use std::fmt;

use super::Error;
use bytes::Bytes;

/// Strip padding from the given payload.
///
/// It is assumed that the frame had the padded flag set. This means that the
/// first byte is the length of the padding with that many
/// 0 bytes expected to follow the actual payload.
///
/// # Returns
///
/// A slice of the given payload where the actual one is found and the length
/// of the padding.
///
/// If the padded payload is invalid (e.g. the length of the padding is equal
/// to the total length), returns `None`.
pub fn strip_padding(payload: &mut Bytes) -> Result<u8, Error> {
    let payload_len = payload.len();
    if payload_len == 0 {
        // If this is the case, the frame is invalid as no padding length can be
        // extracted, even though the frame should be padded.
        return Err(Error::TooMuchPadding);
    }

    let pad_len = payload[0] as usize;

    if pad_len >= payload_len {
        // This is invalid: the padding length MUST be less than the
        // total frame size.
        return Err(Error::TooMuchPadding);
    }

    let _ = payload.split_to(1);
    let _ = payload.split_off(payload_len - pad_len - 1);

    Ok(pad_len as u8)
}

pub(super) fn debug_flags<'a, 'f: 'a>(fmt: &'a mut fmt::Formatter<'f>, bits: u8) -> DebugFlags<'a, 'f> {
    let result = write!(fmt, "({:#x}", bits);
    DebugFlags {
        fmt,
        result,
        started: false,
    }
}

pub(super) struct DebugFlags<'a, 'f: 'a> {
    fmt: &'a mut fmt::Formatter<'f>,
    result: fmt::Result,
    started: bool,
}

impl<'a, 'f: 'a> DebugFlags<'a, 'f> {
    pub(super) fn flag_if(&mut self, enabled: bool, name: &str) -> &mut Self {
        if enabled {
            self.result = self.result.and_then(|()| {
                let prefix = if self.started {
                    " | "
                } else {
                    self.started = true;
                    ": "
                };

                write!(self.fmt, "{}{}", prefix, name)
            });
        }
        self
    }

    pub(super) fn finish(&mut self) -> fmt::Result {
        self.result.and_then(|()| {
            write!(self.fmt, ")")
        })
    }
}