package main

import (
	"bufio"
	"os"
	"strconv"
	"strings"
	"sync"
	"time"
)

type NetworkMetrics struct {
	Interface      string  `json:"interface"`
	ReceiveBytes   uint64  `json:"receiveBytes"`
	TransmitBytes  uint64  `json:"transmitBytes"`
	ReceivePerSec  float64 `json:"receivePerSec"`
	TransmitPerSec float64 `json:"transmitPerSec"`
}

type networkSnapshot struct {
	at       time.Time
	received uint64
	sent     uint64
}

type networkCollector struct {
	mu       sync.Mutex
	path     string
	previous networkSnapshot
}

func newNetworkCollector(path string) (*networkCollector, error) {
	snapshot, _, err := readNetworkSnapshot(path)
	if err != nil {
		return nil, err
	}
	return &networkCollector{path: path, previous: snapshot}, nil
}

func (c *networkCollector) collect() (NetworkMetrics, error) {
	c.mu.Lock()
	defer c.mu.Unlock()

	current, interfaces, err := readNetworkSnapshot(c.path)
	if err != nil {
		return NetworkMetrics{}, err
	}
	elapsed := current.at.Sub(c.previous.at).Seconds()
	var receivedRate, sentRate float64
	if elapsed > 0 && current.received >= c.previous.received && current.sent >= c.previous.sent {
		receivedRate = float64(current.received-c.previous.received) / elapsed
		sentRate = float64(current.sent-c.previous.sent) / elapsed
	}
	c.previous = current
	return NetworkMetrics{
		Interface: strings.Join(interfaces, ", "), ReceiveBytes: current.received,
		TransmitBytes: current.sent, ReceivePerSec: receivedRate, TransmitPerSec: sentRate,
	}, nil
}

func readNetworkSnapshot(path string) (networkSnapshot, []string, error) {
	file, err := os.Open(path)
	if err != nil {
		return networkSnapshot{}, nil, err
	}
	defer file.Close()

	var received, sent uint64
	var interfaces []string
	scanner := bufio.NewScanner(file)
	for scanner.Scan() {
		line := strings.TrimSpace(scanner.Text())
		name, values, found := strings.Cut(line, ":")
		if !found || strings.TrimSpace(name) == "lo" {
			continue
		}
		fields := strings.Fields(values)
		if len(fields) < 9 {
			continue
		}
		rx, errRX := strconv.ParseUint(fields[0], 10, 64)
		tx, errTX := strconv.ParseUint(fields[8], 10, 64)
		if errRX == nil && errTX == nil {
			received += rx
			sent += tx
			interfaces = append(interfaces, strings.TrimSpace(name))
		}
	}
	return networkSnapshot{at: time.Now(), received: received, sent: sent}, interfaces, scanner.Err()
}