feat(occurrences): add occurrences function with tests (#187)

Making my first pull request: add occurrences function.

Author: lwd369

src/index.js

@@ -67,6 +67,7 @@ import tail from './tail'
 import makeObjectIterable from './makeObjectIterable'
 import fibonacciSum from './fibonacciSum'
 import lcm from './lcm'
+import occurrences from './occurrences'
 
 export {
   reverseArrayInPlace,
@@ -138,4 +139,5 @@ export {
   makeObjectIterable,
   fibonacciSum,
   lcm,
+  occurrences,
 }

src/occurrences.js

@@ -0,0 +1,35 @@
+/**
+ * Original Source: https://stackoverflow.com/questions/4009756/how-to-count-string-occurrence-in-string
+ *
+ * Function that count occurrences of a substring in a string
+ *
+ * @param {String} string               The string
+ * @param {String} subString            The sub string to search for
+ * @param {Boolean} [allowOverlapping]  Optional. (Default:false)
+ */
+
+function occurrences(string, subString, allowOverlapping) {
+  string += ''
+  subString += ''
+  if (subString.length <= 0) {
+    return (string.length + 1)
+  }
+
+  let n = 0
+  let pos = 0
+  const step = allowOverlapping ? 1 : subString.length
+  let flag = true
+
+  while (flag) {
+    pos = string.indexOf(subString, pos)
+    if (pos >= 0) {
+      ++n
+      pos += step
+    } else {
+      flag = false
+    }
+  }
+  return n
+}
+
+export default occurrences

test/occurrences.test.js

@@ -0,0 +1,43 @@
+import test from 'ava'
+import { occurrences } from '../src'
+
+test('empty substring', t => {
+  t.deepEqual(occurrences('', ''), 1)
+  t.deepEqual(occurrences('abc', ''), 4)
+})
+
+test('single occurences', t => {
+  t.deepEqual(occurrences('foo', 'foo'), 1)
+  t.deepEqual(occurrences('blahfooblah', 'foo'), 1)
+  t.deepEqual(occurrences('foo', 'f'), 1)
+})
+
+test('multiple occurrences', t => {
+  t.deepEqual(occurrences('foofoofoofoo', 'foo'), 4)
+  t.deepEqual(occurrences('foofoofoofoo', 'foofoo'), 2)
+  t.deepEqual(occurrences('blafooblahfooblah', 'foo'), 2)
+  t.deepEqual(occurrences('foofoofooooofo', 'foo'), 3)
+})
+
+test('no occurrences', t => {
+  t.deepEqual(occurrences('', 'foo'), 0)
+  t.deepEqual(occurrences('abc', 'foo'), 0)
+  t.deepEqual(occurrences('boo', 'foo'), 0)
+})
+
+test('overlap', t => {
+  t.deepEqual(occurrences('', '', true), 1)
+  t.deepEqual(occurrences('abc', '', true), 4)
+  t.deepEqual(occurrences('foofoofoofoo', 'foofoo', true), 3)
+  t.deepEqual(occurrences('blafooblahfooblah', 'foo', true), 2)
+  t.deepEqual(occurrences('foofoofooooofo', 'foo', true), 3)
+})
+
+test('overlap no occurrences', t => {
+  t.deepEqual(occurrences('', 'foo', true), 0);
+  t.deepEqual(occurrences('abc', 'foo', true), 0);
+  t.deepEqual(occurrences('boo', 'foo', true), 0);
+  t.deepEqual(occurrences('fooofooofooofoo', 'foofoo', true), 0);
+  t.deepEqual(occurrences('blafobooblahfoboblah', 'foo', true), 0);
+  t.deepEqual(occurrences('fofofofaooooofo', 'foo', true), 0);
+})