//! Shrub geometry: the class header.
//!
//! Spec: `docs/plan/level-assets.md`.
//!
//! Shrub is the third class-geometry family, and it is not the moby one. All
//! 358 shrub spans on the disc have a leading word of 0 or one too large to
//! index a table into, so none of them contain a moby-style part table -- the
//! rule `vertex_offset + 16 * vertex_count == next_part`, which holds for
//! 43,467 of 43,467 moby parts, holds for **zero** of them.
//!
//! What the shrub header *does* have is two fields that self-identify exactly,
//! on all 358 spans:
//!
//! - `+0x24` equals the owning class entry's own `o_class`.
//! - `+0x2c` is a data offset that lands inside the span.
//!
//! Those two are what this parses. The body past the data offset is not
//! decoded: the run of bytes there does not tile at any stride that divides the
//! tail consistently with the count at `+0x28`.
//!
//! Note the class entry size differs here too: shrub entries are 0x30 bytes,
//! not 0x20, because they carry billboard texture fields the other two tables
//! do not have.

use crate::buf::{Buf, Error, ErrorKind, Result};

/// The measured size of a shrub class entry.
pub const SHRUB_CLASS_ENTRY_BYTES: usize = 0x30;

/// The decoded part of a shrub class header.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct ShrubHeader {
    /// Four floats at `+0x00..+0x10`. All finite on all 358 spans, but they do
    /// not behave like a `min.xyz / max.xyz` bounding box -- that ordering only
    /// holds on 146 of 358. Not named because it is not established.
    pub bounds: [f32; 4],
    /// A positive float at `+0x10`, 3.0 to 20.0 across the disc.
    pub size: f32,
    /// A float at `+0x14`. Exactly 0.0 on 342 of the disc's 358 spans; the
    /// other 16 hold the bit pattern `04000000`, a denormal of about 2.8e-45.
    ///
    /// The Emotion Engine has no denormals, so hardware reads all 358 of these
    /// as zero. That is the only reading the game itself ever sees, and it is
    /// why this is safe to treat as a flag rather than a magnitude -- see
    /// `AGENTS.md` on reproducing the FPU model rather than correcting it.
    pub reserved: f32,
    /// The class this geometry belongs to; equals the class entry's
    /// `o_class` on all 358 spans on the disc.
    pub o_class: u32,
    /// A count. Meaning not established.
    pub count: u32,
    /// Offset of the geometry data from the start of the span. Lands inside
    /// the span on all 358 spans on the disc.
    pub data_offset: u32,
}

/// Bytes per body record.
///
/// Each record is 6 varying bytes followed by 2 constant ones. Found by
/// grouping sibling spans and byte-diffing them rather than by stride-fitting,
/// which failed three times: the varying payload is **not** sized by the
/// `count` field, and was a constant 144 bytes per group whether `count` was
/// 1, 2, 3, 5 or 6.
pub const SHRUB_BODY_RECORD_BYTES: usize = 8;

/// The two constant bytes closing each body record.
pub const SHRUB_RECORD_TAIL: [u8; 2] = [0, 0];

/// A shrub span: its header plus the bytes the header points at.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct Shrub<'a> {
    pub header: ShrubHeader,
    /// The whole span, starting at `+0x24` with the class number.
    pub raw: &'a [u8],
    /// The bytes at `data_offset`, running to `span_end`. Not yet decoded.
    pub data: &'a [u8],
    /// Whether `span_end` came from the next class rather than from the end of
    /// the lump. False for the last class in each level, whose span contains
    /// whatever follows shrub.
    pub span_end_is_a_successor: bool,
}

impl<'a> Shrub<'a> {
    /// The body's 8-byte records.
    ///
    /// The payload of each record is six bytes whose encoding is **not**
    /// established. It is not half-floats -- decoding as a `half3` gives
    /// values like 245.25 and -168.375 -- and a VIF reading of it was
    /// retracted after a control put it on the random baseline. Handed back
    /// raw rather than guessed at.
    pub fn body_records(&self) -> impl Iterator<Item = &'a [u8]> {
        self.data.chunks_exact(SHRUB_BODY_RECORD_BYTES)
    }

