CelSprite: Unify width storage

Reduces the size of a CelSprite by taking advantage of the fact that
16-bit pointers are aligned, so the last bit is always 0.

Source/engine/cel_sprite.hpp

@@ -3,6 +3,7 @@
 #include <memory>
 #include <utility>
 
+#include "utils/pointer_value_union.hpp"
 #include "utils/stdcompat/cstddef.hpp"
 
 namespace devilution {
@@ -23,7 +24,7 @@ public:
 
 	CelSprite(const byte *data, const uint16_t *widths)
 	    : data_ptr_(data)
-	    , widths_(widths)
+	    , width_(widths)
 	{
 	}
 
@@ -41,7 +42,7 @@ public:
 
 	[[nodiscard]] uint16_t Width(std::size_t frame = 1) const
 	{
-		return widths_ == nullptr ? width_ : widths_[frame];
+		return width_.HoldsPointer() ? width_.AsPointer()[frame] : width_.AsValue();
 	}
 
 	[[nodiscard]] bool operator==(CelSprite other) const
@@ -55,8 +56,7 @@ public:
 
 private:
 	const byte *data_ptr_;
-	uint16_t width_ = 0;
-	const uint16_t *widths_ = nullptr; // unowned
+	PointerOrValue<uint16_t> width_;
 };
 
 /**

Source/utils/pointer_value_union.hpp

@@ -0,0 +1,50 @@
+#pragma once
+
+#include <cstdint>
+
+namespace devilution {
+
+/**
+ * @brief A tagged union of a pointer and a value in the space of a single pointer.
+ *
+ * Requires the type T to have alignment > 1.
+ * Internally, uses the last bit to hold the tag:
+ * 0 if it is a pointer, guaranteed by T's alignment requirements.
+ * 1 if it is a value.
+ */
+template <typename T>
+class PointerOrValue {
+	static_assert(alignof(T) > 1, "requires alignof > 1");
+	static_assert(sizeof(T) < sizeof(T *), "type too large");
+
+public:
+	explicit PointerOrValue(const T *ptr)
+	    : repr_(reinterpret_cast<uintptr_t>(ptr))
+	{
+	}
+
+	explicit PointerOrValue(T val)
+	    : repr_((static_cast<uintptr_t>(val) << 1) | 1)
+	{
+	}
+
+	[[nodiscard]] bool HoldsPointer() const
+	{
+		return (repr_ & 1) == 0;
+	}
+
+	[[nodiscard]] const T *AsPointer() const
+	{
+		return reinterpret_cast<const T *>(repr_);
+	}
+
+	[[nodiscard]] T AsValue() const
+	{
+		return repr_ >> 1;
+	}
+
+private:
+	uintptr_t repr_;
+};
+
+} // namespace devilution