AK: Add an overload of String::find_byte_offset for StringView

AK/String.cpp

@@ -8,6 +8,7 @@
 #include <AK/Checked.h>
 #include <AK/FlyString.h>
 #include <AK/Format.h>
+#include <AK/MemMem.h>
 #include <AK/String.h>
 #include <AK/StringBuilder.h>
 #include <AK/Utf8View.h>
@@ -328,6 +329,21 @@ Optional<size_t> String::find_byte_offset(u32 code_point, size_t from_byte_offse
     return {};
 }
 
+Optional<size_t> String::find_byte_offset(StringView substring, size_t from_byte_offset) const
+{
+    auto view = bytes_as_string_view();
+    if (from_byte_offset >= view.length())
+        return {};
+
+    auto index = memmem_optional(
+        view.characters_without_null_termination() + from_byte_offset, view.length() - from_byte_offset,
+        substring.characters_without_null_termination(), substring.length());
+
+    if (index.has_value())
+        return *index + from_byte_offset;
+    return {};
+}
+
 bool String::operator==(String const& other) const
 {
     if (is_short_string())

AK/String.h

@@ -133,6 +133,7 @@ public:
     ErrorOr<Vector<String>> split(u32 separator, SplitBehavior = SplitBehavior::Nothing) const;
 
     Optional<size_t> find_byte_offset(u32 code_point, size_t from_byte_offset = 0) const;
+    Optional<size_t> find_byte_offset(StringView substring, size_t from_byte_offset = 0) const;
 
     [[nodiscard]] bool operator==(String const&) const;
     [[nodiscard]] bool operator!=(String const& other) const { return !(*this == other); }

Tests/AK/TestString.cpp

@@ -320,8 +320,11 @@ TEST_CASE(find_byte_offset)
 {
     {
         String string {};
-        auto index = string.find_byte_offset(0);
-        EXPECT(!index.has_value());
+        auto index1 = string.find_byte_offset(0);
+        EXPECT(!index1.has_value());
+
+        auto index2 = string.find_byte_offset(""sv);
+        EXPECT(!index2.has_value());
     }
     {
         auto string = MUST(String::from_utf8("foo"sv));
@@ -338,6 +341,21 @@ TEST_CASE(find_byte_offset)
         auto index4 = string.find_byte_offset('b');
         EXPECT(!index4.has_value());
     }
+    {
+        auto string = MUST(String::from_utf8("foo"sv));
+
+        auto index1 = string.find_byte_offset("fo"sv);
+        EXPECT_EQ(index1, 0u);
+
+        auto index2 = string.find_byte_offset("oo"sv);
+        EXPECT_EQ(index2, 1u);
+
+        auto index3 = string.find_byte_offset("o"sv, *index2 + 1);
+        EXPECT_EQ(index3, 2u);
+
+        auto index4 = string.find_byte_offset("fooo"sv);
+        EXPECT(!index4.has_value());
+    }
     {
         auto string = MUST(String::from_utf8("ωΣωΣω"sv));
 
@@ -356,6 +374,24 @@ TEST_CASE(find_byte_offset)
         auto index5 = string.find_byte_offset(0x03C9U, 6);
         EXPECT_EQ(index5, 8u);
     }
+    {
+        auto string = MUST(String::from_utf8("ωΣωΣω"sv));
+
+        auto index1 = string.find_byte_offset("ω"sv);
+        EXPECT_EQ(index1, 0u);
+
+        auto index2 = string.find_byte_offset("Σ"sv);
+        EXPECT_EQ(index2, 2u);
+
+        auto index3 = string.find_byte_offset("ω"sv, 2);
+        EXPECT_EQ(index3, 4u);
+
+        auto index4 = string.find_byte_offset("Σ"sv, 4);
+        EXPECT_EQ(index4, 6u);
+
+        auto index5 = string.find_byte_offset("ω"sv, 6);
+        EXPECT_EQ(index5, 8u);
+    }
 }
 
 TEST_CASE(repeated)