package main

import (
	"encoding/json"
	"fmt"
	"io"
	"log"
	"net/http"
	"os"
	"path/filepath"
	"time"
)

type VoltageBatch struct {
	Voltages []float64 `json:"voltages"`
}

var dataFile *os.File
var berlinTZ *time.Location

func init() {
	var err error
	berlinTZ, err = time.LoadLocation("Europe/Berlin")
	if err != nil {
		log.Fatal(err)
	}
}

func initDataFile() {
	_ = os.MkdirAll("data", 0755)

	filename := filepath.Join(
		"data",
		fmt.Sprintf("voltages_%s.csv",
			time.Now().In(berlinTZ).Format("20060102_150405")),
	)

	f, err := os.OpenFile(filename,
		os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0644)
	if err != nil {
		log.Fatal(err)
	}

	dataFile = f
	dataFile.WriteString("timestamp,voltage\n")
}

func handler(w http.ResponseWriter, r *http.Request) {
	if r.Method != http.MethodPost {
		http.Error(w, "POST only", http.StatusMethodNotAllowed)
		return
	}

	body, err := io.ReadAll(r.Body)
	if err != nil {
		http.Error(w, "read error", http.StatusBadRequest)
		return
	}
	defer r.Body.Close()

	var batch VoltageBatch
	if err := json.Unmarshal(body, &batch); err != nil {
		http.Error(w, "invalid JSON", http.StatusBadRequest)
		return
	}

	start := time.Now().In(berlinTZ)

	for i, v := range batch.Voltages {
		ts := start.Add(time.Duration(i) * 33 * time.Millisecond)
		dataFile.WriteString(
			fmt.Sprintf("%s,%.6f\n",
				ts.Format("2006-01-02 15:04:05.000"), v),
		)
	}

	w.WriteHeader(http.StatusOK)
	w.Write([]byte("ok"))
}

func main() {
	initDataFile()
	defer dataFile.Close()

	http.HandleFunc("/data", handler)

	log.Fatal(http.ListenAndServe(":8080", nil))
}