep is a recommendation, not a field.** The
/// selector is the **highest byte rate** among the equal-duration survivors, on
/// the Decoder's advice. Nothing on the disc says which one the game plays, and
/// on `ADV` this picks the **quieter** of the two — −8.3 dBFS against 0.0. It is
/// stated in the manifest with that consequence so the choice is reversible; a
/// capture of the intro with dialogue audible settles it.
///
/// ## Chunk 0 is dropped, and it is a DUPLICATE rather than a truncation
///
/// This comment first guessed it was `BGM_103`'s third sub-wave — a bank header
/// — and a census over all 95 regions showed that is a *different structure*:
/// 78 open with a 10 240 B bank header, 17 with a leading headerless stream at
/// the disc's own `1392 mod 2048` data offset, and **the chunk count
/// discriminates neither**. The byte-span test then found it is **this movie's
/// own dialogue, 17 of 17** — not an in-mission line, which was the standing
/// hypothesis.
///
/// Which raised the real question: is dropping it a truncation? **No.** Measured
/// here with a decoder the RE container does not have — sliding envelope
/// correlation, overhang allowed, normalised over the overlap: **r = 0.998**
/// (`ADV`) and **0.932** (`S00A`), against controls of 1.000 (self) and 0.289 (a
/// different movie), with both lags placing chunk 0 **flush against the end** of
/// the kept stream. Confirmed in the sample domain at 16.7 / 23.2 dB of
/// residual. It is the tail of the take, presented again.
///
/// So the selection rule is stated in terms of what was measured — *the longest
/// duration, ties broken by byte rate, minus anything digitally silent* — and
/// every dropped chunk is named in the manifest with its length and peak.
/// * **Mono**, with the fold chosen from the stream's own declared channel
///   count rather than by passing `-ac 1` and hoping. A voice track that is
///   already mono is passed through untouched.
/// * **No sync offset.** The voice plays from the video's first frame, so the
///   runtime needs no delay and none is authored. The decoded length is
///   recorded beside the movie's own length in the manifest so a disagreement
///   is visible rather than absorbed.
pub fn export_voice<S: DiscSource + ?Sized>(
    source: &S,
    out: &Path,
    movie: &str,
    video_duration_s: Option<f32>,
    presentation: Presentation,
) -> Result<Option<Exported>> {
    use sylpheed_formats::slb::VoiceLang;

    // English only: MISSION §7 puts localisation beyond English out of scope.
    // The language is a parameter of the resolution, not of the file layout, so
    // adding Japanese later is a second call and a second file, not a re-think.
    let lang = VoiceLang::English;
    let region = media::resolve_movie_voice_region(source, movie, lang);
    let Some((start, end)) = region else {
        return Ok(None);
    };
    let riffs = media::voice_region_riffs(source, start, end)
        .map_err(anyhow::Error::msg)
        .with_context(|| format!("decode the {movie} voice region"))?;
    if riffs.is_empty() {
        return Ok(None);
    }

    let dir = out.join("audio/voice");
    std::fs::create_dir_all(&dir)?;
    let mut all = Vec::new();
    for (i, riff) in riffs.iter().enumerate() {
        all.push(stage_riff(&dir, &format!("{movie}.{i}"), riff)?);
    }

