Create new class for DateRangePicker state internals

packages/datetime/src/common/dateUtils.ts

@@ -5,6 +5,8 @@
  * and https://github.com/palantir/blueprint/blob/master/PATENTS
  */
 
+import { Months } from "./months";
+
 export type DateRange = [Date | undefined, Date | undefined];
 
 export function areEqual(date1: Date, date2: Date) {
@@ -107,3 +109,21 @@ export function getDateTime(date: Date, time: Date) {
             time.getHours(), time.getMinutes(), time.getSeconds(), time.getMilliseconds());
     }
 }
+
+export function getDatePreviousMonth(date: Date): Date {
+    const monthIsJanuary = date.getMonth() === Months.JANUARY;
+    if (monthIsJanuary) {
+        return new Date(date.getFullYear() - 1, Months.DECEMBER);
+    } else {
+        return new Date(date.getFullYear(), date.getMonth() - 1);
+    }
+}
+
+export function getDateNextMonth(date: Date): Date {
+    const monthIsDecember = date.getMonth() === Months.DECEMBER;
+    if (monthIsDecember) {
+        return new Date(date.getFullYear() + 1, Months.JANUARY);
+    } else {
+        return new Date(date.getFullYear(), date.getMonth() + 1);
+    }
+}

packages/datetime/src/common/monthAndYear.ts

@@ -0,0 +1,74 @@
+/*
+ * Copyright 2016 Palantir Technologies, Inc. All rights reserved.
+ * Licensed under the BSD-3 License as modified (the “License”); you may obtain a copy
+ * of the license at https://github.com/palantir/blueprint/blob/master/LICENSE
+ * and https://github.com/palantir/blueprint/blob/master/PATENTS
+ */
+
+import { getDateNextMonth, getDatePreviousMonth } from "./dateUtils";
+
+export class MonthAndYear {
+    private date: Date;
+
+    constructor (month?: number, year?: number) {
+        if (month !== null && year !== null) {
+            this.date = new Date(year, month);
+        } else {
+            this.date = new Date();
+        }
+    }
+
+    public clone(): MonthAndYear {
+        return new MonthAndYear(this.getMonth(), this.getYear());
+    }
+
+    public getFullDate(): Date {
+        return this.date;
+    }
+
+    public getMonth(): number {
+        return this.date.getMonth();
+    }
+
+    public getYear(): number {
+        return this.date.getFullYear();
+    }
+
+    public getPreviousMonth(): MonthAndYear {
+        const previousMonth = getDatePreviousMonth(this.date);
+        return new MonthAndYear(previousMonth.getMonth(), previousMonth.getFullYear());
+    }
+
+    public getNextMonth(): MonthAndYear {
+        const nextMonth = getDateNextMonth(this.date);
+        return new MonthAndYear(nextMonth.getMonth(), nextMonth.getFullYear());
+    }
+
+    public isBefore(displayMonth: MonthAndYear): boolean {
+        return compareMonthAndYear(this, displayMonth) < 0;
+    }
+
+    public isAfter(displayMonth: MonthAndYear): boolean {
+        return compareMonthAndYear(this, displayMonth) > 0;
+    }
+
+    public isSame(displayMonth: MonthAndYear): boolean {
+        return compareMonthAndYear(this, displayMonth) === 0;
+    }
+}
+
+// returns -ve if left < right
+// returns +ve if left > right
+// returns 0 if left === right
+function compareMonthAndYear(firstMonthAndYear: MonthAndYear, secondMonthAndYear: MonthAndYear): number {
+    const firstMonth = firstMonthAndYear.getMonth();
+    const firstYear = firstMonthAndYear.getYear();
+    const secondMonth = secondMonthAndYear.getMonth();
+    const secondYear = secondMonthAndYear.getYear();
+
+    if (firstYear === secondYear) {
+        return firstMonth - secondMonth;
+    } else {
+        return firstYear - secondYear;
+    }
+}