package main

import (
	"fmt"
	"io/fs"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"time"
)

// clockTicks is USER_HZ, the unit of utime and stime in /proc/<pid>/stat: 100 on every Linux the mesh runs.
const clockTicks = 100.0

// ProcessCPU is one process of a unit and its share of CPU over the window (cpu_top with unit set).
type ProcessCPU struct {
	PID      int     `json:"pid"`
	Command  string  `json:"command"`   // comm, then the first 120 characters of the command line
	CPUPct   float64 `json:"cpu_pct"`   // % of one CPU: 100 = one CPU busy the whole window
	SharePct float64 `json:"share_pct"` // this process's share of the unit's busy time
	Threads  int     `json:"threads"`
	raw      float64
}

// procTicks is a process's utime+stime ticks and thread count from its /proc/<pid>/stat, whose comm
// field may hold spaces and parentheses, so the fields are counted from the last ")".
func parseProcStat(s string) (comm string, ticks uint64, threads int, start string, ok bool) {
	l, r := strings.Index(s, "("), strings.LastIndex(s, ")")
	if l < 0 || r < l {
		return "", 0, 0, "", false
	}
	f := strings.Fields(s[r+1:])
	if len(f) < 20 {
		return "", 0, 0, "", false
	}
	ut, e1 := strconv.ParseUint(f[11], 10, 64)
	st, e2 := strconv.ParseUint(f[12], 10, 64)
	th, e3 := strconv.Atoi(f[17])
	if e1 != nil || e2 != nil || e3 != nil {
		return "", 0, 0, "", false
	}
	return s[l+1 : r], ut + st, th, f[19], true
}

// cgroupOfUnit is the cgroup path (relative to /sys/fs/cgroup) a unit name or a path names.
func (m *Machine) cgroupOfUnit(unit string) (string, error) {
	base := m.path("/sys/fs/cgroup")
	if strings.Contains(unit, "/") {
		rel := strings.Trim(strings.TrimPrefix(unit, "/sys/fs/cgroup"), "/")
		if strings.Contains(rel, "..") || filepath.Clean("/"+rel) != "/"+rel {
			return "", fmt.Errorf("cgroup %q is not a path under /sys/fs/cgroup", unit)
		}
		if rel == "" || !m.exists("/sys/fs/cgroup/"+rel) {
			return "", fmt.Errorf("no cgroup %q", unit)
		}
		return rel, nil
	}
	var found []string
	filepath.WalkDir(base, func(p string, d fs.DirEntry, err error) error {
		if err == nil && d.IsDir() && d.Name() == unit {
			rel, _ := filepath.Rel(base, p)
			found = append(found, filepath.ToSlash(rel))
		}
		return nil
	})
	switch len(found) {
	case 0:
		return "", fmt.Errorf("no unit %q in the cgroup tree", unit)
	case 1:
		return found[0], nil
	}
	return "", fmt.Errorf("unit %q is %d cgroups (%s): give the cgroup path", unit, len(found), strings.Join(found, ", "))
}

type procReading struct {
	ticks   uint64
	comm    string
	threads int
	pid     int
	start   string // starttime, to tell a reused pid
}

// unitProcs reads every process in a cgroup and its child cgroups: its ticks, name and threads.
func (m *Machine) unitProcs(cgroup string) map[int]procReading {
	out := map[int]procReading{}
	base := m.path("/sys/fs/cgroup/" + cgroup)
	filepath.WalkDir(base, func(p string, d fs.DirEntry, err error) error {
		if err != nil || d.IsDir() || d.Name() != "cgroup.procs" {
			return nil
		}
		for _, f := range strings.Fields(m.read(strings.TrimPrefix(p, filepath.Clean(m.Root)))) {
			pid, err := strconv.Atoi(f)
			if err != nil {
				continue
			}
			comm, ticks, th, start, ok := parseProcStat(m.read("/proc/" + f + "/stat"))
			if ok {
				out[pid] = procReading{ticks: ticks, comm: comm, threads: th, pid: pid, start: start}
			}
		}
		return nil
	})
	return out
}

// commandLine is a process's program path (argv[0], cut to 120 characters), never its arguments:
// they can carry a password.
func (m *Machine) commandLine(pid int) string {
	s, _, _ := strings.Cut(m.read("/proc/"+strconv.Itoa(pid)+"/cmdline"), "\x00")
	s, _, _ = strings.Cut(s, " ") // a process that rewrites its title puts its whole command line here
	s, _, _ = strings.Cut(s, "\t")
	s = strings.TrimSpace(s)
	if r := []rune(s); len(r) > 120 {
		s = string(r[:120])
	}
	return s
}

// topProcessCPU ranks a unit's processes by CPU between two readings; a process missing from either
// reading (started or ended in the window) is skipped.
func (m *Machine) topProcessCPU(a, b map[int]procReading, window time.Duration, limit int) []ProcessCPU {
	var out []ProcessCPU
	var sum float64
	for pid, end := range b {
		start, ok := a[pid]
		if !ok || end.start != start.start || end.ticks <= start.ticks {
			continue
		}
		pct := float64(end.ticks-start.ticks) / clockTicks / window.Seconds() * 100
		sum += pct
		out = append(out, ProcessCPU{PID: pid, Command: end.comm, Threads: end.threads, raw: pct})
	}
	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].PID < out[j].PID
	})
	if len(out) > limit {
		out = out[:limit]
	}
	for i := range out {
		out[i].CPUPct = round1(out[i].raw)
		out[i].SharePct = round1(out[i].raw / sum * 100)
		if cl := m.commandLine(out[i].PID); cl != "" {
			out[i].Command += ": " + cl
		}
	}
	return out
}