use std::fmt::Debug;
use std::ops::Deref;

use byteorder::{BigEndian, ByteOrder};

use crate::errors::HgError;
use crate::revlog::node::Node;
use crate::revlog::{Revision, NULL_REVISION};
use crate::{Graph, GraphError, RevlogIndex, UncheckedRevision};

pub const INDEX_ENTRY_SIZE: usize = 64;

pub struct IndexHeader {
    header_bytes: [u8; 4],
}

#[derive(Copy, Clone)]
pub struct IndexHeaderFlags {
    flags: u16,
}

/// Corresponds to the high bits of `_format_flags` in python
impl IndexHeaderFlags {
    /// Corresponds to FLAG_INLINE_DATA in python
    pub fn is_inline(self) -> bool {
        self.flags & 1 != 0
    }
    /// Corresponds to FLAG_GENERALDELTA in python
    pub fn uses_generaldelta(self) -> bool {
        self.flags & 2 != 0
    }
}

/// Corresponds to the INDEX_HEADER structure,
/// which is parsed as a `header` variable in `_loadindex` in `revlog.py`
impl IndexHeader {
    fn format_flags(&self) -> IndexHeaderFlags {
        // No "unknown flags" check here, unlike in python. Maybe there should
        // be.
        IndexHeaderFlags {
            flags: BigEndian::read_u16(&self.header_bytes[0..2]),
        }
    }

    /// The only revlog version currently supported by rhg.
    const REVLOGV1: u16 = 1;

    /// Corresponds to `_format_version` in Python.
    fn format_version(&self) -> u16 {
        BigEndian::read_u16(&self.header_bytes[2..4])
    }

    const EMPTY_INDEX_HEADER: IndexHeader = IndexHeader {
        // We treat an empty file as a valid index with no entries.
        // Here we make an arbitrary choice of what we assume the format of the
        // index to be (V1, using generaldelta).
        // This doesn't matter too much, since we're only doing read-only
        // access. but the value corresponds to the `new_header` variable in
        // `revlog.py`, `_loadindex`
        header_bytes: [0, 3, 0, 1],
    };

    fn parse(index_bytes: &[u8]) -> Result<IndexHeader, HgError> {
        if index_bytes.is_empty() {
            return Ok(IndexHeader::EMPTY_INDEX_HEADER);
        }
        if index_bytes.len() < 4 {
            return Err(HgError::corrupted(
                "corrupted revlog: can't read the index format header",
            ));
        }
        Ok(IndexHeader {
            header_bytes: {
                let bytes: [u8; 4] =
                    index_bytes[0..4].try_into().expect("impossible");
                bytes
            },
        })
    }
}

/// Abstracts the access to the index bytes since they can be spread between
/// the immutable (bytes) part and the mutable (added) part if any appends
/// happened. This makes it transparent for the callers.
struct IndexData {
    /// Immutable bytes, most likely taken from disk
    bytes: Box<dyn Deref<Target = [u8]> + Send>,
    /// Bytes that were added after reading the index
    added: Vec<u8>,
}

impl IndexData {
    pub fn new(bytes: Box<dyn Deref<Target = [u8]> + Send>) -> Self {
        Self {
            bytes,
            added: vec![],
        }
    }

    pub fn len(&self) -> usize {
        self.bytes.len() + self.added.len()
    }
}

impl std::ops::Index<std::ops::Range<usize>> for IndexData {
    type Output = [u8];

    fn index(&self, index: std::ops::Range<usize>) -> &Self::Output {
        let start = index.start;
        let end = index.end;
        let immutable_len = self.bytes.len();
        if start < immutable_len {
            if end > immutable_len {
                panic!("index data cannot span existing and added ranges");
            }
            &self.bytes[index]
        } else {
            &self.added[start - immutable_len..end - immutable_len]
        }
    }
}

/// A Revlog index
pub struct Index {
    bytes: IndexData,
    /// Offsets of starts of index blocks.
    /// Only needed when the index is interleaved with data.
    offsets: Option<Vec<usize>>,
    uses_generaldelta: bool,
}

impl Debug for Index {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("Index")
            .field("offsets", &self.offsets)
            .field("uses_generaldelta", &self.uses_generaldelta)
            .finish()
    }
}

impl Graph for Index {
    fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError> {
        let err = || GraphError::ParentOutOfRange(rev);
        match self.get_entry(rev) {
            Some(entry) => {
                // The C implementation checks that the parents are valid
                // before returning
                Ok([
                    self.check_revision(entry.p1()).ok_or_else(err)?,
                    self.check_revision(entry.p2()).ok_or_else(err)?,
                ])
            }
            None => Ok([NULL_REVISION, NULL_REVISION]),
        }
    }
}

impl Index {
    /// Create an index from bytes.
    /// Calculate the start of each entry when is_inline is true.
    pub fn new(
        bytes: Box<dyn Deref<Target = [u8]> + Send>,
    ) -> Result<Self, HgError> {
        let header = IndexHeader::parse(bytes.as_ref())?;

        if header.format_version() != IndexHeader::REVLOGV1 {
            // A proper new version should have had a repo/store
            // requirement.
            return Err(HgError::corrupted("unsupported revlog version"));
        }

