Add `add` module

diff --git a/src/__tests__/add-test.js b/src/__tests__/add-test.js
new file mode 100644 (file)
index 0000000..b575b44
--- /dev/null
+++ b/src/__tests__/add-test.js
@@ -0,0 +1,107 @@
+/**
+ * Copyright 2015-present Greg Hurrell. All rights reserved.
+ * Licensed under the terms of the MIT license.
+ */
+
+'use strict';
+
+import add from '../add';
+
+describe('add()', () => {
+  it('adds empty numbers to 0', () => {
+    expect(add('', '', 2)).toBe('0');
+    expect(add('', '', 8)).toBe('0');
+    expect(add('', '', 10)).toBe('0');
+    expect(add('', '', 16)).toBe('0');
+
+    // Whitespace is considered "empty" as well.
+    expect(add(' ', ' ', 2)).toBe('0');
+    expect(add(' ', ' ', 8)).toBe('0');
+    expect(add(' ', ' ', 10)).toBe('0');
+    expect(add(' ', ' ', 16)).toBe('0');
+  });
+
+  it('adds an empty number to a non-empty number', () => {
+    expect(add('', '101', 2)).toBe('101');
+    expect(add('', '755', 8)).toBe('755');
+    expect(add('', '321', 10)).toBe('321');
+    expect(add('', 'ff4', 16)).toBe('ff4');
+  });
+
+  it('adds a non-empty number to an empty number', () => {
+    expect(add('101', '', 2)).toBe('101');
+    expect(add('755', '', 8)).toBe('755');
+    expect(add('321', '', 10)).toBe('321');
+    expect(add('ff4', '', 16)).toBe('ff4');
+  });
+
+  it('adds binary numbers with overflow', () => {
+    // Equal number of digits.
+    expect(add('101', '110', 2)).toBe('1011');
+
+    // More digits in first operand.
+    expect(add('1001', '101', 2)).toBe('1110');
+
+    // More digits in second operand.
+    expect(add('11', '1011', 2)).toBe('1110');
+  });
+
+  it('adds decimal numbers with overflow', () => {
+    // Equal number of digits.
+    expect(add('481', '110', 10)).toBe('591');
+
+    // More digits in first operand.
+    expect(add('1024', '85', 10)).toBe('1109');
+
+    // More digits in second operand.
+    expect(add('12', '4999', 10)).toBe('5011');
+  });
+
+  it('adds hexadecimal numbers with overflow', () => {
+    // Equal number of digits.
+    expect(add('200', 'f39', 16)).toBe('1139');
+
+    // More digits in first operand.
+    expect(add('102a', '805', 16)).toBe('182f');
+
+    // More digits in second operand.
+    expect(add('a2', '4f02', 16)).toBe('4fa4');
+  });
+
+  it('adds hexadecimal numbers case-insensitively', () => {
+    expect(add('feed', 'FACE', 16)).toBe('1f9bb');
+  });
+
+  it('ignores leading and trailing whitespace', () => {
+    expect(add('   429  ', ' 120   ', 10)).toBe('549');
+  });
+
+  it('ignores an optional 0 prefix for octal numbers', () => {
+    expect(add('0755', '002', 8)).toBe('757');
+  });
+
+  it('ignores an optional 0x prefix for hexadecimal numbers', () => {
+    expect(add('0x100', 'ff', 16)).toBe('1ff');
+    expect(add('feff', '0Xff', 16)).toBe('fffe');
+  });
+
+  it('adds arbitrary precision numbers', () => {
+    expect(add(
+      'dff026dc84b91cb70e984582a9ce6515e2100d55',
+      'cf4fee757d570bd5f3d390004312c40867a512d1',
+      16
+    )).toBe('1af4015520210288d026bd582ece1291e49b52026');
+
+  });
+
+  it('complains if given a digit that is out-of-range for the base', () => {
+    expect(() => add('10', '20', 2))
+      .toThrow('Invalid digit `2` for base `2`');
+    expect(() => add('558', '444', 8))
+      .toThrow('Invalid digit `8` for base `8`');
+    expect(() => add('404', '3*', 8))
+      .toThrow('Invalid digit `*` for base `10`');
+    expect(() => add('feedface', 'groundbeef', 16))
+      .toThrow('Invalid digit `g` for base `16`');
+  });
+});

diff --git a/src/add.js b/src/add.js
new file mode 100644 (file)
index 0000000..43656fb
--- /dev/null
+++ b/src/add.js
@@ -0,0 +1,65 @@
+/**
+ * Copyright 2015-present Greg Hurrell. All rights reserved.
+ * Licensed under the terms of the MIT license.
+ *
+ * @flow
+ */
+
+/**
+ * Strips the leading prefix from `number` in `base` and returns the remaining
+ * part of the string.
+ *
+ * - "0x" (or "0X") for hexadecimal numbers.
+ * - "0" for octal numbers.
+ *
+ * For all bases, leading whitespace is stripped.
+ */
+function stripPrefix(number: string, base: number): string {
+  if (base === 16) {
+    return number.replace(/^\s*0x/i, '');
+  } else if (base === 8) {
+    return number.replace(/^\s*0/, '');
+  } else {
+    return number.replace(/^\s*/, '');
+  }
+}
+
+/**
+ * Breaks the string repsentation of `number` in `base` into an array of decimal
+ * digits (from least significant to most significant) for easier manipulation.
+ *
+ * For example, the hexadecimal representation `"40fa"` becomes `[10, 15, 0,
+ * 4]`.
+ */
+function getDigits(number: string, base: number): Array<number> {
+  return stripPrefix(number, base).trim().split('').reverse().map(digit => {
+    const result = parseInt(digit, base);
+    if (isNaN(result)) {
+      throw new Error('Invalid digit `' + digit + '` for base `' + base + '`');
+    }
+    return result;
+  });
+}
+
+/**
+ * Adds two numbers `a` and `b`, both in `base` and returns the answer as a
+ * string representation in `base`.
+ */
+export default function add(a: string, b: string, base: number): string {
+  const aDigits = getDigits(a, base);
+  const bDigits = getDigits(b, base);
+  let result = '';
+  let carry = 0;
+  for (
+    let i = 0, max = Math.max(aDigits.length, bDigits.length);
+    i < max || carry;
+    i++
+  ) {
+    const aDigit = aDigits[i] || 0;
+    const bDigit = bDigits[i] || 0;
+    const sum = aDigit + bDigit + carry;
+    result = (sum % base).toString(base) + result;
+    carry = Math.floor(sum / base);
+  }
+  return result ? result : '0';
+}