p5.Vector reflect() unexpectedly modifies surface normal argument

[nbogie]

Most appropriate sub-area of p5.js?

• Accessibility
• Color
• Core/Environment/Rendering
• Data
• DOM
• Events
• Image
• IO
• Math
• Typography
• Utilities
• WebGL
• Build process
• Unit testing
• Internationalization
• Friendly errors
• Other (specify if possible)

p5.js version

1.9.4

Web browser and version

all

Operating system

all

Steps to reproduce this

Snippet:

function setup() {
	instanceDemo();
	staticDemo();
}

function instanceDemo() {
	const surfaceNormal = createVector(1, 0);
	const incidentVector = createVector(1, 1);
	
	incidentVector.reflect(surfaceNormal);

	//We'd hope surfaceNormal remains [1, 0, 0], but it'll be [2, 0, 0];
	console.log(surfaceNormal.toString());
}

function staticDemo() {
	const surfaceNormal = createVector(1, 0);
	const incidentVector = createVector(1, 1);
	
	p5.Vector.reflect(incidentVector, surfaceNormal);

	//We'd hope surfaceNormal remains [1, 0, 0], but it'll be [2, 0, 0];
	console.log(surfaceNormal.toString());
}

Expected behaviour:

I'd expect the surface normal argument to be unchanged by the calculation, in both instance and static cases.

Actual behaviour:

The surface normal argument is mutated during the calculation, in both instance and static cases.

Misc:

Here's a (simplification of a) real usage where the bug caught me out: https://openprocessing.org/sketch/2295422

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[nbogie]

I'd like to contribute a fix and regression tests, if it turns out that that's wanted!

Looks like the code has been this way since addition of the functions in v1.0.0

[limzykenneth]

It seems like on this line the surface normal is multiplied by two using the instance method which modifies the vector where it probably should use a static method that does not modify the original instead.

@nbogie You can go ahead with a fix. Thanks!