use std::marker::Unpin;
use tokio::io::{AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt};

use super::{hash, verify_checksum, DecodeError, EncodeError};

/// Decode a framed message into a `Vec<u8>`.
///
/// # Errors
/// An error may be returned if:
/// * The given `data` does not contain enough data to parse a length,
/// * The given `data` does not contain the given length's worth of data,
/// * The given `data` has a checksum that does not match what we build locally.
pub async fn try_decode_from(
    readable: &mut (impl AsyncRead + Unpin),
) -> Result<Vec<u8>, DecodeError> {
    let len = readable.read_u32().await?;

    let mut data = Vec::with_capacity(len as usize);
    readable.read_exact(&mut data[..]).await?;

    let content = verify_checksum(&data[..])?;

    // Note: Optimizing this isn't *too* practical, we could probably pop the first 32 bytes off
    // the front of the Vec, but it might not even be worth it as opposed to one reallocation.
    Ok(content.to_vec())
}

/// Encode a &[u8] into a framed message
///
/// # Errors
/// An error may be returned if:
/// * The given `data` is more than [`u32::MAX`] bytes. This is a constraint on a protocol level.
/// * The resulting data was unable to be written to the given `writable`.
pub async fn try_encode_to(
    data: &[u8],
    writable: &mut (impl AsyncWrite + Unpin),
) -> Result<(), EncodeError> {
    let hash = hash(data);
    let len = u32::try_from(hash.len() + data.len())
        .map_err(|_| EncodeError::InputTooLarge(hash.len() + data.len()))?;
    writable.write_u32(len).await?;
    writable.write_all(&hash[..]).await?;
    writable.write_all(data).await?;
    Ok(())
}