        // This is only correct because we know version is REVLOGV1.
        // In v2 we always use generaldelta, while in v0 we never use
        // generaldelta. Similar for [is_inline] (it's only used in v1).
        let uses_generaldelta = header.format_flags().uses_generaldelta();

        if header.format_flags().is_inline() {
            let mut offset: usize = 0;
            let mut offsets = Vec::new();

            while offset + INDEX_ENTRY_SIZE <= bytes.len() {
                offsets.push(offset);
                let end = offset + INDEX_ENTRY_SIZE;
                let entry = IndexEntry {
                    bytes: &bytes[offset..end],
                    offset_override: None,
                };

                offset += INDEX_ENTRY_SIZE + entry.compressed_len() as usize;
            }

            if offset == bytes.len() {
                Ok(Self {
                    bytes: IndexData::new(bytes),
                    offsets: Some(offsets),
                    uses_generaldelta,
                })
            } else {
                Err(HgError::corrupted("unexpected inline revlog length"))
            }
        } else {
            Ok(Self {
                bytes: IndexData::new(bytes),
                offsets: None,
                uses_generaldelta,
            })
        }
    }

    pub fn uses_generaldelta(&self) -> bool {
        self.uses_generaldelta
    }

    /// Value of the inline flag.
    pub fn is_inline(&self) -> bool {
        self.offsets.is_some()
    }

    /// Return a slice of bytes if `revlog` is inline. Panic if not.
    pub fn data(&self, start: usize, end: usize) -> &[u8] {
        if !self.is_inline() {
            panic!("tried to access data in the index of a revlog that is not inline");
        }
        &self.bytes[start..end]
    }

    /// Return number of entries of the revlog index.
    pub fn len(&self) -> usize {
        if let Some(offsets) = &self.offsets {
            offsets.len()
        } else {
            self.bytes.len() / INDEX_ENTRY_SIZE
        }
    }

    /// Returns `true` if the `Index` has zero `entries`.
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Return the index entry corresponding to the given revision if it
    /// exists.
    pub fn get_entry(&self, rev: Revision) -> Option<IndexEntry> {
        if rev == NULL_REVISION {
            return None;
        }
        Some(if let Some(offsets) = &self.offsets {
            self.get_entry_inline(rev, offsets)
        } else {
            self.get_entry_separated(rev)
        })
    }

    fn get_entry_inline(
        &self,
        rev: Revision,
        offsets: &[usize],
    ) -> IndexEntry {
        let start = offsets[rev.0 as usize];
        let end = start + INDEX_ENTRY_SIZE;
        let bytes = &self.bytes[start..end];

        // See IndexEntry for an explanation of this override.
        let offset_override = Some(end);

        IndexEntry {
            bytes,
            offset_override,
        }
    }

    fn get_entry_separated(&self, rev: Revision) -> IndexEntry {
        let start = rev.0 as usize * INDEX_ENTRY_SIZE;
        let end = start + INDEX_ENTRY_SIZE;
        let bytes = &self.bytes[start..end];

        // Override the offset of the first revision as its bytes are used
        // for the index's metadata (saving space because it is always 0)
        let offset_override = if rev == Revision(0) { Some(0) } else { None };

        IndexEntry {
            bytes,
            offset_override,
        }
    }
}

impl super::RevlogIndex for Index {
    fn len(&self) -> usize {
        self.len()
    }

    fn node(&self, rev: Revision) -> Option<&Node> {
        self.get_entry(rev).map(|entry| entry.hash())
    }
}

