From ed35a00247e2c50ffb59cc3f137c987fc2ca8dae Mon Sep 17 00:00:00 2001
From: Douglas Christopher Wilson <doug@somethingdoug.com>
Date: Sun, 24 Aug 2014 21:40:41 -0400
Subject: [PATCH] Add weak ETag generation

---
 HISTORY.md   |  1 +
 README.md    | 20 ++++++++++---------
 index.js     | 54 +++++++++++++++++++++++++++++++++++++++++++++-------
 package.json |  3 +++
 test/test.js | 32 ++++++++++++++++++++++++++-----
 5 files changed, 89 insertions(+), 21 deletions(-)

diff --git a/HISTORY.md b/HISTORY.md
index 94568e5..da0c298 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -2,6 +2,7 @@ unreleased
 ==========
 
   * Add fast-path for empty entity
+  * Add weak ETag generation
   * Shrink size of generated ETags
 
 1.0.1 / 2014-08-24
diff --git a/README.md b/README.md
index 502f22c..da36124 100644
--- a/README.md
+++ b/README.md
@@ -20,23 +20,25 @@ $ npm install etag
 var etag = require('etag')
 ```
 
-### etag(str)
+### etag(entity, [options])
 
-Generate a strong ETag for the given string. This string should be the
-complete body and is assumed to be UTF-8.
+Generate a strong ETag for the given entity. This should be the complete
+body of the entity. Both strings are `Buffer`s are accepted. By default,
+a string will generate a weak ETag while a `Buffer` will generate a strong
+ETag (this can be overwritten by `options.weak`).
 
 ```js
 res.setHeader('ETag', etag(body))
 ```
 
-### etag(buf)
+#### Options
 
-Generate a strong ETag for the given `Buffer`. This buffer should be the
-complete body.
+`etag` accepts these properties in the options object.
 
-```js
-res.setHeader('ETag', etag(buf))
-```
+##### weak
+
+Specifies if a "strong" or a "weak" ETag will be generated. The ETag can only
+really be a strong as the given input.
 
 ## Testing
 
diff --git a/index.js b/index.js
index fd9b651..ad5c437 100644
--- a/index.js
+++ b/index.js
@@ -14,17 +14,20 @@ module.exports = etag
  * Module dependencies.
  */
 
+var crc = require('crc').crc32
 var crypto = require('crypto')
 
 /**
  * Create a simple ETag.
  *
  * @param {string|Buffer} entity
+ * @param {object} [options]
+ * @param {boolean} [options.weak]
  * @return {String}
  * @api public
  */
 
-function etag(entity) {
+function etag(entity, options) {
   if (entity == null) {
     throw new TypeError('argument entity is required')
   }
@@ -35,14 +38,51 @@ function etag(entity) {
     throw new TypeError('argument entity must be string or Buffer')
   }
 
-  if (entity.length === 0) {
-    // fast-path empty body
-    return '"1B2M2Y8AsgTpgAmY7PhCfg=="'
+  var buf = !isBuffer
+    ? new Buffer(entity, 'utf8')
+    : entity
+  var weak = options && typeof options.weak === 'boolean'
+    ? options.weak
+    : !isBuffer
+
+  return weak
+    ? 'W/"' + weakhash(buf) + '"'
+    : '"' + stronghash(buf) + '"'
+}
+
+/**
+ * Generate a strong hash.
+ *
+ * @param {Buffer} entity
+ * @return {String}
+ * @api private
+ */
+
+function stronghash(buf) {
+  if (buf.length === 0) {
+    // fast-path empty
+    return '1B2M2Y8AsgTpgAmY7PhCfg=='
   }
 
-  var hash = crypto
+  return crypto
     .createHash('md5')
-    .update(entity, 'utf8')
+    .update(buf)
     .digest('base64')
-  return '"' + hash + '"'
+}
+
+/**
+ * Generate a weak hash.
+ *
+ * @param {Buffer} entity
+ * @return {String}
+ * @api private
+ */
+
+function weakhash(buf) {
+  if (buf.length === 0) {
+    // fast-path empty
+    return '0-0'
+  }
+
+  return buf.length.toString(16) + '-' + crc(buf).toString(16)
 }
diff --git a/package.json b/package.json
index 7d74250..6d68618 100644
--- a/package.json
+++ b/package.json
@@ -13,6 +13,9 @@
     "res"
   ],
   "repository": "jshttp/etag",
+  "dependencies": {
+    "crc": "2.1.1"
+  },
   "devDependencies": {
     "istanbul": "0.3.0",
     "mocha": "~1.21.4"
diff --git a/test/test.js b/test/test.js
index 8e2e35b..bb5bb67 100644
--- a/test/test.js
+++ b/test/test.js
@@ -12,21 +12,21 @@ describe('etag(entity)', function () {
   })
 
   describe('when "entity" is a string', function () {
-    it('should generate an ETag', function () {
-      assert.equal(etag('beep boop'), '"Z34SGyQ2IB7YzB7HMkCjrQ=="')
+    it('should generate a weak ETag', function () {
+      assert.equal(etag('beep boop'), 'W/"9-7f3ee715"')
     })
 
     it('should work containing Unicode', function () {
-      assert.equal(etag('论'), '"aW9HeLTk2Yt6lf7zJYElgw=="')
+      assert.equal(etag('论'), 'W/"3-438093ff"')
     })
 
     it('should work for empty string', function () {
-      assert.equal(etag(''), '"1B2M2Y8AsgTpgAmY7PhCfg=="')
+      assert.equal(etag(''), 'W/"0-0"')
     })
   })
 
   describe('when "entity" is a Buffer', function () {
-    it('should generate an ETag', function () {
+    it('should generate a strong ETag', function () {
       assert.equal(etag(new Buffer([1, 2, 3])), '"Uonfc331cyb83SJZevsfrA=="')
     })
 
@@ -34,4 +34,26 @@ describe('etag(entity)', function () {
       assert.equal(etag(new Buffer(0)), '"1B2M2Y8AsgTpgAmY7PhCfg=="')
     })
   })
+
+  describe('with "weak" option', function () {
+    describe('when "false"', function () {
+      it('should generate a strong ETag for a string', function () {
+        assert.equal(etag('beep boop', {weak: false}), '"Z34SGyQ2IB7YzB7HMkCjrQ=="')
+      })
+
+      it('should generate a strong ETag for a Buffer', function () {
+        assert.equal(etag(new Buffer([1, 2, 3]), {weak: false}), '"Uonfc331cyb83SJZevsfrA=="')
+      })
+    })
+
+    describe('when "true"', function () {
+      it('should generate a strong ETag for a string', function () {
+        assert.equal(etag('beep boop', {weak: true}), 'W/"9-7f3ee715"')
+      })
+
+      it('should generate a strong ETag for a Buffer', function () {
+        assert.equal(etag(new Buffer([1, 2, 3]), {weak: true}), 'W/"3-55bc801d"')
+      })
+    })
+  })
 })