package main

import (
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"time"
)

// cgroupCPU is a cgroup's processor time, from its cpu.stat.
type cgroupCPU struct {
	usageUsec uint64
}

// parseCPUStat reads a cgroup's cpu.stat ("usage_usec 123\nuser_usec …\nsystem_usec …"), readable by
// every account, unlike /proc/<pid>/stat for another account's process.
func parseCPUStat(s string) cgroupCPU {
	var c cgroupCPU
	for _, line := range strings.Split(s, "\n") {
		k, v, ok := strings.Cut(line, " ")
		if !ok {
			continue
		}
		if k == "usage_usec" {
			c.usageUsec, _ = strconv.ParseUint(strings.TrimSpace(v), 10, 64)
		}
	}
	return c
}

// unitCPU is every unit's processor time, by its cgroup's path. A cgroup whose cpu.stat cannot be
// read is left out, never an error for the whole answer.
func (m *Machine) unitCPU() map[string]cgroupCPU {
	out := map[string]cgroupCPU{}
	for _, u := range m.unitPaths() {
		text := m.read("/sys/fs/cgroup/" + u + "/cpu.stat")
		if text == "" {
			continue
		}
		out[u] = parseCPUStat(text)
	}
	return out
}

// MachineCPU is the machine's CPU in use over the window, from /proc/stat's cpu line. CPUsBusy is
// rounded only for display; raw, unexported, is the same count unrounded, for topCPU's share and
// unaccounted math (review round 1 of novox/hq issue 741: a small busy time that rounds CPUsBusy to
// 0.0 must not zero out every unit's share along with it).
type MachineCPU struct {
	CPUs     int     `json:"cpus"`
	BusyPct  float64 `json:"busy_pct"`  // % of all CPUs together
	CPUsBusy float64 `json:"cpus_busy"` // as a count of CPUs, e.g. 7.5 of 12, rounded for display
	raw      float64
}

// machineCPUOver is the machine's CPU in use between two readings of /proc/stat: everything but idle
// and iowait, counted against the window's total ticks over every CPU.
func machineCPUOver(a, b cpuTimes) MachineCPU {
	total := float64(b.total() - a.total())
	if total <= 0 {
		return MachineCPU{CPUs: b.cpus}
	}
	idle := sub(b.idle, a.idle) + sub(b.iowait, a.iowait)
	busy := total - idle
	if busy < 0 {
		busy = 0
	}
	raw := busy / total * float64(b.cpus)
	return MachineCPU{CPUs: b.cpus, BusyPct: round1(busy / total * 100), CPUsBusy: round1(raw), raw: raw}
}

// CPUUsage is a unit's or a container's share of CPU between two readings of its cgroup's cpu.stat.
// Cgroup is its cgroup's path, relative to /sys/fs/cgroup, said beside Unit always (review round 1 of
// novox/hq issue 741): two cgroups can share a leaf name (an outer "init.scope" and a user session's
// "user.slice/user@1000.service/init.scope"), and the path is how they are told apart.
type CPUUsage struct {
	Unit      string  `json:"unit,omitempty"`
	Cgroup    string  `json:"cgroup,omitempty"`
	Container string  `json:"container,omitempty"`
	CPUPct    float64 `json:"cpu_pct"`   // % of one CPU: 100 = one CPU busy the whole window
	SharePct  float64 `json:"share_pct"` // this unit's share of the machine's busy time
	raw       float64
}

// topCPU ranks the units, or the containers, with the most CPU time between two readings of
// cpu.stat, busiest first. unaccountedPct is the machine's busy time no unit accounts for, as a
// share of it (never below 0): the sum is always taken over every unit, whichever by ranks by, so a
// limit on the list never changes it.
func (m *Machine) topCPU(a, b map[string]cgroupCPU, window time.Duration, by string, limit int, cpusBusy float64) ([]CPUUsage, float64) {
	windowUsec := window.Seconds() * 1e6
	type entry struct {
		path, unit, container string
		pct                   float64
	}
	var entries []entry
	var names map[string]string
	var sumPct float64
	for path, end := range b {
		start, ok := a[path]
		if !ok {
			continue
		}
		d := sub(end.usageUsec, start.usageUsec)
		if d == 0 {
			continue
		}
		pct := d / windowUsec * 100
		sumPct += pct
		unit := filepath.Base(path)
		var container string
		if id := containerID.FindStringSubmatch("/" + path); id != nil {
			if names == nil {
				names = m.containerNames()
			}
			container = id[1][:12]
			if n, ok := names[id[1]]; ok {
				container = n
			}
		}
		entries = append(entries, entry{path: path, unit: unit, container: container, pct: pct})
	}

	// Units are keyed by their cgroup's own path, never by its leaf name (filepath.Base): two cgroups can
	// share a leaf name (an outer "init.scope" and a user session's nested one), and keying by the name
	// alone would merge them into one row (review round 1 of novox/hq issue 741). Containers stay keyed
	// by their name, as before.
	groups := map[string]*CPUUsage{}
	for _, e := range entries {
		key := e.path
		if by == "container" {
			if e.container == "" {
				continue
			}
			key = e.container
		}
		g, ok := groups[key]
		if !ok {
			g = &CPUUsage{Unit: e.unit, Cgroup: e.path, Container: e.container}
			if by == "container" {
				g.Unit, g.Cgroup = "", ""
			}
			groups[key] = g
		}
		g.raw += e.pct
	}
	out := make([]CPUUsage, 0, len(groups))
	for _, g := range groups {
		out = append(out, *g)
	}
	sort.Slice(out, func(i, j int) bool {
		if out[i].raw != out[j].raw {
			return out[i].raw > out[j].raw
		}
		return out[i].Unit+out[i].Cgroup+out[i].Container < out[j].Unit+out[j].Cgroup+out[j].Container
	})
	if len(out) > limit {
		out = out[:limit]
	}
	for i := range out {
		out[i].CPUPct = round1(out[i].raw)
		if cpusBusy > 0 {
			out[i].SharePct = round1(out[i].raw / (cpusBusy * 100) * 100)
		}
	}

	unaccounted := cpusBusy*100 - sumPct
	if unaccounted < 0 {
		unaccounted = 0
	}
	unaccountedPct := 0.0
	if cpusBusy > 0 {
		unaccountedPct = round1(unaccounted / (cpusBusy * 100) * 100)
	}
	return out, unaccountedPct
}