use crate::common::errors::FrameError;

pub fn checksum(payload: &[u8]) -> u8 {
    let mut cs: u8 = 0x01;
    for &b in payload {
        cs ^= b;
    }
    cs
}

pub fn build_frame(cmd: u8, args: &[u8]) -> Vec<u8> {
    let mut payload_body = Vec::with_capacity(1 + args.len());
    payload_body.push(cmd);
    payload_body.extend_from_slice(args);

    let length = payload_body.len() as u8;

    let mut inner = Vec::with_capacity(3 + payload_body.len());
    inner.push(0x00);
    inner.push(0x01);
    inner.push(length);
    inner.extend_from_slice(&payload_body);

    let cs = checksum(&inner);

    let mut frame = Vec::with_capacity(2 + inner.len() + 3);
    frame.push(0x10);
    frame.push(0x02);
    frame.extend_from_slice(&inner);
    frame.push(0x10);
    frame.push(0x03);
    frame.push(cs);
    frame
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Frame {
    pub command: u8,
    pub payload: Vec<u8>,
}

pub fn extract_frame(data: &[u8]) -> Result<Frame, FrameError> {
    if data.len() < 8 {
        return Err(FrameError::new(format!(
            "Frame too short: expected at least 8 bytes, got {}",
            data.len()
        )));
    }

    if data[0] != 0x10 || data[1] != 0x02 {
        return Err(FrameError::new(format!(
            "Invalid frame header: {:#04x} {:#04x}",
            data[0], data[1]
        )));
    }

    if data[2] == 0x00 {
        return Err(FrameError::new("Not a response frame"));
    }

    if data[3] != 0x00 {
        return Err(FrameError::new(format!(
            "Invalid device id byte: {:#04x}",
            data[3]
        )));
    }

    let footer = find_footer(data)
        .ok_or_else(|| FrameError::new("Missing frame footer"))?;

    let expected = checksum(&data[2..footer]);
    let received = data[data.len() - 1];

    if expected != received {
        return Err(FrameError::new(format!(
            "Invalid checksum: expected 0x{:02X}, received 0x{:02X}",
            expected, received
        )));
    }

    Ok(Frame {
        command: data[5],
        payload: data[6..footer].to_vec(),
    })
}

fn find_footer(data: &[u8]) -> Option<usize> {
    if data.len() < 2 {
        return None;
    }

    (2..data.len() - 1)
        .find(|&i| data[i] == 0x10 && data[i + 1] == 0x03)
}

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

    #[test]
    fn test_checksum() {
        let payload = [0x00, 0x01, 0x03, 0x10, 0x20, 0x30];

        // 0x01 ^ 0x00 ^ 0x01 ^ 0x03 ^ 0x10 ^ 0x20 ^ 0x30
        assert_eq!(checksum(&payload), 0x03);
    }

    #[test]
    fn test_build_frame() {
        let frame = build_frame(0x05, &[0xAA, 0xBB]);

        let expected_without_checksum = vec![
            0x10, 0x02, // header
            0x00, 0x01, 0x03, // device + version + length
            0x05, 0xAA, 0xBB, // command + args
            0x10, 0x03, // footer
        ];

        assert_eq!(&frame[..frame.len() - 1], expected_without_checksum);

        let expected_checksum = checksum(&frame[2..frame.len() - 3]);
        assert_eq!(frame[frame.len() - 1], expected_checksum);
    }

    #[test]
    fn test_extract_frame_invalid_header() {
        let mut frame = build_frame(0x01, &[]);
        frame[0] = 0x11;

        let result = extract_frame(&frame);

        assert!(result.is_err());
    }

    #[test]
    fn test_extract_frame_invalid_checksum() {
        let mut frame = build_frame(0x01, &[]);
        let last = frame.len() - 1;

        frame[last] ^= 0xFF;

        let result = extract_frame(&frame);

        assert!(result.is_err());
    }

    #[test]
    fn test_extract_frame_not_response() {
        let mut frame = build_frame(0x01, &[]);

        // data[2] == 0 means request frame according to your parser
        frame[2] = 0x00;

        let result = extract_frame(&frame);

        assert!(result.is_err());
    }

    #[test]
    fn test_extract_frame_invalid_device_id() {
        let mut frame = build_frame(0x01, &[]);

        frame[3] = 0xFF;

        let result = extract_frame(&frame);

        assert!(result.is_err());
    }

    #[test]
    fn test_extract_frame_too_short() {
        let data = vec![0x10, 0x02, 0x01];

        let result = extract_frame(&data);

        assert!(result.is_err());
    }
}