package main

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

// Everything the driver counts against a frame the link failed to carry, split
// by direction of travel so each counter belongs to exactly one direction: the
// transmitting interface owns the tx fields and the receiving interface the rx
// ones. Reading both sets off both interfaces would report every drop twice.
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
}