//! `savedata` — the retail save file.
//!
//! The whole save is **545 bytes**. It lives in the console's content tree as a
//! single file and there is no second one, so everything the game remembers
//! between sessions is in here:
//!
//! ```text
//! <content>/<XUID>/535107D4/00000001/game01/savedata
//! ```
//!
//! # Container
//!
//! ```text
//! 'GDHA'  <146-byte header>  <zlib stream, 78 DA>
//! ```
//!
//! Much of the header is **uninitialised memory** — words like `0x828F3DA8` are
//! guest virtual addresses that leaked out of the struct's padding — so it is not
//! byte-reproducible and must not be read as meaningful. Three header fields
//! *are* derived from the payload ([`Header::derived`]), and a handful more are a
//! **summary copy** of payload fields ([`Header::summary`]) — which matters more
//! than it sounds: the in-game Details panel reads the *summary*, not the
//! payload, so editing a payload field alone leaves a stale panel beside it. A
//! probe that judges "did that field change the display?" without patching the
//! mirror cannot tell *"wrong field"* from *"the panel never reads the payload"*.
//!
//! # Payload
//!
//! A chunk stream, read off the title's own serializer at `0x822C00E8` (its
//! writer primitive is `0x821885A8(stream, buf, len)`) rather than guessed, and
//! verified by a byte-identical round-trip:
//!
//! ```text
//! 'GDAA'                              payload magic
//! u32 len, char[len]                  current game phase, e.g. "GP_BUNK"
//! 'GHAD' + 122 bytes                  the progress block
//! u32 16, 16 x ('SHAB' + 5 x u32)     the per-stage record table
//! u32 4, "BUNK", 'NETA', u32          trailer
//! ```
//!
//! The in-memory struct is written field-by-field with no packing changes, so a
//! payload offset **is** the offset in the live save object: GHAD at `save+8`,
//! the record table at `save+136`, and the serializer's next access after the
//! table is `lwz r11,456(save)` — exactly `136 + 16*20`.
//!
//! # Confidence
//!
//! The container and the chunk layout are ✅ `CONFIRMED`. Individual GHAD fields
//! are **not** uniformly known — eleven are still ❔ — so every field carries its
//! own [`Confidence`] and a viewer is expected to show it rather than present the
//! lot as solved. See `docs/re/structures/savegame-format.md`.

use std::io::Read;

/// How well a field is understood. Mirrors the `docs/re` convention.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Confidence {
    /// Named and verified against the game.
    Confirmed,
    /// A well-supported reading, not proven.
    Probable,
    /// Unidentified.
    Unknown,
    /// A reading that was tested and **refuted** — recorded so it is not retried.
    Refuted,
}

/// How to render a GHAD word.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum FieldKind {
    /// Plain 32-bit count / enum / id.
    U32,
    /// 64-bit word.
    U64,
    /// Milliseconds, displayed as `m:ss.mmm`.
    Millis,
    /// Percent.
    Percent,
    /// Raw bytes.
    Raw,
    /// The per-item Arsenal development blob.
    DevelopBlob,
}

/// One documented field of the GHAD progress block.
#[derive(Debug, Clone, Copy)]
pub struct FieldSpec {
    /// Offset within the 122-byte GHAD block.
    pub offset: usize,
    /// Byte width.
    pub size: usize,
    pub kind: FieldKind,
    pub confidence: Confidence,
    /// Short name, or `""` when the field is unidentified.
    pub name: &'static str,
    /// What is known — including what was ruled out.
    pub note: &'static str,
}