    // Classify before mixing. XMA declares no duration, so each chunk is decoded
    // and timed -- the only way to tell a stem from the leading region, and the
    // measurement that showed concatenation to be wrong here.
    let probed: Vec<(f32, f32)> = all.iter().map(|p| decoded_chunk(p)).collect();
    let lengths: Vec<f32> = probed.iter().map(|&(d, _)| d).collect();
    // A DIGITALLY SILENT chunk is dropped before anything else, and that is
    // arithmetic rather than a judgement about content: it contributes nothing
    // to a mix, and counting it in the 1/n normalisation costs 6.02 dB for
    // nothing. `S00A`'s second full-length chunk is exactly this -- 4 497 300
    // samples of zeroes, peak -inf -- and summing it is why that movie's voice
    // came out at -16.2 dBFS against a source peaking at -4.2.
    let silent: Vec<usize> = (0..all.len()).filter(|&i| probed[i].1 <= -90.0).collect();
    let longest = (0..all.len())
        .filter(|i| !silent.contains(i))
        .map(|i| lengths[i])
        .fold(0.0f32, f32::max);
    // A tie at 1 ms. The two stems agree to six decimals and the chunk that is
    // not one of them misses by tens of seconds, so nothing sits near this
    // bound: it separates the measured cases without being a tuned threshold.
    // ONE STREAM, NOT A SUM -- and this is the third reading of these chunks, each
    // one refuted by a measurement rather than by an argument.
    //
    // They were concatenated (359 s for a 137 s movie), then summed as HANDOFF
    // Q10's two stems (refuted here: `S00A`'s second is silence, `ADV`'s is
    // 0.60x the first). The Decoder then decoded the shape disc-wide -- counting
    // stream starts inside every inter-descriptor span gives 258 spans with ONE
    // stream and 28 with THREE, and nothing with two or any other number, so a
    // region carries **three presentations of one take**, not a mix. Summing a
    // take with a scaled copy of itself adds ~4 dB and colours it.
    //
    // WHICH of the equal-duration survivors is a CHOICE, and it lives in
    // `authored/audio.json` rather than here -- see [`Presentation`]. It was
    // `highest_rate` on the Decoder's recommendation until that was withdrawn as
    // self-contradictory. `loudest` is a PER-ASSET CONTENT choice and nothing
    // more: the disc masters its other audio near full scale, and it puts the
    // two cutscenes' dialogue at comparable levels.
    //
    // ⚠️ A structural argument for it — `ADV`'s higher-rate stream is dual-mono,
    // so its extra bytes encode a duplicated channel rather than fidelity — was
    // offered here and **does not generalise**. The channel measurement is
    // `ADV`'s and stands; the inference was tested disc-wide over the 28
    // three-stream cues and the size ratio runs 0.0778 to 2.9163. Neither rule
    // has a structural argument behind it, which is exactly why the choice is
    // authored rather than derived.
    let tied: Vec<usize> = (0..all.len())
        .filter(|&i| !silent.contains(&i) && (longest - lengths[i]).abs() < 0.001)
        .collect();
    let chosen = match presentation {
        Presentation::HighestRate => tied.iter().copied().max_by_key(|&i| riffs[i].len()),
        Presentation::Loudest => tied
            .iter()
            .copied()
            .max_by(|&a, &b| probed[a].1.total_cmp(&probed[b].1)),
    };
    let keep: Vec<usize> = chosen.into_iter().collect();
    let dropped: Vec<String> = (0..all.len())
        .filter(|i| !keep.contains(i))
        .map(|i| {
            format!(
                "chunk {i} ({:.3} s, {} B, peak {})",
                lengths[i],
                riffs[i].len(),
                if silent.contains(&i) {
                    "SILENT".to_string()
                } else {
                    format!("{:.1} dBFS", probed[i].1)
                }
            )
        })
        .collect();
    if keep.is_empty() {
        return Ok(None);
    }
    let staged: Vec<PathBuf> = keep.iter().map(|&i| all[i].clone()).collect();

    // The fold averages the channels that CARRY SIGNAL, not the channels the
    // stream declares.
    //
    // This function's first version averaged all declared channels, and the doc
    // comment above it warned in as many words that "a stereo matrix applied to
    // a mono voice track is not an error, it is a -6 dB attenuation that nothing
    // reports". It then did exactly that: **channel 2 of both voice streams is
    // digitally silent** -- peak -inf over the whole file, on `ADV` and on
    // `S00A` -- so this is a mono recording carried in a nominally stereo
    // stream, and averaging it with silence cost 5.94 dB. Checking the declared
    // count is not checking the content, and only the content is the fold.
    //
    // Same principle as the silent-chunk drop above, one level down: a silent
    // input contributes nothing to an average and counting it in the divisor is
    // arithmetic, not a mixing decision. `sylpheed-viewer`'s `pan=mono|c0=c0`
    // reaches the right answer here for a reason it does not state.
    let live = live_channels(&staged[0]);
    let channels = probe_channels(&staged[0]).unwrap_or(1);
    let fold = if live.len() <= 1 && channels <= 1 {
        String::new()
    } else if live.len() == 1 {
        format!(",pan=mono|c0=c{}", live[0])
    } else {
        let g = 1.0 / live.len() as f64;
        let terms: Vec<String> = live.iter().map(|c| format!("{g:.6}*c{c}")).collect();
        format!(",pan=mono|c0={}", terms.join("+"))
    };

