//! The asset containers under `G/` on the disc.
//!
//! Each `G/*.WAD` file begins with a typed header whose only universally
//! applicable fields are:
//!
//! ```text
//! offset  size  field
//! 0x00    4     header size in bytes
//! 0x04    4     sector count of this container
//! 0x08..  ...   a table of {offset, size} pairs, both in SECTORS
//! ```
//!
//! The pair table is the important part. Sectors chain: entry `n`'s offset plus
//! its size is entry `n+1`'s offset, which is how the extractor recovers the
//! layout without a per-container parser. The pairs begin at `0x08` and run to
//! `header_size`, giving `(header_size - 8) / 8` entries.
//!
//! The payload behind each pair is **not** WAD-compressed. Insomniac's WAD LZ77
//! container is used elsewhere on the disc (inside level cores and MPEG
//! streams); these global containers store their sections raw and let the
//! table do the addressing.
//!
//! Which pair means what depends on the container — `ARMOR.WAD` is nine armor
//! entries then six wrench entries, each with a mesh and a texture range — but
//! the geometry above holds for every global on this disc, so the extractor
//! stays generic and per-container meaning is recovered later.
//!
//! Spec: `docs/formats/global-wad.md`.

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

/// Disc logical block size. All container offsets are in these units.
pub const SECTOR_SIZE: u32 = 2048;

/// A `{offset, size}` pair, both in sectors.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct SectorRange {
    pub offset: i32,
    pub size: i32,
}

impl SectorRange {
    /// Byte offset of the extent. `None` when the range is absent.
    pub fn byte_offset(&self) -> Option<u64> {
        if self.size <= 0 || self.offset < 0 {
            return None;
        }
        Some(self.offset as u64 * SECTOR_SIZE as u64)
    }

    /// Byte length of the extent.
    pub fn byte_len(&self) -> u64 {
        if self.size <= 0 {
            0
        } else {
            self.size as u64 * SECTOR_SIZE as u64
        }
    }

    pub fn is_present(&self) -> bool {
        self.size > 0
    }
}

/// A `{offset, size}` pair where the offset is in sectors and the size is in
/// **bytes**.
///
/// The MPEG containers use this form: their entries are video and subtitle
/// streams whose length is not sector-aligned, so a sector count would round
/// and lose the tail. See `docs/formats/global-wad.md`.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct SectorByteRange {
    pub offset: i32,
    pub size: i32,
}

impl SectorByteRange {
    pub fn byte_offset(&self) -> Option<u64> {
        if self.size <= 0 || self.offset < 0 {
            return None;
        }
        Some(self.offset as u64 * SECTOR_SIZE as u64)
    }

    pub fn byte_len(&self) -> u64 {
        if self.size <= 0 {
            0
        } else {
            self.size as u64
        }
    }

    pub fn is_present(&self) -> bool {
        self.size > 0
    }
}

/// How a container's table should be read.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum RangeKind {
    /// Both fields are sector counts.
    Sectors,
    /// Offset in sectors, size in bytes. Used by every indexed container on
    /// this disc except the scene containers.
    SectorBytes,
    /// A flat array of sector numbers, not pairs. The scene containers are
    /// this: each value points at a sector, and the length of each extent comes
    /// from the difference to the next entry. The last entry is bounded by the
    /// end of the container.
    SectorList,
}

impl RangeKind {
    /// Is this container's table a flat list rather than pairs?
    pub fn is_list(&self) -> bool {
        matches!(self, RangeKind::SectorList)
    }
}

impl RangeKind {
    /// Classify a container from its header size.
    ///
    /// On this disc **every** indexed container uses the sector/byte form: a
    /// table of `{offset in sectors, size in bytes}` pairs. This was measured
    /// rather than assumed. For all 96 containers that carry a pair table, the
    /// sector/byte reading places every entry inside the file; the
    /// sector/sector reading leaves entries running past the end of the file,
    /// and on the 27 level containers and the 27 audio containers it places
    /// *none* of them correctly.
    ///
    /// The header size is the reliable discriminator because it is the field
    /// every container agrees on, and it distinguishes the level containers
    /// (0x60) from audio (0x1018), scene (0x137c) and the rest.
    pub fn for_header(header_size: u32) -> RangeKind {
        match header_size {
            // The per-level scene containers hold a flat sector list rather
            // than pairs: 0x137c is the header size the 27 SCENE<n>.WAD files
            // share, and (0x137c - 8) is not a multiple of 8, so the bytes
            // cannot be read as pairs at all.
            0x137c => RangeKind::SectorList,
            // No container on this disc uses the sector/sector form. Kept so a
            // future disc revision can be checked rather than assumed.
            _ => RangeKind::SectorBytes,
        }
    }