/// The GHAD block's layout, read off `0x822BF678`: ten u32, one u64
/// (`ld r11,40(r30)`), four u32, a raw 4-byte field and a raw 54-byte blob.
/// 10·4 + 8 + 4·4 + 4 + 54 = **122**, exactly what the file carries — the layout
/// is closed, with nothing unaccounted for.
pub const GHAD_LAYOUT: &[FieldSpec] = &[
    FieldSpec { offset: 0,  size: 4,  kind: FieldKind::U32,     confidence: Confidence::Unknown,   name: "",              note: "" },
    FieldSpec { offset: 4,  size: 4,  kind: FieldKind::Millis,  confidence: Confidence::Confirmed, name: "FlightTime",    note: "Milliseconds. 324773 ms against the panel's `Flight Time 000:05:24`." },
    FieldSpec { offset: 8,  size: 4,  kind: FieldKind::Percent, confidence: Confidence::Confirmed, name: "ClearRatio",    note: "Percent. Not a stage counter: developing one Arsenal weapon stepped it 5 → 6, so it counts collection too." },
    FieldSpec { offset: 12, size: 4,  kind: FieldKind::U32,     confidence: Confidence::Probable,  name: "TimesCleared",  note: "Mirrored to header +0x28; the panel prints `Times Cleared`." },
    FieldSpec { offset: 16, size: 4,  kind: FieldKind::U32,     confidence: Confidence::Unknown,   name: "",              note: "" },
    FieldSpec { offset: 20, size: 4,  kind: FieldKind::U32,     confidence: Confidence::Unknown,   name: "",              note: "" },
    FieldSpec { offset: 24, size: 4,  kind: FieldKind::U32,     confidence: Confidence::Confirmed, name: "Points",        note: "The spendable balance. Separated from +28 by a develop differential: spending 4000 P moved only this field (4101 → 101)." },
    FieldSpec { offset: 28, size: 4,  kind: FieldKind::U32,     confidence: Confidence::Probable,  name: "PointsTotal?",  note: "Not the displayed Points — it did NOT move when 4000 P was spent. A lifetime/earned total is the obvious read; unproven until a save is taken after earning." },
    FieldSpec { offset: 32, size: 4,  kind: FieldKind::U32,     confidence: Confidence::Unknown,   name: "",              note: "" },
    FieldSpec { offset: 36, size: 4,  kind: FieldKind::U32,     confidence: Confidence::Refuted,   name: "",              note: "Tested as difficulty and as stage; probe saves refuted both." },
    FieldSpec { offset: 40, size: 8,  kind: FieldKind::U64,     confidence: Confidence::Unknown,   name: "",              note: "" },
    FieldSpec { offset: 48, size: 4,  kind: FieldKind::U32,     confidence: Confidence::Probable,  name: "GameStatus",    note: "Mirrored to header +0x18; 0 renders `At Standby`, matching the screen's STATE_STAND_BY / STATE_STAGE_CLEAR / STATE_GAME_CLEAR list." },
    FieldSpec { offset: 52, size: 4,  kind: FieldKind::U32,     confidence: Confidence::Confirmed, name: "Stage",         note: "1-based stage number. Set it to 5 (with the header mirror) and the game reads `STAGE 05 — Star System Escape`, loads it, and flies it." },
    FieldSpec { offset: 56, size: 4,  kind: FieldKind::U32,     confidence: Confidence::Refuted,   name: "",              note: "See +36 — refuted." },
    FieldSpec { offset: 60, size: 4,  kind: FieldKind::U32,     confidence: Confidence::Unknown,   name: "",              note: "" },
    FieldSpec { offset: 64, size: 4,  kind: FieldKind::Raw,     confidence: Confidence::Unknown,   name: "",              note: "The trailer's u32 carries the same value." },
    FieldSpec { offset: 68, size: 54, kind: FieldKind::DevelopBlob, confidence: Confidence::Confirmed, name: "DevelopState", note: "One byte per Arsenal item. Index space is strings.tbl's item order, cut items included, closing at 53." },
];

/// Size of the GHAD progress block.
pub const GHAD_SIZE: usize = 122;
/// Records in the per-stage table.
pub const RECORD_COUNT: usize = 16;
/// `u32`s per record, after the `SHAB` tag.
pub const RECORD_FIELDS: usize = 5;
/// Length of the Arsenal development blob.
pub const DEVELOP_BLOB_LEN: usize = 54;

/// State of one Arsenal item in the development blob.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum DevelopState {
    /// `0` — not yet available.
    Locked,
    /// `2` — available to develop now. **Re-derived at load**, not stored state.
    Developable,
    /// `4` — owned.
    Developed,
    /// Anything else, which no save has yet shown.
    Other(u8),
}

impl DevelopState {
    pub fn from_byte(b: u8) -> Self {
        match b {
            0 => Self::Locked,
            2 => Self::Developable,
            4 => Self::Developed,
            other => Self::Other(other),
        }
    }
}

