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/*
 * =========================================================================================
 * Copyright © 2013-2017 the kamon project 
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
 * except in compliance with the License. You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the
 * License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
 * either express or implied. See the License for the specific language governing permissions
 * and limitations under the License.
 * =========================================================================================
 */

package kamon.system.process

import kamon.Kamon
import kamon.system.{Metric, MetricBuilder, SigarMetricBuilder}
import org.hyperic.sigar.{ProcCpu, Sigar}
import org.slf4j.Logger

import scala.util.Try

/**
 *  Process Cpu usage metrics, as reported by Sigar:
 *    - user:  Process cpu user time.
 *    - total: Process cpu time (sum of User and Sys).
 *    - system: Process cpu kernel time.
 */
object ProcessCpuMetrics extends MetricBuilder("process.cpu") with SigarMetricBuilder {
  def build(sigar: Sigar, metricName: String, logger: Logger) = new Metric {
    val processCpuMetric = Kamon.histogram(metricName)

    val processUserCpuMetric = processCpuMetric.refine(Map("component" -> "system-metrics", "mode" -> "user"))
    val processSystemCpuMetric = processCpuMetric.refine(Map("component" -> "system-metrics", "mode" -> "system"))
    val processTotalCpuMetric = processCpuMetric.refine(Map("component" -> "system-metrics", "mode" -> "total"))

    val pid = sigar.getPid
    val totalCores = sigar.getCpuInfoList.headOption.map(_.getTotalCores.toLong).getOrElse(1L)

    var lastProcCpu: ProcCpu = sigar.getProcCpu(pid)
    var currentLoad: Long = 0

    /**
      * While CPU usage time updates not very often, We have introduced a simple heuristic, that supposes that the load is the same as previous,
      * while CPU usage time doesn't update. But supposing that it could be zero load for a process for some time,
      * We used an arbitrary duration of 2000 milliseconds, after which the same CPU usage time value become legal, and it is supposed that the load is really zero.
      *
      * @see [[http://stackoverflow.com/questions/19323364/using-sigar-api-to-get-jvm-cpu-usage "StackOverflow: Using Sigar API to get JVM Cpu usage"]]
      */
    override def update(): Unit = {
      def percentUsage(delta: Long, timeDiff: Long): Long = Try(100 * delta / timeDiff / totalCores).getOrElse(0L)

      def positiveSubtraction(left: Long, right: Long): Long = {
        val result = left - right
        if (result < 0L) 0L else result
      }

      val currentProcCpu = sigar.getProcCpu(pid)
      val totalDiff = positiveSubtraction(currentProcCpu.getTotal, lastProcCpu.getTotal)
      val userDiff = positiveSubtraction(currentProcCpu.getUser, lastProcCpu.getUser)
      val systemDiff = positiveSubtraction(currentProcCpu.getSys, lastProcCpu.getSys)
      val timeDiff = currentProcCpu.getLastTime - lastProcCpu.getLastTime

      if (totalDiff == 0L) {
        if (timeDiff > 2000L) currentLoad = 0L
        if (currentLoad == 0L) lastProcCpu = currentProcCpu
      } else {
        val totalPercent = percentUsage(totalDiff, timeDiff)
        val userPercent = percentUsage(userDiff, timeDiff)
        val systemPercent = percentUsage(systemDiff, timeDiff)

        processUserCpuMetric.record(userPercent)
        processSystemCpuMetric.record(systemPercent)
        processTotalCpuMetric.record(userPercent + systemPercent)

        currentLoad = totalPercent
        lastProcCpu = currentProcCpu
      }
    }
  }
}