feat(tip): add personalized payment success experience

src/app/tip/tip-screen.helpers.ts

@@ -4,6 +4,11 @@ import {
   type TipCoffeeType,
 } from "@/lib/tip/tip_coffee";
 
+export interface TipSuccessCopy {
+  readonly title: string;
+  readonly body: readonly string[];
+}
+
 export function findTipCoffeePlan(
   plans: readonly PaymentPlan[],
   coffeeType: TipCoffeeType,
@@ -29,3 +34,46 @@ export function formatTipPrice(
   if (currency.length > 0) return `${currency} ${formattedAmount}`;
   return formattedAmount;
 }
+
+export function formatEnglishOrdinal(value: number): string {
+  const remainder100 = value % 100;
+  if (remainder100 >= 11 && remainder100 <= 13) return `${value}th`;
+
+  switch (value % 10) {
+    case 1:
+      return `${value}st`;
+    case 2:
+      return `${value}nd`;
+    case 3:
+      return `${value}rd`;
+    default:
+      return `${value}th`;
+  }
+}
+
+export function resolveTipSuccessCopy(
+  tipCount: number | null,
+  thankYouMessage: string | null,
+): TipSuccessCopy {
+  if (tipCount === 1) {
+    return {
+      title: "Did you really just buy me a coffee?",
+      body: [
+        "That honestly made me smile.",
+        "Thank you. I'll definitely think of you while I enjoy it.",
+      ],
+    };
+  }
+
+  if (tipCount !== null && tipCount > 1 && thankYouMessage) {
+    return {
+      title: `This is the ${formatEnglishOrdinal(tipCount)} coffee you've treated me to.`,
+      body: [thankYouMessage],
+    };
+  }
+
+  return {
+    title: "Thank you. Your coffee made me smile.",
+    body: [],
+  };
+}