    /// Classify by ISO file name, for callers that only have the path.
    pub fn for_container(name: &str) -> RangeKind {
        let stem = name.rsplit('/').next().unwrap_or(name);
        let stem = stem.trim_end_matches(".WAD").to_ascii_uppercase();
        if stem.starts_with("SCENE")
            && stem.len() > 5
            && stem[5..].chars().all(|c| c.is_ascii_digit())
        {
            // SCENE0..SCENE26 are the per-level containers. SCENE.WAD (no
            // digits) is the global one and does use pairs.
            return RangeKind::SectorList;
        }
        RangeKind::SectorBytes
    }
}

/// A parsed global container header.
#[derive(Debug, Clone)]
pub struct GlobalWad {
    pub header_size: u32,
    /// Declared sector count of the whole container.
    pub sector_count: u32,
    /// The pair table, in declaration order.
    pub entries: Vec<SectorRange>,
    /// How the pair table was read.
    pub kind: RangeKind,
}

impl GlobalWad {
    /// Parse a container header, reading the table as sector-count pairs.
    pub fn parse(bytes: &[u8]) -> Result<GlobalWad> {
        GlobalWad::parse_with(bytes, RangeKind::SectorBytes)
    }

    /// Parse a container header with an explicit range interpretation.
    ///
    /// Both interpretations read the same 8 bytes per entry; they differ only in
    /// how the size field is scaled, which is a property of the container
    /// rather than of the bytes.
    pub fn parse_with(bytes: &[u8], kind: RangeKind) -> Result<GlobalWad> {
        let mut b = Buf::new(bytes);
        let header_size = b.u32("container header size")? as usize;
        let sector_count = b.u32("container sector count")?;

        if header_size < 8 || header_size > bytes.len() {
            return Err(Error {
                offset: 0,
                context: "container header",
                kind: ErrorKind::InvalidLength("container header size"),
            });
        }
        // A pair table must divide evenly into 8-byte entries; a remainder
        // means we have guessed the layout wrong and should not read garbage.
        // A sector list is a flat run of 4-byte values and needs no such check.
        let table_bytes = header_size - 8;
        if !kind.is_list() && table_bytes % 8 != 0 {
            return Err(Error {
                offset: 0,
                context: "container header",
                kind: ErrorKind::Invalid("pair table is not a whole number of entries"),
            });
        }

        let mut entries = Vec::with_capacity(table_bytes / 8);
        if kind.is_list() {
            // Each value is the first sector of one extent; the size runs to the
            // next value, and the last runs to the end of the container.
            let mut sectors = Vec::with_capacity(table_bytes / 4);
            while b.pos() + 4 <= header_size {
                sectors.push(b.i32("extent sector")?);
            }
            let total_sectors = (bytes.len() as u32).div_ceil(SECTOR_SIZE);
            for (i, &start) in sectors.iter().enumerate() {
                let end = sectors.get(i + 1).copied().unwrap_or(total_sectors as i32);
                entries.push(SectorRange {
                    offset: start,
                    size: end - start,
                });
            }
        } else {
            while b.pos() + 8 <= header_size {
                let offset = b.i32("entry offset")?;
                let size = b.i32("entry size")?;
                entries.push(SectorRange { offset, size });
            }
        }

        Ok(GlobalWad {
            header_size: header_size as u32,
            sector_count,
            entries,
            kind,
        })
    }

    /// Entries that point at real data, as `(index, byte offset, byte length)`.
    pub fn present(&self) -> impl Iterator<Item = (usize, u64, u64)> + '_ {
        self.entries
            .iter()
            .copied()
            .enumerate()
            .filter_map(|(i, r)| {
                let len = match self.kind {
                    RangeKind::Sectors => r.byte_len(),
                    RangeKind::SectorList => r.byte_len(),
                    RangeKind::SectorBytes => SectorByteRange {
                        offset: r.offset,
                        size: r.size,
                    }
                    .byte_len(),
                };
                let off = r.byte_offset()?;
                if len == 0 {
                    return None;
                }
                Some((i, off, len))
            })
    }
}

