Add ListAt to the gesture utilities

BUG=b:271591258
TEST=USE="coverage" FEATURES="test noclean" emerge-brya chromeos-base/gestures

Change-Id: Ie95a888ddccb16255db75f3e99436896ef367016
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/gestures/+/4352449
Code-Coverage: Harry Cutts <hcutts@chromium.org>
Auto-Submit: Denis Brockus <dbrockus@chromium.org>
Commit-Queue: Harry Cutts <hcutts@chromium.org>
Reviewed-by: Harry Cutts <hcutts@chromium.org>
Tested-by: Denis Brockus <dbrockus@chromium.org>

2 files changed, 62 insertions, 0 deletions

diff --git a/include/util.h b/include/util.h
index 2e87a7b..9feb2d5 100644
--- a/include/util.h
+++ b/include/util.h
@@ -5,7 +5,9 @@
 #ifndef GESTURES_UTIL_H_
 #define GESTURES_UTIL_H_
 
+#include <list>
 #include <map>
+#include <optional>
 #include <set>
 
 #include <math.h>
@@ -105,6 +107,26 @@ inline bool SetContainsValue(const Set& the_set,
   return the_set.find(elt) != the_set.end();
 }
 
+template<typename R, typename L>
+R ListAt(L& the_list, int offset) {
+  // Traverse to the appropriate offset
+  if (offset < 0) {
+    // negative offset is from end to begin
+    for (auto iter = the_list.rbegin(); iter != the_list.rend(); ++iter) {
+      if (++offset == 0)
+        return R(*iter);
+    }
+  } else {
+    // positive offset is from begin to end
+    for (auto iter = the_list.begin(); iter != the_list.end(); ++iter) {
+      if (offset-- == 0)
+        return R(*iter);
+    }
+  }
+  // Invalid offset
+  return std::nullopt;
+}
+
 }  // namespace gestures
 
 #endif  // GESTURES_UTIL_H_
diff --git a/src/util_unittest.cc b/src/util_unittest.cc
index 5d0a435..9784d70 100644
--- a/src/util_unittest.cc
+++ b/src/util_unittest.cc
@@ -21,4 +21,44 @@ TEST(UtilTest, DistSqTest) {
   EXPECT_FLOAT_EQ(DistSqXY(fs[0], 4, 6), 25);
 }
 
+TEST(UtilTest, ListAtTest) {
+  const int kMaxElements = 3;
+  struct element {
+    int x;
+  };
+
+  typedef std::list<element> ElemListType;
+  typedef std::optional<std::reference_wrapper<element>> OptionalRefElem;
+
+  struct ElemList : public ElemListType {
+    OptionalRefElem at(int offset) {
+      return ListAt<OptionalRefElem, ElemListType>(*this, offset);
+    }
+  } list;
+
+  for (auto i = 0; i < kMaxElements; ++i) {
+    auto& elem = list.emplace_back();
+    elem.x = i;
+  }
+  EXPECT_EQ(list.at(-1)->get().x, list.at(list.size() - 1)->get().x);
+
+  for (auto i = 0; i < kMaxElements; ++i) {
+    for (auto j = 0; j < kMaxElements; ++j) {
+      if (i == j) {
+        EXPECT_EQ(list.at(i)->get().x, list.at(j)->get().x);
+        EXPECT_EQ(&(list.at(i)->get()), &(list.at(j)->get()));
+      } else {
+        EXPECT_NE(list.at(i)->get().x, list.at(j)->get().x);
+        EXPECT_NE(&(list.at(i)->get()), &(list.at(j)->get()));
+      }
+    }
+  }
+
+  int list_count = list.size();
+  EXPECT_EQ(list.at(list_count), std::nullopt);
+  EXPECT_NE(list.at(list_count - 1), std::nullopt);
+  EXPECT_EQ(list.at(-list_count - 1), std::nullopt);
+  EXPECT_NE(list.at(-list_count), std::nullopt);
+}
+
 }  // namespace gestures