Support move-only key types in the Max/MinElementBy routines

It is also slightly faster for non-trivial key types.
commit_hash:4104cabfe8dc9a51174034c62aae25be16b57bf9

2 files changed, 14 insertions, 4 deletions

diff --git a/util/generic/algorithm.h b/util/generic/algorithm.h
index a770ab3033..d1f48c479d 100644
--- a/util/generic/algorithm.h
+++ b/util/generic/algorithm.h
@@ -21,11 +21,11 @@ namespace NPrivate {
         auto bestValue = func(*begin);
         auto bestPos = begin;
 
-        for (auto i = ++begin; i != end; ++i) {
-            auto curValue = func(*i);
+        for (++begin; begin != end; ++begin) {
+            auto curValue = func(*begin);
             if (pred(curValue, bestValue)) {
-                bestValue = curValue;
-                bestPos = i;
+                bestValue = std::move(curValue);
+                bestPos = begin;
             }
         }
 
diff --git a/util/generic/algorithm_ut.cpp b/util/generic/algorithm_ut.cpp
index 51b6e12264..8609026797 100644
--- a/util/generic/algorithm_ut.cpp
+++ b/util/generic/algorithm_ut.cpp
@@ -752,6 +752,16 @@ Y_UNIT_TEST_SUITE(TAlgorithm) {
         MinElementBy(empty, functor);
     }
 
+    Y_UNIT_TEST(MinMaxElementMovableKeys) {
+        const TString strings[] = {"one", "two", "three", "four"};
+        struct TMoveOnlyKey: TString, TMoveOnly {
+            using TString::TString;
+        };
+        auto keyFn = [](TString s) { return TMoveOnlyKey{std::move(s)}; };
+        UNIT_ASSERT_STRINGS_EQUAL(*MaxElementBy(strings, keyFn), "two");
+        UNIT_ASSERT_STRINGS_EQUAL(*MinElementBy(strings, keyFn), "four");
+    }
+
     Y_UNIT_TEST(TestApplyToMany) {
         int res = 0;
         ApplyToMany([&res](auto v) { res += v; }, 1, 2, 3, 4, 5);