From 732e838ab08b0cff3ccc129920ac3a14d8505cd4 Mon Sep 17 00:00:00 2001
From: Ryan Smith <3045513+ryansmith94@users.noreply.github.com>
Date: Thu, 5 Jan 2023 17:04:23 +0000
Subject: [PATCH] fix: Fixes issue for large numbers - closing #1

---
 src/baseroo.test.ts |  8 ++++++++
 src/baseroo.ts      | 25 +++++++++++++++++++------
 2 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/src/baseroo.test.ts b/src/baseroo.test.ts
index 7ee4b18d..e63401b9 100644
--- a/src/baseroo.test.ts
+++ b/src/baseroo.test.ts
@@ -21,3 +21,11 @@ test('convertBase should convert an empty input string to 0', () => {
 	const expectedOutput = '0'
 	assert.strictEqual(actualOutput, expectedOutput)
 })
+
+test('convertBase should convert BigInts too', () => {
+	const input =
+		'4ef57aa335b86bce90cd99144be26fa47645c36624eeb54ae153bc67861f9a7ad96e23e0d200348bd6a442ef96bd04a2c'
+	const interimOutput = convertBase(input, 16, 10)
+	const actualOutput = convertBase(interimOutput, 10, 16)
+	assert.strictEqual(actualOutput, input)
+})
diff --git a/src/baseroo.ts b/src/baseroo.ts
index 7ecc2d3e..9b938a22 100644
--- a/src/baseroo.ts
+++ b/src/baseroo.ts
@@ -6,25 +6,38 @@ export class InvalidDigitError extends BaseError {
 	}
 }
 
+function bigIntPow(x: bigint, y: bigint): bigint {
+	const ZERO = BigInt(0)
+	if (y === ZERO) return BigInt(1)
+	const TWO = BigInt(2)
+	const p2 = bigIntPow(x, y / TWO)
+	if (y % TWO === ZERO) return p2 * p2
+	return x * p2 * p2
+}
+
 export function convertBase(value: string, fromBase: number, toBase: number): string {
 	const range = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ+/'.split('')
 	const fromRange = range.slice(0, fromBase)
 	const toRange = range.slice(0, toBase)
 
+	const fromBaseBig = BigInt(fromBase)
+	const toBaseBig = BigInt(toBase)
+
 	let decValue = value
 		.split('')
 		.reverse()
-		.reduce(function (carry: number, digit: string, index: number): number {
-			if (fromRange.indexOf(digit) === -1) {
+		.reduce(function (carry: bigint, digit: string, index: number): bigint {
+			const fromIndex = fromRange.indexOf(digit)
+			if (fromIndex === -1) {
 				throw new InvalidDigitError(digit, fromBase)
 			}
-			return (carry += fromRange.indexOf(digit) * Math.pow(fromBase, index))
-		}, 0)
+			return carry + BigInt(fromIndex) * bigIntPow(fromBaseBig, BigInt(index))
+		}, BigInt(0))
 
 	let newValue = ''
 	while (decValue > 0) {
-		newValue = toRange[decValue % toBase] + newValue
-		decValue = (decValue - (decValue % toBase)) / toBase
+		newValue = toRange[Number(decValue % toBaseBig)] + newValue
+		decValue = (decValue - (decValue % toBaseBig)) / toBaseBig
 	}
 	return newValue || '0'
 }