//! Decoding a channel a band of rows at a time: [`ChannelData::row_decoder`].
//!
//! [`ChannelData::decode`] returns the whole plane, so turning a layer into tiles holds every
//! channel's plane plus an interleaved copy at once: for a 20000×15000 RGBA layer that is 2.4 GB
//! on top of the tiles. A [`RowDecoder`] hands out any band of rows instead. RLE and raw channels
//! are read band by band (a channel still in its [`Stored`](crate::Stored) source reads only the
//! band's bytes: RLE's per-row byte counts give each row's offset); ZIP streams cannot be entered
//! part way, so a ZIP channel is inflated once, when the decoder is made.

use std::borrow::Cow;

use crate::compression::{Compression, MAX_DECODED_BYTES, PlaneLayout, decode_planes, packbits};
use crate::error::{PsdError, Result};
use crate::header::Version;
use crate::layer::ChannelData;
use crate::source::Stored;

/// Where a channel's encoded bytes are.
enum Bytes<'a> {
    Memory(&'a [u8]),
    Source(&'a Stored),
}

impl Bytes<'_> {
    fn len(&self) -> u64 {
        match self {
            Bytes::Memory(b) => b.len() as u64,
            Bytes::Source(s) => s.len,
        }
    }

    fn get(&self, at: u64, len: u64) -> Result<Cow<'_, [u8]>> {
        match self {
            Bytes::Memory(b) => {
                let range = usize::try_from(at).ok().zip(usize::try_from(len).ok()).and_then(|(a, n)| b.get(a..a.checked_add(n)?));
                range.map(Cow::Borrowed).ok_or(PsdError::UnexpectedEof {
                    offset: usize::try_from(at).unwrap_or(usize::MAX),
                    needed: usize::try_from(len).unwrap_or(usize::MAX),
                })
            }
            Bytes::Source(s) => {
                if at.checked_add(len).is_none_or(|end| end > s.len) {
                    return Err(PsdError::UnexpectedEof {
                        offset: usize::try_from(at).unwrap_or(usize::MAX),
                        needed: usize::try_from(len).unwrap_or(usize::MAX),
                    });
                }
                s.read_part(at, len).map(Cow::Owned)
            }
        }
    }
}

enum Kind<'a> {
    /// Rows at `row * row_bytes`.
    Raw(Bytes<'a>),
    /// `offsets[r]..offsets[r + 1]` are row `r`'s PackBits bytes (offsets from the start of the
    /// data, after the count table).
    Rle { bytes: Bytes<'a>, offsets: Vec<u64> },
    /// The whole decoded plane.
    Decoded(Vec<u8>),
    /// No data and no rows.
    Empty,
}

/// Decodes bands of rows of one channel: see the [module docs](self).
pub struct RowDecoder<'a> {
    kind: Kind<'a>,
    row_bytes: usize,
    height: usize,
}

impl ChannelData {
    /// A decoder of this channel's rows, for a `width × height` plane. Fails where
    /// [`Self::decode`] would on everything it can check without decoding the rows (sizes, the
    /// RLE count table, the whole of a ZIP stream); a damaged row fails when its band is decoded.
    pub fn row_decoder(&self, width: usize, height: usize, depth: u16, version: Version) -> Result<RowDecoder<'_>> {
        let layout = PlaneLayout { planes: 1, width, height, depth, version };
        let bytes = match &self.stored {
            Some(s) => Bytes::Source(s),
            None => Bytes::Memory(&self.data),
        };
        RowDecoder::new(self.compression, bytes, &layout, || self.encoded())
    }
}

