#![allow(clippy::cast_possible_truncation, clippy::cast_sign_loss, clippy::cast_precision_loss, clippy::cast_possible_wrap)]
//! Length-prefixed binary framing and JSON (de)serialization helpers for
//! the IPC wire protocol.
//!
//! Flow: `write_frame`/`read_frame` handle the raw byte-level framing
//! (4-byte big-endian length header + payload) over any `Read`/`Write`;
//! `serialize_frame`/`deserialize_frame` handle the JSON layer on top.
//! `Connection` (see `conn.rs`) composes both layers for a full send/receive.
//!
//! Why: a fixed-size length prefix lets the reader know exactly how many
//! bytes to pull before attempting to parse, avoiding partial-JSON reads
//! over a stream socket.

use std::io::{Read, Write};
use anyhow::Result;

/// Upper bound on a single frame's byte size (64 MiB), enforced on both
/// the write and read paths to bound memory use and reject malformed or
/// malicious oversized length headers.
pub(crate) const MAX_FRAME_SIZE: usize = 64 * 1024 * 1024;

/// Write `data` as a length-prefixed frame: 4-byte big-endian length
/// followed by the raw bytes, then flush.
///
/// Why: rejects frames over `MAX_FRAME_SIZE` to bound memory use on the
/// reading side before any bytes are read.
pub fn write_frame<W: Write>(writer: &mut W, data: &[u8]) -> Result<()> {
    let len = data.len();
    if len > MAX_FRAME_SIZE {
        anyhow::bail!("frame too large: {len} bytes exceeds 64 MiB limit");
    }
    let len_bytes = (len as u32).to_be_bytes();
    writer.write_all(&len_bytes)?;
    writer.write_all(data)?;
    writer.flush()?;
    Ok(())
}

/// Read one length-prefixed frame written by `write_frame`.
///
/// Flow: read 4-byte length header → on clean EOF before any bytes,
/// return `Ok(None)` (peer closed) → validate against `MAX_FRAME_SIZE`
/// → read the payload.
///
/// Return: `Ok(None)` signals a graceful connection close, distinct
/// from an `Err` mid-frame I/O failure.
pub fn read_frame<R: Read>(reader: &mut R) -> Result<Option<Vec<u8>>> {
    let mut len_buf = [0u8; 4];
    match reader.read_exact(&mut len_buf) {
        Ok(()) => {}
        Err(e) if e.kind() == std::io::ErrorKind::UnexpectedEof => return Ok(None),
        Err(e) => return Err(e.into()),
    }
    let len = u32::from_be_bytes(len_buf) as usize;
    if len > MAX_FRAME_SIZE {
        anyhow::bail!("frame too large: {len} bytes exceeds 64 MiB limit");
    }
    let mut buf = vec![0u8; len];
    reader.read_exact(&mut buf)?;
    Ok(Some(buf))
}

/// Serialize `value` to JSON bytes, rejecting output over `MAX_FRAME_SIZE`.
pub fn serialize_frame<T: serde::Serialize>(value: &T) -> Result<Vec<u8>> {
    let json = serde_json::to_vec(value)?;
    if json.len() > MAX_FRAME_SIZE {
        anyhow::bail!("serialized frame too large: {} bytes", json.len());
    }
    Ok(json)
}

/// Deserialize a frame's raw JSON bytes into `T`.
pub fn deserialize_frame<'a, T: serde::Deserialize<'a>>(data: &'a [u8]) -> Result<T> {
    Ok(serde_json::from_slice(data)?)
}

#[cfg(test)]
mod tests {
    use super::*;

    /// Write a value, read it back, and verify exact equality.
    fn roundtrip_bytes(data: &[u8]) {
        let mut buf: Vec<u8> = Vec::new();
        write_frame(&mut buf, data).unwrap();
        let read_back = read_frame(&mut buf.as_slice())
            .unwrap()
            .expect("expected Some(frame)");
        assert_eq!(read_back, data);
    }

    #[test]
    fn test_write_read_roundtrip_empty() {
        roundtrip_bytes(b"");
    }

    #[test]
    fn test_write_read_roundtrip_small_text() {
        roundtrip_bytes(b"hello world");
    }

    #[test]
    fn test_write_read_roundtrip_binary() {
        roundtrip_bytes(&[0x00, 0xFF, 0xAB, 0xCD, 0x01, 0x02, 0x03]);
    }

    #[test]
    fn test_write_read_roundtrip_large() {
        let data = vec![0x42u8; 100_000];
        roundtrip_bytes(&data);
    }

    #[test]
    fn test_write_rejects_too_large_frame() {
        let oversized = vec![0u8; MAX_FRAME_SIZE + 1];
        let mut buf = Vec::new();
        let result = write_frame(&mut buf, &oversized);
        assert!(result.is_err());
        let err = result.unwrap_err().to_string();
        assert!(err.contains("too large") || err.contains("64 MiB"));
    }

    #[test]
    fn test_read_rejects_too_large_header() {
        // Manually craft a 4-byte length header that exceeds MAX_FRAME_SIZE
        let len = (MAX_FRAME_SIZE as u32).wrapping_add(1);
        let header = len.to_be_bytes();
        let mut buf = Vec::from(&header[..]);
        buf.extend_from_slice(b"dummy");
        let result = read_frame(&mut buf.as_slice());
        assert!(result.is_err());
        let err = result.unwrap_err().to_string();
        assert!(err.contains("too large"));
    }

    #[test]
    fn test_read_empty_buf_returns_none() {
        let empty: &[u8] = &[];
        let result = read_frame(&mut &empty[..]).unwrap();
        assert!(result.is_none(), "expected None for empty reader");
    }

    #[test]
    fn test_read_partial_header_returns_none() {
        // Only 2 bytes of the 4-byte header → EOF
        let partial: &[u8] = &[0x00, 0x01];
        let result = read_frame(&mut &partial[..]).unwrap();
        assert!(result.is_none(), "expected None for partial header");
    }

    #[test]
    fn test_read_truncated_payload_returns_err() {
        let mut buf = Vec::new();
        let header = (10u32).to_be_bytes();
        buf.extend_from_slice(&header);
        buf.extend_from_slice(b"abc"); // only 3 of 10 bytes
        let result = read_frame(&mut buf.as_slice());
        assert!(result.is_err(), "truncated payload should error");
    }

    #[test]
    fn test_serialize_deserialize_roundtrip() {
        use serde::{Deserialize, Serialize};

        #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
        struct Msg {
            id: u32,
            content: String,
            tags: Vec<String>,
        }

        let original = Msg {
            id: 42,
            content: "hello world".into(),
            tags: vec!["foo".into(), "bar".into()],
        };

        let bytes = serialize_frame(&original).unwrap();
        let deserialized: Msg = deserialize_frame(&bytes).unwrap();
        assert_eq!(original, deserialized);
    }

    #[test]
    fn test_serialize_rejects_oversized_value() {
        let huge = vec![0u8; MAX_FRAME_SIZE + 1];
        let result = serialize_frame(&huge);
        assert!(result.is_err());
    }

    #[test]
    fn test_deserialize_malformed_json_errors() {
        let bad_json = b"this is not json";
        let result: Result<String> = deserialize_frame(bad_json);
        assert!(result.is_err());
    }
}