#[derive(Debug)]
pub struct IndexEntry<'a> {
    bytes: &'a [u8],
    /// Allows to override the offset value of the entry.
    ///
    /// For interleaved index and data, the offset stored in the index
    /// corresponds to the separated data offset.
    /// It has to be overridden with the actual offset in the interleaved
    /// index which is just after the index block.
    ///
    /// For separated index and data, the offset stored in the first index
    /// entry is mixed with the index headers.
    /// It has to be overridden with 0.
    offset_override: Option<usize>,
}

impl<'a> IndexEntry<'a> {
    /// Return the offset of the data.
    pub fn offset(&self) -> usize {
        if let Some(offset_override) = self.offset_override {
            offset_override
        } else {
            let mut bytes = [0; 8];
            bytes[2..8].copy_from_slice(&self.bytes[0..=5]);
            BigEndian::read_u64(&bytes[..]) as usize
        }
    }

    pub fn flags(&self) -> u16 {
        BigEndian::read_u16(&self.bytes[6..=7])
    }

    /// Return the compressed length of the data.
    pub fn compressed_len(&self) -> u32 {
        BigEndian::read_u32(&self.bytes[8..=11])
    }

    /// Return the uncompressed length of the data.
    pub fn uncompressed_len(&self) -> i32 {
        BigEndian::read_i32(&self.bytes[12..=15])
    }

    /// Return the revision upon which the data has been derived.
    pub fn base_revision_or_base_of_delta_chain(&self) -> UncheckedRevision {
        // TODO Maybe return an Option when base_revision == rev?
        //      Requires to add rev to IndexEntry

        BigEndian::read_i32(&self.bytes[16..]).into()
    }

    pub fn link_revision(&self) -> UncheckedRevision {
        BigEndian::read_i32(&self.bytes[20..]).into()
    }

    pub fn p1(&self) -> UncheckedRevision {
        BigEndian::read_i32(&self.bytes[24..]).into()
    }

    pub fn p2(&self) -> UncheckedRevision {
        BigEndian::read_i32(&self.bytes[28..]).into()
    }

    /// Return the hash of revision's full text.
    ///
    /// Currently, SHA-1 is used and only the first 20 bytes of this field
    /// are used.
    pub fn hash(&self) -> &'a Node {
        (&self.bytes[32..52]).try_into().unwrap()
    }
}

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

    #[cfg(test)]
    #[derive(Debug, Copy, Clone)]
    pub struct IndexEntryBuilder {
        is_first: bool,
        is_inline: bool,
        is_general_delta: bool,
        version: u16,
        offset: usize,
        compressed_len: usize,
        uncompressed_len: usize,
        base_revision_or_base_of_delta_chain: Revision,
        link_revision: Revision,
        p1: Revision,
        p2: Revision,
        node: Node,
    }

    #[cfg(test)]
    impl IndexEntryBuilder {
        #[allow(clippy::new_without_default)]
        pub fn new() -> Self {
            Self {
                is_first: false,
                is_inline: false,
                is_general_delta: true,
                version: 1,
                offset: 0,
                compressed_len: 0,
                uncompressed_len: 0,
                base_revision_or_base_of_delta_chain: Revision(0),
                link_revision: Revision(0),
                p1: NULL_REVISION,
                p2: NULL_REVISION,
                node: NULL_NODE,
            }
        }

        pub fn is_first(&mut self, value: bool) -> &mut Self {
            self.is_first = value;
            self
        }

        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 with_offset(&mut self, value: usize) -> &mut Self {
            self.offset = value;
            self
        }

        pub fn with_compressed_len(&mut self, value: usize) -> &mut Self {
            self.compressed_len = value;
            self
        }

        pub fn with_uncompressed_len(&mut self, value: usize) -> &mut Self {
            self.uncompressed_len = value;
            self
        }

        pub fn with_base_revision_or_base_of_delta_chain(
            &mut self,
            value: Revision,
        ) -> &mut Self {
            self.base_revision_or_base_of_delta_chain = value;
            self
        }

        pub fn with_link_revision(&mut self, value: Revision) -> &mut Self {
            self.link_revision = value;
            self
        }

        pub fn with_p1(&mut self, value: Revision) -> &mut Self {
            self.p1 = value;
            self
        }

        pub fn with_p2(&mut self, value: Revision) -> &mut Self {
            self.p2 = value;
            self
        }

        pub fn with_node(&mut self, value: Node) -> &mut Self {
            self.node = value;
            self
        }

