From f52a182afe3af442ecd8ecf441124a6489f831a3 Mon Sep 17 00:00:00 2001
From: Klaus Aehlig <klaus.aehlig@huawei.com>
Date: Mon, 30 May 2022 12:38:36 +0200
Subject: 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.
---
 doc/concepts/expressions.org                           |  5 +++++
 src/buildtool/build_engine/expression/evaluator.cpp    | 15 +++++++++++++++
 .../build_engine/expression/expression.test.cpp        | 18 ++++++++++++++++++
 3 files changed, 38 insertions(+)

diff --git a/doc/concepts/expressions.org b/doc/concepts/expressions.org
index 50737413..895b5e0a 100644
--- a/doc/concepts/expressions.org
+++ b/doc/concepts/expressions.org
@@ -211,6 +211,11 @@ those) argument(s) to obtain the final result.
   first non-negative integers. For example, ~{"type": "range",
   "$1": "3"}~ evaluates to ~["0", "1", "2"]~.
 
+- ~"enumerate"~ The argument has to be a list. The result is a map
+  containing one entry for each element of the list; the key is the
+  decimal representation of the position in the list (starting from
+  ~0~) and the value is the element.
+
 - ~"++"~ 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 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},
diff --git a/test/buildtool/build_engine/expression/expression.test.cpp b/test/buildtool/build_engine/expression/expression.test.cpp
index cbc8c7bf..11ff7a43 100644
--- a/test/buildtool/build_engine/expression/expression.test.cpp
+++ b/test/buildtool/build_engine/expression/expression.test.cpp
@@ -890,6 +890,24 @@ TEST_CASE("Expression Evaluation", "[expression]") {  // NOLINT
         CHECK(result == Expression::FromJson(R"("esXcXape me XX")"_json));
     }
 
+    SECTION("enumerate expression") {
+        auto expr = Expression::FromJson(R"(
+          { "type": "enumerate"
+          , "$1": ["foo", "bar", "baz"]
+          }
+        )"_json);
+        REQUIRE(expr);
+
+        auto result = expr.Evaluate(env, fcts);
+        REQUIRE(result);
+        CHECK(result == Expression::FromJson(R"(
+          { "0": "foo"
+          , "1": "bar"
+          , "2": "baz"
+          }
+        )"_json));
+    }
+
     SECTION("keys expression") {
         auto expr = Expression::FromJson(R"(
             { "type": "keys"
-- 
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