nix-util: Add concatMapStrings

(cherry picked from commit 583a852c8a6b7461dad3635337a7d2fe6f205fd3)
Change-Id: I38e9174d800339cae26bcdbebb0e27008a96839e

lix/libutil/strings.hh

@@ -5,6 +5,7 @@
 #include "lix/libutil/types.hh"
 
 #include <vector>
+#include <boost/container/small_vector.hpp>
 
 namespace nix {
 
@@ -34,7 +35,6 @@ MakeError(FormatError, Error);
  */
 template<class C> C tokenizeString(std::string_view s, std::string_view separators = " \t\n\r");
 
-
 /**
  * Concatenate the given strings with a separator between the
  * elements.
@@ -63,6 +63,22 @@ auto concatStrings(Parts && ... parts)
 }
 
 
+/**
+ * Apply a function to the `iterable`'s items and concat them with `separator`.
+ */
+template<class C, class F>
+std::string concatMapStringsSep(std::string_view separator, const C & iterable, F fn)
+{
+    boost::container::small_vector<std::string, 64> strings;
+    strings.reserve(iterable.size());
+    for (const auto & elem : iterable) {
+        strings.push_back(fn(elem));
+    }
+    return concatStringsSep(separator, strings);
+}
+
+
+
 /**
  * Add quotes around a collection of strings.
  */

tests/unit/libutil/tests.cc

@@ -667,4 +667,43 @@ namespace nix {
         ASSERT_EQ(filterANSIEscapes("f𐍈𐍈bär", true, 4), "f𐍈𐍈b");
     }
 
+    /* ----------------------------------------------------------------------------
+     * concatMapStringsSep
+     * --------------------------------------------------------------------------*/
+    TEST(concatMapStringsSep, empty)
+    {
+        Strings strings;
+
+        ASSERT_EQ(concatMapStringsSep(",", strings, [](const std::string & s) { return s; }), "");
+    }
+
+    TEST(concatMapStringsSep, justOne)
+    {
+        Strings strings;
+        strings.push_back("this");
+
+        ASSERT_EQ(concatMapStringsSep(",", strings, [](const std::string & s) { return s; }), "this");
+    }
+
+    TEST(concatMapStringsSep, two)
+    {
+        Strings strings;
+        strings.push_back("this");
+        strings.push_back("that");
+
+        ASSERT_EQ(concatMapStringsSep(",", strings, [](const std::string & s) { return s; }), "this,that");
+    }
+
+    TEST(concatMapStringsSep, map)
+    {
+        std::map<std::string, std::string> strings;
+        strings["this"] = "that";
+        strings["1"] = "one";
+
+        ASSERT_EQ(
+            concatMapStringsSep(
+                ", ", strings, [](const std::pair<std::string, std::string> & s) { return s.first + " -> " + s.second; }),
+            "1 -> one, this -> that");
+    }
+
 }