From 5a42b7a40fd758b619bf3600bde593e827d59e2c Mon Sep 17 00:00:00 2001
From: Sam Atkins <sam@ladybird.org>
Date: Thu, 11 Sep 2025 15:58:10 +0100
Subject: [PATCH] LibWeb/CSS: Implement product_of_two_unit_maps()

Helper function for combining two UnitMaps.
---
 Libraries/LibWeb/CSS/NumericType.cpp | 17 +++++++++++++++++
 Libraries/LibWeb/CSS/NumericType.h   |  1 +
 2 files changed, 18 insertions(+)

diff --git a/Libraries/LibWeb/CSS/NumericType.cpp b/Libraries/LibWeb/CSS/NumericType.cpp
index 4fa0304c8ff..6358dd9b425 100644
--- a/Libraries/LibWeb/CSS/NumericType.cpp
+++ b/Libraries/LibWeb/CSS/NumericType.cpp
@@ -16,6 +16,23 @@
 
 namespace Web::CSS {
 
+// https://drafts.css-houdini.org/css-typed-om-1/#product-of-two-unit-maps
+UnitMap product_of_two_unit_maps(UnitMap const& units1, UnitMap const& units2)
+{
+    // 1. Let result be a copy of units1.
+    auto result = units1;
+
+    // 2. For each unit → power in units2:
+    for (auto const& [unit, power] : units2) {
+        // 1. If result[unit] exists, increment result[unit] by power.
+        // 2. Otherwise, set result[unit] to power.
+        result.ensure(unit) += power;
+    }
+
+    // 3. Return result.
+    return result;
+}
+
 Optional<NumericType::BaseType> NumericType::base_type_from_value_type(ValueType value_type)
 {
     switch (value_type) {
diff --git a/Libraries/LibWeb/CSS/NumericType.h b/Libraries/LibWeb/CSS/NumericType.h
index 67a25e89149..c69752214d1 100644
--- a/Libraries/LibWeb/CSS/NumericType.h
+++ b/Libraries/LibWeb/CSS/NumericType.h
@@ -14,6 +14,7 @@
 namespace Web::CSS {
 
 using UnitMap = HashMap<FlyString, i32>;
+UnitMap product_of_two_unit_maps(UnitMap const&, UnitMap const&);
 
 // https://drafts.css-houdini.org/css-typed-om-1/#cssnumericvalue-type
 class NumericType {