[PATCH 4 of 7 V4] sparse-revlog: add a `trim_endidx` function in C

[Boris Feld]

# HG changeset patch
# User Boris Feld <boris.feld at octobus.net>
# Date 1541785523 -3600
#      Fri Nov 09 18:45:23 2018 +0100
# Node ID 7b040bdc7eda9183cc751aa396cf08e2db7b191e
# Parent  5bb00d137c14d311c91c7a4f32f58378cd9195ec
# EXP-Topic sparse-perf
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 7b040bdc7eda
sparse-revlog: add a `trim_endidx` function in C

We are about to implement a native version of `slicechunktodensity`. For
clarity, we introduce the helper functions first.

This function implement a subpart of the python function `_trimchunk` in
`mercurial/revlogutils/deltas.py`. Handling of actual Python objects is left
to the caller function.

diff --git a/mercurial/cext/revlog.c b/mercurial/cext/revlog.c
--- a/mercurial/cext/revlog.c
+++ b/mercurial/cext/revlog.c
@@ -1089,6 +1089,24 @@ index_segment_span(indexObject *self, Py
 	return (end_offset - start_offset) + (int64_t)end_size;
 }
 
+/* returns revs[startidx:endidx] without empty trailing revs */
+static Py_ssize_t trim_endidx(indexObject *self, Py_ssize_t *revs,
+                              Py_ssize_t startidx, Py_ssize_t endidx)
+{
+	int length;
+	while (endidx > 1 && endidx > startidx) {
+		length = index_get_length(self, revs[endidx - 1]);
+		if (length < 0) {
+			return -1;
+		}
+		if (length != 0) {
+			break;
+		}
+		endidx -= 1;
+	}
+	return endidx;
+}
+
 static inline int nt_level(const char *node, Py_ssize_t level)
 {
 	int v = node[level >> 1];