From b558b4dba6532ef56fef0a230fb51346ca841976 Mon Sep 17 00:00:00 2001
From: Jelle Raaijmakers <jelle@ladybird.org>
Date: Tue, 10 Jun 2025 15:11:08 +0200
Subject: [PATCH] AK: Add `Span<T>::index_of(ReadonlySpan)`

This will be used for case-sensitive substring index matches in a later
commit.
---
 AK/Span.h             | 18 ++++++++++++++++++
 Tests/AK/TestSpan.cpp | 15 +++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/AK/Span.h b/AK/Span.h
index 6a1644ebdf1..b7afbe59491 100644
--- a/AK/Span.h
+++ b/AK/Span.h
@@ -307,6 +307,24 @@ public:
         return { data(), size() };
     }
 
+    Optional<size_t> index_of(ReadonlySpan<T> other, size_t start_offset = 0) const
+    {
+        Checked maximum_offset { start_offset };
+        maximum_offset += other.size();
+        if (maximum_offset.has_overflow() || maximum_offset.value() > size())
+            return {};
+
+        if (other.is_empty())
+            return start_offset;
+
+        for (size_t index = start_offset; index <= size() - other.size(); ++index) {
+            if (TypedTransfer<T>::compare(data() + index, other.data(), other.size()))
+                return index;
+        }
+
+        return {};
+    }
+
     template<typename TUnaryPredicate>
     Optional<T&> last_matching(TUnaryPredicate const& predicate)
     {
diff --git a/Tests/AK/TestSpan.cpp b/Tests/AK/TestSpan.cpp
index 6644d78d651..fab9cf4916d 100644
--- a/Tests/AK/TestSpan.cpp
+++ b/Tests/AK/TestSpan.cpp
@@ -180,3 +180,18 @@ TEST_CASE(compare_different_constness)
 
     EXPECT_EQ(array, vector.span());
 }
+
+TEST_CASE(index_of)
+{
+    constexpr Array haystack { 1, 2, 3, 4, 5 };
+    constexpr Array needle_1 { 2, 3, 4 };
+    constexpr Array needle_2 { 2, 4 };
+    constexpr Array<int, 0> needle_3 {};
+
+    EXPECT_EQ(1u, haystack.span().index_of(needle_1.span()).value());
+    EXPECT(!haystack.span().index_of(needle_1.span(), 2).has_value());
+    EXPECT(!haystack.span().index_of(needle_2.span()).has_value());
+    EXPECT_EQ(0u, haystack.span().index_of(needle_3.span()));
+    EXPECT_EQ(1u, haystack.span().index_of(needle_3.span(), 1));
+    EXPECT(!haystack.span().index_of(needle_3.span(), 16).has_value());
+}