Concurrent Shopping Cart

main.go

package main

import (
	"log/slog"
	"os"
)

func main() {
	logger := slog.New(slog.NewTextHandler(os.Stdout, nil))
	app := &App{
		cart:    make(map[int]Value),
		logger:  logger,
		version: 0,
	}
	err := app.serve()
	if err != nil {
		logger.Error(err.Error())
		os.Exit(1)
	}
}

server.go

package main

import (
	"encoding/json"
	"log/slog"
	"net/http"
)

type App struct {
	cart    map[int]Value
	logger  *slog.Logger
	version int
}

type Value []string

type Request struct {
	ClientID int      `json:"clientID,omitempty"`
	Items    []string `json:"items,omitempty"`
	Version  int      `json:"version,omitempty"`
}

type Response struct {
	Values  map[int]Value `json:"values,omitempty"`
	Version int           `json:"version,omitempty"`
}

func (app *App) serve() error {
	http.HandleFunc("GET /cart", app.handleGet)
	http.HandleFunc("PUT /cart", app.handlePut)
	app.logger.Info("starting server on port 3000...")
	err := http.ListenAndServe(":3000", nil)
	if err != nil {
		return err
	}
	app.logger.Info("stopped server.")
	return nil
}

func (app *App) handleGet(w http.ResponseWriter, r *http.Request) {
	app.logger.Info("handling GET...")
	res := Response{
		Values:  app.cart,
		Version: app.version,
	}
	writeJSON(w, 200, res)
}

func (app *App) handlePut(w http.ResponseWriter, r *http.Request) {
	app.logger.Info("handling PUT...")
	var req Request
	err := json.NewDecoder(r.Body).Decode(&req)
	if err != nil {
		w.WriteHeader(400)
		return
	}

	if _, found := app.cart[req.ClientID]; found {
		if req.Version < app.version {
			app.cart[req.ClientID] = req.Items
			app.version++
		}
	} else {
		if len(app.cart) == 0 {
			app.version = 1
		} else {
			app.version++
		}
		app.cart[req.ClientID] = req.Items
	}
	res := Response{
		Values:  app.cart,
		Version: app.version,
	}
	writeJSON(w, 200, res)
}

func writeJSON(w http.ResponseWriter, status int, data any) {
	bytes, err := json.Marshal(data)
	if err != nil {
		w.WriteHeader(500)
		return
	}
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)
	w.Write(bytes)
}