//! Level textures: 8-bit palette indices plus a 256-entry CLUT.
//!
//! Spec: `docs/formats/texture.md`.
//!
//! Every level texture is a rectangular grid of **palette indices**, not
//! pixels: `width * height` bytes at `core_data[textures_base + data_offset]`.
//! The colour comes from a CLUT of 256 RGBA entries (1024 bytes) in GS RAM at
//! `palette * 0x100`. Both halves are indexed by the same byte, so nothing
//! about the layout is inferred — it is a straight lookup.
//!
//! Two conventions have to be respected, and both are PS2 hardware details
//! rather than choices:
//!
//! * **CSM1 CLUT order.** Within each 32-entry group the two middle 8-entry
//!   blocks are swapped. The remap is an involution, so the same function
//!   converts either way.
//! * **Alpha runs 0..=0x80, not 0..=255.** `0x80` is fully opaque, and
//!   anything above it clamps to opaque.

use crate::buf::{Error, ErrorKind, Result};
use crate::level_core::TextureEntry;

/// PS2 CSM1 CLUT order to linear: within each 32-entry group, swap the two
/// middle 8-entry blocks. An involution.
pub fn clut_index(i: u32) -> u32 {
    if ((i >> 3) & 1) != ((i >> 4) & 1) {
        i ^ 0x18
    } else {
        i
    }
}

/// PS2 alpha 0..=0x80 to 0..=0xff. `0x80` is opaque; above that clamps.
pub fn scale_alpha(a: u8) -> u8 {
    if a < 0x80 {
        a * 2
    } else {
        0xff
    }
}

/// A decoded RGBA8 image.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Texture {
    pub width: u32,
    pub height: u32,
    /// `width * height * 4` bytes, row-major from the top.
    pub rgba: Vec<u8>,
}

/// One stored image: palette indices plus the CLUT, exactly as on disc.
#[derive(Debug, Clone, Copy)]
pub struct IndexedImage<'a> {
    pub width: u32,
    pub height: u32,
    pub indices: &'a [u8],
    /// 1024 bytes, 256 RGBA entries in CSM1 order.
    pub clut: &'a [u8],
}

impl IndexedImage<'_> {
    /// Look every index up in the CLUT.
    pub fn decode(&self) -> Result<Texture> {
        decode_indexed8(self.indices, self.width, self.height, self.clut)
    }
}

/// Decode `width * height` palette indices against a 256-entry CLUT.
pub fn decode_indexed8(indices: &[u8], width: u32, height: u32, clut: &[u8]) -> Result<Texture> {
    let n = width as usize * height as usize;
    if indices.len() < n {
        return Err(Error {
            offset: 0,
            context: "indexed texture data too small",
            kind: ErrorKind::OutOfBounds {
                needed: n,
                available: indices.len(),
            },
        });
    }
    if clut.len() < 1024 {
        return Err(Error {
            offset: 0,
            context: "texture palette too small",
            kind: ErrorKind::OutOfBounds {
                needed: 1024,
                available: clut.len(),
            },
        });
    }
    let mut pal = [[0u8; 4]; 256];
    for (i, p) in pal.iter_mut().enumerate() {
        let at = clut_index(i as u32) as usize * 4;
        let e = &clut[at..at + 4];
        *p = [e[0], e[1], e[2], scale_alpha(e[3])];
    }
    let mut rgba = Vec::with_capacity(n * 4);
    for &ix in &indices[..n] {
        rgba.extend_from_slice(&pal[ix as usize]);
    }
    Ok(Texture {
        width,
        height,
        rgba,
    })
}

/// The base level of a tfrag texture: `width * height` indices at `data_offset`.
pub fn base_image<'a>(
    textures: &'a [u8],
    gs_ram: &'a [u8],
    e: &TextureEntry,
) -> Result<IndexedImage<'a>> {
    let (px, clut) = pixels_and_clut(textures, gs_ram, e)?;
    let (w, h) = (e.width as usize, e.height as usize);
    Ok(IndexedImage {
        width: w as u32,
        height: h as u32,
        indices: px,
        clut,
    })
}

