package main

import (
	"os"
	"strconv"
	"strings"
	"sync/atomic"
	"time"
)

// Split by direction of travel so each counter belongs to exactly one
// direction: the transmitting interface owns the tx fields and the receiving one
// the rx fields. Reading both sets off both interfaces double counts every drop.
var (
	nicTxFields = []string{
		"tx_errors", "tx_dropped", "tx_fifo_errors",
		"tx_carrier_errors", "tx_aborted_errors", "tx_window_errors",
		"collisions",
	}
	nicRxFields = []string{
		"rx_errors", "rx_dropped", "rx_crc_errors", "rx_missed_errors",
		"rx_length_errors", "rx_over_errors", "rx_frame_errors", "rx_fifo_errors",
	}
)

type nicCounters struct {
	tx          uint64
	rx          uint64
	carrierDown uint64
}

func readUint(path string) (uint64, bool) {
	b, err := os.ReadFile(path)
	if err != nil {
		return 0, false
	}
	v, err := strconv.ParseUint(strings.TrimSpace(string(b)), 10, 64)
	if err != nil {
		return 0, false
	}
	return v, true
}

func sumFields(base string, fields []string) uint64 {
	var total uint64
	for _, f := range fields {
		if v, ok := readUint(base + "/statistics/" + f); ok {
			total += v
		}
	}
	return total
}

func readNIC(ifname string) nicCounters {
	base := "/sys/class/net/" + ifname
	c := nicCounters{
		tx: sumFields(base, nicTxFields),
		rx: sumFields(base, nicRxFields),
	}
	c.carrierDown, _ = readUint(base + "/carrier_down_count")
	return c
}

// A poll is thirty-odd sysfs attributes and each one costs microseconds of open
// and read, so it runs here rather than on the goroutine that has a frame to
// draw every sixteen milliseconds. Off that path the interval can be short
// enough that an unplugged cable shows up as a link error while the hand is
// still on it.
const nicInterval = 200 * time.Millisecond

type nicPoller struct {
	txName, rxName string
	total          *atomic.Uint64
	raw            uint64
}

func (p *nicPoller) read() uint64 {
	tx := readNIC(p.txName)
	rx := readNIC(p.rxName)
	return tx.tx + rx.rx + tx.carrierDown
}

// Sysfs nic counters run from boot and restart from zero whenever the driver
// resets its statistics, so only their forward motion is accumulated. Taking
// raw differences instead charges a boot's worth of errors to the first sample
// and turns a reset into a near-2^64 underflow.
func (p *nicPoller) poll() {
	raw := p.read()
	if raw > p.raw {
		p.total.Add(raw - p.raw)
	}
	p.raw = raw
}

// Whatever the interfaces counted before now is not ours, so the first poll has
// something to measure forward motion from and adds nothing.
func (p *nicPoller) prime() {
	p.raw = p.read()
}

// Held until the test starts, since polling before the baseline is primed would
// charge everything the interfaces counted since boot to the first sample.
func (p *nicPoller) run(done *atomic.Bool, startTx <-chan struct{}) {
	<-startTx

	tick := time.NewTicker(nicInterval)
	defer tick.Stop()

	for !done.Load() {
		<-tick.C
		p.poll()
	}
}