use std::borrow::Cow;

use std::fs::File;

use std::io::Read;

use std::ops::Deref;

use std::path::Path;

use byteorder::{BigEndian, ByteOrder};

use flate2::read::ZlibDecoder;

use memmap::{Mmap, MmapOptions};

use micro_timer::timed;

use zstd;

use super::index::Index;

use super::patch;

use crate::revlog::Revision;

pub enum RevlogError {

    IoError(std::io::Error),

    UnsuportedVersion(u16),

    InvalidRevision,

    Corrupted,

    UnknowDataFormat(u8),

}

fn mmap_open(path: &Path) -> Result<Mmap, std::io::Error> {

    let file = File::open(path)?;

    let mmap = unsafe { MmapOptions::new().map(&file) }?;

    Ok(mmap)

}

/// Read only implementation of revlog.

pub struct Revlog {

    /// When index and data are not interleaved: bytes of the revlog index.

    /// When index and data are interleaved: bytes of the revlog index and

    /// data.

    index_bytes: Box<dyn Deref<Target = [u8]> + Send>,

    /// When index and data are not interleaved: bytes of the revlog data

    data_bytes: Option<Box<dyn Deref<Target = [u8]> + Send>>,

}

impl Revlog {

    /// Open a revlog index file.

    ///

    /// It will also open the associated data file if index and data are not

    /// interleaved.

    #[timed]

    pub fn open(index_path: &Path) -> Result<Self, RevlogError> {

        let index_mmap =

            mmap_open(&index_path).map_err(RevlogError::IoError)?;

        let version = get_version(&index_mmap);

        if version != 1 {

            return Err(RevlogError::UnsuportedVersion(version));

        }

        let is_inline = is_inline(&index_mmap);

        let index_bytes = Box::new(index_mmap);

        // TODO load data only when needed //

        // type annotation required

        // won't recognize Mmap as Deref<Target = [u8]>

        let data_bytes: Option<Box<dyn Deref<Target = [u8]> + Send>> =

            if is_inline {

                None

            } else {

                let data_path = index_path.with_extension("d");

                let data_mmap =

                    mmap_open(&data_path).map_err(RevlogError::IoError)?;

                Some(Box::new(data_mmap))

            };

        Ok(Revlog {

            index_bytes,

            data_bytes,

        })

    }

    /// Return the full data associated to a revision.

    ///

    /// All entries required to build the final data out of deltas will be

    /// retrieved as needed, and the deltas will be applied to the inital

    /// snapshot to rebuild the final data.

    #[timed]

    pub fn get_rev_data(&self, rev: Revision) -> Result<Vec<u8>, RevlogError> {

        // Todo return -> Cow

        let mut entry = self.get_entry(rev)?;

        let mut delta_chain = vec![];

        while let Some(base_rev) = entry.base_rev {

            delta_chain.push(entry);

            entry = self

                .get_entry(base_rev)

                .map_err(|_| RevlogError::Corrupted)?;

        }

        } else {

        }

    }

    /// Build the full data of a revision out its snapshot

    /// and its deltas.

    #[timed]

    fn build_data_from_deltas(

        snapshot: RevlogEntry,

        deltas: &[RevlogEntry],

    ) -> Result<Vec<u8>, RevlogError> {

        let snapshot = snapshot.data()?;

        let deltas = deltas

            .iter()

            .rev()

            .map(RevlogEntry::data)

            .collect::<Result<Vec<Cow<'_, [u8]>>, RevlogError>>()?;

        let patches: Vec<_> =

            deltas.iter().map(|d| patch::PatchList::new(d)).collect();

        let patch = patch::fold_patch_lists(&patches);

        Ok(patch.apply(&snapshot))

    }

    /// Return the revlog index.

    fn index(&self) -> Index {

        let is_inline = self.data_bytes.is_none();

        Index::new(&self.index_bytes, is_inline)

    }

    /// Return the revlog data.

    fn data(&self) -> &[u8] {

        match self.data_bytes {

            Some(ref data_bytes) => &data_bytes,

            None => &self.index_bytes,

        }

    }

    /// Get an entry of the revlog.

    fn get_entry(&self, rev: Revision) -> Result<RevlogEntry, RevlogError> {

        let index = self.index();

        let index_entry =

            index.get_entry(rev).ok_or(RevlogError::InvalidRevision)?;

        let start = index_entry.offset();

        let end = start + index_entry.compressed_len();

        let entry = RevlogEntry {

            rev,

            bytes: &self.data()[start..end],

            compressed_len: index_entry.compressed_len(),

            uncompressed_len: index_entry.uncompressed_len(),

            base_rev: if index_entry.base_revision() == rev {

                None

            } else {

                Some(index_entry.base_revision())

            },

        };

        Ok(entry)

    }

}

/// The revlog entry's bytes and the necessary informations to extract

/// the entry's data.