/// Every mip level of a tfrag texture, base level first.
///
/// All tfrag textures are square, so the levels halve evenly. Levels 0 and 1
/// live in the texture block; levels 2 and 3 are resident in GS RAM and are
/// addressed by the entry's `mipmap` and `pad` fields.
pub fn mip_images<'a>(
    textures: &'a [u8],
    gs_ram: &'a [u8],
    e: &TextureEntry,
) -> Result<Vec<IndexedImage<'a>>> {
    let (px0, clut) = pixels_and_clut(textures, gs_ram, e)?;
    let (w, h) = (e.width as usize, e.height as usize);
    let levels = e.ty as usize;
    if !(1..=4).contains(&levels) {
        return Err(Error {
            offset: 0,
            context: "tfrag texture level count outside 1..=4",
            kind: ErrorKind::InvalidLength("tfrag texture level count"),
        });
    }
    let sub = |data: &'a [u8], at: usize, need: usize, what: &'static str| -> Result<&'a [u8]> {
        let end = at
            .checked_add(need)
            .filter(|&e| e <= data.len())
            .ok_or(Error {
                offset: at,
                context: what,
                kind: ErrorKind::OutOfBounds {
                    needed: need,
                    available: data.len().saturating_sub(at),
                },
            })?;
        Ok(&data[at..end])
    };
    let mut out = Vec::with_capacity(levels);
    for level in 0..levels {
        let (lw, lh) = (w >> level, h >> level);
        if lw == 0 || lh == 0 {
            return Err(Error {
                offset: 0,
                context: "mip level smaller than one texel",
                kind: ErrorKind::InvalidLength("mip level size"),
            });
        }
        let need = lw * lh;
        let indices = match level {
            0 => px0,
            1 => sub(
                textures,
                e.data_offset as usize + w * h,
                need,
                "mip 1 pixels",
            )?,
            2 if e.mipmap >= 0 => sub(gs_ram, e.mipmap as usize * 0x100, need, "mip 2 pixels")?,
            3 if e.pad >= 0 => sub(gs_ram, e.pad as usize * 0x100, need, "mip 3 pixels")?,
            _ => {
                return Err(Error {
                    offset: 0,
                    context: "mip level without a gs_ram block",
                    kind: ErrorKind::InvalidLength("mip level"),
                })
            }
        };
        out.push(IndexedImage {
            width: lw as u32,
            height: lh as u32,
            indices,
            clut,
        });
    }
    Ok(out)
}

/// Decode the whole mip chain.
pub fn decode_tfrag_mips(textures: &[u8], gs_ram: &[u8], e: &TextureEntry) -> Result<Vec<Texture>> {
    mip_images(textures, gs_ram, e)?
        .iter()
        .map(IndexedImage::decode)
        .collect()
}

fn pixels_and_clut<'a>(
    textures: &'a [u8],
    gs_ram: &'a [u8],
    e: &TextureEntry,
) -> Result<(&'a [u8], &'a [u8])> {
    if e.width <= 0 || e.height <= 0 || e.data_offset < 0 || e.palette < 0 {
        return Err(Error {
            offset: 0,
            context: "texture entry has no pixels",
            kind: ErrorKind::Invalid("texture entry has no pixels"),
        });
    }
    let need = e.width as usize * e.height as usize;
    let end = (e.data_offset as usize)
        .checked_add(need)
        .filter(|&x| x <= textures.len())
        .ok_or(Error {
            offset: e.data_offset as usize,
            context: "texture pixels run past the texture block",
            kind: ErrorKind::OutOfBounds {
                needed: need,
                available: textures.len().saturating_sub(e.data_offset.max(0) as usize),
            },
        })?;
    let clut_at = e.palette as usize * 0x100;
    let clut_end = clut_at
        .checked_add(1024)
        .filter(|&x| x <= gs_ram.len())
        .ok_or(Error {
            offset: clut_at,
            context: "texture palette runs past gs_ram",
            kind: ErrorKind::OutOfBounds {
                needed: 1024,
                available: gs_ram.len().saturating_sub(clut_at),
            },
        })?;
    Ok((
        &textures[e.data_offset as usize..end],
        &gs_ram[clut_at..clut_end],
    ))
}

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

    #[test]
    fn clut_order_swaps_the_middle_blocks() {
        // Bits 3 and 4 differ inside a 32-entry group, so the remap applies.
        assert_eq!(clut_index(0x00), 0x00);
        assert_eq!(clut_index(0x08), 0x10);
        assert_eq!(clut_index(0x10), 0x08);
        assert_eq!(clut_index(0x1f), 0x1f);
        // Outside that pattern the index is unchanged.
        assert_eq!(clut_index(0x07), 0x07);
        assert_eq!(clut_index(0x18), 0x18);
        // The remap is an involution, so it round-trips.
        for i in 0..256u32 {
            assert_eq!(clut_index(clut_index(i)), i);
        }
    }

