add test

ui/app/utils/common-prefix.js

@@ -2,22 +2,23 @@ import { get } from '@ember/object';
 
 export default function(arr, attribute = 'id') {
   let content = arr || [];
-  // this assumes an already sorted array, if we have a match,
-  // it will have matched the first one so we also want the last one
+  // this assumes an already sorted array
+  // if the array is sorted, we want to compare the first and last
+  // item in the array - if they share a prefix, all of the items do
   let firstString = get(content[0], attribute);
   let lastString = get(content[arr.length - 1], attribute);
 
   // the longest the shared prefix could be is the length of the match
   let targetLength = firstString.length;
   let prefixLength = 0;
   // walk the two strings, and if they match at the current length,
-  // increment the prefix and try again
+  // increment the prefixLength and try again
   while (
     prefixLength < targetLength &&
     firstString.charAt(prefixLength) === lastString.charAt(prefixLength)
   ) {
     prefixLength++;
   }
-  // slice the prefix from the match
+  // slice the prefix from the first item
   return firstString.substring(0, prefixLength);
 }

ui/tests/unit/utils/common-prefix-test.js

@@ -0,0 +1,27 @@
+import commonPrefix from 'vault/utils/common-prefix';
+import { module, test } from 'qunit';
+
+module('Unit | Util | common prefix', function() {
+  test('it returns empty string if there are no common prefixes', function(assert) {
+    let secrets = ['asecret', 'secret2', 'secret3'].map(s => ({id: s}));
+    let returned = commonPrefix(secrets);
+    assert.equal(returned, '', 'returns an empty string');
+  });
+
+  test('it returns the longest prefix', function(assert) {
+    let secrets = ['secret1', 'secret2', 'secret3'].map(s => ({id: s}));
+    let returned = commonPrefix(secrets);
+    assert.equal(returned, 'secret', 'finds secret prefix');
+    let greetings = ['hello-there', 'hello-hi', 'hello-howdy'].map(s => ({id: s}));
+    returned = commonPrefix(greetings);
+    assert.equal(returned, 'hello-', 'finds hello- prefix');
+  });
+
+
+  test('it can compare an attribute that is not "id" to calculate the longest prefix', function(assert) {
+    let secrets = ['secret1', 'secret2', 'secret3'].map(s => ({name: s}));
+    let returned = commonPrefix(secrets, 'name');
+    assert.equal(returned, 'secret', 'finds secret prefix from name attribute');
+  });
+});
+