[libc] Add a method `find_last_of` to StringView.

[jeffbailey]

Thanks! One comment within.

libc/test/src/__support/CPP/stringview_test.cpp

203

I apparently missed this in find_first_of, too, but it would be nice to have a test on an empty string to make sure that's handled correctly.

[sivachandra]

Add tests for empty string views.

libc/src/__support/CPP/StringView.h

size_t find_first_of(const char c, size_t From = 0) const noexcept {
     while (!S.empty()) {
       if (S.front() == c)
         return size() - S.size();
       S = S.drop_front();
     }
     return npos;
   }
 
+  // Search for the last character matching the character
+  //
+  // Return the index of the last character equal to the |c| before End.
+  size_t find_last_of(const char c, size_t End = npos) const {
+    End = End > size() ? size() : End + 1;
+    StringView S = drop_back(size() - End);
+    while (!S.empty()) {
+      if (S.back() == c)
+        return S.size() - 1;
+      S = S.drop_back();
+    }
+    return npos;
+  }
+
   // Search for the first character satisfying the predicate Function
   //
   // Returns The index of the first character satisfying Function starting from
   // From, or npos if not found.
   template <typename F> size_t find_if(F Function, size_t From = 0) const {
     StringView S = drop_front(From);
     while (!S.empty()) {
       if (Function(S.front()))

libc/test/src/__support/CPP/stringview_test.cpp

TEST(LlvmLibcStringViewTest, FindFirstOf) {
   ASSERT_TRUE(Tmp.find_first_of('a', 0) == 0);
   ASSERT_TRUE(Tmp.find_first_of('b', 1) == 1);
   ASSERT_TRUE(Tmp.find_first_of('a', 1) == 3);
   ASSERT_TRUE(Tmp.find_first_of('a', 42) == StringView::npos);
   ASSERT_FALSE(Tmp.find_first_of('c') == 1);
   ASSERT_FALSE(Tmp.find_first_of('c', 0) == 1);
   ASSERT_FALSE(Tmp.find_first_of('c', 1) == 1);
 }
+
+TEST(LlvmLibcStringViewTest, FindLastOf) {
+  StringView Tmp("abada");
+
+  ASSERT_EQ(Tmp.find_last_of('a'), size_t(4));
+  ASSERT_EQ(Tmp.find_last_of('a', 123), size_t(4));
+  ASSERT_EQ(Tmp.find_last_of('a', 5), size_t(4));
+  ASSERT_EQ(Tmp.find_last_of('a', 4), size_t(4));
+  ASSERT_EQ(Tmp.find_last_of('a', 3), size_t(2));
+  ASSERT_EQ(Tmp.find_last_of('a', 2), size_t(2));
+  ASSERT_EQ(Tmp.find_last_of('a', 1), size_t(0));
+  ASSERT_EQ(Tmp.find_last_of('a', 0), size_t(0));
+
+  ASSERT_EQ(Tmp.find_last_of('b'), size_t(1));
+  ASSERT_EQ(Tmp.find_last_of('b', 123), size_t(1));
+  ASSERT_EQ(Tmp.find_last_of('b', 5), size_t(1));
+  ASSERT_EQ(Tmp.find_last_of('b', 4), size_t(1));
+  ASSERT_EQ(Tmp.find_last_of('b', 3), size_t(1));
+  ASSERT_EQ(Tmp.find_last_of('b', 2), size_t(1));
+  ASSERT_EQ(Tmp.find_last_of('b', 1), size_t(1));
+  ASSERT_EQ(Tmp.find_last_of('b', 0), StringView::npos);
+
+  ASSERT_EQ(Tmp.find_last_of('d'), size_t(3));
+  ASSERT_EQ(Tmp.find_last_of('d', 123), size_t(3));
+  ASSERT_EQ(Tmp.find_last_of('d', 5), size_t(3));
+  ASSERT_EQ(Tmp.find_last_of('d', 4), size_t(3));
+  ASSERT_EQ(Tmp.find_last_of('d', 3), size_t(3));
+  ASSERT_EQ(Tmp.find_last_of('d', 2), StringView::npos);
+  ASSERT_EQ(Tmp.find_last_of('d', 1), StringView::npos);
+  ASSERT_EQ(Tmp.find_last_of('d', 0), StringView::npos);
+
+  ASSERT_EQ(Tmp.find_last_of('e'), StringView::npos);
+  ASSERT_EQ(Tmp.find_last_of('e', 123), StringView::npos);
+  ASSERT_EQ(Tmp.find_last_of('e', 5), StringView::npos);
+  ASSERT_EQ(Tmp.find_last_of('e', 4), StringView::npos);
+  ASSERT_EQ(Tmp.find_last_of('e', 3), StringView::npos);
+  ASSERT_EQ(Tmp.find_last_of('e', 2), StringView::npos);
+  ASSERT_EQ(Tmp.find_last_of('e', 1), StringView::npos);
+  ASSERT_EQ(Tmp.find_last_of('e', 0), StringView::npos);
+}