impl crate::image_data::ImageData {
    /// A decoder of the merged image's rows: its planes one after another, so plane `p`'s row
    /// `r` is row `p × height + r`. As [`ChannelData::row_decoder`]; only a ZIP merged image is
    /// decoded whole (and so only one within `MAX_DECODED_BYTES`).
    pub fn row_decoder(&self, header: &crate::header::Header) -> Result<RowDecoder<'_>> {
        let layout = Self::layout(header);
        let bytes = match &self.stored {
            Some(s) => Bytes::Source(s),
            None => Bytes::Memory(&self.data),
        };
        RowDecoder::new(Some(self.compression), bytes, &layout, || self.encoded())
    }
}

impl<'a> RowDecoder<'a> {
    fn new(
        compression: Option<Compression>,
        bytes: Bytes<'a>,
        layout: &PlaneLayout,
        encoded: impl FnOnce() -> Result<Cow<'a, [u8]>>,
    ) -> Result<Self> {
        // The whole-plane cap applies to a whole decode (ZIP) only: rows are decoded a band at a
        // time, each band within the cap.
        let total = layout.total_bytes()?;
        let row_bytes = layout.row_bytes();
        let height = layout.rows()?;
        let kind = match compression {
            None if total == 0 => Kind::Empty,
            None => return Err(PsdError::invalid("channel has no data")),
            Some(Compression::Raw) => {
                if bytes.len() < total {
                    return Err(PsdError::UnexpectedEof {
                        offset: usize::try_from(bytes.len()).unwrap_or(usize::MAX),
                        needed: usize::try_from(total - bytes.len()).unwrap_or(usize::MAX),
                    });
                }
                Kind::Raw(bytes)
            }
            Some(Compression::Rle) => {
                let size: u64 = if layout.version.is_psb() { 4 } else { 2 };
                let table_len = (height as u64).saturating_mul(size);
                if table_len > bytes.len() {
                    return Err(PsdError::UnexpectedEof { offset: 0, needed: usize::try_from(table_len - bytes.len()).unwrap_or(usize::MAX) });
                }
                let table = bytes.get(0, table_len)?;
                let mut offsets = Vec::new();
                offsets.try_reserve_exact(height + 1).map_err(|_| PsdError::OutOfMemory("not enough memory for the RLE row table"))?;
                let mut at = table_len;
                offsets.push(at);
                for c in table.chunks_exact(size as usize) {
                    let n = match *c {
                        [a, b] => u64::from(u16::from_be_bytes([a, b])),
                        [a, b, c, d] => u64::from(u32::from_be_bytes([a, b, c, d])),
                        _ => 0,
                    };
                    at = at.saturating_add(n);
                    offsets.push(at);
                }
                let data_len = bytes.len() - table_len;
                // As `decode_planes` checks: PackBits expands at most 64x.
                if total > data_len.saturating_mul(64) {
                    return Err(PsdError::Decompress("RLE data too short for declared size".into()));
                }
                if at > bytes.len() {
                    return Err(PsdError::UnexpectedEof {
                        offset: usize::try_from(bytes.len()).unwrap_or(usize::MAX),
                        needed: usize::try_from(at - bytes.len()).unwrap_or(usize::MAX),
                    });
                }
                Kind::Rle { bytes, offsets }
            }
            Some(c) => Kind::Decoded(decode_planes(c, &encoded()?, layout)?),
        };
        Ok(RowDecoder { kind, row_bytes, height })
    }
}

impl RowDecoder<'_> {
    /// Bytes per decoded row.
    pub fn row_bytes(&self) -> usize {
        self.row_bytes
    }

    /// Number of rows.
    pub fn height(&self) -> usize {
        self.height
    }