/// One `SHAB` record — a per-stage result, record 0 being Stage 01.
///
/// These are **not** the UI's 20 save slots, which is what the count of 16
/// first suggested.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct StageRecord {
    /// 🟡 difficulty.
    pub a: u32,
    /// ❔ a points figure — equals GHAD `+24`/`+28`, so not the displayed high
    /// score (which is computed at display time and stored nowhere).
    pub b: u32,
    /// ✅ best clear time in milliseconds — 324 773 against the screen's
    /// `05:24.77`.
    pub best_time_ms: u32,
    /// Windows FILETIME, high then low word.
    pub time_hi: u32,
    pub time_lo: u32,
}

impl StageRecord {
    /// Whether the record holds a result at all.
    pub fn is_used(&self) -> bool {
        self.a != 0 || self.b != 0 || self.best_time_ms != 0
    }

    /// The record's FILETIME as 100 ns ticks since 1601, or `None` if unset.
    pub fn filetime_ticks(&self) -> Option<u64> {
        let t = (u64::from(self.time_hi) << 32) | u64::from(self.time_lo);
        (t != 0).then_some(t)
    }
}

/// The GDHA container header.
#[derive(Debug, Clone)]
pub struct Header {
    /// The header bytes, verbatim.
    pub bytes: Vec<u8>,
}

/// A header word that mirrors a payload field, with where it came from.
#[derive(Debug, Clone, Copy)]
pub struct Mirror {
    pub header_offset: usize,
    pub value: u32,
    /// The GHAD offset this copies, if it is a GHAD field.
    pub ghad_offset: Option<usize>,
    pub name: &'static str,
}

impl Header {
    fn be32(&self, o: usize) -> u32 {
        if o + 4 > self.bytes.len() {
            return 0;
        }
        u32::from_be_bytes([
            self.bytes[o],
            self.bytes[o + 1],
            self.bytes[o + 2],
            self.bytes[o + 3],
        ])
    }

    /// The summary copy the in-game Details panel actually reads.
    ///
    /// This is the trap the format doc records: a payload edit that leaves these
    /// stale shows no change on the panel, which is *not* evidence that the
    /// payload field was the wrong one.
    pub fn summary(&self) -> Vec<Mirror> {
        vec![
            Mirror { header_offset: 0x14, value: self.be32(0x14), ghad_offset: Some(52), name: "Stage" },
            Mirror { header_offset: 0x1c, value: self.be32(0x1c), ghad_offset: Some(24), name: "Points" },
            Mirror { header_offset: 0x20, value: self.be32(0x20), ghad_offset: Some(4),  name: "FlightTime" },
            Mirror { header_offset: 0x24, value: self.be32(0x24), ghad_offset: Some(8),  name: "ClearRatio" },
            Mirror { header_offset: 0x28, value: self.be32(0x28), ghad_offset: Some(12), name: "TimesCleared" },
        ]
    }

    /// The three header fields derived from the payload — all that stands between
    /// a parse and a hand-written save the title will load.
    pub fn derived(&self) -> Vec<Mirror> {
        vec![
            Mirror { header_offset: 0x30, value: self.be32(0x30), ghad_offset: None, name: "deflate length + 10" },
            Mirror { header_offset: 0x8c, value: u32::from(u16::from_be_bytes([self.bytes.get(0x8c).copied().unwrap_or(0), self.bytes.get(0x8d).copied().unwrap_or(0)])), ghad_offset: None, name: "payload length" },
            Mirror { header_offset: 0x8e, value: self.be32(0x8e), ghad_offset: None, name: "adler32(payload)" },
        ]
    }
}

/// A fully parsed save.
#[derive(Debug, Clone)]
pub struct SaveGame {
    pub header: Header,
    /// The inflated payload, verbatim — the round-trip compares against this.
    pub payload: Vec<u8>,
    /// Current game phase, e.g. `GP_BUNK`. One of the title's screen ids.
    pub phase: String,
    /// The 122-byte progress block.
    pub ghad: Vec<u8>,
    /// The per-stage record table.
    pub records: Vec<StageRecord>,
    /// Trailer: `(name, tag, value)`.
    pub trailer: (String, String, u32),
}

