add "mod" function to FlxMath (#3341)

* add mod function

* add mod test

* update mod docs

* update mod docs 2

flixel/math/FlxMath.hx

@@ -573,4 +573,23 @@ class FlxMath
 	{
 		return (n > 0) ? n : -n;
 	}
+
+	/**
+	 * Performs a modulo operation to calculate the remainder of `a` divided by `b`.
+	 * 
+	 * The definition of "remainder" varies by implementation;
+	 * this one is similar to GLSL or Python in that it uses Euclidean division, which always returns positive,
+	 * while Haxe's `%` operator uses signed truncated division.
+	 * 
+	 * For example, `-5 % 3` returns `-2` while `FlxMath.mod(-5, 3)` returns `1`.
+	 * 
+	 * @param a The dividend.
+	 * @param b The divisor.
+	 * @return `a mod b`.
+	 */
+	public static inline function mod(a:Float, b:Float):Float
+	{
+		b = Math.abs(b);
+		return a - b * Math.floor(a / b);
+	}
 }

tests/unit/src/flixel/math/FlxMathTest.hx

@@ -102,4 +102,16 @@ class FlxMathTest extends FlxTest
 		// #2126
 		Assert.areEqual(1521730678.942, FlxMath.roundDecimal(1521730678.942, 3));
 	}
+
+	@Test
+	function testMod()
+	{
+		Assert.areEqual(0, FlxMath.mod(8, 4));
+		Assert.areEqual(3, FlxMath.mod(8, 5));
+		Assert.areEqual(0.355, FlxMath.mod(0.941, 0.586));
+		Assert.areEqual(1, FlxMath.mod(-5, 3));
+		Assert.areEqual(1, FlxMath.mod(-5, -3));
+		Assert.areEqual(0.128, FlxMath.mod(-0.834, 0.481));
+		Assert.areEqual(2.5, FlxMath.mod(9, -6.5));
+	}
 }