    /// Rows `r0..r1` as planar big-endian samples (`(r1 - r0) × row_bytes` bytes).
    pub fn rows(&self, r0: usize, r1: usize) -> Result<Vec<u8>> {
        if r0 > r1 || r1 > self.height {
            return Err(PsdError::invalid(format!("rows {r0}..{r1} outside 0..{}", self.height)));
        }
        let n = (r1 - r0).checked_mul(self.row_bytes).ok_or(PsdError::LimitExceeded("band size overflow"))?;
        if n as u64 > MAX_DECODED_BYTES {
            return Err(PsdError::LimitExceeded("decoded channel data exceeds MAX_DECODED_BYTES"));
        }
        let mut out = Vec::new();
        out.try_reserve_exact(n).map_err(|_| PsdError::OutOfMemory("not enough memory for the decoded channel data"))?;
        match &self.kind {
            Kind::Empty => {}
            Kind::Raw(bytes) => out.extend_from_slice(&bytes.get(r0 as u64 * self.row_bytes as u64, n as u64)?),
            Kind::Decoded(plane) => out
                .extend_from_slice(plane.get(r0 * self.row_bytes..r1 * self.row_bytes).ok_or_else(|| PsdError::invalid("decoded plane too short"))?),
            Kind::Rle { bytes, offsets } => {
                let (Some(&start), Some(&end)) = (offsets.get(r0), offsets.get(r1)) else {
                    return Err(PsdError::invalid("RLE row table too short"));
                };
                let band = bytes.get(start, end - start)?;
                for r in r0..r1 {
                    let (Some(&a), Some(&b)) = (offsets.get(r), offsets.get(r + 1)) else {
                        return Err(PsdError::invalid("RLE row table too short"));
                    };
                    let row = band.get((a - start) as usize..(b - start) as usize).ok_or_else(|| PsdError::invalid("RLE row outside its band"))?;
                    packbits::decode_into(row, self.row_bytes, &mut out)?;
                }
            }
        }
        Ok(out)
    }
}

#[cfg(test)]
mod tests {
    use crate::source::Source;
    use crate::{PsdFile, testgen};

    /// Every channel of every generated file, decoded in uneven bands from memory and from a
    /// source, is its whole-plane decode.
    #[test]
    fn bands_join_up_to_the_whole_plane() {
        let mut checked = 0;
        for case in testgen::all_cases() {
            let bytes = case.file.to_bytes().unwrap();
            let eager = PsdFile::from_bytes(&bytes).unwrap();
            let lazy = PsdFile::from_source(Source::new(bytes)).unwrap();
            let (d, v) = (eager.header.depth, eager.header.version);
            for file in [&eager, &lazy] {
                for rec in file.layers() {
                    for ch in &rec.channels {
                        let (w, h) = rec.channel_rect(ch.id).size().unwrap();
                        let whole = ch.decode(w, h, d, v);
                        let dec = ch.row_decoder(w, h, d, v);
                        let (Ok(whole), Ok(dec)) = (whole, dec) else { continue };
                        let mut joined = Vec::new();
                        let mut r = 0;
                        for step in [1, 3, 2, 7].iter().cycle() {
                            if r >= h {
                                break;
                            }
                            let r1 = (r + step).min(h);
                            joined.extend(dec.rows(r, r1).unwrap());
                            r = r1;
                        }
                        assert_eq!(joined, whole, "{}", case.name);
                        assert!(dec.rows(0, h + 1).is_err());
                        checked += 1;
                    }
                }
            }
        }
        assert!(checked > 100, "{checked}");
    }

    #[test]
    fn a_damaged_rle_row_fails_its_band() {
        use crate::Compression;
        use crate::layer::ChannelData;
        // Two rows of 4 bytes: a valid row, then one that ends early.
        let mut data = vec![0, 2, 0, 2];
        data.extend([0xFD, 7]); // header -3: 1 - (-3) = 4 copies of 7
        data.extend([0x01, 9]); // a literal of 2 bytes, only 1 present
        let ch = ChannelData::new(0, Some(Compression::Rle), data);
        let dec = ch.row_decoder(4, 2, 8, crate::Version::Psd).unwrap();
        assert_eq!(dec.rows(0, 1).unwrap(), vec![7; 4]);
        assert!(dec.rows(1, 2).is_err());
        assert!(ch.decode(4, 2, 8, crate::Version::Psd).is_err());
    }
}