    let ogg = dir.join(format!("{movie}.ogg"));
    let mut argv: Vec<String> = ["-hide_banner", "-loglevel", "error", "-y"]
        .iter()
        .map(|s| s.to_string())
        .collect();
    for s in &staged {
        argv.push("-i".into());
        argv.push(s.display().to_string());
    }
    // One input, so no mix and no normalising coefficient: the stream reaches the
    // Ogg at the level the disc has it, and the only filter is the mono fold.
    let filter = format!("[0:a]anull{fold}[a]");
    argv.push("-filter_complex".into());
    argv.push(filter);
    argv.push("-map".into());
    argv.push("[a]".into());
    argv.extend(
        ["-c:a", "libvorbis", "-q:a", VORBIS_Q, &ogg.display().to_string()]
            .iter()
            .map(|s| s.to_string()),
    );
    let command = format!("ffmpeg {}", argv.join(" "));
    run_ffmpeg(&argv, &ogg)?;
    let (peak, dur) = measure(&ogg);
    for s in &all {
        std::fs::remove_file(s).ok();
    }

    let against = match (dur, video_duration_s) {
        (Some(d), Some(v)) => format!(
            " Decoded length {d:.3} s against the movie's {v:.3} s (delta {:+.3} s); \
             NOT trimmed to fit -- a clamp would hide a resolution error, and the \
             runtime stops the voice when the video ends.",
            d - v
        ),
        _ => String::new(),
    };

    Ok(Some(Exported {
        name: movie.to_string(),
        file: format!("audio/voice/{movie}.ogg"),
        command,
        why: format!(
            "DECODED, not authored: the movie manifest in tables.pak binds {movie} to a \
             voice cue, and sylpheed_formats::media::resolve_movie_voice_region walks \
             movie -> token -> sound id -> byte region [{start}, {end}) of the continuous \
             voice stream. NOT matched by filename: RT01A's voice lives inside \
             VOICE_ADV.slb, so the name is right for this movie by luck and wrong for \
             others. Of {} region chunk(s), exactly ONE is kept -- not summed. A region \
             carries THREE PRESENTATIONS OF ONE TAKE, decoded disc-wide by counting stream \
             starts inside every inter-descriptor span: 258 spans hold one stream and 28 hold \
             three, and nothing holds two. This exporter read these chunks wrongly twice \
             before that landed -- first concatenating them (359 s for a 137 s movie), then \
             summing them as HANDOFF Q10's two stems, which its own measurements refuted: \
             S00A's second full-length chunk is DIGITAL SILENCE and ADV's is 0.60x the first \
             with 26.8 dB of residual. Summing a take with a scaled copy of itself adds ~4 dB \
             and colours it. 🟡 WHICH of the equal-duration survivors is kept is a CHOICE, \
             not a decoded field -- authored/audio.json voice.presentation = {:?}, with its \
             why. Nothing on the disc ranks the presentations: wEncodeOptions, channel count \
             and channel mask are byte-identical across them. One capture of the movie with \
             dialogue audible deletes that entry.{} Folded to mono from the {} of {channels} declared \
             channel(s) that carry signal -- channel 2 of both voice streams is digitally \
             silent, and averaging it in cost 5.94 dB until this was measured rather than \
             read off the declared count.{against}",
            riffs.len(),
            match presentation {
                Presentation::Loudest => "loudest",
                Presentation::HighestRate => "highest_rate",
            },
            if dropped.is_empty() {
                String::new()
            } else {
                format!(
                    " DROPPED, and NOT as junk -- the leading chunk is DECODED to be this \
                     movie's OWN dialogue, 17 of 17 regions (docs/re/structures/\
                     voice-region-leading-chunk.md; an earlier note here wrongly equated it \
                     with BGM_103's third sub-wave, which a disc-wide census showed is a \
                     different structure). This port then measured it to be the TAIL of the \
                     kept stream -- sliding envelope correlation r=0.998 (ADV) and 0.932 \
                     (S00A), the lag placing it flush against that stream's end, against \
                     controls of 1.000 self and 0.289 for a different movie -- so dropping it \
                     removes a DUPLICATE, not dialogue. Dropped: {}.",
                    dropped.join(", ")
                )
            },
            live.len()
        ),
        peak_dbfs: peak,
        duration_s: dur,
        kind: "voice",
        name_match: None,
        loop_mode: None,
        sub_waves: riffs.len(),
    }))
}