    /// How many whole records the body holds.
    pub fn body_record_count(&self) -> usize {
        self.data.len() / SHRUB_BODY_RECORD_BYTES
    }

    /// Whether every whole record ends in the constant pair.
    ///
    /// This is how a class's body is delimited: on all 24 levels that have
    /// shrub geometry, the last shrub class's body ends exactly where its
    /// trailing `00 00` pair stops, always on a record boundary.
    pub fn body_is_well_formed(&self) -> bool {
        let records: Vec<&[u8]> = self.body_records().collect();
        !records.is_empty() && records.iter().all(|r| r[6..8] == SHRUB_RECORD_TAIL)
    }
}

impl ShrubHeader {
    /// Parse a shrub span starting at `offset` within `core_data`.
    ///
    /// `o_class` is passed in so the self-identifying field can be checked
    /// rather than merely reported: a span whose `+0x24` disagrees with its
    /// own class entry is a different thing entirely, and quietly returning
    /// the mismatched value would hide that.
    /// `span_end` bounds the class span within `core`. It matters: without it
    /// `data` runs to the end of the lump and swallows whatever follows, which
    /// is how a per-class check ends up measuring its neighbours.
    pub fn region<'a>(
        core: &'a [u8],
        offset: i32,
        o_class: i32,
        span_end: usize,
    ) -> Result<Option<Shrub<'a>>> {
        if offset <= 0 {
            return Ok(None);
        }
        let start = offset as usize;
        let head = core.get(start..).filter(|s| s.len() >= 0x30).ok_or(Error {
            offset: start,
            context: "shrub span is too short to hold a header",
            kind: ErrorKind::OutOfBounds {
                needed: 0x30,
                available: core.len().saturating_sub(start),
            },
        })?;
        let mut fb = Buf::new(&head[..16]);
        let bounds = [
            fb.f32("shrub bounds 0")?,
            fb.f32("shrub bounds 1")?,
            fb.f32("shrub bounds 2")?,
            fb.f32("shrub bounds 3")?,
        ];
        let mut fs = Buf::new(&head[0x10..0x18]);
        let size = fs.f32("shrub size")?;
        let reserved = fs.f32("shrub reserved")?;
        if !bounds.iter().all(|v| v.is_finite()) || !size.is_finite() || !reserved.is_finite() {
            return Err(Error {
                offset: start,
                context: "shrub header holds a non-finite float",
                kind: ErrorKind::Invalid("shrub header float"),
            });
        }
        // Zero, or the denormal the PS2 would flush to zero anyway. Anything
        // else means the header is not laid out the way this thinks it is.
        if reserved != 0.0 && !reserved.is_subnormal() {
            return Err(Error {
                offset: start + 0x14,
                context: "shrub reserved word is neither zero nor denormal",
                kind: ErrorKind::Invalid("shrub reserved"),
            });
        }

        let mut b = Buf::new(&head[0x24..]);
        let span_class = b.u32("shrub o_class")?;
        let header = ShrubHeader {
            bounds: [0.0; 4],
            size: 0.0,
            reserved: 0.0,
            o_class: span_class,
            count: b.u32("shrub count")?,
            data_offset: b.u32("shrub data offset")?,
        };

        if header.o_class != o_class.max(0) as u32 {
            return Err(Error {
                offset: start + 0x24,
                context: "shrub span does not name its own class",
                kind: ErrorKind::Invalid("shrub o_class disagrees with the class entry"),
            });
        }

        let at = header.data_offset as usize;
        let stop = span_end.clamp(start, core.len());
        let data = core.get(start + at..stop).ok_or(Error {
            offset: start + at,
            context: "shrub data offset runs past the lump",
            kind: ErrorKind::OutOfBounds {
                needed: at,
                available: core.len().saturating_sub(start),
            },
        })?;
        Ok(Some(Shrub {
            span_end_is_a_successor: true,
            header: ShrubHeader {
                bounds,
                size,
                reserved,
                o_class: header.o_class,
                count: header.count,
                data_offset: header.data_offset,
            },
            raw: &core[start..],
            data,
        }))
    }
}

