Built-in expressions: add enumerate

Add a function transforming a list into a map. In this way, artifacts
collected positionally in a list can easily be realized as a stage
used for input to an action or output of a target.

1 files changed, 15 insertions, 0 deletions

diff --git a/src/buildtool/build_engine/expression/evaluator.cpp b/src/buildtool/build_engine/expression/evaluator.cpp
index cfd0be31..76ad3076 100644
--- a/src/buildtool/build_engine/expression/evaluator.cpp
+++ b/src/buildtool/build_engine/expression/evaluator.cpp
@@ -139,6 +139,20 @@ auto Values(ExpressionPtr const& d) -> ExpressionPtr {
     return ExpressionPtr{d->Map().Values()};
 }
 
+auto Enumerate(ExpressionPtr const& expr) -> ExpressionPtr {
+    if (not expr->IsList()) {
+        throw Evaluator::EvaluationError{fmt::format(
+            "enumerate expects list but instead got: {}.", expr->ToString())};
+    }
+    auto result = Expression::map_t::underlying_map_t{};
+    size_t count = 0;
+    for (auto const& entry : expr->List()) {
+        result[fmt::format("{}", count)] = entry;
+        count++;
+    }
+    return ExpressionPtr{Expression::map_t{result}};
+}
+
 auto NubRight(ExpressionPtr const& expr) -> ExpressionPtr {
     if (not expr->IsList()) {
         throw Evaluator::EvaluationError{fmt::format(
@@ -849,6 +863,7 @@ auto const kBuiltInFunctions =
                           {"json_encode", JsonEncodeExpr},
                           {"escape_chars", EscapeCharsExpr},
                           {"keys", UnaryExpr(Keys)},
+                          {"enumerate", UnaryExpr(Enumerate)},
                           {"values", UnaryExpr(Values)},
                           {"lookup", LookupExpr},
                           {"empty_map", EmptyMapExpr},