//go:build linux

package stats

import (
	"bufio"
	"os"
	"strconv"
	"strings"
)

func (r *Reporter) SystemCPUPercent() float64 {
	idle, total, ok := readProcStatCPUSample()
	if !ok {
		return 0
	}

	r.mu.Lock()
	defer r.mu.Unlock()

	if !r.hasSample {
		r.lastIdle = idle
		r.lastTotal = total
		r.hasSample = true
		return 0
	}

	idleDelta := float64(idle - r.lastIdle)
	totalDelta := float64(total - r.lastTotal)
	r.lastIdle = idle
	r.lastTotal = total

	return cpuBusyPercentFromDeltas(idleDelta, totalDelta)
}

func readProcStatCPUSample() (idle, total uint64, ok bool) {
	f, err := os.Open("/proc/stat")
	if err != nil {
		return 0, 0, false
	}
	defer f.Close()

	sc := bufio.NewScanner(f)
	if !sc.Scan() {
		return 0, 0, false
	}
	return parseProcStatCPU(sc.Text())
}

func parseProcStatCPU(line string) (idle, total uint64, ok bool) {
	fields := strings.Fields(line)
	if len(fields) < 5 || fields[0] != "cpu" {
		return 0, 0, false
	}
	var values []uint64
	for _, f := range fields[1:] {
		v, err := strconv.ParseUint(f, 10, 64)
		if err != nil {
			return 0, 0, false
		}
		values = append(values, v)
	}
	for _, v := range values {
		total += v
	}
	// idle + iowait (index 3 and 4 when present)
	idle = values[3]
	if len(values) > 4 {
		idle += values[4]
	}
	return idle, total, true
}