expression: add new built in "reverse"

While a foldl is enough to implement a reverse functionality,
adding it as a built in allows doing so in linear time.

3 files changed, 29 insertions, 0 deletions

diff --git a/doc/concepts/expressions.md b/doc/concepts/expressions.md
index c66de8bc..dcd567af 100644
--- a/doc/concepts/expressions.md
+++ b/doc/concepts/expressions.md
@@ -253,6 +253,9 @@ those) argument(s) to obtain the final result.
   a map with the members of the list as keys, and all values being
   `true`.
 
+- `"reverse"` The argument has to be a list. The result is a new list
+  with the entries in reverse order.
+
  - `"++"` The argument has to be a list of lists. The result is
    the concatenation of those lists.
 
diff --git a/src/buildtool/build_engine/expression/evaluator.cpp b/src/buildtool/build_engine/expression/evaluator.cpp
index 431792e8..1d207b3b 100644
--- a/src/buildtool/build_engine/expression/evaluator.cpp
+++ b/src/buildtool/build_engine/expression/evaluator.cpp
@@ -185,6 +185,16 @@ auto Set(ExpressionPtr const& expr) -> ExpressionPtr {
     return ExpressionPtr{Expression::map_t{result}};
 }
 
+auto Reverse(ExpressionPtr const& expr) -> ExpressionPtr {
+    if (not expr->IsList()) {
+        throw Evaluator::EvaluationError{fmt::format(
+            "reverse expects list but instead got: {}.", expr->ToString())};
+    }
+    auto reverse_result = Expression::list_t(expr->List());
+    std::reverse(reverse_result.begin(), reverse_result.end());
+    return ExpressionPtr{reverse_result};
+}
+
 auto NubRight(ExpressionPtr const& expr) -> ExpressionPtr {
     if (not expr->IsList()) {
         throw Evaluator::EvaluationError{fmt::format(
@@ -957,6 +967,7 @@ auto const kBuiltInFunctions =
                           {"keys", UnaryExpr(Keys)},
                           {"enumerate", UnaryExpr(Enumerate)},
                           {"set", UnaryExpr(Set)},
+                          {"reverse", UnaryExpr(Reverse)},
                           {"values", UnaryExpr(Values)},
                           {"lookup", LookupExpr},
                           {"empty_map", EmptyMapExpr},
diff --git a/test/buildtool/build_engine/expression/expression.test.cpp b/test/buildtool/build_engine/expression/expression.test.cpp
index 7f3df7c5..242c0f4a 100644
--- a/test/buildtool/build_engine/expression/expression.test.cpp
+++ b/test/buildtool/build_engine/expression/expression.test.cpp
@@ -942,6 +942,21 @@ TEST_CASE("Expression Evaluation", "[expression]") {  // NOLINT
         )"_json));
     }
 
+    SECTION("reverse expression") {
+        auto expr = Expression::FromJson(R"(
+          { "type": "reverse"
+          , "$1": ["foo", "bar", "baz"]
+          }
+        )"_json);
+        REQUIRE(expr);
+
+        auto result = expr.Evaluate(env, fcts);
+        REQUIRE(result);
+        CHECK(result == Expression::FromJson(R"(
+           ["baz", "bar", "foo"]
+        )"_json));
+    }
+
     SECTION("keys expression") {
         auto expr = Expression::FromJson(R"(
             { "type": "keys"