[FEATURE] Support arithmetic operators in CSS functions

src/Value/Value.php

@@ -164,7 +164,16 @@ public static function parsePrimitiveValue(ParserState $oParserState)
         } elseif ($oParserState->comes("U+")) {
             $oValue = self::parseUnicodeRangeValue($oParserState);
         } else {
-            $oValue = self::parseIdentifierOrFunction($oParserState);
+            $sNextChar = $oParserState->peek(1);
+            try {
+                $oValue = self::parseIdentifierOrFunction($oParserState);
+            } catch (UnexpectedTokenException $e) {
+                if (\in_array($sNextChar, ['+', '-', '*', '/'], true)) {
+                    $oValue = $oParserState->consume(1);
+                } else {
+                    throw $e;
+                }
+            }
         }
         $oParserState->consumeWhiteSpace();
         return $oValue;

tests/Value/ValueTest.php

@@ -0,0 +1,84 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Tests\Value;
+
+use PHPUnit\Framework\TestCase;
+use Sabberworm\CSS\Parsing\ParserState;
+use Sabberworm\CSS\Settings;
+use Sabberworm\CSS\Value\Value;
+
+/**
+ * @covers \Sabberworm\CSS\Value\Value
+ */
+final class ValueTest extends TestCase
+{
+    /**
+     * @return array<string, array{0: string}>
+     */
+    public static function provideArithmeticOperator(): array
+    {
+        $units = ['+', '-', '*', '/'];
+
+        return \array_combine(
+            $units,
+            \array_map(
+                function (string $unit): array {
+                    return [$unit];
+                },
+                $units
+            )
+        );
+    }
+
+    /**
+     * @test
+     *
+     * @dataProvider provideArithmeticOperator
+     */
+    public function parsesArithmeticInFunctions(string $operator): void
+    {
+        $subject = Value::parseValue(new ParserState('max(300px, 50vh ' . $operator . ' 10px);', Settings::create()));
+
+        self::assertSame('max(300px,50vh ' . $operator . ' 10px)', (string) $subject);
+    }
+
+    /**
+     * @test
+     */
+    public function parsesArithmeticWithMultipleOperatorsInFunctions(): void
+    {
+        $subject = Value::parseValue(new ParserState('calc(300px + 10% + 10vw);', Settings::create()));
+
+        self::assertSame('calc(300px + 10% + 10vw)', (string) $subject);
+    }
+
+    /**
+     * @return array<string, array{0: string, 1: string}>
+     */
+    public static function provideMalformedLengthOperands(): array
+    {
+        return [
+            'LHS missing value' => ['vh', '10px'],
+            'RHS missing value' => ['50vh', 'px'],
+            'LHS missing unit' => ['50', '10px'],
+            'RHS missing unit' => ['50vh', '10'],
+        ];
+    }
+
+    /**
+     * @test
+     *
+     * @dataProvider provideMalformedLengthOperands
+     */
+    public function parsesArithmeticWithMalformedOperandsInFunctions(string $leftOperand, string $rightOperand): void
+    {
+        $subject = Value::parseValue(new ParserState(
+            'max(300px, ' . $leftOperand . ' + ' . $rightOperand . ');',
+            Settings::create()
+        ));
+
+        self::assertSame('max(300px,' . $leftOperand . ' + ' . $rightOperand . ')', (string) $subject);
+    }
+}