package stats

import (
	"bytes"
	"os"
	"path/filepath"
	"runtime"
	"strconv"
	"strings"
	"time"
)

// Server statistics from /proc: named's process and the host. They need no
// privileges: /proc/PID/stat and /proc/PID/status are readable by every
// user (unless /proc is mounted with hidepid).

// clockTicks is USER_HZ, the unit of the CPU times in /proc/PID/stat. It
// is 100 on every Linux architecture Jarl runs on.
const clockTicks = 100

// procRoot is where /proc is (tests point it elsewhere).
var procRoot = "/proc"

// namedProc is a sample of named's process.
type namedProc struct {
	PID     int
	RSS     int64 // bytes
	Threads int64
	CPU     int64 // user + system time, in clock ticks
	At      time.Time
}

// findNamed returns the PID of named: the process named "named" whose
// command line mentions mainConfig, or else the oldest one (the lowest
// PID). 0 when there is none.
func findNamed(mainConfig string) int {
	entries, err := os.ReadDir(procRoot)
	if err != nil {
		return 0
	}
	best := 0
	for _, e := range entries {
		pid, err := strconv.Atoi(e.Name())
		if err != nil {
			continue
		}
		comm, err := os.ReadFile(filepath.Join(procRoot, e.Name(), "comm"))
		if err != nil || strings.TrimSpace(string(comm)) != "named" {
			continue
		}
		if mainConfig != "" {
			if cmd, err := os.ReadFile(filepath.Join(procRoot, e.Name(), "cmdline")); err == nil && bytes.Contains(cmd, []byte(mainConfig)) {
				return pid
			}
		}
		if best == 0 || pid < best {
			best = pid
		}
	}
	return best
}

// readNamedProc samples a process.
func readNamedProc(pid int, now time.Time) (*namedProc, bool) {
	dir := filepath.Join(procRoot, strconv.Itoa(pid))
	stat, err := os.ReadFile(filepath.Join(dir, "stat"))
	if err != nil {
		return nil, false
	}
	// The command name is in parentheses and may contain spaces: the
	// fields that follow start after the last ')'.
	i := bytes.LastIndexByte(stat, ')')
	if i < 0 {
		return nil, false
	}
	f := strings.Fields(string(stat[i+1:]))
	// f[0] is field 3 (state); utime and stime are fields 14 and 15.
	if len(f) < 13 {
		return nil, false
	}
	utime, _ := strconv.ParseInt(f[11], 10, 64)
	stime, _ := strconv.ParseInt(f[12], 10, 64)
	p := &namedProc{PID: pid, CPU: utime + stime, At: now}
	status, err := os.ReadFile(filepath.Join(dir, "status"))
	if err != nil {
		return nil, false
	}
	for _, line := range strings.Split(string(status), "\n") {
		k, v, ok := strings.Cut(line, ":")
		if !ok {
			continue
		}
		switch k {
		case "VmRSS":
			p.RSS = kib(v)
		case "Threads":
			p.Threads, _ = strconv.ParseInt(strings.TrimSpace(v), 10, 64)
		}
	}
	return p, true
}

// kib parses a "1234 kB" value from /proc into bytes.
func kib(v string) int64 {
	n, _ := strconv.ParseInt(strings.TrimSpace(strings.TrimSuffix(strings.TrimSpace(v), "kB")), 10, 64)
	return n * 1024
}

// Host is the machine's memory, load and uptime.
type Host struct {
	MemTotal     int64     `json:"mem_total"`
	MemAvailable int64     `json:"mem_available"`
	Load         []float64 `json:"load"` // 1, 5 and 15 minutes
	CPUs         int       `json:"cpus"`
	Uptime       int64     `json:"uptime"` // seconds
}

func readHost() *Host {
	h := &Host{CPUs: runtime.NumCPU()}
	if b, err := os.ReadFile(filepath.Join(procRoot, "meminfo")); err == nil {
		for _, line := range strings.Split(string(b), "\n") {
			k, v, _ := strings.Cut(line, ":")
			switch k {
			case "MemTotal":
				h.MemTotal = kib(v)
			case "MemAvailable":
				h.MemAvailable = kib(v)
			}
		}
	}
	if b, err := os.ReadFile(filepath.Join(procRoot, "loadavg")); err == nil {
		for _, f := range strings.Fields(string(b))[:min(3, len(strings.Fields(string(b))))] {
			x, _ := strconv.ParseFloat(f, 64)
			h.Load = append(h.Load, x)
		}
	}
	if b, err := os.ReadFile(filepath.Join(procRoot, "uptime")); err == nil {
		if f := strings.Fields(string(b)); len(f) > 0 {
			x, _ := strconv.ParseFloat(f[0], 64)
			h.Uptime = int64(x)
		}
	}
	return h
}