// Copyright 2019-2020 Georges Racinet <georges.racinet@octobus.net>

//

// This software may be used and distributed according to the terms of the

// GNU General Public License version 2 or any later version.

//! Minimal `RevlogIndex`, readable from standard Mercurial file format

use hg::*;

use memmap2::*;

use std::fs::File;

use std::ops::Deref;

use std::path::Path;

use std::slice;

pub struct Index {

    data: Box<dyn Deref<Target = [IndexEntry]> + Send>,

}

/// A fixed sized index entry. All numbers are big endian

#[repr(C)]

pub struct IndexEntry {

    not_used_yet: [u8; 24],

    p1: Revision,

    p2: Revision,

    node: Node,

    unused_node: [u8; 12],

}

pub const INDEX_ENTRY_SIZE: usize = 64;

impl IndexEntry {

    fn parents(&self) -> [Revision; 2] {

        [self.p1, self.p2]

    }

}

impl RevlogIndex for Index {

    fn len(&self) -> usize {

        self.data.len()

    }

    fn node(&self, rev: Revision) -> Option<&Node> {

        if rev == NULL_REVISION {

            return None;

        }

        Some(&self.data[rev.0 as usize].node)

    }

}

impl Graph for &Index {

    fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError> {

        let [p1, p2] = self.data[rev.0 as usize].parents();

        let len = (*self).len();

        if p1 < NULL_REVISION

            || p2 < NULL_REVISION

            || p1.0 as usize >= len

            || p2.0 as usize >= len

        {

            return Err(GraphError::ParentOutOfRange(rev));

        }

        Ok([p1, p2])

    }

}

struct IndexMmap(Mmap);

impl Deref for IndexMmap {

    type Target = [IndexEntry];

    fn deref(&self) -> &[IndexEntry] {

        let ptr = self.0.as_ptr() as *const IndexEntry;

        // Any misaligned data will be ignored.

        debug_assert_eq!(

            self.0.len() % std::mem::align_of::<IndexEntry>(),

            0,

            "Misaligned data in mmap"

        );

        unsafe { slice::from_raw_parts(ptr, self.0.len() / INDEX_ENTRY_SIZE) }

    }

}

impl Index {

    pub fn load_mmap(path: impl AsRef<Path>) -> Self {

        let file = File::open(path).unwrap();

        let msg = "Index file is missing, or missing permission";

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

        Self {

            data: Box::new(IndexMmap(mmap)),

        }

    }

}