Implement TastyHash based on PJW hash.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/tasty/TastyPickler.scala

@@ -6,7 +6,6 @@ package tasty
 import TastyFormat._
 import collection.mutable
 import TastyBuffer._
-import util.CityHash
 import core.Symbols.Symbol
 import ast.tpd
 import Decorators._
@@ -26,8 +25,8 @@ class TastyPickler {
       buf.length + natSize(buf.length)
     }
 
-    val uuidLow: Long = CityHash.bytesHash(nameBuffer.bytes)
-    val uuidHi: Long = sections.iterator.map(x => CityHash.bytesHash(x._2.bytes)).fold(0L)(_ ^ _)
+    val uuidLow: Long = pjwHash64(nameBuffer.bytes)
+    val uuidHi: Long = sections.iterator.map(x => pjwHash64(x._2.bytes)).fold(0L)(_ ^ _)
 
     val headerBuffer = {
       val buf = new TastyBuffer(header.length + 24)
@@ -72,4 +71,23 @@ class TastyPickler {
   var addrOfSym: Symbol => Option[Addr] = (_ => None)
 
   val treePkl = new TreePickler(this)
+
+  /** Returns a non-cryptographic 64-bit hash of the array.
+   *
+   *  from https://en.wikipedia.org/wiki/PJW_hash_function#Implementation
+   */
+  private def pjwHash64(data: Array[Byte]): Long = {
+    var h = 0L
+    var high = 0L
+    var i = 0
+    while (i < data.length) {
+      h = (h << 4) + data(i)
+      high = h & 0xF0000000L
+      if (high != 0)
+        h ^= high >> 24
+      h &= ~high
+      i += 1
+    }
+    h
+  }
 }