darkomen 0.5.0

Warhammer: Dark Omen library and CLI in Rust
Documentation 
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use std::{
    ffi::CStr,
    fmt,
    io::{Error as IoError, Read, Seek},
};

use glam::Vec3;

use super::*;

/// The format ID used in all .M3D files. The last part probably stands for "3D
/// model".
pub(crate) const FORMAT: &str = "PD3M";

const HEADER_SIZE_BYTES: usize = 24;
const TEXTURE_DESCRIPTOR_SIZE_BYTES: usize = 96;
pub(crate) const TEXTURE_DESCRIPTOR_PATH_SIZE_BYTES: usize = 64;
const VECTOR_SIZE_BYTES: usize = 12;
const OBJECT_HEADER_SIZE_BYTES: usize = 52 + VECTOR_SIZE_BYTES;
const OBJECT_FACE_SIZE_BYTES: usize = 16 + VECTOR_SIZE_BYTES;
const OBJECT_VERTEX_SIZE_BYTES: usize = (2 * VECTOR_SIZE_BYTES) + 20;

#[derive(Debug)]
pub enum DecodeError {
    IoError(IoError),
    InvalidFormat(String),
    InvalidString,
}

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

impl From<IoError> for DecodeError {
    fn from(error: IoError) -> Self {
        DecodeError::IoError(error)
    }
}

impl fmt::Display for DecodeError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            DecodeError::IoError(e) => write!(f, "IO error: {e}"),
            DecodeError::InvalidFormat(s) => write!(f, "invalid format: {s}"),
            DecodeError::InvalidString => write!(f, "invalid string"),
        }
    }
}

pub struct Decoder<R>
where
    R: Read + Seek,
{
    reader: R,
}

impl<R: Read + Seek> Decoder<R> {
    pub fn new(reader: R) -> Self {
        Decoder { reader }
    }

    pub fn decode(&mut self) -> Result<M3d, DecodeError> {
        let header = self.decode_header()?;

        let texture_descriptors = self.read_texture_descriptors(header.texture_count)?;

        let objects = self.read_objects(header.object_count)?;

        Ok(M3d {
            header,
            texture_descriptors,
            objects,
        })
    }

    fn decode_header(&mut self) -> Result<Header, DecodeError> {
        let mut buf = [0; HEADER_SIZE_BYTES];
        self.reader.read_exact(&mut buf)?;

        if &buf[0..4] != FORMAT.as_bytes() {
            return Err(DecodeError::InvalidFormat(
                String::from_utf8_lossy(&buf[0..4]).to_string(),
            ));
        }

        Ok(Header {
            _magic: u32::from_le_bytes(buf[4..8].try_into().unwrap()),
            _version: u32::from_le_bytes(buf[8..12].try_into().unwrap()),
            _crc: u32::from_le_bytes(buf[12..16].try_into().unwrap()),
            _not_crc: u32::from_le_bytes(buf[16..20].try_into().unwrap()),
            texture_count: u16::from_le_bytes(buf[20..22].try_into().unwrap()),
            object_count: u16::from_le_bytes(buf[22..24].try_into().unwrap()),
        })
    }

    fn read_texture_descriptors(
        &mut self,
        count: u16,
    ) -> Result<Vec<M3dTextureDescriptor>, DecodeError> {
        let mut descriptors = Vec::with_capacity(count as usize);

        for _ in 0..count {
            descriptors.push(self.read_texture_descriptor()?);
        }

        Ok(descriptors)
    }

    fn read_texture_descriptor(&mut self) -> Result<M3dTextureDescriptor, DecodeError> {
        let mut buf = [0; TEXTURE_DESCRIPTOR_SIZE_BYTES];
        self.reader.read_exact(&mut buf)?;

        let path_buf = &buf[0..TEXTURE_DESCRIPTOR_PATH_SIZE_BYTES];
        let (path_buf, path_residual_bytes) = Self::split_at_null(path_buf);

        let file_name_buf = &buf[TEXTURE_DESCRIPTOR_PATH_SIZE_BYTES..];
        let (file_name_buf, file_name_residual_bytes) = Self::split_at_null(file_name_buf);

        Ok(M3dTextureDescriptor {
            path: self.read_string(path_buf)?,
            path_residual_bytes: Self::extract_residual_bytes(path_residual_bytes),
            file_name: self.read_string(file_name_buf)?,
            file_name_residual_bytes: Self::extract_residual_bytes(file_name_residual_bytes),
        })
    }

    fn read_objects(&mut self, count: u16) -> Result<Vec<Object>, DecodeError> {
        let mut objects = Vec::with_capacity(count as usize);

        for _ in 0..count {
            objects.push(self.read_object()?);
        }

        Ok(objects)
    }

    fn read_object(&mut self) -> Result<Object, DecodeError> {
        let mut buf = [0; OBJECT_HEADER_SIZE_BYTES];
        self.reader.read_exact(&mut buf)?;

        let name_buf = &buf[0..32];
        let (name_buf, name_remainder) = name_buf
            .iter()
            .enumerate()
            .find(|(_, &b)| b == 0)
            .map(|(i, _)| name_buf.split_at(i + 1))
            .unwrap_or((name_buf, &[]));

        let vertex_count = u16::from_le_bytes(buf[48..50].try_into().unwrap());
        let face_count = u16::from_le_bytes(buf[50..52].try_into().unwrap());

        let mut faces = Vec::with_capacity(face_count as usize);
        for _ in 0..face_count {
            faces.push(self.read_face()?);
        }

        let mut vertices = Vec::with_capacity(vertex_count as usize);
        for _ in 0..vertex_count {
            vertices.push(self.read_vertex()?);
        }

