view rust/hg-core/examples/nodemap/index.rs @ 50979:4c5f6e95df84

rust: make `Revision` a newtype

This change is the one we've been building towards during this series.
The aim is to make `Revision` mean more than a simple integer, holding
the information that it is valid for a given revlog index.
While this still allows for programmer error, since creating a revision
directly and querying a different index with a "checked" revision are
still possible, the friction created by the newtype will hopefully make
us think twice about which type to use.

Enough of the Rust ecosystem relies on the newtype pattern to be
efficiently optimized away (even compiler in codegen tests¹), so I'm not
worried about this being a fundamental problem.

[1] https://github.com/rust-lang/rust/blob/7a70647f195f6b0a0f1ebd72b1542ba91a32f43a/tests/codegen/vec-in-place.rs#L47

// Copyright 2019-2020 Georges Racinet <[e-mail]>
//
// 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)),
        }
    }
}