// SPDX-License-Identifier: AGPL-3.0-or-later

//! This module contains the [BufReader] type.

use log::error;
use positioned_io::Size;
use std::io::{self, Cursor, Read, Seek, SeekFrom};

use crate::{bterr, Decompose, Result, Sectored, Split, EMPTY_SLICE};

pub use private::BufReader;

mod private {
    use crate::{ReadExt, SeekFromExt, SizeExt};

    use super::*;

    /// Nearly identical to [std::io::BufReader] but allows a buffer to be reused between
    /// instantiations.
    pub struct BufReader<T> {
        cursor: Cursor<Vec<u8>>,
        reader: T,
    }

    impl<T> BufReader<T> {
        /// Create a new [BufReader] which contains the given buffer and reader type.
        pub fn with_buf(buf: Vec<u8>, reader: T) -> Result<BufReader<T>> {
            Ok(Self {
                cursor: Self::make_cursor(buf)?,
                reader,
            })
        }

        /// Returns a reference to the inner reader.
        pub fn get_ref(&self) -> &T {
            &self.reader
        }

        /// Extracts the buffer from this [BufReader]. The [BufReader] which is returned contains
        /// an empty buffer.
        pub fn take_buf(mut self) -> (Self, Vec<u8>) {
            let buf = self.cursor.into_inner();
            self.cursor = Cursor::new(Vec::new());
            (self, buf)
        }

        fn make_cursor(buf: Vec<u8>) -> Result<Cursor<Vec<u8>>> {
            // If buf is zero-length then a call to read will loop forever.
            if buf.is_empty() {
                return Err(bterr!("the given vector must be non-empty"));
            }
            let mut cursor = Cursor::new(buf);
            cursor.seek(SeekFrom::End(0)).unwrap();
            Ok(cursor)
        }

        /// Returns true if all bytes have been read from the cursor.
        fn cursor_is_empty(&self) -> bool {
            let cursor_len = self.cursor.get_ref().len() as u64;
            let cursor_pos = self.cursor.position();
            cursor_pos >= cursor_len
        }
    }

    impl<T: Sectored> BufReader<T> {
        /// Creates a new [BufReader] containing the given reader type and a new buffer.
        pub fn new(reader: T) -> Result<BufReader<T>> {
            let sect_sz = reader.sector_sz();
            Ok(Self {
                cursor: Self::make_cursor(vec![0u8; sect_sz])?,
                reader,
            })
        }
    }

    impl<T: Seek> BufReader<T> {
        /// Calculates the current position in this stream.
        pub fn pos(&mut self) -> io::Result<u64> {
            let inner_pos = self.reader.stream_position()?;
            // Because the inner stream is ahead of this stream, the current position is the
            // position of the inner stream minus the number of bytes remaining in the cursor.
            let remaining = self.cursor.get_ref().len() as u64 - self.cursor.position();
            let pos = inner_pos - remaining;
            Ok(pos)
        }
    }

    impl<T: Read> BufReader<T> {
        /// Refills the cursor by reading from the underlying stream.
        fn refill(&mut self) -> Result<()> {
            self.cursor.rewind()?;
            let vec = self.cursor.get_mut();
            let read = self.reader.fill_buf(vec)?;
            if read == 0 || read == vec.len() {
                Ok(())
            } else {
                Err(bterr!("unexpected number of bytes read: {read}"))
            }
        }
    }

    impl<T: Read> Read for BufReader<T> {
        fn read(&mut self, mut buf: &mut [u8]) -> std::io::Result<usize> {
            if buf.len() == self.sector_sz() && self.cursor_is_empty() {
                return self.reader.read(buf);
            }
            let buf_len_start = buf.len();
            while !buf.is_empty() {
                let read = match self.cursor.read(buf) {
                    Ok(read) => read,
                    Err(err) => {
                        if buf_len_start == buf.len() {
                            return Err(err);
                        } else {
                            error!("{err}");
                            break;
                        }
                    }
                };
                buf = &mut buf[read..];
                if self.cursor_is_empty() {
                    if let Err(err) = self.refill() {
                        if buf_len_start == buf.len() {
                            return Err(err.into());
                        } else {
                            error!("error occurred in BufReader::refill: {err}");
                            break;
                        }
                    }
                }
            }
            Ok(buf_len_start - buf.len())
        }
    }

