package main

import (
	"fmt"
	"os"
	"strconv"
	"strings"
)

var nicFields = []string{
	"rx_packets", "tx_packets",
	"rx_errors", "tx_errors",
	"rx_dropped", "tx_dropped",
	"rx_crc_errors", "rx_missed_errors",
	"rx_length_errors", "rx_over_errors",
	"rx_frame_errors", "rx_fifo_errors",
	"collisions",
}

type nicCounters struct {
	stats          map[string]uint64
	carrierChanges uint64
	carrierUp      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 readNIC(ifname string) nicCounters {
	c := nicCounters{stats: make(map[string]uint64, len(nicFields))}
	base := "/sys/class/net/" + ifname
	for _, f := range nicFields {
		if v, ok := readUint(base + "/statistics/" + f); ok {
			c.stats[f] = v
		}
	}
	c.carrierChanges, _ = readUint(base + "/carrier_changes")
	c.carrierUp, _ = readUint(base + "/carrier_up_count")
	c.carrierDown, _ = readUint(base + "/carrier_down_count")
	return c
}

func (c nicCounters) diff(prev nicCounters) string {
	var parts []string
	for _, f := range nicFields {
		now, ok := c.stats[f]
		if !ok {
			continue
		}
		if d := now - prev.stats[f]; d != 0 && !strings.HasSuffix(f, "_packets") {
			parts = append(parts, fmt.Sprintf("%s=%d", f, d))
		}
	}
	if d := c.carrierChanges - prev.carrierChanges; d != 0 {
		parts = append(parts, fmt.Sprintf("carrier_changes=%d", d))
	}
	if d := c.carrierDown - prev.carrierDown; d != 0 {
		parts = append(parts, fmt.Sprintf("carrier_down=%d", d))
	}
	return strings.Join(parts, " ")
}