// cindex.rs

//

// Copyright 2018 Georges Racinet <gracinet@anybox.fr>

//

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

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

//! Bindings to use the Index defined by the parsers C extension

//!

//! Ideally, we should use an Index entirely implemented in Rust,

//! but this will take some time to get there.

use cpython::{PyClone, PyObject, PyResult, Python};

use hg::{Graph, GraphError, Revision, WORKING_DIRECTORY_REVISION};

use libc::c_int;

#[repr(C)]

pub struct Revlog_CAPI {

    index_parents: unsafe extern "C" fn(

        index: *mut revlog_capi::RawPyObject,

        rev: c_int,

        ps: *mut [c_int; 2],

    ) -> c_int,

}

py_capsule!(

    from mercurial.cext.parsers import revlog_CAPI

        as revlog_capi for Revlog_CAPI);

/// A `Graph` backed up by objects and functions from revlog.c

///

/// This implementation of the `Graph` trait, relies on (pointers to)

/// - the C index object (`index` member)

/// - the `index_get_parents()` function (`parents` member)

///

/// # Safety

///

/// The C index itself is mutable, and this Rust exposition is **not

/// protected by the GIL**, meaning that this construct isn't safe with respect

/// to Python threads.

///

/// All callers of this `Index` must acquire the GIL and must not release it

/// while working.

///

/// # TODO find a solution to make it GIL safe again.

///

/// This is non trivial, and can wait until we have a clearer picture with

/// more Rust Mercurial constructs.

///

/// One possibility would be to a `GILProtectedIndex` wrapper enclosing

/// a `Python<'p>` marker and have it be the one implementing the

/// `Graph` trait, but this would mean the `Graph` implementor would become

/// likely to change between subsequent method invocations of the `hg-core`

/// objects (a serious change of the `hg-core` API):

/// either exposing ways to mutate the `Graph`, or making it a non persistent

/// parameter in the relevant methods that need one.

///

/// Another possibility would be to introduce an abstract lock handle into

/// the core API, that would be tied to `GILGuard` / `Python<'p>`

/// in the case of the `cpython` crate bindings yet could leave room for other

/// mechanisms in other contexts.

pub struct Index {

    index: PyObject,

    capi: &'static Revlog_CAPI,

}

impl Index {

    pub fn new(py: Python, index: PyObject) -> PyResult<Self> {

        Ok(Index {

            index: index,

            capi: unsafe { revlog_capi::retrieve(py)? },

        })

    }

}

impl Clone for Index {

    fn clone(&self) -> Self {

        let guard = Python::acquire_gil();

        Index {

            index: self.index.clone_ref(guard.python()),

            capi: self.capi,

        }

    }

}

impl Graph for Index {

    /// wrap a call to the C extern parents function

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

        if rev == WORKING_DIRECTORY_REVISION {

            return Err(GraphError::WorkingDirectoryUnsupported);

        }

        let mut res: [c_int; 2] = [0; 2];

        let code = unsafe {

            (self.capi.index_parents)(

                self.index.as_ptr(),

                rev as c_int,

                &mut res as *mut [c_int; 2],

            )

        };

        match code {

            0 => Ok(res),

            _ => Err(GraphError::ParentOutOfRange(rev)),

        }

    }

}