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frontend/src/lib/format.ts

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+/**
+ * Format a number as a price string with 2 decimal places and thousands separators.
+ * e.g. 1234.5 → "1,234.50"
+ */
+export function formatPrice(n: number): string {
+  return n.toLocaleString('en-US', {
+    minimumFractionDigits: 2,
+    maximumFractionDigits: 2,
+  });
+}
+
+/**
+ * Format a number as a percentage string with 2 decimal places.
+ * e.g. 12.345 → "12.35%"
+ */
+export function formatPercent(n: number): string {
+  return `${n.toLocaleString('en-US', {
+    minimumFractionDigits: 2,
+    maximumFractionDigits: 2,
+  })}%`;
+}
+
+/**
+ * Format a large number with K/M/B suffix.
+ * Values >= 1_000_000_000 → "1.23B"
+ * Values >= 1_000_000     → "456.7M"
+ * Values >= 1_000         → "12.3K"
+ * Values < 1_000          → plain number, no suffix
+ */
+export function formatLargeNumber(n: number): string {
+  const abs = Math.abs(n);
+  const sign = n < 0 ? '-' : '';
+
+  if (abs >= 1_000_000_000) {
+    return `${sign}${(abs / 1_000_000_000).toFixed(2).replace(/\.?0+$/, '')}B`;
+  }
+  if (abs >= 1_000_000) {
+    return `${sign}${(abs / 1_000_000).toFixed(1).replace(/\.?0+$/, '')}M`;
+  }
+  if (abs >= 1_000) {
+    return `${sign}${(abs / 1_000).toFixed(1).replace(/\.?0+$/, '')}K`;
+  }
+  return n.toString();
+}
+
+/**
+ * Format an ISO date string as a short date.
+ * e.g. "2025-01-15T14:30:00Z" → "Jan 15, 2025"
+ */
+export function formatDate(d: string): string {
+  const date = new Date(d);
+  return date.toLocaleDateString('en-US', {
+    month: 'short',
+    day: 'numeric',
+    year: 'numeric',
+  });
+}
+
+/**
+ * Format an ISO date string as a date with time.
+ * e.g. "2025-01-15T14:30:00Z" → "Jan 15, 2025 2:30 PM"
+ */
+export function formatDateTime(d: string): string {
+  const date = new Date(d);
+  return `${date.toLocaleDateString('en-US', {
+    month: 'short',
+    day: 'numeric',
+    year: 'numeric',
+  })} ${date.toLocaleTimeString('en-US', {
+    hour: 'numeric',
+    minute: '2-digit',
+    hour12: true,
+  })}`;
+}