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frontend/src/lib/format.test.ts

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+import { describe, expect, it } from 'vitest';
+
+import {
+  buildPercentileScale,
+  calculateHistogramMean,
+  formatFilterValue,
+  formatTransactionDate,
+  parseInputValue,
+  roundedPercentages,
+} from './format';
+
+describe('format utilities', () => {
+  it('formats compact filter values and transaction dates', () => {
+    expect(formatFilterValue(1250)).toBe('1.3k');
+    expect(formatFilterValue(1_250_000)).toBe('1.3M');
+    expect(formatFilterValue(1250, true)).toBe('1250');
+    expect(formatTransactionDate(2024.5)).toBe('Jul 2024');
+  });
+
+  it('parses user-entered compact numeric values', () => {
+    expect(parseInputValue('£1.25M', { prefix: '£', step: 5000 })).toBe(1_250_000);
+    expect(parseInputValue('45 sqm', { suffix: ' sqm' })).toBe(45);
+    expect(parseInputValue('2.5万')).toBe(25_000);
+    expect(parseInputValue('not a number')).toBeNull();
+  });
+
+  it('rounds percentages so displayed values sum to 100', () => {
+    expect(roundedPercentages([1, 1, 1], 3)).toEqual([34, 33, 33]);
+    expect(roundedPercentages([1, 2, 3], 6, 1)).toEqual([16.7, 33.3, 50]);
+    expect(roundedPercentages([5, 5], 0)).toEqual([0, 0]);
+  });
+
+  it('maps histogram percentiles and weighted means consistently', () => {
+    const histogram = { min: 0, p1: 10, p99: 90, max: 100, counts: [10, 80, 10] };
+    const scale = buildPercentileScale(histogram);
+
+    expect(scale.toValue(0)).toBe(0);
+    expect(scale.toValue(50)).toBeCloseTo(50);
+    expect(scale.toPercentile(50)).toBeCloseTo(50);
+    expect(calculateHistogramMean(histogram)).toBeCloseTo(50);
+  });
+});