refactor(calculate): extract safe percentage rounding logic into a single reusable helper function

lib/calculate.test.ts

@@ -2,6 +2,7 @@ import { describe, it, expect } from 'vitest';
 import {
   calculateStreak,
   calculateMonthlyStats,
+  calculateSafePercentage,
   calculateWrappedStats,
   aggregateCalendars,
   isStreakAlive,
@@ -2821,3 +2822,14 @@ describe('DST transition boundary handling', () => {
     expect(result.currentStreak).toBe(1);
   });
 });
+
+describe('calculateSafePercentage utility metric verification', () => {
+  it('safely yields 0 when denominator total is 0', () => {
+    expect(calculateSafePercentage(10, 0)).toBe(0);
+  });
+
+  it('correctly rounds regular integer percentages', () => {
+    expect(calculateSafePercentage(1, 3)).toBe(33); // 33.333... rounds to 33
+    expect(calculateSafePercentage(2, 3)).toBe(67); // 66.666... rounds to 67
+  });
+});

lib/calculate.ts

@@ -7,6 +7,19 @@ import type {
   MonthlyStats,
 } from '../types';
 
+/* ==========================================================================
+ * UTILITY FUNCTIONS
+ * ========================================================================== */
+
+/**
+ * Safely calculates and rounds a percentage fraction to prevent NaN or
+ * Infinity division values when the total denominator resolves to zero.
+ */
+export function calculateSafePercentage(part: number, total: number): number {
+  if (total === 0) return 0;
+  return Math.round((part / total) * 100);
+}
+
 /* ==========================================================================
  * STREAK & CALENDAR CALCULATIONS
  * ========================================================================== */
@@ -91,6 +104,7 @@ export function getLocalTodayStr(now: Date, timezone: string): string {
     return now.toISOString().split('T')[0];
   }
 }
+
 export function isStreakAlive(
   today?: { contributionCount: number } | null,
   yesterday?: { contributionCount: number } | null
@@ -532,15 +546,14 @@ export function calculateWrappedStats(calendar?: ContributionCalendar | null) {
       ? 'N/A'
       : Object.keys(monthCounts).reduce((a, b) => (monthCounts[a] > monthCounts[b] ? a : b));
 
+  const total = weekendCommits + weekdayCommits;
+
   return {
     totalContributions: calendar.totalContributions || 0,
     mostActiveDate: mostActiveDay.date,
     highestDailyCount: mostActiveDay.count,
     busiestMonth: busiestMonthStr,
-    weekendRatio: (() => {
-      const total = weekendCommits + weekdayCommits;
-      return total > 0 ? Math.round((weekendCommits / total) * 100) : 0;
-    })(),
+    weekendRatio: calculateSafePercentage(weekendCommits, total),
   };
 }