use bstr::ByteSlice;

use crate::store::file::{log, log::iter::decode::LineNumber};

///
pub mod decode {
    use crate::store::file::log;

    /// The error returned by items in the [forward][super::forward()] and [reverse][super::reverse()] iterators
    #[derive(Debug)]
    pub struct Error {
        inner: log::line::decode::Error,
        line: LineNumber,
    }

    impl std::fmt::Display for Error {
        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
            write!(f, "In line {}: {}", self.line, self.inner)
        }
    }

    impl<'a> std::error::Error for Error {}

    impl Error {
        pub(crate) fn new(err: log::line::decode::Error, line: LineNumber) -> Self {
            Error { line, inner: err }
        }
    }

    #[derive(Debug)]
    pub(crate) enum LineNumber {
        FromStart(usize),
        FromEnd(usize),
    }

    impl std::fmt::Display for LineNumber {
        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
            let (line, suffix) = match self {
                LineNumber::FromStart(line) => (line, ""),
                LineNumber::FromEnd(line) => (line, " from the end"),
            };
            write!(f, "{}{}", line + 1, suffix)
        }
    }
}

/// Returns a forward iterator over the given `lines`, starting from the first line in the file and ending at the last.
///
/// Note that `lines` are an entire reflog file.
///
/// This iterator is useful when the ref log file is going to be rewritten which forces processing of the entire file.
/// It will continue parsing even if individual log entries failed to parse, leaving it to the driver to decide whether to
/// abort or continue.
pub fn forward(lines: &[u8]) -> impl Iterator<Item = Result<log::Line<'_>, decode::Error>> {
    lines.as_bstr().lines().enumerate().map(|(ln, line)| {
        log::Line::from_bytes(line).map_err(|err| decode::Error::new(err, decode::LineNumber::FromStart(ln)))
    })
}

/// An iterator yielding parsed lines in a file in reverse.
#[allow(dead_code)]
pub struct Reverse<'a, F> {
    buf: &'a mut [u8],
    count: usize,
    read_and_pos: Option<(F, u64)>,
    last_nl_pos: Option<usize>,
}

/// An iterator over entries of the `log` file in reverse, using `buf` as sliding window.
///
/// Note that `buf` must be big enough to capture typical line length or else partial lines will be parsed and probably fail
/// in the process.
///
/// This iterator is very expensive in terms of I/O operations and shouldn't be used to read more than the last few entries of the log.
/// Use a forward iterator instead for these cases.
///
/// It will continue parsing even if individual log entries failed to parse, leaving it to the driver to decide whether to
/// abort or continue.
pub fn reverse<F>(mut log: F, buf: &mut [u8]) -> std::io::Result<Reverse<'_, F>>
where
    F: std::io::Read + std::io::Seek,
{
    let pos = log.seek(std::io::SeekFrom::End(0))?;
    Ok(Reverse {
        buf,
        count: 0,
        read_and_pos: Some((log, pos)),
        last_nl_pos: None,
    })
}

impl<'a, F> Iterator for Reverse<'a, F>
where
    F: std::io::Read + std::io::Seek,
{
    type Item = std::io::Result<Result<log::mutable::Line, decode::Error>>;

    fn next(&mut self) -> Option<Self::Item> {
        match (self.last_nl_pos.take(), self.read_and_pos.take()) {
            // Initial state - load first data block
            (None, Some((mut read, pos))) => {
                let npos = pos.saturating_sub(self.buf.len() as u64);
                if let Err(err) = read.seek(std::io::SeekFrom::Start(npos)) {
                    return Some(Err(err));
                }

                let n = (pos - npos) as usize;
                if n == 0 {
                    return None;
                }
                let buf = &mut self.buf[..n];
                if let Err(err) = read.read_exact(buf) {
                    return Some(Err(err));
                };

                let last_byte = *buf.last().expect("we have read non-zero bytes before");
                self.last_nl_pos = Some(if last_byte != b'\n' { buf.len() } else { buf.len() - 1 });
                self.read_and_pos = Some((read, npos));
                self.next()
            }
            // Has data block and can extract lines from it, load new blocks as needed
            (Some(end), Some(read_and_pos)) => match self.buf[..end].rfind_byte(b'\n') {
                Some(start) => {
                    self.read_and_pos = Some(read_and_pos);
                    self.last_nl_pos = Some(start);
                    let buf = &self.buf[start + 1..end];
                    let res = Some(Ok(log::Line::from_bytes(buf)
                        .map_err(|err| decode::Error::new(err, LineNumber::FromEnd(self.count)))
                        .map(Into::into)));
                    self.count += 1;
                    res
                }
                None => {
                    let (mut read, last_read_pos) = read_and_pos;
                    if last_read_pos == 0 {
                        let buf = &self.buf[..end];
                        Some(Ok(log::Line::from_bytes(buf)
                            .map_err(|err| decode::Error::new(err, LineNumber::FromEnd(self.count)))
                            .map(Into::into)))
                    } else {
                        let npos = last_read_pos.saturating_sub((self.buf.len() - end) as u64);
                        if npos == last_read_pos {
                            return Some(Err(std::io::Error::new(
                                std::io::ErrorKind::Other,
                                "buffer too small for line size",
                            )));
                        }
                        let n = (last_read_pos - npos) as usize;
                        self.buf.copy_within(0..end, n);
                        if let Err(err) = read.seek(std::io::SeekFrom::Start(npos)) {
                            return Some(Err(err));
                        }
                        if let Err(err) = read.read_exact(&mut self.buf[..n]) {
                            return Some(Err(err));
                        }
                        self.read_and_pos = Some((read, npos));
                        self.last_nl_pos = Some(n + end);
                        self.next()
                    }
                }
            },
            // depleted
            (None, None) => None,
            (Some(_), None) => unreachable!("BUG: Invalid state: we never discard only our file, always both."),
        }
    }
}