From bb8456691b72aacdd045e94a57a6dcf5241ef87d Mon Sep 17 00:00:00 2001
From: wraitii <wraitii@wildfiregames.com>
Date: Mon, 9 Nov 2020 13:25:50 +0000
Subject: [PATCH] Avoid overflow in UnitMotion.

ComputeTargetPosition called Dot() with large enough vectors that it
overflowed. Avoid that by not actually doing the full dot product.

Reported by: Itms
Fixes #5852

Differential Revision: https://code.wildfiregames.com/D3061
This was SVN commit r24152.
---
 source/maths/FixedVector2D.h                     | 11 +++++++++++
 source/simulation2/components/CCmpUnitMotion.cpp |  2 +-
 source/simulation2/helpers/Geometry.cpp          |  8 ++++----
 3 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/source/maths/FixedVector2D.h b/source/maths/FixedVector2D.h
index 87c8fd4916..100c42597e 100644
--- a/source/maths/FixedVector2D.h
+++ b/source/maths/FixedVector2D.h
@@ -204,6 +204,7 @@ public:
 
 	/**
 	 * Compute the dot product of this vector with another.
+	 * Likely to overflow if both vectors are large-ish (around the 200 range).
 	 */
 	fixed Dot(const CFixedVector2D& v) const
 	{
@@ -219,6 +220,16 @@ public:
 		return ret;
 	}
 
+	/**
+	 * @return -1, 0 or 1 if this and @v face respectively opposite directions, perpendicular, or same directions.
+	 */
+	int RelativeOrientation(const CFixedVector2D& v) const
+	{
+		i64 x = MUL_I64_I32_I32(X.GetInternalValue(), v.X.GetInternalValue());
+		i64 y = MUL_I64_I32_I32(Y.GetInternalValue(), v.Y.GetInternalValue());
+		return x > -y ? 1 : x < -y ? -1 : 0;
+	}
+
 	CFixedVector2D Perpendicular() const
 	{
 		return CFixedVector2D(Y, -X);
diff --git a/source/simulation2/components/CCmpUnitMotion.cpp b/source/simulation2/components/CCmpUnitMotion.cpp
index 92722eead2..b8635700fe 100644
--- a/source/simulation2/components/CCmpUnitMotion.cpp
+++ b/source/simulation2/components/CCmpUnitMotion.cpp
@@ -1149,7 +1149,7 @@ bool CCmpUnitMotion::ComputeTargetPosition(CFixedVector2D& out, const MoveReques
 
 			// Check if we anticipate the target to go through us, in which case we shouldn't anticipate
 			// (or e.g. units fleeing might suddenly turn around towards their attacker).
-			if ((out - cmpPosition->GetPosition2D()).Dot(tempPos - cmpPosition->GetPosition2D()) >= fixed::Zero())
+			if ((out - cmpPosition->GetPosition2D()).RelativeOrientation(tempPos - cmpPosition->GetPosition2D()) >= 0)
 				out = tempPos;
 		}
 	}
diff --git a/source/simulation2/helpers/Geometry.cpp b/source/simulation2/helpers/Geometry.cpp
index f7563969ad..4be531c8ce 100644
--- a/source/simulation2/helpers/Geometry.cpp
+++ b/source/simulation2/helpers/Geometry.cpp
@@ -342,13 +342,13 @@ static bool SquareSAT(const CFixedVector2D& a, const CFixedVector2D& axis, const
 	fixed hh = halfSize.Y;
 
 	CFixedVector2D p = axis.Perpendicular();
-	if (p.Dot((u.Multiply(hw) + v.Multiply(hh)) - a) <= fixed::Zero())
+	if (p.RelativeOrientation(u.Multiply(hw) + v.Multiply(hh) - a) <= 0)
 		return true;
-	if (p.Dot((u.Multiply(hw) - v.Multiply(hh)) - a) <= fixed::Zero())
+	if (p.RelativeOrientation(u.Multiply(hw) - v.Multiply(hh) - a) <= 0)
 		return true;
-	if (p.Dot((-u.Multiply(hw) - v.Multiply(hh)) - a) <= fixed::Zero())
+	if (p.RelativeOrientation(-u.Multiply(hw) - v.Multiply(hh) - a) <= 0)
 		return true;
-	if (p.Dot((-u.Multiply(hw) + v.Multiply(hh)) - a) <= fixed::Zero())
+	if (p.RelativeOrientation(-u.Multiply(hw) + v.Multiply(hh) - a) <= 0)
 		return true;
 
 	return false;