/// What went wrong reading a save.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum SaveError {
    NotGdha,
    NoZlibStream,
    Inflate(String),
    NotGdaa,
    Truncated(&'static str),
    /// A chunk tag was not where the serializer puts it.
    BadTag { expected: &'static str, at: usize },
    /// Bytes left over after the trailer — the layout is supposed to close
    /// exactly, so this means the parse is wrong, not that the file has extras.
    TrailingBytes(usize),
}

impl std::fmt::Display for SaveError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::NotGdha => write!(f, "not a GDHA container"),
            Self::NoZlibStream => write!(f, "no zlib stream in the container"),
            Self::Inflate(e) => write!(f, "inflate failed: {e}"),
            Self::NotGdaa => write!(f, "payload is not GDAA"),
            Self::Truncated(what) => write!(f, "payload truncated reading {what}"),
            Self::BadTag { expected, at } => write!(f, "expected {expected} at {at:#x}"),
            Self::TrailingBytes(n) => write!(f, "{n} bytes left after the trailer"),
        }
    }
}

impl std::error::Error for SaveError {}

fn be32(b: &[u8], o: usize) -> Option<u32> {
    b.get(o..o + 4)
        .map(|s| u32::from_be_bytes([s[0], s[1], s[2], s[3]]))
}

/// Split a `GDHA` container into its header and inflated payload.
pub fn unwrap_container(raw: &[u8]) -> Result<(Header, Vec<u8>), SaveError> {
    if raw.len() < 4 || &raw[0..4] != b"GDHA" {
        return Err(SaveError::NotGdha);
    }
    let at = raw
        .windows(2)
        .position(|w| w == [0x78, 0xDA])
        .ok_or(SaveError::NoZlibStream)?;
    let mut out = Vec::new();
    flate2::read::ZlibDecoder::new(&raw[at..])
        .read_to_end(&mut out)
        .map_err(|e| SaveError::Inflate(e.to_string()))?;
    Ok((
        Header {
            bytes: raw[..at].to_vec(),
        },
        out,
    ))
}

/// Parse an inflated payload.
pub fn parse_payload(payload: &[u8]) -> Result<SaveGame, SaveError> {
    if payload.len() < 4 || &payload[0..4] != b"GDAA" {
        return Err(SaveError::NotGdaa);
    }
    let mut o = 4usize;
    let nlen = be32(payload, o).ok_or(SaveError::Truncated("phase length"))? as usize;
    o += 4;
    let phase = payload
        .get(o..o + nlen)
        .map(|s| String::from_utf8_lossy(s).to_string())
        .ok_or(SaveError::Truncated("phase name"))?;
    o += nlen;

    if payload.get(o..o + 4) != Some(b"GHAD") {
        return Err(SaveError::BadTag { expected: "GHAD", at: o });
    }
    o += 4;
    let ghad = payload
        .get(o..o + GHAD_SIZE)
        .ok_or(SaveError::Truncated("GHAD block"))?
        .to_vec();
    o += GHAD_SIZE;

    let count = be32(payload, o).ok_or(SaveError::Truncated("record count"))? as usize;
    o += 4;
    let mut records = Vec::with_capacity(count);
    for _ in 0..count {
        if payload.get(o..o + 4) != Some(b"SHAB") {
            return Err(SaveError::BadTag { expected: "SHAB", at: o });
        }
        o += 4;
        let mut v = [0u32; RECORD_FIELDS];
        for slot in v.iter_mut() {
            *slot = be32(payload, o).ok_or(SaveError::Truncated("record"))?;
            o += 4;
        }
        records.push(StageRecord {
            a: v[0],
            b: v[1],
            best_time_ms: v[2],
            time_hi: v[3],
            time_lo: v[4],
        });
    }

    let tlen = be32(payload, o).ok_or(SaveError::Truncated("trailer length"))? as usize;
    o += 4;
    let tname = payload
        .get(o..o + tlen)
        .map(|s| String::from_utf8_lossy(s).to_string())
        .ok_or(SaveError::Truncated("trailer name"))?;
    o += tlen;
    let ttag = payload
        .get(o..o + 4)
        .map(|s| String::from_utf8_lossy(s).to_string())
        .ok_or(SaveError::Truncated("trailer tag"))?;
    o += 4;
    let tval = be32(payload, o).ok_or(SaveError::Truncated("trailer value"))?;
    o += 4;
    if o != payload.len() {
        return Err(SaveError::TrailingBytes(payload.len() - o));
    }

    Ok(SaveGame {
        header: Header { bytes: Vec::new() },
        payload: payload.to_vec(),
        phase,
        ghad,
        records,
        trailer: (tname, ttag, tval),
    })
}