#[derive(Debug)]

pub struct RevlogEntry<'a> {

    rev: Revision,

    bytes: &'a [u8],

    compressed_len: usize,

    uncompressed_len: usize,

    base_rev: Option<Revision>,

}

impl<'a> RevlogEntry<'a> {

    /// Extract the data contained in the entry.

    pub fn data(&self) -> Result<Cow<'_, [u8]>, RevlogError> {

        if self.bytes.is_empty() {

            return Ok(Cow::Borrowed(&[]));

        }

        match self.bytes[0] {

            // Revision data is the entirety of the entry, including this

            // header.

            b'\0' => Ok(Cow::Borrowed(self.bytes)),

            // Raw revision data follows.

            b'u' => Ok(Cow::Borrowed(&self.bytes[1..])),

            // zlib (RFC 1950) data.

            b'x' => Ok(Cow::Owned(self.uncompressed_zlib_data())),

            // zstd data.

            b'\x28' => Ok(Cow::Owned(self.uncompressed_zstd_data())),

            format_type => Err(RevlogError::UnknowDataFormat(format_type)),

        }

    }

    fn uncompressed_zlib_data(&self) -> Vec<u8> {

        let mut decoder = ZlibDecoder::new(self.bytes);

        if self.is_delta() {

            let mut buf = Vec::with_capacity(self.compressed_len);

            decoder.read_to_end(&mut buf).expect("corrupted zlib data");

            buf

        } else {

            let mut buf = vec![0; self.uncompressed_len];

            decoder.read_exact(&mut buf).expect("corrupted zlib data");

            buf

        }

    }

    fn uncompressed_zstd_data(&self) -> Vec<u8> {

        if self.is_delta() {

            let mut buf = Vec::with_capacity(self.compressed_len);

            zstd::stream::copy_decode(self.bytes, &mut buf)

                .expect("corrupted zstd data");

            buf

        } else {

            let mut buf = vec![0; self.uncompressed_len];

            let len = zstd::block::decompress_to_buffer(self.bytes, &mut buf)

                .expect("corrupted zstd data");

            assert_eq!(len, self.uncompressed_len, "corrupted zstd data");

            buf

        }

    }

    /// Tell if the entry is a snapshot or a delta

    /// (influences on decompression).

    fn is_delta(&self) -> bool {

        self.base_rev.is_some()

    }

}

/// Value of the inline flag.

pub fn is_inline(index_bytes: &[u8]) -> bool {

    match &index_bytes[0..=1] {

        [0, 0] | [0, 2] => false,

        _ => true,

    }

}

/// Format version of the revlog.

pub fn get_version(index_bytes: &[u8]) -> u16 {

    BigEndian::read_u16(&index_bytes[2..=3])

}

#[cfg(test)]

mod tests {

    use super::*;

    use super::super::index::IndexEntryBuilder;

    #[cfg(test)]

    pub struct RevlogBuilder {

        version: u16,

        is_general_delta: bool,

        is_inline: bool,

        offset: usize,

        index: Vec<Vec<u8>>,

        data: Vec<Vec<u8>>,

    }

    #[cfg(test)]

    impl RevlogBuilder {

        pub fn new() -> Self {

            Self {

                version: 2,

                is_inline: false,

                is_general_delta: true,

                offset: 0,

                index: vec![],

                data: vec![],

            }

        }

        pub fn with_inline(&mut self, value: bool) -> &mut Self {

            self.is_inline = value;

            self

        }

        pub fn with_general_delta(&mut self, value: bool) -> &mut Self {

            self.is_general_delta = value;

            self

        }

        pub fn with_version(&mut self, value: u16) -> &mut Self {

            self.version = value;

            self

        }

        pub fn push(

            &mut self,

            mut index: IndexEntryBuilder,

            data: Vec<u8>,

        ) -> &mut Self {

            if self.index.is_empty() {

                index.is_first(true);

                index.with_general_delta(self.is_general_delta);

                index.with_inline(self.is_inline);

                index.with_version(self.version);

            } else {

                index.with_offset(self.offset);

            }