Synchronize Luhn exercise with canonical specs

Canonical specs for Luhn exercise were changed, and tracks implementing that exercise should synchronize wih them.

Closes #260

Author: rchavarria

exercises/luhn/example.js

@@ -1,57 +1,34 @@
-export default class Luhn {
-
-  constructor(number) {
-    this.checkDigit = number % 10;
-    this.addends = Luhn.calculateAddends(number);
-    this.checksum = Luhn.calculateChecksum(this.addends);
-    this.valid = Luhn.determineIfValid(this.checksum);
-  }
-
-  static calculateAddends(number) {
-    const numberAsString = '' + number + '',
-      numbers = [...numberAsString],
-      addends = [];
-
-    for (let i = 0; i < numbers.length; i++) {
-      const index = numbers.length - 1 - i;
-      let currentAddend = parseInt(numbers[index], 10);
-
-      if ((i + 1) % 2 === 0) {
-        currentAddend = currentAddend * 2;
-        if (currentAddend > 10) {
-          currentAddend = currentAddend - 9;
-        }
+function isValid(number) {
+  number = number.replace(/\s/g, '');
+  const digits = [...number];
+
+  const sum = digits
+    // convert to integers
+    .map(d => parseInt(d, 10))
+    // double even positions (odd indexes)
+    .map((d, i) => {
+      if (i % 2 !== 0) {
+        return d * 2;
       }
-      addends.push(currentAddend);
-    }
-    return addends.reverse();
-  }
-
-  static calculateChecksum(numbers) {
-    let sum = 0;
-    for (let i = 0; i < numbers.length; i++) {
-      sum += numbers[i];
-    }
-    return sum;
-  }
+      return d;
+    })
+    // limit to digits less than 10
+    .map(d => {
+      if (d > 9) {
+        return d - 9;
+      }
+      return d;
+    })
+    // sum all digits
+    .reduce((d, acc) => d + acc, 0);
 
-  static determineIfValid(sum) {
-    return sum % 10 === 0;
-  }
+  return sum > 0 && sum % 10 === 0;
+}
 
-  static create(number) {
-    let finalNumber = number * 10,
-      luhnNumber = new Luhn(finalNumber),
-      index = 0;
+export default class Luhn {
 
-    while (!luhnNumber.valid) {
-      finalNumber = number * 10 + index;
-      luhnNumber = new Luhn(finalNumber);
-      if (luhnNumber.valid) {
-        break;
-      }
-      index += 1;
-    }
-    return finalNumber;
+  constructor(number) {
+    this.valid = isValid(number);
   }
+
 }

exercises/luhn/luhn.spec.js

@@ -2,59 +2,34 @@ import Luhn from './luhn';
 
 describe('Luhn',() => {
 
-  it('check digit',() => {
-    const luhn = new Luhn(34567);
-    expect(luhn.checkDigit).toEqual(7);
-  });
-
-  xit('check digit again',() => {
-    const luhn = new Luhn(91370);
-    expect(luhn.checkDigit).toEqual(0);
-  });
-
-  xit('addends',() => {
-    const luhn = new Luhn(12121);
-    expect(luhn.addends).toEqual([1, 4, 1, 4, 1]);
-  });
-
-  xit('too large added',() => {
-    const luhn = new Luhn(8631);
-    expect(luhn.addends).toEqual([7, 6, 6, 1]);
-  });
-
-  xit('checksum',() => {
-    const luhn = new Luhn(4913);
-    expect(luhn.checksum).toEqual(22);
-  });
-
-  xit('checksum again',() => {
-    const luhn = new Luhn(201773);
-    expect(luhn.checksum).toEqual(21);
+  it('single digit strings can not be valid', () => {
+    const luhn = new Luhn('1');
+    expect(luhn.valid).toEqual(false);
   });
 
-  xit('invalid number',() => {
-    const luhn = new Luhn(738);
+  xit('A single zero is invalid', () => {
+    const luhn = new Luhn('0');
     expect(luhn.valid).toEqual(false);
   });
 
-  xit('invalid number',() => {
-    const luhn = new Luhn(8739567);
+  xit('valid Canadian SIN', () => {
+    const luhn = new Luhn('046 454 286');
     expect(luhn.valid).toEqual(true);
   });
 
-  xit('create valid number',() => {
-    const number = Luhn.create(123);
-    expect(number).toEqual(1230);
+  xit('invalid Canadian SIN', () => {
+    const luhn = new Luhn('046 454 287');
+    expect(luhn.valid).toEqual(false);
   });
 
-  xit('create other valid number',() => {
-    const number = Luhn.create(873956);
-    expect(number).toEqual(8739567);
+  xit('invalid credit card', () => {
+    const luhn = new Luhn('8273 1232 7352 0569');
+    expect(luhn.valid).toEqual(false);
   });
 
-  xit('create yet another valid number',() => {
-    const number = Luhn.create(837263756);
-    expect(number).toEqual(8372637564);
+  xit('valid strings with a non-digit added become invalid', () => {
+    const luhn = new Luhn('046a 454 286');
+    expect(luhn.valid).toEqual(false);
   });
 
 });