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//! misc stuff
use alloc::{boxed::Box, vec, vec::Vec};
use core::{convert::TryFrom, marker::PhantomData};

use crate::codec::{Codec, Decode, Encode};
use crate::io::{Read, Seek, SeekFrom, Write};

/// A raw value for a certain codec.
///
/// Contains the raw, unprocessed data for a single item for that particular codec
#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord)]
pub struct RawValue<C> {
    data: Box<[u8]>,
    _p: PhantomData<C>,
}

impl<C> RawValue<C> {
    fn new(data: Box<[u8]>) -> Self {
        Self {
            data,
            _p: PhantomData,
        }
    }
}

impl<C> AsRef<[u8]> for RawValue<C> {
    fn as_ref(&self) -> &[u8] {
        &self.data
    }
}

impl<C> From<RawValue<C>> for Box<[u8]> {
    fn from(value: RawValue<C>) -> Self {
        value.data
    }
}

impl<C> From<RawValue<C>> for Vec<u8> {
    fn from(value: RawValue<C>) -> Self {
        value.data.into()
    }
}

/// trait to implement to skip a single item at the current position
pub trait SkipOne: Codec {
    /// assuming r is at the start of an item, advance r to the end
    fn skip<R: Read + Seek>(&self, r: &mut R) -> anyhow::Result<()>;
}

impl<C: Codec + SkipOne> Decode<C> for RawValue<C> {
    fn decode<R: Read + Seek>(c: C, r: &mut R) -> anyhow::Result<Self> {
        let p0 = r.seek(SeekFrom::Current(0)).map_err(anyhow::Error::msg)?;
        c.skip(r)?;
        let p1 = r.seek(SeekFrom::Current(0)).map_err(anyhow::Error::msg)?;
        // seeking backward is not allowed
        anyhow::ensure!(p1 > p0);
        // this will fail if usize is 4 bytes and an item is > 32 bit of length
        let len = usize::try_from(p1 - p0).map_err(anyhow::Error::msg)?;
        r.seek(SeekFrom::Start(p0)).map_err(anyhow::Error::msg)?;
        let mut buf = vec![0u8; len];
        r.read_exact(&mut buf).map_err(anyhow::Error::msg)?;
        Ok(Self::new(buf.into()))
    }
}

impl<C: Codec> Encode<C> for RawValue<C> {
    fn encode<W: Write>(&self, _: C, w: &mut W) -> anyhow::Result<()> {
        w.write_all(&self.data).map_err(anyhow::Error::msg)?;
        Ok(())
    }
}

/// Allows to ignore a single item
#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Default)]
pub struct IgnoredAny;

impl<C: Codec + SkipOne> Decode<C> for IgnoredAny {
    fn decode<R: Read + Seek>(c: C, r: &mut R) -> anyhow::Result<Self> {
        c.skip(r)?;
        Ok(Self)
    }
}