it downloads and saves the fares data from NRDP, what more do you want

.gitignore

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+config.toml
+*.zip

config.toml.example

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+nrdp_username = "nrdp@email.goes.here"
+nrdp_password = "password!"

dtd_download.go

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+package main
+
+import (
+	"encoding/json"
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"net/url"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/BurntSushi/toml"
+)
+
+const DEFAULT_NRDP_BASE_URL = "https://opendata.nationalrail.co.uk"
+const DEFAULT_USER_AGENT = "interfare/0.1 (interfare@eta.st)"
+
+type Config struct {
+	BaseURL      string `toml:"base_url"`
+	NrdpUsername string `toml:"nrdp_username"`
+	NrdpPassword string `toml:"nrdp_password"`
+	UserAgent    string `toml:"user_agent"`
+}
+
+func loadConfig(path string) (Config, error) {
+	var ret Config
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return ret, fmt.Errorf("failed to read config file %s: %w", path, err)
+	}
+	_, err = toml.Decode(string(data), &ret)
+	if err != nil {
+		return ret, fmt.Errorf("failed to decode config file %s: %w", path, err)
+	}
+	if ret.BaseURL == "" {
+		ret.BaseURL = DEFAULT_NRDP_BASE_URL
+	}
+	if ret.UserAgent == "" {
+		ret.UserAgent = DEFAULT_USER_AGENT
+	}
+	if ret.NrdpUsername == "" || ret.NrdpPassword == "" {
+		return ret, fmt.Errorf("must specify `nrdp_username` and `nrdp_password` in config file")
+	}
+	return ret, nil
+}
+
+type NrdpClient struct {
+	cfg         Config
+	apiToken    string
+	tokenExpiry time.Time
+}
+
+func NewNrdpClient(cfg Config) NrdpClient {
+	var ret NrdpClient
+	ret.cfg = cfg
+	return ret
+}
+
+// EnsureToken checks that the NrdpClient's API token has not expired, and calls /authenticate to get a new one
+// if it has.
+func (cli *NrdpClient) EnsureToken() error {
+	if cli.tokenExpiry.After(time.Now()) {
+		// don't need to do this
+		return nil
+	}
+	uri := fmt.Sprintf("%s/authenticate", cli.cfg.BaseURL)
+	form := url.Values{}
+	form.Add("username", cli.cfg.NrdpUsername)
+	form.Add("password", cli.cfg.NrdpPassword)
+	req, err := http.NewRequest("POST", uri, strings.NewReader(form.Encode()))
+	if err != nil {
+		return fmt.Errorf("failed to build token request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.Header.Set("User-Agent", cli.cfg.UserAgent)
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return fmt.Errorf("token request failed: %w", err)
+	}
+	body := make(map[string]interface{})
+	err = json.NewDecoder(resp.Body).Decode(&body)
+	if err != nil {
+		return fmt.Errorf("failed to decode token response json: %w", err)
+	}
+	token, ok := body["token"]
+	if !ok {
+		return fmt.Errorf("token response did not contain token; body %#v", body)
+	}
+	tok, ok := token.(string)
+	if !ok {
+		return fmt.Errorf("token response was not string; body %#v", body)
+	}
+	cli.apiToken = tok
+	cli.tokenExpiry = time.Now().Add(1 * time.Hour)
+	return nil
+}
+
+// FetchFile obtains the named file (one of "fares", "routeing", or "timetable") from NRDP
+// and returns an interface to read it.
+func (cli *NrdpClient) FetchFile(name string) (io.ReadCloser, error) {
+	if err := cli.EnsureToken(); err != nil {
+		return nil, fmt.Errorf("failed to ensure token: %w", err)
+	}
+	var version string
+	switch name {
+	case "routeing":
+	case "fares":
+		version = "2.0"
+	case "timetable":
+		version = "3.0"
+	default:
+		panic(fmt.Sprintf("invalid argument %s to FetchFile", name))
+	}
+	uri := fmt.Sprintf("%s/api/staticfeeds/%s/%s", cli.cfg.BaseURL, version, name)
+	req, err := http.NewRequest("GET", uri, nil)
+	if err != nil {
+		return nil, fmt.Errorf("failed to build file request: %w", err)
+	}
+	req.Header.Set("X-Auth-Token", cli.apiToken)
+	req.Header.Set("User-Agent", cli.cfg.UserAgent)
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("file http get failed: %w", err)
+	}
+	return resp.Body, nil
+}
+
+var configPath = flag.String("config", "./config.toml", "Path to the configuration file to use.")
+
+func main() {
+	flag.Parse()
+	cfg, err := loadConfig(*configPath)
+	if err != nil {
+		log.Fatalf("failed to load config: %s", err)
+	}
+	client := NewNrdpClient(cfg)
+	err = client.EnsureToken()
+	if err != nil {
+		log.Fatalf("failed to get token: %s", err)
+	}
+	out, err := os.Create("./fares.zip")
+	if err != nil {
+		log.Fatalf("failed to create ./fares.zip: %s", err)
+	}
+	faresData, err := client.FetchFile("fares")
+	if err != nil {
+		log.Fatalf("failed to fetch fares data: %s", err)
+	}
+	written, err := io.Copy(out, faresData)
+	if err != nil {
+		log.Fatalf("failed to write fares data: %s", err)
+	}
+	log.Printf("wrote %d bytes of fares data", written)
+}

go.mod

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+module go.eta.st/interfare
+
+go 1.18
+
+require github.com/BurntSushi/toml v1.1.0

go.sum

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+github.com/BurntSushi/toml v1.1.0 h1:ksErzDEI1khOiGPgpwuI7x2ebx/uXQNw7xJpn9Eq1+I=
+github.com/BurntSushi/toml v1.1.0/go.mod h1:CxXYINrC8qIiEnFrOxCa7Jy5BFHlXnUU2pbicEuybxQ=