Further support Monzo OAuth2.0 login flow

I'm now pulling the authorization code off of Monzo's request to my redirect
URI. I intend to use exchange that code for an access and refresh token. Once I
have these two items, I should be able to interact with Monzo's API much more
easily.

monzo-ynab/main.go

@@ -15,6 +15,8 @@ import (
 var (
 	clientId     = os.Getenv("client_id")
 	clientSecret = os.Getenv("client_secret")
+	accessToken  = nil
+	refreshToken = nil
 )
 
 const (
@@ -22,14 +24,52 @@ const (
 	redirectUri = "http://localhost:8080/authorize"
 )
 
+func getAccessCode(string authCode) {
+	form := map[string]string{
+		"grant_type":    "authorization_code",
+		"client_id":     client_id,
+		"client_secret": client_secret,
+		"redirect_uri":  redirectUri,
+		"code":          authCode,
+	}
+	json := map[string]string{
+		"access_token":  "access_token",
+		"client_id":     "client_id",
+		"expires_in":    21600,
+		"refresh_token": "refresh_token",
+		"token_type":    "Bearer",
+		"user_id":       "user_id",
+	}
+
+	// TODO: Handle retry with backoff logic here.
+	resp, error := http.Post("https://api.monzo.com/oauth2/token", form.Form(), json.Json())
+	if err != nil {
+		log.Fatal("Could not exchange authorization code for an access token.")
+	}
+
+	resp.Body()
+}
+
 func handleRedirect(w http.ResponseWriter, r *http.Request) {
-	fmt.Println(r)
-	fmt.Fprintf(w, "Ackified")
+	// assert that `r.state` is the same as `state`.
+	params := r.URL.Query()
+
+	reqState := params["state"][0]
+	reqCode := params["code"][0]
+
+	if reqState != state {
+		log.Fatal(fmt.Sprintf("Value for state returned by Monzo does not equal our state. %s != %s", reqState, state))
+	}
+
+	go getAccessCode(reqCode)
+
+	fmt.Printf("Received the authorization code from Monzo: %s", reqCode)
+	fmt.Fprintf(w, fmt.Sprintf("Authorization code: %s", reqCode))
 }
 
 func authorizeClient() {
 	url :=
-		fmt.Sprintf("https://auth.monzo.com/?client_id=%s&redirect_uri=%s&response_type=code&state=:state",
+		fmt.Sprintf("https://auth.monzo.com/?client_id=%s&redirect_uri=%s&response_type=code&state=%s",
 			clientId, redirectUri, state)
 	exec.Command("google-chrome", url).Start()
 }