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

    /// Where the span sits in the fake lump. Offset 0 means "absent", so the
    /// span cannot start there.
    const BASE: usize = 0x10;

    /// A span whose `+0x24` names class 1072 and whose data starts at 0x30.
    fn span() -> Vec<u8> {
        let mut v = vec![0u8; BASE + 0x40];
        v[BASE + 0x24..BASE + 0x28].copy_from_slice(&1072u32.to_le_bytes());
        v[BASE + 0x28..BASE + 0x2c].copy_from_slice(&5u32.to_le_bytes());
        v[BASE + 0x2c..BASE + 0x30].copy_from_slice(&0x30u32.to_le_bytes());
        v[BASE + 0x30..].fill(0xab);
        v
    }

    #[test]
    fn reads_the_self_identifying_header() {
        let c = span();
        let s = ShrubHeader::region(&c, BASE as i32, 1072, c.len())
            .unwrap()
            .expect("present");
        assert_eq!(s.header.o_class, 1072);
        assert_eq!(s.header.count, 5);
        assert_eq!(s.header.data_offset, 0x30);
        assert!(s.data.iter().all(|&b| b == 0xab));
    }

    #[test]
    fn a_span_naming_the_wrong_class_is_rejected() {
        let c = span();
        let err = ShrubHeader::region(&c, BASE as i32, 999, c.len()).unwrap_err();
        assert!(format!("{err:?}").contains("disagrees"));
    }

    #[test]
    fn a_zero_offset_means_absent_not_present() {
        let c = span();
        assert!(ShrubHeader::region(&c, 0, 1072, c.len()).unwrap().is_none());
        assert!(ShrubHeader::region(&c, -1, 1072, c.len())
            .unwrap()
            .is_none());
    }

    #[test]
    fn body_records_split_into_eights_and_check_their_tail() {
        let c = span();
        let s = ShrubHeader::region(&c, BASE as i32, 1072, c.len())
            .unwrap()
            .expect("present");
        // The fixture fills 0xab, so every record's tail is wrong.
        assert!(!s.body_is_well_formed());
        assert_eq!(s.body_record_count(), (0x40 - 0x30) / 8);
        // Fix the tails and it is well formed.
        let mut c2 = span();
        for i in 0..((0x40 - 0x30) / 8) {
            c2[BASE + 0x30 + i * 8 + 6] = 0;
            c2[BASE + 0x30 + i * 8 + 7] = 0;
        }
        let s2 = ShrubHeader::region(&c2, BASE as i32, 1072, c2.len())
            .unwrap()
            .expect("present");
        assert!(s2.body_is_well_formed());
        assert_eq!(s2.body_records().count(), s2.body_record_count());
    }

    #[test]
    fn a_body_shorter_than_one_record_is_not_well_formed() {
        let mut c = span();
        // Point the data at the last four bytes of the span.
        c[BASE + 0x2c..BASE + 0x30].copy_from_slice(&(0x3cu32).to_le_bytes());
        let s = ShrubHeader::region(&c, BASE as i32, 1072, c.len())
            .unwrap()
            .expect("present");
        assert_eq!(s.body_record_count(), 0);
        assert!(!s.body_is_well_formed());
    }

    #[test]
    fn out_of_range_is_an_error_not_a_panic() {
        // Offset 0 is absent, not an error -- an empty lump is just empty.
        assert!(ShrubHeader::region(&[], 0, 1, 0).unwrap().is_none());
        assert!(ShrubHeader::region(&[], 4, 1, 8).is_err());
        // Data offset past the end of the lump.
        let mut c = span();
        c[BASE + 0x2c..BASE + 0x30].copy_from_slice(&9_000u32.to_le_bytes());
        assert!(ShrubHeader::region(&c, BASE as i32, 1072, c.len()).is_err());
        // Span starting near the end, with no room for a header.
        assert!(ShrubHeader::region(&c, (c.len() - 4) as i32, 1072, c.len()).is_err());
    }
}