package main

import (
	"fmt"
	"strconv"
	"strings"
	"time"
)

// The policy, as constants until the mesh has settings a module can read (novox/hq issue 168). The
// values are the predecessor's, made explicit, and two of its behaviours are changed on purpose:
//
//   - **Sustained, not momentary.** The predecessor boosted on one five-second sample above 50 %: a
//     compile's first second, a browser's tab restore. Here the load must stay above the line for
//     SustainSamples samples in a row, and below the lower line as long, before the profile moves.
//   - **Waiting on a disk is not load.** iowait is counted as idle: a machine stalled on its SSD does
//     not get faster with a higher power limit, only hotter.
const (
	ProfileOnBattery = "Quiet"
	ProfileOnAC      = "Balanced"
	ProfileUnderLoad = "Performance"

	CPUHighPercent = 50.0 // on mains, sustained at or above this boosts to ProfileUnderLoad
	CPULowPercent  = 20.0 // and sustained at or below this goes back to ProfileOnAC

	SampleEvery    = 10 * time.Second // CPU is sampled only on mains; on battery nothing is sampled
	SustainSamples = 3                // 30 s above CPUHighPercent to boost
	RelaxSamples   = 3                // 30 s below CPULowPercent to relax

	// SafetyRecheck is how often the power source is read when no event has said it changed: the
	// kernel's event is the trigger, and this only covers one lost across a suspend.
	SafetyRecheck = 5 * time.Minute

	// DefaultHold is how long a profile chosen through the profile tool is kept before the switcher
	// may move it again. A change of power source ends a hold at once.
	DefaultHold = 60 * time.Minute

	// ChargeLimitPercent is the battery charge limit the module asserts through asusd at start.
	ChargeLimitPercent = 80
)

// Policy is the switcher's memory of recent load: how many samples in a row were above the upper line
// or below the lower one, and whether it is boosted.
type Policy struct {
	Boosted      bool      `json:"boosted"`
	Above        int       `json:"samples_above"`
	Below        int       `json:"samples_below"`
	Recent       []float64 `json:"recent_cpu_percent"`
	BoostedSince time.Time `json:"boosted_since,omitempty"`
}

// Observe takes one CPU sample (busy percent since the previous one) taken on mains.
func (p *Policy) Observe(cpu float64, at time.Time) {
	p.Recent = append(p.Recent, round1(cpu))
	if len(p.Recent) > 6 {
		p.Recent = p.Recent[len(p.Recent)-6:]
	}
	switch {
	case cpu >= CPUHighPercent:
		p.Above++
		p.Below = 0
		if !p.Boosted && p.Above >= SustainSamples {
			p.Boosted = true
			p.BoostedSince = at
		}
	case cpu <= CPULowPercent:
		p.Below++
		p.Above = 0
		if p.Boosted && p.Below >= RelaxSamples {
			p.Boosted = false
			p.BoostedSince = time.Time{}
		}
	default:
		// Between the lines: no direction is sustained, and the profile stays where it is.
		p.Above, p.Below = 0, 0
	}
}

// Reset forgets the load, for a change of power source.
func (p *Policy) Reset() { *p = Policy{} }

// Decision is what the switcher would choose, and why.
type Decision struct {
	Profile string `json:"profile"`
	Reason  string `json:"reason"`
}

// Decide is the policy: battery → ProfileOnBattery; mains → ProfileOnAC, or ProfileUnderLoad while
// boosted.
func (p *Policy) Decide(src Source) Decision {
	if !src.OnAC {
		return Decision{ProfileOnBattery, "on battery (" + src.Reason + ")"}
	}
	if p.Boosted {
		return Decision{ProfileUnderLoad, fmt.Sprintf("on mains (%s) and CPU load sustained at or above %s%% for %d samples of %s",
			src.Reason, strconv.FormatFloat(CPUHighPercent, 'f', -1, 64), SustainSamples, SampleEvery)}
	}
	return Decision{ProfileOnAC, fmt.Sprintf("on mains (%s), and CPU load not sustained at or above %s%%",
		src.Reason, strconv.FormatFloat(CPUHighPercent, 'f', -1, 64))}
}

// CPUTimes is the first line of /proc/stat: total and idle jiffies (iowait counted as idle).
type CPUTimes struct{ Total, Idle uint64 }

// ParseProcStat reads the aggregate cpu line of /proc/stat.
func ParseProcStat(text string) (CPUTimes, error) {
	line := strings.SplitN(text, "\n", 2)[0]
	f := strings.Fields(line)
	if len(f) < 6 || f[0] != "cpu" {
		return CPUTimes{}, fmt.Errorf("/proc/stat does not start with the cpu line")
	}
	var t CPUTimes
	for i, s := range f[1:] {
		if i >= 8 { // user nice system idle iowait irq softirq steal; guest is already in user
			break
		}
		n, err := strconv.ParseUint(s, 10, 64)
		if err != nil {
			return CPUTimes{}, fmt.Errorf("/proc/stat: %v", err)
		}
		t.Total += n
		if i == 3 || i == 4 {
			t.Idle += n
		}
	}
	return t, nil
}

// Busy is the percentage of time not idle between two readings.
func Busy(before, after CPUTimes) (float64, bool) {
	if after.Total <= before.Total {
		return 0, false
	}
	total := float64(after.Total - before.Total)
	idle := float64(after.Idle - before.Idle)
	return (total - idle) / total * 100, true
}