[payment] Implement a pro-ration helper method

Author: jankeromnes

components/ee/payment-endpoint/src/chargebee/upgrade-helper.ts

@@ -6,6 +6,7 @@
 
 import { injectable, inject } from "inversify";
 import { ChargebeeProvider } from "./chargebee-provider";
+import { Chargebee as chargebee } from "./chargebee-types";
 import { LogContext, log } from "@gitpod/gitpod-protocol/lib/util/logging";
 
 @injectable()
@@ -22,24 +23,75 @@ export class UpgradeHelper {
      * @param description
      * @param upgradeTimestamp
      */
-    async chargeForUpgrade(userId: string, chargebeeSubscriptionId: string, amountInCents: number, description: string, upgradeTimestamp: string) {
+    async chargeForUpgrade(
+        userId: string,
+        chargebeeSubscriptionId: string,
+        amountInCents: number,
+        description: string,
+        upgradeTimestamp: string,
+    ) {
         const logContext: LogContext = { userId };
-        const logPayload = { chargebeeSubscriptionId: chargebeeSubscriptionId, amountInCents, description, upgradeTimestamp };
+        const logPayload = {
+            chargebeeSubscriptionId: chargebeeSubscriptionId,
+            amountInCents,
+            description,
+            upgradeTimestamp,
+        };
 
         await new Promise<void>((resolve, reject) => {
-            log.info(logContext, 'Charge on Upgrade: Upgrade detected.', logPayload);
-            this.chargebeeProvider.subscription.add_charge_at_term_end(chargebeeSubscriptionId, {
-                amount: amountInCents,
-                description
-            }).request(function (error: any, result: any) {
-                if (error) {
-                    log.error(logContext, 'Charge on Upgrade: error', error, logPayload);
-                    reject(error);
-                } else {
-                    log.info(logContext, 'Charge on Upgrade: successful', logPayload);
-                    resolve();
-                }
-            });
+            log.info(logContext, "Charge on Upgrade: Upgrade detected.", logPayload);
+            this.chargebeeProvider.subscription
+                .add_charge_at_term_end(chargebeeSubscriptionId, {
+                    amount: amountInCents,
+                    description,
+                })
+                .request(function (error: any, result: any) {
+                    if (error) {
+                        log.error(logContext, "Charge on Upgrade: error", error, logPayload);
+                        reject(error);
+                    } else {
+                        log.info(logContext, "Charge on Upgrade: successful", logPayload);
+                        resolve();
+                    }
+                });
         });
     }
-}
+
+    // Returns a ratio between 0 and 1:
+    //     0 means we've just finished the term
+    //     1 means we still have the entire term left
+    getCurrentTermRemainingRatio(chargebeeSubscription: chargebee.Subscription): number {
+        if (!chargebeeSubscription.next_billing_at) {
+            throw new Error("subscription.next_billing_at must be set.");
+        }
+        const now = new Date();
+        const nextBilling = new Date(chargebeeSubscription.next_billing_at * 1000);
+        const remainingMs = nextBilling.getTime() - now.getTime();
+
+        const getBillingCycleMs = (unit: chargebee.BillingPeriodUnit): number => {
+            const previousBilling = new Date(nextBilling.getTime());
+            switch (unit) {
+                case "day":
+                    previousBilling.setDate(nextBilling.getDate() - 1);
+                    break;
+                case "week":
+                    previousBilling.setDate(nextBilling.getDate() - 7);
+                    break;
+                case "month":
+                    previousBilling.setMonth(nextBilling.getMonth() - 1);
+                    break;
+                case "year":
+                    previousBilling.setFullYear(nextBilling.getFullYear() - 1);
+                    break;
+            }
+            return nextBilling.getTime() - previousBilling.getTime();
+        };
+
+        const billingCycleMs =
+            typeof chargebeeSubscription.billing_period === "number" && chargebeeSubscription.billing_period_unit
+                ? chargebeeSubscription.billing_period * getBillingCycleMs(chargebeeSubscription.billing_period_unit)
+                : 1 * getBillingCycleMs("month");
+
+        return remainingMs / billingCycleMs;
+    }
+}