    impl<T: Seek + Read + Size> Seek for BufReader<T> {
        fn seek(&mut self, seek_from: std::io::SeekFrom) -> std::io::Result<u64> {
            let pos = self.pos()?;
            let new_pos = seek_from.abs(|| Ok(pos), || self.reader.size_or_err())?;
            let sect_sz = self.sector_sz64();
            let buf_pos = new_pos % sect_sz;
            let index = pos / sect_sz;
            let new_index = new_pos / sect_sz;
            if index != new_index {
                // Seek to the new position and invalidate the buffer.
                self.reader.seek(SeekFrom::Start(sect_sz * new_index))?;
                self.cursor.seek(SeekFrom::End(0))?;
            }
            if buf_pos != 0 {
                // If the buffer position is not at the end then we must refill it.
                self.refill()?;
                self.cursor.seek(SeekFrom::Start(buf_pos))?;
            }
            Ok(new_pos)
        }
    }

    impl<U, T: AsRef<U>> AsRef<U> for BufReader<T> {
        fn as_ref(&self) -> &U {
            self.reader.as_ref()
        }
    }

    impl<U, T: AsMut<U>> AsMut<U> for BufReader<T> {
        fn as_mut(&mut self) -> &mut U {
            self.reader.as_mut()
        }
    }

    impl<T: Size> Size for BufReader<T> {
        fn size(&self) -> std::io::Result<Option<u64>> {
            self.reader.size()
        }
    }

    impl<T> Sectored for BufReader<T> {
        fn sector_sz(&self) -> usize {
            self.cursor.get_ref().len()
        }
    }

    impl<T> Decompose<T> for BufReader<T> {
        fn into_inner(self) -> T {
            self.reader
        }
    }

    impl<T> Split<BufReader<&'static [u8]>, T> for BufReader<T> {
        fn split(self) -> (BufReader<&'static [u8]>, T) {
            let reader = BufReader {
                cursor: self.cursor,
                reader: EMPTY_SLICE,
            };
            (reader, self.reader)
        }

        fn combine(left: BufReader<&'static [u8]>, right: T) -> Self {
            BufReader {
                cursor: left.cursor,
                reader: right,
            }
        }
    }
}

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

    use crate::test_helpers::{
        random_indices, read_check, read_indices, write_fill, write_indices, Randomizer,
        SectoredCursor,
    };

    #[test]
    fn can_read() {
        const EXPECTED: [u8; 32] = [1u8; 32];
        let mut reader = BufReader::new(SectoredCursor::new(EXPECTED, EXPECTED.len())).unwrap();

        let mut actual = [0u8; EXPECTED.len()];
        reader.read(actual.as_mut()).expect("read failed");

        assert_eq!(EXPECTED, actual);
    }

    /// Tests that the inner [Read] only sees calls to `read` with sector sized buffers.
    #[test]
    fn inner_sees_only_sector_sized_reads() {
        const SECT_SZ: usize = 32;
        const CHUNK_SZ: usize = 8;
        const CHUNKS: usize = SECT_SZ / CHUNK_SZ;
        let data = std::iter::successors(Some(1u8), |prev| Some(*prev + 1))
            .map(|e| [e; CHUNK_SZ])
            .take(CHUNKS)
            .fold(Vec::with_capacity(SECT_SZ), |mut prev, curr| {
                prev.extend_from_slice(&curr);
                prev
            });
        // SectoredCursor will panic if it's given a buffer that isn't exactly SECT_SZ bytes long.
        let mut reader =
            BufReader::new(SectoredCursor::new(data, SECT_SZ).require_sect_sz(true)).unwrap();

        let mut actual = [0u8; CHUNK_SZ];
        for k in 1..(CHUNKS + 1) {
            let expected = [k as u8; CHUNK_SZ];
            reader.read(&mut actual).expect("read failed");
            assert_eq!(expected, actual);
        }
    }

    #[test]
    fn sequential_read() {
        const SECT_SZ: usize = 16;
        const SECT_CT: usize = 8;
        let mut cursor = SectoredCursor::new(Vec::new(), SECT_SZ);
        write_fill(&mut cursor, SECT_SZ, SECT_CT);
        cursor.rewind().unwrap();
        let mut reader = BufReader::new(cursor).unwrap();
        read_check(&mut reader, SECT_SZ, SECT_CT);
    }

