diff mercurial/cext/revlog.c @ 37968:0304f22497fa

revlog: use node tree (native code) for shortest() calculation

I want to rewrite revlog.shortest() to disambiguate only among hex
nodeids and then disambiguate the result with revnums at a higher
level (in scmutil). However, that would slow down `hg log -T
'{shortest(node,1)}\n'` from 5.0s to 6.8s, which I wasn't sure would
be acceptable. So this patch makes revlog.shortest() use the node tree
for finding the length of the shortest prefix that's unambiguous among
nodeids. Once that has been found, it makes it longer until it is also
not ambiguous with a revnum.

This speeds up `hg log -T '{shortest(node,1)}\n'` from 5.0s to 4.0s.

Differential Revision: https://phab.mercurial-scm.org/D3499

--- a/mercurial/cext/revlog.c	Mon May 07 16:49:31 2018 -0700
+++ b/mercurial/cext/revlog.c	Wed May 02 23:17:58 2018 -0700
@@ -1259,6 +1259,55 @@
 	return nt_find(self, node, nodelen, 1);
 }
 
+/*
+ * Find the length of the shortest unique prefix of node.
+ *
+ * Return values:
+ *
+ *   -3: error (exception set)
+ *   -2: not found (no exception set)
+ * rest: length of shortest prefix
+ */
+static int nt_shortest(indexObject *self, const char *node)
+{
+	int level, off;
+
+	if (nt_init(self) == -1)
+		return -3;
+	if (nt_populate(self) == -1)
+		return -3;
+
+	for (level = off = 0; level < 40; level++) {
+		int k, v;
+		nodetree *n = &self->nt[off];
+		k = nt_level(node, level);
+		v = n->children[k];
+		if (v < 0) {
+			const char *n;
+			v = -(v + 1);
+			n = index_node(self, v);
+			if (memcmp(node, n, 20) != 0)
+				/*
+				 * Found a unique prefix, but it wasn't for the
+				 * requested node (i.e the requested node does
+				 * not exist).
+				 */
+				return -2;
+			return level + 1;
+		}
+		if (v == 0)
+			return -2;
+		off = v;
+	}
+	/*
+	 * The node was still not unique after 40 hex digits, so this won't
+	 * happen. Also, if we get here, then there's a programming error in
+	 * this file that made us insert a node longer than 40 hex digits.
+	 */
+	PyErr_SetString(PyExc_Exception, "broken node tree");
+	return -3;
+}
+
 static PyObject *index_partialmatch(indexObject *self, PyObject *args)
 {
 	const char *fullnode;
@@ -1307,6 +1356,29 @@
 	return PyBytes_FromStringAndSize(fullnode, 20);
 }
 
+static PyObject *index_shortest(indexObject *self, PyObject *args)
+{
+	Py_ssize_t nodelen;
+	PyObject *val;
+	char *node;
+	int length;
+
+	if (!PyArg_ParseTuple(args, "O", &val))
+		return NULL;
+	if (node_check(val, &node, &nodelen) == -1)
+		return NULL;
+
+	self->ntlookups++;
+	length = nt_shortest(self, node);
+	if (length == -3)
+		return NULL;
+	if (length == -2) {
+		raise_revlog_error();
+		return NULL;
+	}
+	return PyInt_FromLong(length);
+}
+
 static PyObject *index_m_get(indexObject *self, PyObject *args)
 {
 	Py_ssize_t nodelen;
@@ -1995,6 +2067,8 @@
 	 "insert an index entry"},
 	{"partialmatch", (PyCFunction)index_partialmatch, METH_VARARGS,
 	 "match a potentially ambiguous node ID"},
+	{"shortest", (PyCFunction)index_shortest, METH_VARARGS,
+	 "find length of shortest hex nodeid of a binary ID"},
 	{"stats", (PyCFunction)index_stats, METH_NOARGS,
 	 "stats for the index"},
 	{NULL} /* Sentinel */