[iter] Implement hb_reduce
diff --git a/src/hb-iter.hh b/src/hb-iter.hh
index b212aa3..04261bd 100644
--- a/src/hb-iter.hh
+++ b/src/hb-iter.hh
@@ -348,6 +348,34 @@
{ return hb_filter_iter_factory_t<Pred, Proj> (p, f); }
} hb_filter HB_UNUSED;
+template <typename Redu, typename TValue>
+struct hb_reduce_t
+{
+ hb_reduce_t (Redu r, TValue init_value) : r (r), init_value (init_value) {}
+
+ template <typename Iter,
+ hb_enable_if (hb_is_iterator (Iter))>
+ TValue
+ operator () (Iter it) const
+ {
+ TValue value = init_value;
+ for (; it; ++it)
+ value = r (*it, value);
+ return value;
+ }
+
+ private:
+ Redu r;
+ TValue init_value;
+};
+static const struct
+{
+ template <typename Redu, typename TValue> hb_reduce_t<Redu, TValue>
+ operator () (Redu&& r, TValue init_value) const
+ { return hb_reduce_t<Redu, TValue> (r, init_value); }
+} hb_reduce HB_UNUSED;
+
+
/* hb_zip() */
template <typename A, typename B>