/// Parse a whole `savedata` file.
pub fn parse(raw: &[u8]) -> Result<SaveGame, SaveError> {
    let (header, payload) = unwrap_container(raw)?;
    let mut save = parse_payload(&payload)?;
    save.header = header;
    Ok(save)
}

impl SaveGame {
    /// Re-serialize the payload.
    ///
    /// This is the correctness check for the whole layout: the serializer writes
    /// the struct field-by-field with no packing, so a correct parse must
    /// reproduce the payload **byte for byte**. See [`SaveGame::round_trips`].
    pub fn serialize_payload(&self) -> Vec<u8> {
        let mut out = Vec::with_capacity(self.payload.len());
        out.extend_from_slice(b"GDAA");
        out.extend_from_slice(&(self.phase.len() as u32).to_be_bytes());
        out.extend_from_slice(self.phase.as_bytes());
        out.extend_from_slice(b"GHAD");
        out.extend_from_slice(&self.ghad);
        out.extend_from_slice(&(self.records.len() as u32).to_be_bytes());
        for r in &self.records {
            out.extend_from_slice(b"SHAB");
            for v in [r.a, r.b, r.best_time_ms, r.time_hi, r.time_lo] {
                out.extend_from_slice(&v.to_be_bytes());
            }
        }
        let (name, tag, val) = &self.trailer;
        out.extend_from_slice(&(name.len() as u32).to_be_bytes());
        out.extend_from_slice(name.as_bytes());
        out.extend_from_slice(tag.as_bytes());
        out.extend_from_slice(&val.to_be_bytes());
        out
    }

    /// Whether re-serializing reproduces the payload exactly.
    pub fn round_trips(&self) -> bool {
        self.serialize_payload() == self.payload
    }

    /// Read a GHAD field as an unsigned integer. `None` for the raw/blob fields.
    pub fn ghad_value(&self, spec: &FieldSpec) -> Option<u64> {
        match spec.size {
            4 => be32(&self.ghad, spec.offset).map(u64::from),
            8 => self
                .ghad
                .get(spec.offset..spec.offset + 8)
                .map(|s| u64::from_be_bytes(s.try_into().unwrap())),
            _ => None,
        }
    }

    /// Raw bytes of a GHAD field.
    pub fn ghad_bytes(&self, spec: &FieldSpec) -> &[u8] {
        self.ghad
            .get(spec.offset..spec.offset + spec.size)
            .unwrap_or(&[])
    }

    /// The per-item Arsenal development state.
    ///
    /// Index space is `strings.tbl`'s item order — the Arsenal's display order
    /// *plus* the cut items only the localisation file lists — closing at 53.
    /// `weapon.tbl`'s id list is **not** the index space; that it is also 54 long
    /// is a coincidence, and the two agree only to index 32.
    pub fn develop_state(&self) -> Vec<DevelopState> {
        self.ghad
            .get(68..68 + DEVELOP_BLOB_LEN)
            .map(|b| b.iter().copied().map(DevelopState::from_byte).collect())
            .unwrap_or_default()
    }

    /// Convenience: the 1-based stage number (GHAD `+52`).
    pub fn stage(&self) -> Option<u32> {
        be32(&self.ghad, 52)
    }

    /// Convenience: the spendable points balance (GHAD `+24`).
    pub fn points(&self) -> Option<u32> {
        be32(&self.ghad, 24)
    }

    /// Convenience: total flight time in milliseconds (GHAD `+4`).
    pub fn flight_time_ms(&self) -> Option<u32> {
        be32(&self.ghad, 4)
    }

    /// Convenience: clear ratio in percent (GHAD `+8`).
    pub fn clear_ratio_pct(&self) -> Option<u32> {
        be32(&self.ghad, 8)
    }
}

/// Format a millisecond count the way the game's panels do (`m:ss.mmm`).
pub fn fmt_millis(ms: u32) -> String {
    let (m, rem) = (ms / 60_000, ms % 60_000);
    format!("{}:{:02}.{:03}", m, rem / 1000, rem % 1000)
}

#[cfg(test)]
mod tests {
    use super::*;

