AK: Implement Span::starts_with()

Useful for checking for contents at the start of a span.

2 files changed, 24 insertions, 0 deletions

diff --git a/AK/Span.h b/AK/Span.h
index 19f4d59710..ed13319201 100644
--- a/AK/Span.h
+++ b/AK/Span.h
@@ -176,6 +176,14 @@ public:
         return false;
     }
 
+    bool constexpr starts_with(Span<const T> other) const
+    {
+        if (size() < other.size())
+            return false;
+
+        return TypedTransfer<T>::compare(data(), other.data(), other.size());
+    }
+
     ALWAYS_INLINE constexpr const T& at(size_t index) const
     {
         VERIFY(index < this->m_size);
diff --git a/Tests/AK/TestSpan.cpp b/Tests/AK/TestSpan.cpp
index 2668133c4e..8affd79584 100644
--- a/Tests/AK/TestSpan.cpp
+++ b/Tests/AK/TestSpan.cpp
@@ -123,3 +123,19 @@ TEST_CASE(span_from_c_string)
     const char* str = "Serenity";
     [[maybe_unused]] ReadonlyBytes bytes { str, strlen(str) };
 }
+
+TEST_CASE(starts_with)
+{
+    const char* str = "HeyFriends!";
+    ReadonlyBytes bytes { str, strlen(str) };
+    const char* str_hey = "Hey";
+    ReadonlyBytes hey_bytes { str_hey, strlen(str_hey) };
+    EXPECT(bytes.starts_with(hey_bytes));
+    const char* str_nah = "Nah";
+    ReadonlyBytes nah_bytes { str_nah, strlen(str_nah) };
+    EXPECT(!bytes.starts_with(nah_bytes));
+
+    const u8 hey_array[3] = { 'H', 'e', 'y' };
+    ReadonlyBytes hey_bytes_u8 { hey_array, 3 };
+    EXPECT(bytes.starts_with(hey_bytes_u8));
+}