use tokio::io::{AsyncBufReadExt, AsyncReadExt, BufReader};

/// Reads an MCP (NDJSON or LSP Content-Length prefixed) message from a buffered async reader.
/// Returns the raw JSON string payload if successful, or None on EOF or error.
pub async fn read_mcp_message<R: tokio::io::AsyncRead + Unpin>(
    stdin: &mut BufReader<R>,
) -> Option<String> {
    let mut length = 0;
    let mut line = String::new();
    loop {
        line.clear();
        if stdin.read_line(&mut line).await.unwrap_or(0) == 0 {
            return None;
        }

        if line.starts_with('{') {
            let len = line.trim_end().len();
            line.truncate(len);
            return Some(line);
        }

        let line = line.trim_end();
        if line.is_empty() {
            break;
        }

        if line.len() >= 15 && line[..15].eq_ignore_ascii_case("content-length:") {
            length = line[15..].trim().parse().unwrap_or(0);
        }
    }

    if length == 0 {
        return None;
    }

    let mut buffer = vec![0; length];
    if stdin.read_exact(&mut buffer).await.is_err() {
        return None;
    }

    String::from_utf8(buffer).ok()
}