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diff --git a/gfx/2d/BaseCoord.h b/gfx/2d/BaseCoord.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef MOZILLA_GFX_BASECOORD_H_
+#define MOZILLA_GFX_BASECOORD_H_
+
+#include <ostream>
+
+#include "mozilla/Attributes.h"
+#include "mozilla/MathAlgorithms.h"
+
+namespace mozilla::gfx {
+
+/**
+ * Do not use this class directly. Subclass it, pass that subclass as the
+ * Sub parameter, and only use that subclass. This allows methods to safely
+ * cast 'this' to 'Sub*'.
+ */
+template <class T, class Sub>
+struct BaseCoord {
+ T value;
+
+ // Constructors
+ constexpr BaseCoord() : value(0) {}
+ explicit constexpr BaseCoord(T aValue) : value(aValue) {}
+
+ // Note that '=' isn't defined so we'll get the
+ // compiler generated default assignment operator
+
+ friend constexpr Sub Abs(BaseCoord aCoord) { return Abs(aCoord.value); }
+
+ constexpr operator T() const { return value; }
+
+ friend constexpr bool operator==(Sub aA, Sub aB) {
+ return aA.value == aB.value;
+ }
+ friend constexpr bool operator!=(Sub aA, Sub aB) {
+ return aA.value != aB.value;
+ }
+
+ friend constexpr Sub operator+(Sub aA, Sub aB) {
+ return Sub(aA.value + aB.value);
+ }
+ friend constexpr Sub operator-(Sub aA, Sub aB) {
+ return Sub(aA.value - aB.value);
+ }
+ friend constexpr Sub operator*(Sub aCoord, T aScale) {
+ return Sub(aCoord.value * aScale);
+ }
+ friend constexpr Sub operator*(T aScale, Sub aCoord) {
+ return Sub(aScale * aCoord.value);
+ }
+ friend constexpr Sub operator/(Sub aCoord, T aScale) {
+ return Sub(aCoord.value / aScale);
+ }
+ // 'scale / coord' is intentionally omitted because it doesn't make sense.
+
+ constexpr Sub& operator+=(Sub aCoord) {
+ value += aCoord.value;
+ return *static_cast<Sub*>(this);
+ }
+ constexpr Sub& operator-=(Sub aCoord) {
+ value -= aCoord.value;
+ return *static_cast<Sub*>(this);
+ }
+ constexpr Sub& operator*=(T aScale) {
+ value *= aScale;
+ return *static_cast<Sub*>(this);
+ }
+ constexpr Sub& operator/=(T aScale) {
+ value /= aScale;
+ return *static_cast<Sub*>(this);
+ }
+
+ // Since BaseCoord is implicitly convertible to its value type T, we need
+ // mixed-type operator overloads to avoid ambiguities at mixed-type call
+ // sites. As we transition more of our code to strongly-typed classes, we
+ // may be able to remove some or all of these overloads.
+ friend constexpr bool operator==(Sub aA, T aB) { return aA.value == aB; }
+ friend constexpr bool operator==(T aA, Sub aB) { return aA == aB.value; }
+ friend constexpr bool operator!=(Sub aA, T aB) { return aA.value != aB; }
+ friend constexpr bool operator!=(T aA, Sub aB) { return aA != aB.value; }
+ friend constexpr T operator+(Sub aA, T aB) { return aA.value + aB; }
+ friend constexpr T operator+(T aA, Sub aB) { return aA + aB.value; }
+ friend constexpr T operator-(Sub aA, T aB) { return aA.value - aB; }
+ friend constexpr T operator-(T aA, Sub aB) { return aA - aB.value; }
+
+ constexpr Sub operator-() const { return Sub(-value); }
+
+ friend std::ostream& operator<<(std::ostream& aStream,
+ const BaseCoord<T, Sub>& aCoord) {
+ return aStream << aCoord.value;
+ }
+};
+
+} // namespace mozilla::gfx
+
+#endif /* MOZILLA_GFX_BASECOORD_H_ */