    #[test]
    fn alpha_is_scaled_from_eight_bit_halves() {
        assert_eq!(scale_alpha(0), 0);
        assert_eq!(scale_alpha(0x40), 0x80);
        assert_eq!(scale_alpha(0x7f), 0xfe);
        assert_eq!(scale_alpha(0x80), 0xff);
        assert_eq!(scale_alpha(0xff), 0xff);
    }

    /// A 2x1 image whose two texels select distinct CLUT entries.
    fn fixture() -> (Vec<u8>, Vec<u8>) {
        let mut clut = vec![0u8; 1024];
        for i in 0..256u32 {
            let at = clut_index(i) as usize * 4;
            clut[at] = i as u8;
            clut[at + 1] = 255 - i as u8;
            clut[at + 2] = 7;
            clut[at + 3] = 0x80;
        }
        (vec![3, 200], clut)
    }

    #[test]
    fn decodes_indices_through_the_clut() {
        let (idx, clut) = fixture();
        let t = decode_indexed8(&idx, 2, 1, &clut).unwrap();
        assert_eq!(t.rgba.len(), 8);
        // Entry 3 is red, entry 200 has a flipped red and green.
        assert_eq!(&t.rgba[0..4], &[3, 252, 7, 0xff]);
        assert_eq!(&t.rgba[4..8], &[200, 55, 7, 0xff]);
    }

    #[test]
    fn short_data_is_an_error_not_a_panic() {
        let (_idx, clut) = fixture();
        assert!(decode_indexed8(&[0, 1], 4, 4, &clut).is_err());
        assert!(decode_indexed8(&[0; 16], 2, 2, &clut[..512]).is_err());
        assert!(decode_indexed8(&[0; 3], 2, 1, &clut).is_ok());
    }

    #[test]
    fn a_square_entry_halves_through_its_mip_chain() {
        // 64x64 base, mip 1 immediately after it, mips 2 and 3 in gs_ram.
        let mut textures = vec![0u8; 64 * 64 * 2];
        for (i, b) in textures.iter_mut().enumerate() {
            *b = (i % 251) as u8;
        }
        let mut gs = vec![0u8; 0x4000];
        for (i, b) in gs.iter_mut().enumerate().take(0x3000) {
            *b = (i % 253) as u8;
        }
        let e = TextureEntry {
            data_offset: 0,
            width: 64,
            height: 64,
            ty: 4,
            palette: 8,
            mipmap: 16,
            pad: 24,
        };
        let mips = mip_images(&textures, &gs, &e).unwrap();
        assert_eq!(mips.len(), 4);
        let sizes: Vec<(u32, u32)> = mips.iter().map(|m| (m.width, m.height)).collect();
        assert_eq!(sizes, [(64, 64), (32, 32), (16, 16), (8, 8)]);
        // Level 0 is the base block; level 1 follows it immediately.
        assert_eq!(mips[0].indices, &textures[0..64 * 64]);
        assert_eq!(mips[1].indices, &textures[64 * 64..64 * 64 + 32 * 32]);
        // Levels 2 and 3 come from gs_ram at `mipmap * 0x100` and `pad * 0x100`.
        assert_eq!(mips[2].indices, &gs[16 * 0x100..16 * 0x100 + 16 * 16]);
        assert_eq!(mips[3].indices, &gs[24 * 0x100..24 * 0x100 + 8 * 8]);
    }

    #[test]
    fn an_absent_high_mip_is_an_error() {
        let textures = vec![0u8; 64 * 64];
        let gs = vec![0u8; 0x4000];
        let e = TextureEntry {
            data_offset: 0,
            width: 64,
            height: 64,
            ty: 4,
            palette: 8,
            mipmap: 16,
            pad: -1,
        };
        assert!(mip_images(&textures, &gs, &e).is_err());
    }

    #[test]
    fn a_palette_past_gs_ram_is_an_error() {
        let textures = vec![0u8; 64 * 64];
        let gs = vec![0u8; 0x100];
        let e = TextureEntry {
            data_offset: 0,
            width: 64,
            height: 64,
            ty: 3,
            palette: 900,
            mipmap: -1,
            pad: -1,
        };
        assert!(base_image(&textures, &gs, &e).is_err());
    }
}