Estimate remeaning download time

app/src/main/java/org/koitharu/kotatsu/utils/progress/Progress.kt

@@ -6,7 +6,7 @@ import kotlinx.parcelize.Parcelize
 @Parcelize
 data class Progress(
 	val value: Int,
-	val total: Int
+	val total: Int,
 ) : Parcelable, Comparable<Progress> {
 
 	override fun compareTo(other: Progress): Int {

app/src/main/java/org/koitharu/kotatsu/utils/progress/TimeLeftEstimator.kt

@@ -0,0 +1,47 @@
+package org.koitharu.kotatsu.utils.progress
+
+import android.os.SystemClock
+import kotlin.math.roundToInt
+import kotlin.math.roundToLong
+
+private const val MIN_ESTIMATE_TICKS = 4
+private const val NO_TIME = -1L
+
+class TimeLeftEstimator {
+
+	private var times = ArrayList<Int>()
+	private var lastTick: Tick? = null
+
+	fun tick(value: Int, total: Int) {
+		if (total < 0) {
+			emptyTick()
+			return
+		}
+		val tick = Tick(value, total, SystemClock.elapsedRealtime())
+		lastTick?.let {
+			val ticksCount = value - it.value
+			times.add(((tick.time - it.time) / ticksCount.toDouble()).roundToInt())
+		}
+		lastTick = tick
+	}
+
+	fun emptyTick() {
+		lastTick = null
+	}
+
+	fun getEstimatedTimeLeft(): Long {
+		val progress = lastTick ?: return NO_TIME
+		if (times.size < MIN_ESTIMATE_TICKS) {
+			return NO_TIME
+		}
+		val timePerTick = times.average()
+		val ticksLeft = progress.total - progress.value
+		return (ticksLeft * timePerTick).roundToLong()
+	}
+
+	private class Tick(
+		val value: Int,
+		val total: Int,
+		val time: Long,
+	)
+}