SL-16024: Introduce tuple.h with tuple_cons(), tuple_cdr().

These functions allow prepending or removing an item at the left end of an
arbitrary tuple -- for instance, to add a sequence key to a caller's data,
then remove it again when delivering the original tuple.

1 files changed, 84 insertions, 0 deletions

diff --git a/indra/llcommon/tuple.h b/indra/llcommon/tuple.h
new file mode 100644
index 0000000000..bfe7e3c2ba
--- /dev/null
+++ b/indra/llcommon/tuple.h
@@ -0,0 +1,84 @@
+/**
+ * @file   tuple.h
+ * @author Nat Goodspeed
+ * @date   2021-10-04
+ * @brief  A couple tuple utilities
+ * 
+ * $LicenseInfo:firstyear=2021&license=viewerlgpl$
+ * Copyright (c) 2021, Linden Research, Inc.
+ * $/LicenseInfo$
+ */
+
+#if ! defined(LL_TUPLE_H)
+#define LL_TUPLE_H
+
+#include <tuple>
+#include <type_traits>              // std::remove_reference
+#include <utility>                  // std::pair
+
+/**
+ * tuple_cons() behaves like LISP cons: it uses std::tuple_cat() to prepend a
+ * new item of arbitrary type to an existing std::tuple.
+ */
+template <typename First, typename... Rest, typename Tuple_=std::tuple<Rest...>>
+auto tuple_cons(First&& first, Tuple_&& rest)
+{
+    // All we need to do is make a tuple containing 'first', and let
+    // tuple_cat() do the hard part.
+    return std::tuple_cat(std::tuple<First>(std::forward<First>(first)),
+                          std::forward<Tuple_>(rest));
+}
+
+/**
+ * tuple_car() behaves like LISP car: it extracts the first item from a
+ * std::tuple.
+ */
+template <typename... Args, typename Tuple_=std::tuple<Args...>>
+auto tuple_car(Tuple_&& tuple)
+{
+    return std::get<0>(std::forward<Tuple_>(tuple));
+}
+
+/**
+ * tuple_cdr() behaves like LISP cdr: it returns a new tuple containing
+ * everything BUT the first item.
+ */
+// derived from https://stackoverflow.com/a/24046437
+template <typename Tuple, std::size_t... Indices>
+auto tuple_cdr_(Tuple&& tuple, const std::index_sequence<Indices...>)
+{
+    // Given an index sequence from [0..N-1), extract tuple items [1..N)
+    return std::make_tuple(std::get<Indices+1u>(std::forward<Tuple>(tuple))...);
+}
+
+template <typename Tuple>
+auto tuple_cdr(Tuple&& tuple)
+{
+    return tuple_cdr_(
+        std::forward<Tuple>(tuple),
+        // Pass helper function an index sequence one item shorter than tuple
+        std::make_index_sequence<
+            std::tuple_size<
+                // tuple_size doesn't like reference types
+                typename std::remove_reference<Tuple>::type
+            >::value - 1u>
+        ());
+}
+
+/**
+ * tuple_split(), the opposite of tuple_cons(), has no direct analog in LISP.
+ * It returns a std::pair of tuple_car(), tuple_cdr(). We could call this
+ * function tuple_car_cdr(), or tuple_slice() or some such. But tuple_split()
+ * feels more descriptive.
+ */
+template <typename... Args, typename Tuple_=std::tuple<Args...>>
+auto tuple_split(Tuple_&& tuple)
+{
+    // We're not really worried about forwarding multiple times a tuple that
+    // might contain move-only items, because the implementation above only
+    // applies std::get() exactly once to each item.
+    return std::make_pair(tuple_car(std::forward<Tuple_>(tuple)),
+                          tuple_cdr(std::forward<Tuple_>(tuple)));
+}
+
+#endif /* ! defined(LL_TUPLE_H) */