add hashCombine

C++11 added std::hash, but neglected to include a function for
combining multiple hashes.

include/lsst/utils/hashCombine.h

@@ -0,0 +1,36 @@
+#ifndef LSST_UTILS_HASH_COMBINE_H
+#define LSST_UTILS_HASH_COMBINE_H
+
+#include <functional>
+
+namespace lsst {
+namespace utils {
+
+//@{
+/** Combine hashes
+ *
+ * This is provided as a convenience for those who need to hash a composite.
+ * C++11 includes std::hash, but neglects to include a facility for
+ * combining hashes.
+ *
+ * To use it:
+ *
+ *    std::size_t seed = 0;
+ *    result = hashCombine(seed, obj1, obj2, obj3);
+ *
+ * This solution is provided by Matteo Italia
+ * https://stackoverflow.com/a/38140932/834250
+ */
+inline std::size_t hashCombine(std::size_t seed) { return seed; }
+
+template <typename T, typename... Rest>
+std::size_t hashCombine(std::size_t seed, const T& value, Rest... rest) {
+    std::hash<T> hasher;
+    seed ^= hasher(value) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
+    return hashCombine(seed, rest...);
+}
+//@}
+
+}} // namespace lsst::utils
+
+#endif