From 18a40587eaec01a3063214d76a2bbe0e28668056 Mon Sep 17 00:00:00 2001
From: Lenny Maiorani <lenny@colorado.edu>
Date: Wed, 21 Oct 2020 09:39:54 -0400
Subject: [PATCH] HashFunctions: constexpr capability

Problem:
- Hash functions can be `constexpr`, but are not.

Solution:
- Change `inline` keyword to `constexpr`.
- Add `static_assert` tests to ensure the hash functions work in a
  `constexpr` context.
---
 AK/HashFunctions.h             | 16 ++++++++--------
 AK/Tests/TestHashFunctions.cpp | 26 ++++++++++++++++++--------
 2 files changed, 26 insertions(+), 16 deletions(-)

diff --git a/AK/HashFunctions.h b/AK/HashFunctions.h
index 8395f7a1097..a5c8dd1ed71 100644
--- a/AK/HashFunctions.h
+++ b/AK/HashFunctions.h
@@ -28,7 +28,7 @@
 
 #include "Types.h"
 
-inline unsigned int_hash(u32 key)
+constexpr unsigned int_hash(u32 key)
 {
     key += ~(key << 15);
     key ^= (key >> 10);
@@ -39,7 +39,7 @@ inline unsigned int_hash(u32 key)
     return key;
 }
 
-inline unsigned double_hash(u32 key)
+constexpr unsigned double_hash(u32 key)
 {
     key = ~key + (key >> 23);
     key ^= (key << 12);
@@ -49,27 +49,27 @@ inline unsigned double_hash(u32 key)
     return key;
 }
 
-inline unsigned pair_int_hash(u32 key1, u32 key2)
+constexpr unsigned pair_int_hash(u32 key1, u32 key2)
 {
     return int_hash((int_hash(key1) * 209) ^ (int_hash(key2 * 413)));
 }
 
-inline unsigned u64_hash(u64 key)
+constexpr unsigned u64_hash(u64 key)
 {
     u32 first = key & 0xFFFFFFFF;
     u32 last = key >> 32;
     return pair_int_hash(first, last);
 }
 
-inline unsigned ptr_hash(FlatPtr ptr)
+constexpr unsigned ptr_hash(FlatPtr ptr)
 {
     if constexpr (sizeof(ptr) == 8)
-        return u64_hash((u64)ptr);
+        return u64_hash(ptr);
     else
-        return int_hash((u32)ptr);
+        return int_hash(ptr);
 }
 
 inline unsigned ptr_hash(const void* ptr)
 {
-    return ptr_hash((FlatPtr)(ptr));
+    return ptr_hash(FlatPtr(ptr));
 }
diff --git a/AK/Tests/TestHashFunctions.cpp b/AK/Tests/TestHashFunctions.cpp
index 48ecaecfcee..16149743ccb 100644
--- a/AK/Tests/TestHashFunctions.cpp
+++ b/AK/Tests/TestHashFunctions.cpp
@@ -31,30 +31,32 @@
 
 TEST_CASE(int_hash)
 {
-    EXPECT_EQ(int_hash(42), 3564735745u);
-    EXPECT_EQ(int_hash(0), 1177991625u);
+    static_assert(int_hash(42) == 3564735745u);
+    static_assert(int_hash(0) == 1177991625u);
 }
 
 TEST_CASE(double_hash)
 {
-    EXPECT_EQ(double_hash(42), 524450u);
-    EXPECT_EQ(double_hash(0), 12384u);
+    static_assert(double_hash(42) == 524450u);
+    static_assert(double_hash(0) == 12384u);
 }
 
 TEST_CASE(pair_int_hash)
 {
-    EXPECT_EQ(pair_int_hash(42, 17), 339337046u);
-    EXPECT_EQ(pair_int_hash(0, 0), 954888656u);
+    static_assert(pair_int_hash(42, 17) == 339337046u);
+    static_assert(pair_int_hash(0, 0) == 954888656u);
 }
 
 TEST_CASE(u64_hash)
 {
-    EXPECT_EQ(u64_hash(42), 2824066580u);
-    EXPECT_EQ(u64_hash(0), 954888656u);
+    static_assert(u64_hash(42) == 2824066580u);
+    static_assert(u64_hash(0) == 954888656u);
 }
 
 TEST_CASE(ptr_hash)
 {
+    // These tests are not static_asserts because the values are
+    // different and the goal is to bind the behavior.
     if constexpr (sizeof(FlatPtr) == 8) {
         EXPECT_EQ(ptr_hash(FlatPtr(42)), 2824066580u);
         EXPECT_EQ(ptr_hash(FlatPtr(0)), 954888656u);
@@ -70,4 +72,12 @@ TEST_CASE(ptr_hash)
     }
 }
 
+TEST_CASE(constexpr_ptr_hash)
+{
+    // This test does not check the result because the goal is just to
+    // ensure the function can be executed in a constexpr context. The
+    // "ptr_hash" test binds the result.
+    static_assert(ptr_hash(FlatPtr(42)));
+}
+
 TEST_MAIN(HashFunctions)