package probe

import (
	"bufio"
	"bytes"
	"context"
	"errors"
	"fmt"
	"os"
	"strconv"
	"strings"
)

const netDevPath = "/proc/net/dev"

// InterfaceStats contains received and transmitted byte counters.
type InterfaceStats struct {
	Name    string
	RXBytes uint64
	TXBytes uint64
}

// ReadInterfaceStats reads byte counters for a network interface.
func ReadInterfaceStats(ctx context.Context, name string) (InterfaceStats, error) {
	if err := ctx.Err(); err != nil {
		return InterfaceStats{}, fmt.Errorf("checking context before reading interface stats: %w", err)
	}
	data, err := os.ReadFile(netDevPath)
	if err != nil {
		return InterfaceStats{}, fmt.Errorf("reading %s: %w", netDevPath, err)
	}
	if err := ctx.Err(); err != nil {
		return InterfaceStats{}, fmt.Errorf("checking context after reading interface stats: %w", err)
	}
	stats, err := parseNetDev(data, name)
	if err != nil {
		return InterfaceStats{}, err
	}
	return stats, nil
}

func parseNetDev(data []byte, name string) (InterfaceStats, error) {
	scanner := bufio.NewScanner(bytes.NewReader(data))
	for scanner.Scan() {
		line := strings.TrimSpace(scanner.Text())
		if !strings.Contains(line, ":") {
			continue
		}
		parts := strings.SplitN(line, ":", 2)
		if strings.TrimSpace(parts[0]) != name {
			continue
		}
		fields := strings.Fields(parts[1])
		if len(fields) < 16 {
			return InterfaceStats{}, fmt.Errorf("parsing interface %s stats: expected at least 16 fields, got %d", name, len(fields))
		}
		rxBytes, err := strconv.ParseUint(fields[0], 10, 64)
		if err != nil {
			return InterfaceStats{}, fmt.Errorf("parsing interface %s rx bytes: %w", name, err)
		}
		txBytes, err := strconv.ParseUint(fields[8], 10, 64)
		if err != nil {
			return InterfaceStats{}, fmt.Errorf("parsing interface %s tx bytes: %w", name, err)
		}
		return InterfaceStats{Name: name, RXBytes: rxBytes, TXBytes: txBytes}, nil
	}
	if err := scanner.Err(); err != nil {
		return InterfaceStats{}, fmt.Errorf("scanning %s contents: %w", netDevPath, err)
	}
	return InterfaceStats{}, errors.New("interface not found")
}