//! The IPS(24/16) patchfile format is one of the simplest possible patchfile formats.
//! It contains no way to identify the correct target file, and cannot insert or remove bytes.
//!
//! An IPS file starts with the magic number "PATCH"
//! ```console
//! 0000:  50 41 54 43 48           |PATCH|
//! ```
//!
//! Patches are encoded linearly with no padding or alignment, and
//! all numeric values are encoded big-endian. Patches cannot have
//! a length of b"EOF", as "EOF" marks the end of the patchfile.
//! ```console
//! xxxx:  45 4f 46                 |EOF|
//! ```
//!
//! The patchfile matches the following pseudo-grammar
//! ```console
//! IPS         = "PATCH" Patch* "EOF"
//! Patch       = { offset: u24 != "EOF", kind: PatchKind }
//! PatchData   = { len: u16 != 0, data: [u8; len] }        // Plain ol' data
//!             | { _:   u16 == 0, len: u16, byte: u8 }     // Run-length-encoded data
//! ```

use crate::{parse_utils::*, Apply};
use std::io::{self, Read, Seek, SeekFrom, Write};

pub struct IPS {
    pub magic: [u8; 5],
    pub patches: Vec<Patch>,
}

// A single IPS patch
pub struct Patch {
    pub offset: u32,
    pub data: Data,
}

/// The data
pub enum Data {
    /// Run-length-encoded (repeated) data
    RLEnc(u16, u8),

    /// Verbatim data
    Plain(Vec<u8>),
}

impl Apply for IPS {
    fn apply<W: Write + Seek>(&self, writer: &mut W) -> io::Result<()> {
        for patch in &self.patches {
            patch.apply(writer)?;
        }
        Ok(())
    }
}

impl Apply for Patch {
    fn apply<W: Write + Seek>(&self, writer: &mut W) -> io::Result<()> {
        let Self { offset, data } = self;
        writer.seek(SeekFrom::Start(*offset as _))?;
        data.apply(writer)
    }
}

impl Apply for Data {
    fn apply<W: Write + Seek>(&self, writer: &mut W) -> io::Result<()> {
        match self {
            &Data::RLEnc(len, value) => {
                io::copy(&mut io::repeat(value).take(len as _), writer)?;
                Ok(())
            }
            Data::Plain(buf) => writer.write_all(buf),
        }
    }
}

impl IPS {
    /// The expected magic number
    pub const MAGIC: &[u8] = b"PATCH";

    /// Reads an IPS file out of the provided reader.
    ///
    /// Consumes 5 bytes and returns Ok(None) if the reader doesn't yield an IPS file.
    pub fn parse(reader: &mut impl Read) -> io::Result<Option<Self>> {
        let magic = read_bytes(reader)?;
        if Self::MAGIC != magic {
            return Ok(None);
        }

        let mut patches = vec![];
        while let Some(patch) = Patch::parse(reader)? {
            patches.push(patch)
        }

        Ok(Some(Self { magic, patches }))
    }
}

impl Patch {
    pub fn parse(reader: &mut impl Read) -> io::Result<Option<Self>> {
        match offset(reader)? {
            None => Ok(None),
            Some(offset) => {
                let data = Data::parse(reader)?;
                Ok(Some(Self { offset, data }))
            }
        }
    }
}

impl Data {
    pub fn parse(reader: &mut impl Read) -> io::Result<Self> {
        match read16(reader)? {
            0 => {
                let len = read16(reader)?;
                let value = read_bytes::<1>(reader)?[0];

                Ok(Data::RLEnc(len, value))
            }
            len => {
                let mut data = vec![0u8; len as _];
                reader.read_exact(&mut data)?;

                Ok(Data::Plain(data))
            }
        }
    }
}