rust-cpython: binding for headrevs()

rust-cpython: binding for headrevs()

This uses the core `dagops::retain_heads` to give a Rust implementation
to `mercurial.dagop.headrevs`.

Testing happens for now from `test-rust-ancestors.py`
(for quick and minimal change), but it'd made more sense to put the binary
index data elsewhere and to create a new test python module

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-cpython/src/dagops.rs	Thu Jan 10 18:25:18 2019 +0100
@@ -0,0 +1,53 @@
+// dagops.rs
+//
+// Copyright 2019 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.
+
+//! Bindings for the `hg::dagops` module provided by the
+//! `hg-core` package.
+//!
+//! From Python, this will be seen as `mercurial.rustext.dagop`
+use cindex::Index;
+use cpython::{PyDict, PyModule, PyObject, PyResult, Python};
+use crate::conversion::{py_set, rev_pyiter_collect};
+use exceptions::GraphError;
+use hg::dagops;
+use hg::Revision;
+use std::collections::HashSet;
+
+/// Using the the `index`, return heads out of any Python iterable of Revisions
+///
+/// This is the Rust counterpart for `mercurial.dagop.headrevs`
+pub fn headrevs(
+    py: Python,
+    index: PyObject,
+    revs: PyObject,
+) -> PyResult<PyObject> {
+    let mut as_set: HashSet<Revision> = rev_pyiter_collect(py, &revs)?;
+    dagops::retain_heads(&Index::new(py, index)?, &mut as_set)
+        .map_err(|e| GraphError::pynew(py, e))?;
+    py_set(py, &as_set)
+}
+
+/// Create the module, with `__package__` given from parent
+pub fn init_module(py: Python, package: &str) -> PyResult<PyModule> {
+    let dotted_name = &format!("{}.dagop", package);
+    let m = PyModule::new(py, dotted_name)?;
+    m.add(py, "__package__", package)?;
+    m.add(py, "__doc__", "DAG operations - Rust implementation")?;
+    m.add(
+        py,
+        "headrevs",
+        py_fn!(py, headrevs(index: PyObject, revs: PyObject)),
+    )?;
+
+    let sys = PyModule::import(py, "sys")?;
+    let sys_modules: PyDict = sys.get(py, "modules")?.extract(py)?;
+    sys_modules.set_item(py, dotted_name, &m)?;
+    // Example C code (see pyexpat.c and import.c) will "give away the
+    // reference", but we won't because it will be consumed once the
+    // Rust PyObject is dropped.
+    Ok(m)
+}

--- a/rust/hg-cpython/src/lib.rs	Wed Jan 16 16:05:27 2019 +0100
+++ b/rust/hg-cpython/src/lib.rs	Thu Jan 10 18:25:18 2019 +0100
@@ -27,6 +27,7 @@
 pub mod ancestors;
 mod cindex;
 mod conversion;
+pub mod dagops;
 pub mod exceptions;
 
 py_module_initializer!(rustext, initrustext, PyInit_rustext, |py, m| {
@@ -38,6 +39,7 @@
 
     let dotted_name: String = m.get(py, "__name__")?.extract(py)?;
     m.add(py, "ancestor", ancestors::init_module(py, &dotted_name)?)?;
+    m.add(py, "dagop", dagops::init_module(py, &dotted_name)?)?;
     m.add(py, "GraphError", py.get_type::<exceptions::GraphError>())?;
     Ok(())
 });

--- a/tests/test-rust-ancestor.py	Wed Jan 16 16:05:27 2019 +0100
+++ b/tests/test-rust-ancestor.py	Thu Jan 10 18:25:18 2019 +0100
@@ -19,6 +19,7 @@
         LazyAncestors,
         MissingAncestors,
     )
+    from mercurial.rustext import dagop
 
 try:
     from mercurial.cext import parsers as cparsers
@@ -165,6 +166,10 @@
         with self.assertRaises(error.WdirUnsupported):
             list(AncestorsIterator(idx, [node.wdirrev], -1, False))
 
+    def testheadrevs(self):
+        idx = self.parseindex()
+        self.assertEqual(dagop.headrevs(idx, [1, 2, 3]), {3})
+
 if __name__ == '__main__':
     import silenttestrunner
     silenttestrunner.main(__name__)