        pub fn build(&self) -> Vec<u8> {
            let mut bytes = Vec::with_capacity(INDEX_ENTRY_SIZE);
            if self.is_first {
                bytes.extend(&match (self.is_general_delta, self.is_inline) {
                    (false, false) => [0u8, 0],
                    (false, true) => [0u8, 1],
                    (true, false) => [0u8, 2],
                    (true, true) => [0u8, 3],
                });
                bytes.extend(&self.version.to_be_bytes());
                // Remaining offset bytes.
                bytes.extend(&[0u8; 2]);
            } else {
                // Offset stored on 48 bits (6 bytes)
                bytes.extend(&(self.offset as u64).to_be_bytes()[2..]);
            }
            bytes.extend(&[0u8; 2]); // Revision flags.
            bytes.extend(&(self.compressed_len as u32).to_be_bytes());
            bytes.extend(&(self.uncompressed_len as u32).to_be_bytes());
            bytes.extend(
                &self.base_revision_or_base_of_delta_chain.0.to_be_bytes(),
            );
            bytes.extend(&self.link_revision.0.to_be_bytes());
            bytes.extend(&self.p1.0.to_be_bytes());
            bytes.extend(&self.p2.0.to_be_bytes());
            bytes.extend(self.node.as_bytes());
            bytes.extend(vec![0u8; 12]);
            bytes
        }
    }

    pub fn is_inline(index_bytes: &[u8]) -> bool {
        IndexHeader::parse(index_bytes)
            .expect("too short")
            .format_flags()
            .is_inline()
    }

    pub fn uses_generaldelta(index_bytes: &[u8]) -> bool {
        IndexHeader::parse(index_bytes)
            .expect("too short")
            .format_flags()
            .uses_generaldelta()
    }

    pub fn get_version(index_bytes: &[u8]) -> u16 {
        IndexHeader::parse(index_bytes)
            .expect("too short")
            .format_version()
    }

    #[test]
    fn flags_when_no_inline_flag_test() {
        let bytes = IndexEntryBuilder::new()
            .is_first(true)
            .with_general_delta(false)
            .with_inline(false)
            .build();

        assert!(!is_inline(&bytes));
        assert!(!uses_generaldelta(&bytes));
    }

    #[test]
    fn flags_when_inline_flag_test() {
        let bytes = IndexEntryBuilder::new()
            .is_first(true)
            .with_general_delta(false)
            .with_inline(true)
            .build();

        assert!(is_inline(&bytes));
        assert!(!uses_generaldelta(&bytes));
    }

    #[test]
    fn flags_when_inline_and_generaldelta_flags_test() {
        let bytes = IndexEntryBuilder::new()
            .is_first(true)
            .with_general_delta(true)
            .with_inline(true)
            .build();

        assert!(is_inline(&bytes));
        assert!(uses_generaldelta(&bytes));
    }

    #[test]
    fn test_offset() {
        let bytes = IndexEntryBuilder::new().with_offset(1).build();
        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(entry.offset(), 1)
    }

    #[test]
    fn test_with_overridden_offset() {
        let bytes = IndexEntryBuilder::new().with_offset(1).build();
        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: Some(2),
        };

        assert_eq!(entry.offset(), 2)
    }

    #[test]
    fn test_compressed_len() {
        let bytes = IndexEntryBuilder::new().with_compressed_len(1).build();
        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(entry.compressed_len(), 1)
    }

    #[test]
    fn test_uncompressed_len() {
        let bytes = IndexEntryBuilder::new().with_uncompressed_len(1).build();
        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(entry.uncompressed_len(), 1)
    }

    #[test]
    fn test_base_revision_or_base_of_delta_chain() {
        let bytes = IndexEntryBuilder::new()
            .with_base_revision_or_base_of_delta_chain(Revision(1))
            .build();
        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(entry.base_revision_or_base_of_delta_chain(), 1.into())
    }

    #[test]
    fn link_revision_test() {
        let bytes = IndexEntryBuilder::new()
            .with_link_revision(Revision(123))
            .build();

        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(entry.link_revision(), 123.into());
    }

    #[test]
    fn p1_test() {
        let bytes = IndexEntryBuilder::new().with_p1(Revision(123)).build();

        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(entry.p1(), 123.into());
    }

    #[test]
    fn p2_test() {
        let bytes = IndexEntryBuilder::new().with_p2(Revision(123)).build();

        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(entry.p2(), 123.into());
    }

    #[test]
    fn node_test() {
        let node = Node::from_hex("0123456789012345678901234567890123456789")
            .unwrap();
        let bytes = IndexEntryBuilder::new().with_node(node).build();

        let entry = IndexEntry {
            bytes: &bytes,
            offset_override: None,
        };

        assert_eq!(*entry.hash(), node);
    }

    #[test]
    fn version_test() {
        let bytes = IndexEntryBuilder::new()
            .is_first(true)
            .with_version(2)
            .build();

        assert_eq!(get_version(&bytes), 2)
    }
}

#[cfg(test)]
pub use tests::IndexEntryBuilder;