Fix to use decimal references if they are shorter

Previously, in cases such as where a double quote (`"`) was encoded,
and the shortest reference was requested, a hexadecimal reference
was produced even though decimal references would be shorter.

Closes GH-5.
Closes GH-6.

index.js

@@ -3,6 +3,7 @@
 var entities = require('character-entities-html4')
 var legacy = require('character-entities-legacy')
 var hexadecimal = require('is-hexadecimal')
+var decimal = require('is-decimal')
 var alphanumerical = require('is-alphanumerical')
 var dangerous = require('./dangerous.json')
 
@@ -71,14 +72,35 @@ function one(char, next, options) {
   var shortest = options.useShortestReferences
   var omit = options.omitOptionalSemicolons
   var named
+  var code
   var numeric
+  var decimal
 
   if ((shortest || options.useNamedReferences) && own.call(characters, char)) {
     named = toNamed(characters[char], next, omit, options.attribute)
   }
 
   if (shortest || !named) {
-    numeric = toHexReference(char.charCodeAt(0), next, omit)
+    code = char.charCodeAt(0)
+    numeric = toHexReference(code, next, omit)
+
+    // Use the shortest numeric reference when requested.
+    // A simple algorithm would use decimal for all code points under 100, as
+    // those are shorter than hexadecimal:
+    //
+    // * `c` vs `c` (decimal shorter)
+    // * `d` vs `d` (equal)
+    //
+    // However, because we take `next` into consideration when `omit` is used,
+    // And it would be possible that decimals are shorter on bigger values as
+    // well if `next` is hexadecimal but not decimal, we instead compare both.
+    if (shortest) {
+      decimal = toDecimalReference(code, next, omit)
+
+      if (decimal.length < numeric.length) {
+        numeric = decimal
+      }
+    }
   }
 
   if (named && (!shortest || named.length < numeric.length)) {
@@ -110,6 +132,12 @@ function toHexReference(code, next, omit) {
   return omit && next && !hexadecimal(next) ? value : value + ';'
 }
 
+// Transform `code` into a decimal character reference.
+function toDecimalReference(code, next, omit) {
+  var value = '&#' + String(code)
+  return omit && next && !decimal(next) ? value : value + ';'
+}
+
 // Create an expression for `characters`.
 function toExpression(characters) {
   return new RegExp('[' + characters.join('') + ']', 'g')

package.json

@@ -27,6 +27,7 @@
     "character-entities-html4": "^1.0.0",
     "character-entities-legacy": "^1.0.0",
     "is-alphanumerical": "^1.0.0",
+    "is-decimal": "^1.0.2",
     "is-hexadecimal": "^1.0.0"
   },
   "devDependencies": {

test.js

@@ -37,11 +37,22 @@ test('stringifyEntities(value[, options])', function(t) {
   )
 
   t.equal(
-    stringify('alpha © bravo ≠ charlie 𝌆 delta', {useShortestReferences: true}),
-    'alpha © bravo ≠ charlie 𝌆 delta',
+    stringify('alpha © bravo ≠ charlie 𝌆 delta " echo', {
+      useShortestReferences: true
+    }),
+    'alpha © bravo ≠ charlie 𝌆 delta " echo',
     'Should use shortest entities if `useShortestReferences`'
   )
 
+  t.equal(
+    stringify('" "0 "a "z µ µ0 µa µz', {
+      useShortestReferences: true,
+      omitOptionalSemicolons: true
+    }),
+    '&#34 "0 &#34a &#34z &#xB5 µ0 &#181a &#xB5z',
+    'Should pick the shortest numeric reference based on `next` with `omitOptionalSemicolons`'
+  )
+
   t.equal(
     stringify('\'"<>&'),
     '&#x27;&#x22;&#x3C;&#x3E;&#x26;',