From ef9bcd17a5ff6f8818b0c4eecae1f493bb58fb66 Mon Sep 17 00:00:00 2001
From: Danil Chapovalov <danilchap@webrtc.org>
Date: Fri, 17 Jun 2022 10:18:21 +0200
Subject: [PATCH] Make webrtc units division by integer constexpr

std::round is not constexpr until c++23 and force conversion to floating point which is unnecessary for integer devision

For integer division change rounding to 'round down' from 'round to nearest' as less surprising.

Bug: webrtc:13756
Change-Id: I9c2382bafc9ddccb0f54d6e7bf8cac4f2a3175a1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/265863
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37250}
---
 rtc_base/units/unit_base.h           | 18 +++++++++++-------
 rtc_base/units/unit_base_unittest.cc |  5 +++++
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/rtc_base/units/unit_base.h b/rtc_base/units/unit_base.h
index 0cd6a61215..bbf7300a16 100644
--- a/rtc_base/units/unit_base.h
+++ b/rtc_base/units/unit_base.h
@@ -266,14 +266,18 @@ class RelativeUnit : public UnitBase<Unit_T> {
     return UnitBase<Unit_T>::template ToValue<double>() /
            other.template ToValue<double>();
   }
-  template <typename T>
-  constexpr typename std::enable_if<std::is_arithmetic<T>::value, Unit_T>::type
-  operator/(const T& scalar) const {
-    return UnitBase<Unit_T>::FromValue(
-        std::round(UnitBase<Unit_T>::template ToValue<int64_t>() / scalar));
+  template <typename T,
+            typename std::enable_if_t<std::is_floating_point_v<T>>* = nullptr>
+  Unit_T operator/(T scalar) const {
+    return UnitBase<Unit_T>::FromValue(std::llround(this->ToValue() / scalar));
   }
-  constexpr Unit_T operator*(double scalar) const {
-    return UnitBase<Unit_T>::FromValue(std::round(this->ToValue() * scalar));
+  template <typename T,
+            typename std::enable_if_t<std::is_integral_v<T>>* = nullptr>
+  constexpr Unit_T operator/(T scalar) const {
+    return UnitBase<Unit_T>::FromValue(this->ToValue() / scalar);
+  }
+  Unit_T operator*(double scalar) const {
+    return UnitBase<Unit_T>::FromValue(std::llround(this->ToValue() * scalar));
   }
   constexpr Unit_T operator*(int64_t scalar) const {
     return UnitBase<Unit_T>::FromValue(this->ToValue() * scalar);
diff --git a/rtc_base/units/unit_base_unittest.cc b/rtc_base/units/unit_base_unittest.cc
index 9eb0c33ae5..258d7d1268 100644
--- a/rtc_base/units/unit_base_unittest.cc
+++ b/rtc_base/units/unit_base_unittest.cc
@@ -68,6 +68,7 @@ TEST(UnitBaseTest, ConstExpr) {
   static_assert(kTestUnitValue.ToValueOr(0) == kValue, "");
   static_assert(TestUnitAddKilo(kTestUnitValue, 2).ToValue() == kValue + 2000,
                 "");
+  static_assert(TestUnit::FromValue(500) / 2 == TestUnit::FromValue(250));
 }
 
 TEST(UnitBaseTest, GetBackSameValues) {
@@ -216,6 +217,10 @@ TEST(UnitBaseTest, MathOperations) {
   EXPECT_EQ(mutable_delta, TestUnit::FromKilo(kValueA + kValueB));
   mutable_delta -= TestUnit::FromKilo(kValueB);
   EXPECT_EQ(mutable_delta, TestUnit::FromKilo(kValueA));
+
+  // Division by an int rounds towards zero to follow regular int division.
+  EXPECT_EQ(TestUnit::FromValue(789) / 10, TestUnit::FromValue(78));
+  EXPECT_EQ(TestUnit::FromValue(-789) / 10, TestUnit::FromValue(-78));
 }
 
 TEST(UnitBaseTest, InfinityOperations) {