    /// Build a payload with the exact chunk stream the serializer writes.
    fn synth_payload(phase: &str, stage: u32, points: u32) -> Vec<u8> {
        let mut ghad = vec![0u8; GHAD_SIZE];
        ghad[4..8].copy_from_slice(&324_773u32.to_be_bytes()); // flight time
        ghad[8..12].copy_from_slice(&5u32.to_be_bytes()); // clear ratio
        ghad[24..28].copy_from_slice(&points.to_be_bytes());
        ghad[52..56].copy_from_slice(&stage.to_be_bytes());
        ghad[68] = 4; // item 0 developed
        ghad[69] = 2; // item 1 developable
        let mut p = Vec::new();
        p.extend_from_slice(b"GDAA");
        p.extend_from_slice(&(phase.len() as u32).to_be_bytes());
        p.extend_from_slice(phase.as_bytes());
        p.extend_from_slice(b"GHAD");
        p.extend_from_slice(&ghad);
        p.extend_from_slice(&(RECORD_COUNT as u32).to_be_bytes());
        for i in 0..RECORD_COUNT {
            p.extend_from_slice(b"SHAB");
            let vals = if i == 0 {
                [2u32, 4101, 324_773, 0x01DC_0000, 0x1234_5678]
            } else {
                [0, 0, 0, 0, 0]
            };
            for v in vals {
                p.extend_from_slice(&v.to_be_bytes());
            }
        }
        p.extend_from_slice(&4u32.to_be_bytes());
        p.extend_from_slice(b"BUNK");
        p.extend_from_slice(b"NETA");
        p.extend_from_slice(&0x0915_0000u32.to_be_bytes());
        p
    }

    #[test]
    fn parses_the_chunk_stream() {
        let p = synth_payload("GP_BUNK", 2, 4101);
        let s = parse_payload(&p).unwrap();
        assert_eq!(s.phase, "GP_BUNK");
        assert_eq!(s.ghad.len(), GHAD_SIZE);
        assert_eq!(s.records.len(), RECORD_COUNT);
        assert_eq!(s.trailer, ("BUNK".into(), "NETA".into(), 0x0915_0000));
        assert_eq!(s.stage(), Some(2));
        assert_eq!(s.points(), Some(4101));
        assert_eq!(s.flight_time_ms(), Some(324_773));
        assert_eq!(s.clear_ratio_pct(), Some(5));
    }

    #[test]
    fn round_trips_byte_identically() {
        // The layout is closed, so a correct parse must reproduce the payload
        // exactly. This is the property the whole spec rests on.
        let p = synth_payload("GP_BUNK", 2, 4101);
        let s = parse_payload(&p).unwrap();
        assert!(s.round_trips());
        assert_eq!(s.serialize_payload(), p);
    }

    #[test]
    fn record_zero_is_stage_one() {
        let s = parse_payload(&synth_payload("GP_BUNK", 2, 4101)).unwrap();
        assert!(s.records[0].is_used());
        assert!(!s.records[1].is_used());
        assert_eq!(s.records[0].best_time_ms, 324_773);
        assert!(s.records[0].filetime_ticks().is_some());
        assert!(s.records[1].filetime_ticks().is_none());
    }

    #[test]
    fn develop_blob_alphabet() {
        let s = parse_payload(&synth_payload("GP_BUNK", 2, 4101)).unwrap();
        let d = s.develop_state();
        assert_eq!(d.len(), DEVELOP_BLOB_LEN);
        assert_eq!(d[0], DevelopState::Developed);
        assert_eq!(d[1], DevelopState::Developable);
        assert_eq!(d[2], DevelopState::Locked);
    }

    #[test]
    fn ghad_layout_is_closed() {
        // 10*4 + 8 + 4*4 + 4 + 54 = 122, exactly what the file carries. If this
        // ever fails the layout has grown a gap and the round-trip is a lie.
        let total: usize = GHAD_LAYOUT.iter().map(|f| f.size).sum();
        assert_eq!(total, GHAD_SIZE);
        let mut at = 0;
        for f in GHAD_LAYOUT {
            assert_eq!(f.offset, at, "gap before {:#x}", f.offset);
            at += f.size;
        }
    }

    #[test]
    fn rejects_a_non_save() {
        assert_eq!(parse(b"not a save at all").unwrap_err(), SaveError::NotGdha);
        assert_eq!(parse_payload(b"XXXX").unwrap_err(), SaveError::NotGdaa);
    }

    #[test]
    fn trailing_bytes_are_an_error_not_a_shrug() {
        let mut p = synth_payload("GP_BUNK", 2, 4101);
        p.push(0);
        assert_eq!(parse_payload(&p).unwrap_err(), SaveError::TrailingBytes(1));
    }

    #[test]
    fn millis_format_matches_the_panel() {
        assert_eq!(fmt_millis(324_773), "5:24.773");
    }
}