AK/Hex: Decode hex digit in constexpr context

Problem:
- Hex digit decoding is not `constexpr`, but can be.

Solution:
- Move the body of the function to the header and decorate with
  `constexpr`.
- Provide tests for run-time and compile-time evaluation.

AK/Hex.cpp

@@ -35,17 +35,6 @@
 
 namespace AK {
 
-u8 decode_hex_digit(char digit)
-{
-    if (digit >= '0' && digit <= '9')
-        return digit - '0';
-    if (digit >= 'a' && digit <= 'f')
-        return 10 + (digit - 'a');
-    if (digit >= 'A' && digit <= 'F')
-        return 10 + (digit - 'A');
-    return 255;
-}
-
 Optional<ByteBuffer> decode_hex(const StringView& input)
 {
     if ((input.length() % 2) != 0)

AK/Hex.h

@@ -33,7 +33,16 @@
 
 namespace AK {
 
-u8 decode_hex_digit(char);
+constexpr u8 decode_hex_digit(char digit)
+{
+    if (digit >= '0' && digit <= '9')
+        return digit - '0';
+    if (digit >= 'a' && digit <= 'f')
+        return 10 + (digit - 'a');
+    if (digit >= 'A' && digit <= 'F')
+        return 10 + (digit - 'A');
+    return 255;
+}
 
 Optional<ByteBuffer> decode_hex(const StringView&);
 

AK/Tests/CMakeLists.txt

@@ -1,5 +1,3 @@
-
-
 set(AK_TEST_SOURCES
     TestAllOf.cpp
     TestAnyOf.cpp
@@ -26,6 +24,7 @@ set(AK_TEST_SOURCES
     TestHashFunctions.cpp
     TestHashMap.cpp
     TestHashTable.cpp
+    TestHex.cpp
     TestIPv4Address.cpp
     TestIndexSequence.cpp
     TestIntrusiveRedBlackTree.cpp

AK/Tests/TestHex.cpp

@@ -0,0 +1,83 @@
+/*
+ * Copyright (c) 2020, the SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/TestSuite.h>
+
+#include <AK/Hex.h>
+
+TEST_CASE(should_decode_hex_digit)
+{
+  EXPECT_EQ(0u, decode_hex_digit('0'));
+  EXPECT_EQ(1u, decode_hex_digit('1'));
+  EXPECT_EQ(2u, decode_hex_digit('2'));
+  EXPECT_EQ(3u, decode_hex_digit('3'));
+  EXPECT_EQ(4u, decode_hex_digit('4'));
+  EXPECT_EQ(5u, decode_hex_digit('5'));
+  EXPECT_EQ(6u, decode_hex_digit('6'));
+  EXPECT_EQ(7u, decode_hex_digit('7'));
+  EXPECT_EQ(8u, decode_hex_digit('8'));
+  EXPECT_EQ(9u, decode_hex_digit('9'));
+  EXPECT_EQ(10u, decode_hex_digit('a'));
+  EXPECT_EQ(11u, decode_hex_digit('b'));
+  EXPECT_EQ(12u, decode_hex_digit('c'));
+  EXPECT_EQ(13u, decode_hex_digit('d'));
+  EXPECT_EQ(14u, decode_hex_digit('e'));
+  EXPECT_EQ(15u, decode_hex_digit('f'));
+  EXPECT_EQ(10u, decode_hex_digit('A'));
+  EXPECT_EQ(11u, decode_hex_digit('B'));
+  EXPECT_EQ(12u, decode_hex_digit('C'));
+  EXPECT_EQ(13u, decode_hex_digit('D'));
+  EXPECT_EQ(14u, decode_hex_digit('E'));
+  EXPECT_EQ(15u, decode_hex_digit('F'));
+}
+
+TEST_CASE(should_constexpr_decode_hex_digit)
+{
+  static_assert(0u == decode_hex_digit('0'));
+  static_assert(1u == decode_hex_digit('1'));
+  static_assert(2u == decode_hex_digit('2'));
+  static_assert(3u == decode_hex_digit('3'));
+  static_assert(4u == decode_hex_digit('4'));
+  static_assert(5u == decode_hex_digit('5'));
+  static_assert(6u == decode_hex_digit('6'));
+  static_assert(7u == decode_hex_digit('7'));
+  static_assert(8u == decode_hex_digit('8'));
+  static_assert(9u == decode_hex_digit('9'));
+  static_assert(10u == decode_hex_digit('a'));
+  static_assert(11u == decode_hex_digit('b'));
+  static_assert(12u == decode_hex_digit('c'));
+  static_assert(13u == decode_hex_digit('d'));
+  static_assert(14u == decode_hex_digit('e'));
+  static_assert(15u == decode_hex_digit('f'));
+  static_assert(10u == decode_hex_digit('A'));
+  static_assert(11u == decode_hex_digit('B'));
+  static_assert(12u == decode_hex_digit('C'));
+  static_assert(13u == decode_hex_digit('D'));
+  static_assert(14u == decode_hex_digit('E'));
+  static_assert(15u == decode_hex_digit('F'));
+}
+
+TEST_MAIN(Hex)