feat: implement all needed BigNumber methods

lib/BigNumber.js

@@ -3,9 +3,26 @@
 const { jsbn: { BigInteger } } = require('node-forge');
 
 class BigNumber {
-	constructor(value = '') {
+	constructor(value, radix = 10) {
+		if (value === null || value === undefined)
+			throw new Error('value is missing.');
+
 		this._n = new BigInteger(null);
-		this._n.fromInt(value);
+
+		if (value instanceof BigNumber)
+			this._n = value._n;
+		else if (value instanceof BigInteger)
+			this._n = value;
+		else if (typeof value === 'number')
+			this._n.fromInt(value);
+		else if (typeof value === 'string')
+			this._n.fromString(value, radix);
+		else if (Buffer.isBuffer(value))
+			this._n.fromString(value.toString('hex'), 16);
+		else if (Array.isArray(value))
+			this._n.fromString(Buffer.from(value).toString('hex'), 16);
+		else
+			throw new TypeError('Unsupported value type.');
 	}
 
 	toBEBuffer(length = undefined) {
@@ -16,6 +33,34 @@ class BigNumber {
 			arr.unshift(0);
 		return Buffer.from(arr);
 	}
+
+	toBuffer() {
+		return Buffer.from(this._n.toByteArray());
+	}
+
+	toString(radix = 10) {
+		return this._n.toString(radix);
+	}
+
+	and(num) {
+		return new BigNumber(this._n.and(new BigNumber(num)._n));
+	}
+
+	mul(num) {
+		return new BigNumber(this._n.multiply(new BigNumber(num)._n));
+	}
+
+	mod(num) {
+		return new BigNumber(this._n.mod(new BigNumber(num)._n));
+	}
+
+	shrn(num) {
+		return new BigNumber(this._n.shiftRight(new BigNumber(num)._n));
+	}
+
+	static fromBuffer(buf) {
+		return new BigNumber(buf.toString('hex'), 16);
+	}
 }
 
 module.exports = BigNumber;