Concurrency with agent pattern

agent.go

package main

import (
	"fmt"
	"image"
	"image/color"
	"io"
	"os"
	"strings"
	"sync"
	"testing"
)

type Agent[TCommand any, TReply any] interface {
	Tell(TCommand) TReply
}

type commandWithReply[TCommand any, TReply any] struct {
	command TCommand
	reply   chan TReply
}

type StatefulAgent[TState any, TCommand any, TReply any] struct {
	state     TState
	channel   chan commandWithReply[TCommand, TReply]
	processor func(*TState, TCommand) TReply
}

func (agent *StatefulAgent[TState, TCommand, TReply]) Tell(command TCommand) TReply {
	reply := make(chan TReply)
	agent.channel <- commandWithReply[TCommand, TReply]{command, reply}
	return <-reply

}

func (agent *StatefulAgent[TState, TCommand, TReply]) run() {
	for cmd := range agent.channel {
		cmd.reply <- agent.processor(&agent.state, cmd.command)
	}
}

func StartAgent[TState any, TCommand any, TReply any](initialState TState, processor func(*TState, TCommand) TReply) Agent[TCommand, TReply] {
	agent := StatefulAgent[TState, TCommand, TReply]{
		state:     initialState,
		channel:   make(chan commandWithReply[TCommand, TReply]),
		processor: processor,
	}

	go agent.run()

	return &agent
}

type Fetcher interface {
	// Fetch returns the body of URL and
	// a slice of URLs found on that page.
	Fetch(url string) (body string, urls []string, err error)
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher, agent Agent[Command, *string], wg *sync.WaitGroup) {
	defer wg.Done()

	tell := agent.Tell(ExistsCommand{Key: url})
	if tell != nil {
		return
	}

	if depth <= 0 {
		return
	}
	body, urls, err := fetcher.Fetch(url)
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Printf("found: %s %q\n", url, body)

	agent.Tell(AddCommand{Key: url})

	for _, u := range urls {
		wg.Add(1)
		go Crawl(u, depth-1, fetcher, agent, wg)
	}
	return
}

type Command interface{}

type AddCommand struct {
	Key   string
	Value string
}

type ExistsCommand struct {
	Key string
}

func processor(state *map[string]string, command Command) *string {
	switch cmd := command.(type) {
	case AddCommand:
		(*state)[cmd.Key] = cmd.Value
		return &cmd.Value
	case ExistsCommand:
		value, exists := (*state)[cmd.Key]
		if exists {
			return &value
		}

		return nil
	default:
		panic("unknown command")
	}
}

func TestCrawler(t *testing.T) {
	var wg sync.WaitGroup
	agent := StartAgent(make(map[string]string), processor)
	wg.Add(1)
	go Crawl("https://golang.org/", 4, fetcher, agent, &wg)
	wg.Wait()
}

// fakeFetcher is Fetcher that returns canned results.
type fakeFetcher map[string]*fakeResult

type fakeResult struct {
	body string
	urls []string
}

func (f fakeFetcher) Fetch(url string) (string, []string, error) {
	if res, ok := f[url]; ok {
		return res.body, res.urls, nil
	}
	return "", nil, fmt.Errorf("not found: %s", url)
}

// fetcher is a populated fakeFetcher.
var fetcher = fakeFetcher{
	"https://golang.org/": &fakeResult{
		"The Go Programming Language",
		[]string{
			"https://golang.org/pkg/",
			"https://golang.org/cmd/",
		},
	},
	"https://golang.org/pkg/": &fakeResult{
		"Packages",
		[]string{
			"https://golang.org/",
			"https://golang.org/cmd/",
			"https://golang.org/pkg/fmt/",
			"https://golang.org/pkg/os/",
		},
	},
	"https://golang.org/pkg/fmt/": &fakeResult{
		"Package fmt",
		[]string{
			"https://golang.org/",
			"https://golang.org/pkg/",
		},
	},
	"https://golang.org/pkg/os/": &fakeResult{
		"Package os",
		[]string{
			"https://golang.org/",
			"https://golang.org/pkg/",
		},
	},
}