Add Mocha-style multi-line string diffs.

Instead of using eyes for strings, use diff to present
a colored diff of the expected vs. actual results.

Author: reid

lib/assert/error.js

@@ -1,5 +1,52 @@
 var stylize = require('../vows/console').stylize;
 var inspect = require('../vows/console').inspect;
+var diff = require('diff');
+
+/**
+ * Pad the given `str` to `len`.
+ *
+ * @param {String} str
+ * @param {String} len
+ * @return {String}
+ * @api private
+ */
+
+function pad(str, len) {
+  str = String(str);
+  return Array(len - str.length + 1).join(' ') + str;
+}
+
+/**
+ * Color lines for `str`, using the color `name`.
+ *
+ * @param {String} name
+ * @param {String} str
+ * @return {String}
+ * @api private
+ */
+
+function styleLines(str, name) {
+  return str.split('\n').map(function(str){
+    return stylize(str, name);
+  }).join('\n');
+}
+
+/**
+ * Return a character diff for `err`.
+ *
+ * @param {Error} err
+ * @return {String}
+ * @api private
+ */
+
+function errorDiff(err, type) {
+  return diff['diff' + type](err.actual, err.expected).map(function(str){
+    if (/^(\n+)$/.test(str.value)) str.value = Array(++RegExp.$1.length).join('<newline>');
+    if (str.added) return styleLines(str.value, 'green');
+    if (str.removed) return styleLines(str.value, 'red');
+    return str.value;
+  }).join('');
+}
 
 require('assert').AssertionError.prototype.toString = function () {
     var that = this,
@@ -10,14 +57,50 @@ require('assert').AssertionError.prototype.toString = function () {
     }
 
     function parse(str) {
-        var actual = inspect(that.actual, {showHidden: that.actual instanceof Error}),
-            expected;
+        var actual = that.actual,
+            expected = that.expected,
+            msg, len;
+
+        if (
+            'string' === typeof actual &&
+            'string' === typeof expected
+        ) {
+            len = Math.max(actual.length, expected.length);
+
+            if (len < 20) msg = errorDiff(that, 'Chars');
+            else msg = errorDiff(that, 'Words');
+
+            // linenos
+            var lines = msg.split('\n');
+            if (lines.length > 4) {
+                var width = String(lines.length).length;
+                msg = lines.map(function(str, i){
+                    return pad(++i, width) + ' |' + ' ' + str;
+                }).join('\n');
+            }
+
+            // legend
+            msg = '\n'
+                + stylize('actual', 'green')
+                + ' '
+                + stylize('expected', 'red')
+                + '\n\n'
+                + msg
+                + '\n';
+
+            // indent
+            msg = msg.replace(/^/gm, '      ');
+
+            return msg;
+        }
+
+        actual = inspect(actual, {showHidden: actual instanceof Error});
 
-        if (that.expected instanceof Function) {
-            expected = that.expected.name;
+        if (expected instanceof Function) {
+            expected = expected.name;
         }
         else {
-            expected = inspect(that.expected, {showHidden: that.actual instanceof Error});
+            expected = inspect(expected, {showHidden: actual instanceof Error});
         }
 
         return str.replace(/{actual}/g,   actual).

package.json

@@ -1,15 +1,37 @@
 {
-  "name"          : "vows",
-  "description"   : "Asynchronous BDD & continuous integration for node.js",
-  "url"           : "http://vowsjs.org",
-  "keywords"      : ["testing", "spec", "test", "BDD"],
-  "author"        : "Alexis Sellier <self@cloudhead.net>",
-  "contributors"  : [{ "name": "Charlie Robbins", "email": "charlie.robbins@gmail.com" }],
-  "dependencies"  : {"eyes": ">=0.1.6"},
-  "main"          : "./lib/vows",
-  "bin"           : { "vows": "./bin/vows" },
-  "directories"   : { "test": "./test", "bin": "./bin" },
-  "version"       : "0.6.2",
-  "scripts"       : {"test": "./bin/vows --spec"},
-  "engines"       : {"node": ">=0.2.6"}
+  "name": "vows",
+  "description": "Asynchronous BDD & continuous integration for node.js",
+  "url": "http://vowsjs.org",
+  "keywords": [
+    "testing",
+    "spec",
+    "test",
+    "BDD"
+  ],
+  "author": "Alexis Sellier <self@cloudhead.net>",
+  "contributors": [
+    {
+      "name": "Charlie Robbins",
+      "email": "charlie.robbins@gmail.com"
+    }
+  ],
+  "dependencies": {
+    "eyes": ">=0.1.6",
+    "diff": "~1.0.3"
+  },
+  "main": "./lib/vows",
+  "bin": {
+    "vows": "./bin/vows"
+  },
+  "directories": {
+    "test": "./test",
+    "bin": "./bin"
+  },
+  "version": "0.6.2",
+  "scripts": {
+    "test": "./bin/vows --spec"
+  },
+  "engines": {
+    "node": ">=0.2.6"
+  }
 }