    /// Tests that a read which is larger than one sector will be handled correctly.
    #[test]
    fn read_larger_than_one_sector() {
        const SECT_SZ: usize = 4;
        const DATA: [u8; 8] = [0, 1, 2, 3, 4, 5, 6, 7];
        let mut reader = BufReader::new(SectoredCursor::new(DATA, SECT_SZ)).unwrap();

        let mut actual = [0u8; 6];
        reader.read(&mut actual).expect("read failed");

        assert_eq!([0, 1, 2, 3, 4, 5], actual);
    }

    #[test]
    fn random_sector_sized_read() {
        const SECT_SZ: usize = 32;
        const SECT_CT: usize = 10;
        let mut rando = Randomizer::new([7u8; Randomizer::HASH.len()]);
        let indices: Vec<_> = random_indices(&mut rando, SECT_CT).collect();
        let mut cursor = SectoredCursor::new(vec![0u8; SECT_SZ * SECT_CT], SECT_SZ);
        write_indices(&mut cursor, SECT_SZ, indices.iter().cloned());
        let mut reader = BufReader::new(cursor).unwrap();
        read_indices(&mut reader, SECT_SZ, indices.iter().cloned());
    }

    #[test]
    fn seek_with_empty_buffer() {
        const SECT_SZ: usize = 4;
        const DATA: [u8; 2 * SECT_SZ] = [0, 1, 2, 3, 4, 5, 6, 7];
        const EXPECTED: u64 = 3;
        let mut reader = BufReader::new(SectoredCursor::new(DATA, SECT_SZ)).unwrap();

        let new_pos = reader.seek(SeekFrom::Start(EXPECTED)).expect("seek failed");
        assert_eq!(EXPECTED, new_pos);
        let mut actual = [0u8; 1];
        reader.read(&mut actual).expect("read failed");

        assert_eq!(EXPECTED as u8, actual[0]);
    }

    #[test]
    fn seek_to_middle_of_next_sector() {
        const SECT_SZ: usize = 4;
        const DATA: [u8; 2 * SECT_SZ] = [0, 1, 2, 3, 4, 5, 6, 7];
        const EXPECTED: u64 = 5;
        let mut reader = BufReader::new(SectoredCursor::new(DATA, SECT_SZ)).unwrap();
        let mut actual = [0u8; 1];
        // This first read ensures the buffer is filled.
        reader.read(&mut actual).expect("first read failed");

        let new_pos = reader.seek(SeekFrom::Start(EXPECTED)).expect("seek failed");
        assert_eq!(EXPECTED, new_pos);
        reader.read(&mut actual).expect("read failed");

        assert_eq!(EXPECTED as u8, actual[0]);
    }

    #[test]
    fn seek_relative_to_current_position() {
        const SECT_SZ: usize = 4;
        const DATA: [u8; 2 * SECT_SZ] = [0, 1, 2, 3, 4, 5, 6, 7];
        const EXPECTED: u64 = 3;
        let mut reader = BufReader::new(SectoredCursor::new(DATA, SECT_SZ)).unwrap();
        let mut actual = [0u8; SECT_SZ];
        reader.read(&mut actual).expect("first read failed");

        let new_pos = reader.seek(SeekFrom::Current(-1)).expect("seek failed");
        assert_eq!(EXPECTED, new_pos);
        reader.read(&mut actual).expect("read failed");

        assert_eq!(EXPECTED as u8, actual[0]);
    }

    #[test]
    fn seek_relative_to_end() {
        const SECT_SZ: usize = 4;
        const DATA: [u8; 2 * SECT_SZ] = [0, 1, 2, 3, 4, 5, 6, 7];
        const EXPECTED: u64 = 7;
        let mut reader = BufReader::new(SectoredCursor::new(DATA, SECT_SZ)).unwrap();
        let mut actual = [0u8; SECT_SZ];
        reader.read(&mut actual).expect("first read failed");

        let new_pos = reader.seek(SeekFrom::End(-1)).expect("seek failed");
        assert_eq!(EXPECTED, new_pos);
        reader.read(&mut actual).expect("read failed");

        assert_eq!(EXPECTED as u8, actual[0]);
    }
}