package hostcollect

import (
	"sort"
	"strconv"
	"strings"
	"time"

	"github.com/itworx/pulse/internal/host"
)

// cpuTimes holds the three aggregates a utilisation delta needs, all in USER_HZ ticks.
type cpuTimes struct {
	total  uint64
	idle   uint64
	iowait uint64
}

type cpuSample struct {
	at    time.Time
	all   cpuTimes
	cores []cpuTimes
	valid bool
}

// parseProcStat reads the cpu aggregate and per-core lines of /proc/stat.
//
// Column layout (proc(5)): user nice system idle iowait irq softirq steal guest
// guest_nice. guest and guest_nice are deliberately excluded from the total: the
// kernel already counts guest time inside user and nice, so including them again
// inflates the denominator and understates utilisation. Old kernels publish fewer
// columns, so anything past idle is optional.
func parseProcStat(data []byte) (cpuSample, error) {
	sample := cpuSample{}
	type indexedCore struct {
		index int
		times cpuTimes
	}
	cores := make([]indexedCore, 0, 8)
	for _, line := range lines(data) {
		if !strings.HasPrefix(line, "cpu") {
			continue
		}
		fields := strings.Fields(line)
		if len(fields) < 5 {
			continue
		}
		times, ok := parseCPUTimes(fields[1:])
		if !ok {
			continue
		}
		if fields[0] == "cpu" {
			sample.all = times
			sample.valid = true
			continue
		}
		index, err := strconv.Atoi(strings.TrimPrefix(fields[0], "cpu"))
		if err != nil || index < 0 {
			continue
		}
		cores = append(cores, indexedCore{index: index, times: times})
	}
	if !sample.valid {
		return cpuSample{}, errNoCPUAggregate
	}
	sort.Slice(cores, func(i, j int) bool { return cores[i].index < cores[j].index })
	sample.cores = make([]cpuTimes, 0, len(cores))
	for _, core := range cores {
		sample.cores = append(sample.cores, core.times)
	}
	return sample, nil
}

// parseCPUTimes sums the first eight jiffy columns; a non-numeric column makes the
// whole line untrustworthy.
func parseCPUTimes(fields []string) (cpuTimes, bool) {
	times := cpuTimes{}
	limit := len(fields)
	if limit > 8 {
		limit = 8
	}
	for index := 0; index < limit; index++ {
		value, ok := parseUint(fields[index])
		if !ok {
			return cpuTimes{}, false
		}
		times.total += value
		switch index {
		case 3:
			times.idle = value
		case 4:
			times.iowait = value
		}
	}
	return times, true
}

// cpuUsage samples /proc/stat and reports utilisation relative to the previous sample.
// The first call after start (or after a counter reset) reports no percentages at all
// rather than a fabricated value: an absolute counter carries no utilisation.
func (c *Collector) cpuUsage(now time.Time, warnings *warningSet) host.RawCPU {
	data, err := readLimited(c.procPath("stat"), maxFileBytes)
	if err != nil {
		warnings.add(WarningCPUUnavailable)
		return host.RawCPU{}
	}
	current, err := parseProcStat(data)
	if err != nil {
		warnings.add(WarningCPUUnavailable)
		return host.RawCPU{}
	}
	current.at = now

	c.mu.Lock()
	previous := c.cpu
	c.cpu = &current
	c.mu.Unlock()

	if previous == nil || !previous.valid {
		warnings.add(WarningCPUFirstSample)
		return host.RawCPU{}
	}
	busy, iowait, ok := utilisation(previous.all, current.all)
	if !ok {
		warnings.add(WarningCPUCounterReset)
		return host.RawCPU{}
	}
	result := host.RawCPU{TotalPercent: &busy, IOWaitPercent: &iowait}
	if len(previous.cores) != len(current.cores) {
		warnings.add(WarningCPUTopologyChanged)
		return result
	}
	cores := make([]float64, 0, len(current.cores))
	for index := range current.cores {
		corePercent, _, coreOK := utilisation(previous.cores[index], current.cores[index])
		if !coreOK {
			warnings.add(WarningCPUCounterReset)
			return result
		}
		cores = append(cores, corePercent)
	}
	if len(cores) > c.options.HostLimits.MaxCores {
		cores = cores[:c.options.HostLimits.MaxCores]
		warnings.add(WarningCPUCoresTruncated)
	}
	result.PerCore = cores
	return result
}

// utilisation converts two counter readings into busy and iowait percentages. Any
// counter going backwards means the counter wrapped or the source was replaced (a
// container restart, a CPU hot-unplug, a fixture root swap); the delta is then
// meaningless and is reported as unusable instead of as a huge or negative spike.
func utilisation(previous, current cpuTimes) (busyPercent, iowaitPercent float64, ok bool) {
	if current.total < previous.total || current.idle < previous.idle || current.iowait < previous.iowait {
		return 0, 0, false
	}
	totalDelta := current.total - previous.total
	if totalDelta == 0 {
		return 0, 0, false
	}
	idleDelta := current.idle - previous.idle
	iowaitDelta := current.iowait - previous.iowait
	if idleDelta+iowaitDelta > totalDelta {
		return 0, 0, false
	}
	busy := float64(totalDelta-idleDelta-iowaitDelta) / float64(totalDelta) * 100
	iowait := float64(iowaitDelta) / float64(totalDelta) * 100
	return clampPercent(busy), clampPercent(iowait), true
}

func clampPercent(value float64) float64 {
	if value < 0 {
		return 0
	}
	if value > 100 {
		return 100
	}
	return value
}