Replace murmur hash with djb2 hash

In profiling StyleSheet.create, I noticed that much of the time was
spent hashing. So, I found a faster hashing algorithm.

The implementation was taken from:

  https://github.com/darkskyapp/string-hash

According to this StackExchange post, this algorithm doesn't have as
good of randomness, but it has about the same percentage of collisions.
I don't think randomness matters for this application, so I think this
is okay.

  http://softwareengineering.stackexchange.com/a/145633

Using similar methodology to #202, this appears to make StylSheet.create
~15% faster (~220ms to ~185ms).

Author: lencioni

src/util.js

@@ -152,52 +152,23 @@ export const stringifyValue = (
 };
 
 /**
- * JS Implementation of MurmurHash2
+ * JS Implementation of djb2
  *
- * @author <a href="mailto:gary.court@gmail.com">Gary Court</a>
- * @see http://github.com/garycourt/murmurhash-js
- * @author <a href="mailto:aappleby@gmail.com">Austin Appleby</a>
- * @see http://sites.google.com/site/murmurhash/
- *
- * @param {string} str ASCII only
- * @return {string} Base 36 encoded hash result
+ * @see https://github.com/darkskyapp/string-hash
  */
-function murmurhash2_32_gc(str) {
-    let l = str.length;
-    let h = l;
-    let i = 0;
-    let k;
-
-    while (l >= 4) {
-        k = ((str.charCodeAt(i) & 0xff)) |
-            ((str.charCodeAt(++i) & 0xff) << 8) |
-            ((str.charCodeAt(++i) & 0xff) << 16) |
-            ((str.charCodeAt(++i) & 0xff) << 24);
-
-        k = (((k & 0xffff) * 0x5bd1e995) + ((((k >>> 16) * 0x5bd1e995) & 0xffff) << 16));
-        k ^= k >>> 24;
-        k = (((k & 0xffff) * 0x5bd1e995) + ((((k >>> 16) * 0x5bd1e995) & 0xffff) << 16));
-
-        h = (((h & 0xffff) * 0x5bd1e995) + ((((h >>> 16) * 0x5bd1e995) & 0xffff) << 16)) ^ k;
-
-        l -= 4;
-        ++i;
-    }
 
-    /* eslint-disable no-fallthrough */ // forgive existing code
-    switch (l) {
-    case 3: h ^= (str.charCodeAt(i + 2) & 0xff) << 16;
-    case 2: h ^= (str.charCodeAt(i + 1) & 0xff) << 8;
-    case 1: h ^= (str.charCodeAt(i) & 0xff);
-        h = (((h & 0xffff) * 0x5bd1e995) + ((((h >>> 16) * 0x5bd1e995) & 0xffff) << 16));
-    }
-    /* eslint-enable no-fallthrough */
+function djb2Hash(str) {
+  let hash = 5381;
+  let i = str.length;
 
-    h ^= h >>> 13;
-    h = (((h & 0xffff) * 0x5bd1e995) + ((((h >>> 16) * 0x5bd1e995) & 0xffff) << 16));
-    h ^= h >>> 15;
+  while(i) {
+    hash = (hash * 33) ^ str.charCodeAt(--i);
+  }
 
-    return (h >>> 0).toString(36);
+  /* JavaScript does bitwise operations (like XOR, above) on 32-bit signed
+   * integers. Since we want the results to be always positive, convert the
+   * signed int to an unsigned by doing an unsigned bitshift. */
+  return "" + (hash >>> 0);
 }
 
 // Hash a javascript object using JSON.stringify. This is very fast, about 3
@@ -208,7 +179,7 @@ function murmurhash2_32_gc(str) {
 // this to produce consistent hashes browsers need to have a consistent
 // ordering of objects. Ben Alpert says that Facebook depends on this, so we
 // can probably depend on this too.
-export const hashObject = (object /* : ObjectMap */) /* : string */ => murmurhash2_32_gc(JSON.stringify(object));
+export const hashObject = (object /* : ObjectMap */) /* : string */ => djb2Hash(JSON.stringify(object));
 
 
 const IMPORTANT_RE = /^([^:]+:.*?)( !important)?;$/;