/// How many channels a staged `RIFF` declares, per ffprobe.
fn probe_channels(path: &Path) -> Option<u8> {
    let out = Command::new("ffprobe")
        .args([
            "-v", "error", "-select_streams", "a:0",
            "-show_entries", "stream=channels", "-of", "csv=p=0",
        ])
        .arg(path)
        .output()
        .ok()?;
    String::from_utf8_lossy(&out.stdout).trim().parse().ok()
}

/// Seconds a finished media file runs, per ffprobe.
///
/// Exposed so the caller can hand [`export_voice`] the movie's own length: the
/// voice is a separate asset with no shared container to agree with, so the only
/// way a resolution error shows up is a length that does not match the picture.
pub fn probe_duration(path: &Path) -> Option<f32> {
    let out = Command::new("ffprobe")
        .args([
            "-v", "error", "-show_entries", "format=duration",
            "-of", "csv=p=0",
        ])
        .arg(path)
        .output()
        .ok()?;
    String::from_utf8_lossy(&out.stdout).trim().parse().ok()
}

/// Seconds and peak dBFS that one staged XMA `RIFF` decodes to.
///
/// XMA carries no duration in its header, so the chunk is decoded to PCM and the
/// result measured. That is expensive and it is the only instrument that can
/// separate these chunks at all: `ffprobe` on the `RIFF` itself returns `N/A`
/// for duration, which a caller that trusted it would read as zero, and the
/// corpus records `sylpheed-cli audio info` mis-reading the same headers as
/// 16 channels at 4 310 Hz.
///
/// A silent chunk returns `-inf`, which the caller drops.
fn decoded_chunk(riff: &Path) -> (f32, f32) {
    let wav = riff.with_extension("probe.wav");
    let ok = Command::new("ffmpeg")
        .args(["-hide_banner", "-loglevel", "error", "-y", "-i"])
        .arg(riff)
        .arg(&wav)
        .output()
        .map(|o| o.status.success())
        .unwrap_or(false);
    let out = if ok {
        let (peak, dur) = measure(&wav);
        (dur.unwrap_or(0.0), peak.unwrap_or(f32::NEG_INFINITY))
    } else {
        (0.0, f32::NEG_INFINITY)
    };
    let _ = std::fs::remove_file(&wav);
    out
}


/// Which channel indices of a decoded stream are not digitally silent.
///
/// `astats` reports per-channel blocks: a `Channel: N` line followed by that
/// channel's own `Peak level dB`. A channel whose peak is `-inf` carries
/// nothing, and folding it into an average is a pure loss.
///
/// Falls back to "every declared channel is live" if the parse finds nothing,
/// because the failure to prefer is the one that changes no level.
fn live_channels(riff: &Path) -> Vec<usize> {
    let wav = riff.with_extension("chan.wav");
    let ok = Command::new("ffmpeg")
        .args(["-hide_banner", "-loglevel", "error", "-y", "-i"])
        .arg(riff)
        .arg(&wav)
        .output()
        .map(|o| o.status.success())
        .unwrap_or(false);
    let mut live = Vec::new();
    if ok {
        if let Ok(out) = Command::new("ffmpeg")
            .args(["-hide_banner", "-v", "info", "-i"])
            .arg(&wav)
            .args(["-af", "astats", "-f", "null", "-"])
            .output()
        {
            let text = String::from_utf8_lossy(&out.stderr).into_owned();
            let mut current: Option<usize> = None;
            for line in text.lines() {
                if let Some((_, n)) = line.split_once("Channel: ") {
                    // astats numbers channels from 1; `pan` addresses c0 upward.
                    current = n.trim().parse::<usize>().ok().map(|n| n.saturating_sub(1));
                } else if let Some((_, v)) = line.split_once("Peak level dB: ") {
                    if let Some(c) = current.take() {
                        if v.trim().parse::<f32>().map(|p| p > -90.0).unwrap_or(false) {
                            live.push(c);
                        }
                    }
                }
            }
        }
    }
    let _ = std::fs::remove_file(&wav);
    if live.is_empty() {
        live = (0..probe_channels(riff).unwrap_or(1) as usize).collect();
    }
    live
}