add one euro filter

server/core/src/main/java/dev/slimevr/util/OneEuroFilter.kt

@@ -0,0 +1,58 @@
+package com.oneeurofilter
+
+import kotlin.math.PI
+import kotlin.math.abs
+
+fun smoothingFactor(tE: Double, cutoff: Double): Double {
+    val r = 2 * PI * cutoff * tE
+    return r / (r + 1)
+}
+
+fun exponentialSmoothing(a: Double, x: Double, xPrev: Double): Double {
+    return a * x + (1 - a) * xPrev
+}
+
+class OneEuroFilter(
+    x0: Double,
+    dx0: Double = 0.0,
+    private val minCutoff: Double = 1.0,
+    private val beta: Double = 0.0,
+    private val dCutoff: Double = 1.0
+) {
+    private var xPrev: Double = x0
+    private var dxPrev: Double = dx0
+    private var tPrev: Long = System.nanoTime()
+
+    operator fun invoke(x: Double): Double {
+        val t = System.nanoTime()
+        val tE = (t - tPrev) / 1_000_000_000.0 // Convert nanoseconds to seconds
+
+        return if (tE != 0.0) {
+            // The filtered derivative of the signal
+            val aD = smoothingFactor(tE, dCutoff)
+            val dx = (x - xPrev) / tE
+            val dxHat = exponentialSmoothing(aD, dx, dxPrev)
+
+            // The filtered signal
+            val cutoff = minCutoff + beta * abs(dxHat)
+            val a = smoothingFactor(tE, cutoff)
+            val xHat = exponentialSmoothing(a, x, xPrev)
+
+            // Memorize the previous values
+            xPrev = xHat
+            dxPrev = dxHat
+            tPrev = t
+
+            xHat
+        } else {
+            xPrev = x
+            x
+        }
+    }
+
+    fun reset(x0: Double, dx0: Double = 0.0) {
+        xPrev = x0
+        dxPrev = dx0
+        tPrev = System.nanoTime()
+    }
+}