package stats

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

// What the collector reads from the kernel without privileges: the
// interfaces' counters (/sys/class/net), the connection tracking table's
// size, and housecarld's and the host's memory and load (/proc).

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

// ifCounters are an interface's totals since it came up.
type ifCounters struct {
	RxBytes, TxBytes, RxPackets, TxPackets int64
}

// readInterfaces reads every interface's counters but the loopback's.
func readInterfaces(sysNet string) map[string]ifCounters {
	out := map[string]ifCounters{}
	entries, err := os.ReadDir(sysNet)
	if err != nil {
		return out
	}
	for _, e := range entries {
		name := e.Name()
		if name == "lo" {
			continue
		}
		dir := filepath.Join(sysNet, name, "statistics")
		read := func(f string) (int64, bool) {
			b, err := os.ReadFile(filepath.Join(dir, f))
			if err != nil {
				return 0, false
			}
			n, err := strconv.ParseInt(strings.TrimSpace(string(b)), 10, 64)
			return n, err == nil
		}
		var c ifCounters
		var ok bool
		if c.RxBytes, ok = read("rx_bytes"); !ok {
			continue
		}
		c.TxBytes, _ = read("tx_bytes")
		c.RxPackets, _ = read("rx_packets")
		c.TxPackets, _ = read("tx_packets")
		out[name] = c
	}
	return out
}

// readConntrack returns how many connections the kernel tracks, and how
// many it can (ok false without connection tracking).
func readConntrack(proc string) (count, max int64, ok bool) {
	read := func(f string) (int64, bool) {
		b, err := os.ReadFile(filepath.Join(proc, "sys/net/netfilter", f))
		if err != nil {
			return 0, false
		}
		n, err := strconv.ParseInt(strings.TrimSpace(string(b)), 10, 64)
		return n, err == nil
	}
	count, ok1 := read("nf_conntrack_count")
	max, ok2 := read("nf_conntrack_max")
	return count, max, ok1 && ok2
}

// readSelf returns housecarld's resident memory (bytes) and CPU time
// (clock ticks).
func readSelf(proc string) (rss, cpu int64, ok bool) {
	dir := filepath.Join(proc, "self")
	stat, err := os.ReadFile(filepath.Join(dir, "stat"))
	if err != nil {
		return 0, 0, 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 0, 0, 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 0, 0, false
	}
	utime, _ := strconv.ParseInt(f[11], 10, 64)
	stime, _ := strconv.ParseInt(f[12], 10, 64)
	status, err := os.ReadFile(filepath.Join(dir, "status"))
	if err != nil {
		return 0, 0, false
	}
	for _, line := range strings.Split(string(status), "\n") {
		if k, v, ok := strings.Cut(line, ":"); ok && k == "VmRSS" {
			rss = kib(v)
		}
	}
	return rss, utime + stime, true
}

// readHost returns the memory in use (bytes) and the 1-minute load
// average.
func readHost(proc string) (memUsed int64, load float64, ok bool) {
	b, err := os.ReadFile(filepath.Join(proc, "meminfo"))
	if err != nil {
		return 0, 0, false
	}
	var total, avail int64
	for _, line := range strings.Split(string(b), "\n") {
		k, v, _ := strings.Cut(line, ":")
		switch k {
		case "MemTotal":
			total = kib(v)
		case "MemAvailable":
			avail = kib(v)
		}
	}
	if b, err := os.ReadFile(filepath.Join(proc, "loadavg")); err == nil {
		if f := strings.Fields(string(b)); len(f) > 0 {
			load, _ = strconv.ParseFloat(f[0], 64)
		}
	}
	return total - avail, load, total > 0
}

// 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
}