Send grammatically correct messages when new orders arrive

internal/pretix/order_placed.go

@@ -4,6 +4,8 @@ import (
 	"encoding/json"
 	"fmt"
 	"net/http"
+	"strconv"
+	"strings"
 	"time"
 
 	"maunium.net/go/mautrix/event"
@@ -73,6 +75,7 @@ func (s Server) orderPlaced(w http.ResponseWriter, r *http.Request) {
 		Positions	[]struct{
 			Price	string
 		}
+		Total		string
 	}{}
 
 	err = decoder.Decode(&order)
@@ -94,11 +97,89 @@ func (s Server) orderPlaced(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if rowsAffected > 0 && order.Datetime.Add(72 * time.Hour).After(time.Now()) { // pretix retries failed webhooks for up to three days
+		numFree := 0
+		numPaid := 0
+		for _, position := range order.Positions {
+			euros, cents, err := parsePrice(position.Price)
+			if err != nil {
+				continue
+			}
+
+			if euros > 0 || cents > 0 {
+				numPaid++
+			} else {
+				numFree++
+			}
+		}
+
+		var verb, free, conjunction, paid, noun, total string
+		if numFree+numPaid == 1 {
+			verb = "wurde "
+		} else {
+			verb = "wurden "
+		}
+		if numFree == 1 {
+			free = "ein kostenloses "
+		} else if numFree > 1 {
+			free = fmt.Sprint(numFree, " kostenlose ")
+		}
+		if numPaid == 1 {
+			paid = "ein bezahltes "
+		} else if numPaid > 1 {
+			paid = fmt.Sprint(numPaid, " bezahlte ")
+		}
+		if numFree > 0 && numPaid > 0 {
+			conjunction = "und "
+		}
+		if numPaid == 1 || numPaid == 0 && numFree == 1 { // match number of adjective immediately before noun
+			noun = "Ticket "
+		} else {
+			noun = "Tickets "
+		}
+		if totalEuros, totalCents, err := parsePrice(order.Total); err == nil && (totalEuros > 0 || totalCents > 0) {
+			var adverb, fraction string
+			if numPaid > 1 {
+				adverb = "insgesamt "
+			}
+			if totalCents > 0 {
+				fraction = fmt.Sprintf(",%02d", totalCents)
+			}
+			total = fmt.Sprint("für ", adverb, totalEuros, fraction, " Euro ")
+		}
+
 		message := event.MessageEventContent{
 			MsgType:	event.MsgText,
-			Body:		fmt.Sprintf("%+v", order),
+			Body:		fmt.Sprint("Es ", verb, free, conjunction, paid, noun, total, "bestellt."),
 		}
 
 		s.matrix.Broadcast(&message)
 	}
 }
+
+func parsePrice(price string) (euros uint64, cents uint64, err error) {
+	parts := strings.Split(price, ".")
+
+	euros, err = strconv.ParseUint(parts[0], 10, 64)
+	if err != nil {
+		return 0, 0, err
+	}
+
+	if len(parts) > 1 {
+		for len(parts[1]) < 2 {
+			parts[1] += "0"
+		}
+		parts[1] = parts[1][:2]
+
+		parts[1] = strings.TrimLeft(parts[1], "0")
+		if parts[1] == "" {
+			parts[1] = "0"
+		}
+
+		cents, err = strconv.ParseUint(parts[1], 10, 64)
+		if err != nil {
+			return 0, 0, err
+		}
+	}
+
+	return euros, cents, nil
+}