MonotonicClock.kt

package dev.gawluk

import android.os.SystemClock
import java.time.Clock
import java.time.Instant
import java.time.ZoneId
import java.time.temporal.ChronoUnit

fun monotonicClock(zone: ZoneId): Clock = MonotonicClock(zone)

private class MonotonicClock(private val zone: ZoneId) : Clock() {

    private val initialInstant = Clock.system(zone).instant()
    private val initialElapsedNanos: Long = SystemClock.elapsedRealtimeNanos()

    private val initialMillis = System.currentTimeMillis()
    private val initialElapsedMillis: Long = SystemClock.elapsedRealtime()

    override fun getZone(): ZoneId = zone

    override fun withZone(zone: ZoneId): Clock {
        if (zone == this.zone) {
            return this
        }
        return MonotonicClock(zone)
    }

    override fun instant(): Instant {
        val elapsedNanos: Long = SystemClock.elapsedRealtimeNanos() - initialElapsedNanos
        return initialInstant.plus(elapsedNanos, ChronoUnit.NANOS)
    }

    // optimization to avoid Instant object creation
    override fun millis(): Long {
        return initialMillis + (SystemClock.elapsedRealtime() - initialElapsedMillis)
    }
}