        Ok(Object {
            name: self.read_string(name_buf)?,
            name_remainder: name_remainder.to_vec(),
            parent_index: i16::from_le_bytes(buf[32..34].try_into().unwrap()),
            padding: i16::from_le_bytes(buf[34..36].try_into().unwrap()),
            translation: self.read_vector(&buf[36..48])?,
            flags: ObjectFlags::from_bits(u32::from_le_bytes(buf[52..56].try_into().unwrap()))
                .expect("object flags should be valid"),
            unknown1: u32::from_le_bytes(buf[56..60].try_into().unwrap()),
            unknown2: u32::from_le_bytes(buf[60..64].try_into().unwrap()),
            faces,
            vertices,
        })
    }

    fn read_face(&mut self) -> Result<Face, DecodeError> {
        let mut buf = [0; OBJECT_FACE_SIZE_BYTES];
        self.reader.read_exact(&mut buf)?;

        Ok(Face {
            indices: [
                u16::from_le_bytes(buf[0..2].try_into().unwrap()),
                u16::from_le_bytes(buf[2..4].try_into().unwrap()),
                u16::from_le_bytes(buf[4..6].try_into().unwrap()),
            ],
            texture_index: u16::from_le_bytes(buf[6..8].try_into().unwrap()),
            normal: self.read_vector(&buf[8..20])?,
            unknown1: u32::from_le_bytes(buf[20..24].try_into().unwrap()),
            unknown2: u32::from_le_bytes(buf[24..28].try_into().unwrap()),
        })
    }

    fn read_vertex(&mut self) -> Result<Vertex, DecodeError> {
        let mut buf = [0; OBJECT_VERTEX_SIZE_BYTES];
        self.reader.read_exact(&mut buf)?;

        Ok(Vertex {
            position: self.read_vector(&buf[0..12])?,
            normal: self.read_vector(&buf[12..24])?,
            color: UVec4::new(
                buf[24] as u32,
                buf[25] as u32,
                buf[26] as u32,
                buf[27] as u32,
            ),
            uv: Vec2::new(
                f32::from_le_bytes(buf[28..32].try_into().unwrap()),
                f32::from_le_bytes(buf[32..36].try_into().unwrap()),
            ),
            index: u32::from_le_bytes(buf[36..40].try_into().unwrap()),
            unknown1: u32::from_le_bytes(buf[40..44].try_into().unwrap()),
        })
    }

    fn read_vector(&mut self, buf: &[u8]) -> Result<Vec3, DecodeError> {
        let x = f32::from_le_bytes(buf[0..4].try_into().unwrap());
        let y = f32::from_le_bytes(buf[4..8].try_into().unwrap());
        let z = f32::from_le_bytes(buf[8..12].try_into().unwrap());

        Ok(Vec3::new(x, y, z))
    }

    fn read_string(&mut self, buf: &[u8]) -> Result<String, DecodeError> {
        Ok(CStr::from_bytes_until_nul(buf)
            .map_err(|_| DecodeError::InvalidString)?
            .to_string_lossy()
            .into_owned())
    }

    /// Splits a buffer at the first null byte.
    ///
    /// Returns a tuple of (string_buffer, residual_bytes) where string_buffer
    /// includes the null terminator and residual_bytes is everything after.
    fn split_at_null(buf: &[u8]) -> (&[u8], &[u8]) {
        buf.iter()
            .enumerate()
            .find(|(_, &b)| b == 0)
            .map(|(i, _)| buf.split_at(i + 1))
            .unwrap_or((buf, &[]))
    }

    /// Extracts residual bytes from a buffer, trimming trailing zeros.
    ///
    /// Returns [None] if all bytes are zero, otherwise returns the bytes up to
    /// and including the last non-zero byte.
    fn extract_residual_bytes(residual_bytes: &[u8]) -> Option<Vec<u8>> {
        if residual_bytes.iter().all(|&b| b == 0) {
            None
        } else {
            Some(
                residual_bytes
                    .iter()
                    .rposition(|&b| b != 0) // find the last non-zero byte
                    .map(|pos| &residual_bytes[..=pos]) // include the last non-zero byte
                    .unwrap_or(residual_bytes)
                    .to_vec(),
            )
        }
    }
}