annotate rust/hg-core/src/revlog.rs @ 44142:63db6657d280

rust-nodemap: building blocks for nodetree structures

This is similar to `nodetreenode` in `revlog.c`. We give it
a higher level feeling for ease of handling in Rust context
and provide tools for tests and debugging.

The encoding choice is dictated by our ultimate goal in this
series, that is to make an append-only persistent version of
`nodetree`: the 0th Block must be adressed from other Blocks.

Differential Revision: https://phab.mercurial-scm.org/D7787

// Copyright 2018-2020 Georges Racinet <[e-mail]>
//           and Mercurial contributors
//
// This software may be used and distributed according to the terms of the
// GNU General Public License version 2 or any later version.
//! Mercurial concepts for handling revision history

pub mod nodemap;

/// Mercurial revision numbers
///
/// As noted in revlog.c, revision numbers are actually encoded in
/// 4 bytes, and are liberally converted to ints, whence the i32
pub type Revision = i32;

/// Marker expressing the absence of a parent
///
/// Independently of the actual representation, `NULL_REVISION` is guaranteed
/// to be smaller than all existing revisions.
pub const NULL_REVISION: Revision = -1;

/// Same as `mercurial.node.wdirrev`
///
/// This is also equal to `i32::max_value()`, but it's better to spell
/// it out explicitely, same as in `mercurial.node`
pub const WORKING_DIRECTORY_REVISION: Revision = 0x7fffffff;

/// The simplest expression of what we need of Mercurial DAGs.
pub trait Graph {
    /// Return the two parents of the given `Revision`.
    ///
    /// Each of the parents can be independently `NULL_REVISION`
    fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError>;
}

#[derive(Clone, Debug, PartialEq)]
pub enum GraphError {
    ParentOutOfRange(Revision),
    WorkingDirectoryUnsupported,
}