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

    /// Build a synthetic container: header_size, sector_count, then pairs.
    fn synthetic(entries: &[(i32, i32)]) -> Vec<u8> {
        let header_size = (8 + entries.len() * 8) as u32;
        let mut v = Vec::new();
        v.extend_from_slice(&header_size.to_le_bytes());
        v.extend_from_slice(&100u32.to_le_bytes());
        for (o, s) in entries {
            v.extend_from_slice(&o.to_le_bytes());
            v.extend_from_slice(&s.to_le_bytes());
        }
        // Pad the body so byte_offset() reads land inside the buffer.
        v.resize(header_size as usize + 4096, 0);
        v
    }

    #[test]
    fn parses_header_and_pairs() {
        let wad = GlobalWad::parse(&synthetic(&[(1, 72), (73, 51), (124, 80)])).unwrap();
        assert_eq!(wad.header_size, 32);
        assert_eq!(wad.sector_count, 100);
        assert_eq!(wad.entries.len(), 3);
        assert_eq!(
            wad.entries[0],
            SectorRange {
                offset: 1,
                size: 72
            }
        );
        assert_eq!(wad.entries[2].offset, 124);
    }

    #[test]
    fn sector_ranges_convert_to_bytes() {
        let r = SectorRange { offset: 3, size: 2 };
        assert_eq!(r.byte_offset(), Some(3 * 2048));
        assert_eq!(r.byte_len(), 2 * 2048);
        assert!(r.is_present());
    }

    #[test]
    fn absent_ranges_are_not_present() {
        assert!(!SectorRange { offset: 0, size: 0 }.is_present());
        assert!(!SectorRange {
            offset: 5,
            size: -1
        }
        .is_present());
        assert_eq!(SectorRange { offset: 5, size: 0 }.byte_offset(), None);
        assert_eq!(
            SectorRange {
                offset: -5,
                size: 10
            }
            .byte_offset(),
            None
        );
    }

    #[test]
    fn present_filters_zero_sizes() {
        let wad = GlobalWad::parse(&synthetic(&[(1, 4), (5, 0), (5, 4)])).unwrap();
        let present: Vec<usize> = wad.present().map(|(i, _, _)| i).collect();
        assert_eq!(present, vec![0, 2]);
    }

    #[test]
    fn present_scales_sizes_by_the_range_kind() {
        let bytes = synthetic(&[(1, 100)]);

        // The default for this disc treats the second field as a byte count.
        let raw = GlobalWad::parse(&bytes).unwrap();
        assert_eq!(raw.present().next().unwrap().2, 100);

        // The sector-count form scales it up.
        let sectors = GlobalWad::parse_with(&bytes, RangeKind::Sectors).unwrap();
        assert_eq!(sectors.present().next().unwrap().2, 100 * 2048);
    }

    #[test]
    fn rejects_a_header_size_beyond_the_buffer() {
        let mut v = 9999u32.to_le_bytes().to_vec();
        v.resize(16, 0);
        assert!(GlobalWad::parse(&v).is_err());
    }

    #[test]
    fn rejects_a_header_size_below_the_minimum() {
        let v = 4u32.to_le_bytes().to_vec();
        assert!(GlobalWad::parse(&v).is_err());
    }

    #[test]
    fn rejects_a_table_that_does_not_divide_evenly() {
        // header_size 13 -> table of 5 bytes, not a multiple of 8.
        let mut v = 13u32.to_le_bytes().to_vec();
        v.extend_from_slice(&0u32.to_le_bytes());
        v.extend_from_slice(&[0u8; 5]);
        let err = GlobalWad::parse(&v).unwrap_err();
        assert!(matches!(err.kind, ErrorKind::Invalid(_)), "got {err:?}");
    }

    #[test]
    fn empty_table_is_allowed() {
        let mut v = 8u32.to_le_bytes().to_vec();
        v.extend_from_slice(&10u32.to_le_bytes());
        v.resize(4096, 0);
        let wad = GlobalWad::parse(&v).unwrap();
        assert!(wad.entries.is_empty());
    }
}