feat(shared): add signed percent and percentage-point formatters (WealthLedger/main#54)

src/shared/lib/display.test.ts

@@ -3,12 +3,22 @@ import { describe, expect, it } from "vitest";
 import {
 	formatDate,
 	formatMoney,
+	formatPercentagePoints,
+	formatSignedPercent,
 	formatSnapshotDay,
 	formatSnapshotTime,
 	instrumentLogoUrl,
 } from "./display";
 
 describe("shared display helpers", () => {
+	it("formats signed rates as percentages and percentage points without negative zero", () => {
+		expect(formatSignedPercent("0.151")).toBe("+15,1 %");
+		expect(formatSignedPercent("-0.15149")).toBe("−15,1 %");
+		expect(formatSignedPercent("0")).toBe("0,0 %");
+		expect(formatSignedPercent("-0.00049")).toBe("0,0 %");
+		expect(formatPercentagePoints("0.043")).toBe("+4,3 п.п.");
+		expect(formatPercentagePoints("-0.0435")).toBe("−4,4 п.п.");
+	});
 	it("uses padded Russian dates and preserves the UTC date of timestamps", () => {
 		expect(formatDate("2027-09-10")).toBe("10.09.2027");
 		expect(formatDate("2024-02-29")).toBe("29.02.2024");

src/shared/lib/display.ts

@@ -1,3 +1,5 @@
+import { decimalFromString } from "./decimal";
+
 const DECIMAL_PART_COUNT = 2;
 const DEFAULT_MINOR_UNITS = 2;
 
@@ -99,6 +101,29 @@ function formatMoney(
 	return value.startsWith("-") ? `−${formatted.slice(1)}` : `+${formatted}`;
 }
 
+function formatPercentagePoints(rate: string): string {
+	return formatRate(rate, "п.п.");
+}
+
+function formatRate(rate: string, suffix: string): string {
+	const { scale, value } = decimalFromString(rate);
+	const abs = value < 0n ? -value : value;
+	const radix = 10n;
+	const half = 2n;
+	const tenthsPerUnit = 1000n;
+	const denominator = radix ** BigInt(scale);
+	const tenths = (abs * tenthsPerUnit + denominator / half) / denominator;
+	let sign = "";
+	if (tenths !== 0n) sign = value < 0n ? "−" : "+";
+	const formatted = `${formatDecimal(String(tenths / radix))},${tenths % radix}`;
+
+	return `${sign}${formatted} ${suffix}`;
+}
+
+function formatSignedPercent(rate: string): string {
+	return formatRate(rate, "%");
+}
+
 function isZeroDecimal(value: string) {
 	const unsigned = value.startsWith("-") ? value.slice(1) : value;
 
@@ -171,6 +196,8 @@ export {
 	formatDate,
 	formatDecimal,
 	formatMoney,
+	formatPercentagePoints,
+	formatSignedPercent,
 	formatSnapshotDay,
 	formatSnapshotTime,
 	instrumentLogoUrl,

src/shared/lib/index.ts

@@ -12,6 +12,8 @@ export {
 	formatDate,
 	formatDecimal,
 	formatMoney,
+	formatPercentagePoints,
+	formatSignedPercent,
 	formatSnapshotDay,
 	formatSnapshotTime,
 	instrumentLogoUrl,