Add support for comparing case-sensitive

Closes GH-6.

index.js

@@ -4,10 +4,18 @@ var levenshtein = require('levenshtein-edit-distance')
 
 module.exports = similarity
 
-function similarity(a, b) {
-  if (!a || !b || a.length === 0 || b.length === 0) return 0
-  if (a === b) return 1
-  var d = levenshtein(a.toLowerCase(), b.toLowerCase())
-  var longest = Math.max(a.length, b.length)
-  return (longest - d) / longest
+function similarity(a, b, options) {
+  var opts = options || {}
+  var sensitive = opts.sensitive
+  var left = casing(a || '', sensitive)
+  var right = casing(b || '', sensitive)
+  if (left.length === 0 || right.length === 0) return 0
+  if (left === right) return 1
+  var distance = levenshtein(left, right)
+  var longest = Math.max(left.length, right.length)
+  return (longest - distance) / longest
+}
+
+function casing(value, sensitive) {
+  return sensitive ? value : value.toLowerCase()
 }

readme.md

@@ -28,6 +28,16 @@ similarity('es6-shim', 'es6 shim') // 0.875
 similarity('ES6-Shim', 'es6 shim') // 0.875 (case insensitive)
 ```
 
+## API
+
+### `similarity(left, right[, options])`
+
+Get the similarity (`number`) between two values (`string`s),
+where `0` is dissimilar, and `1` is equal.
+
+*   `options.sensitive` (`boolean`, default: `false`)
+    — Turn on (`true`) to treat casing differences as differences
+
 ## See also
 
 Note: This module uses [Levenshtein distance][wiki] to measure similarity, but

test.js

@@ -12,5 +12,10 @@ test('similarity', function(t) {
   t.equal(similarity('chicken', 'chick'), 5 / 7)
   t.equal(similarity('es6-shim', 'es6 shim'), 7 / 8)
   t.equal(similarity('ES6 Shim', 'es6-shim'), 7 / 8, 'case insensitive')
+  t.equal(
+    similarity('ES6 Shim', 'es6-shim', {sensitive: true}),
+    4 / 8,
+    'case sensitive'
+  )
   t.end()
 })