// -=-=- StartHere -=-=-
//
// # Chapter IX: Parallel DNS lookups
//
// The program we see here is _really_ similar to the one we
// discussed in the previous chapter. The main difference
// is the following: rather than performing DNS lookups
// sequentially, we call a function that runs through the
// list of resolvers and run them in parallel.
//
// Again, we are going to use low parallelism for the same
// rationale mentioned in chapter09.
//
// (This file is auto-generated. Do not edit it directly! To apply
// changes you need to modify `./internal/tutorial/measurex/chapter10/main.go`.)
//
// ## main.go
//
// The beginning of the program is pretty much the same.
//
// ```Go
package main

import (
	"context"
	"encoding/json"
	"flag"
	"fmt"
	"net/url"
	"time"

	"github.com/ooni/probe-cli/v3/internal/measurex"
	"github.com/ooni/probe-cli/v3/internal/runtimex"
)

type measurement struct {
	DNS       []*measurex.DNSMeasurement
	Endpoints []*measurex.HTTPEndpointMeasurement
}

func print(v interface{}) {
	data, err := json.Marshal(v)
	runtimex.PanicOnError(err, "json.Marshal failed")
	fmt.Printf("%s\n", string(data))
}

func main() {
	URL := flag.String("url", "https://blog.cloudflare.com/", "URL to fetch")
	timeout := flag.Duration("timeout", 60*time.Second, "timeout to use")
	flag.Parse()
	ctx, cancel := context.WithTimeout(context.Background(), *timeout)
	defer cancel()
	parsed, err := url.Parse(*URL)
	runtimex.PanicOnError(err, "url.Parse failed")
	mx := measurex.NewMeasurerWithDefaultSettings()
	m := &measurement{}
	// ```
	//
	// The bulk of the difference is here. We create
	// a list of DNS resolvers. For each of them, we specify
	// the type and the endpoint address. (There is no
	// endpoint address for the system resolver, therefore
	// we leave its address empty.)
	//
	// ```Go
	resolvers := []*measurex.ResolverInfo{{
		Network: measurex.ResolverUDP,
		Address: "8.8.8.8:53",
	}, {
		Network: measurex.ResolverUDP,
		Address: "8.8.4.4:53",
	}, {
		Network: measurex.ResolverUDP,
		Address: "1.1.1.1:53",
	}, {
		Network: measurex.ResolverUDP,
		Address: "1.0.0.1:53",
	}, {
		Network: measurex.ResolverSystem,
		Address: "",
	}}
	// ```
	//
	// Then we call `LookupURLHostParallel`. This function runs
	// the queries that make sense given the input URL using a
	// pool of (currently three) background goroutines.
	//
	// When I say "queries that make sense", I mostly mean
	// that we only query for HTTPSSvc when the input URL
	// scheme is "https". Otherwise, if it's just "http", it
	// does not make sense to send this query.
	//
	// ```Go
	for dns := range mx.LookupURLHostParallel(ctx, parsed, resolvers...) {
		m.DNS = append(m.DNS, dns)
	}
	// ```
	//
	// The rest of the program is exactly like in chapter09.
	//
	// ```Go
	headers := measurex.NewHTTPRequestHeaderForMeasuring()
	httpEndpoints, err := measurex.AllHTTPEndpointsForURL(parsed, headers, m.DNS...)
	runtimex.PanicOnError(err, "cannot get all the HTTP endpoints")
	cookies := measurex.NewCookieJar()
	for epnt := range mx.HTTPEndpointGetParallel(ctx, cookies, httpEndpoints...) {
		m.Endpoints = append(m.Endpoints, epnt)
	}
	print(m)
}

// ```
//
// ## Running the example program
//
// Let us perform a vanilla run first:
//
// ```bash
// go run -race ./internal/tutorial/measurex/chapter10 | jq
// ```
//
// Take a look at the JSON output. Can you spot that
// DNS queries are run in parallel?
//
// ## Conclusion
//
// We have seen how to run parallel DNS queries.
//
// -=-=- StopHere -=-=-