Minify methods, classes and fields.

sdk/lib/_internal/compiler/implementation/js_backend/minify_namer.dart

+// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+part of js_backend;
+
+/**
+ * Assigns JavaScript identifiers to Dart variables, class-names and members.
+ */
+class MinifyNamer extends Namer {
+  MinifyNamer(Compiler compiler) : super(compiler);
+
+  String get isolateName => 'I';
+  String get isolatePropertiesName => 'p';
+  bool get shouldMinify => true;
+
+  const ALPHABET_CHARACTERS = 52;  // a-zA-Z.
+  const ALPHANUMERIC_CHARACTERS = 62;  // a-zA-Z0-9.
+
+  // You can pass an invalid identifier to this and unlike its non-minifying
+  // counterpart it will never return the proposedName as the new fresh name.
+  String getFreshName(String proposedName, Set<String> usedNames) {
+    var freshName = _getUnusedName(proposedName, usedNames);
+    usedNames.add(freshName);
+    return freshName;
+  }
+
+  // This gets a minified name based on a hash of the proposed name.  This
+  // is slightly less efficient than just getting the next name in a series,
+  // but it means that small changes in the input program will give smallish
+  // changes in the output, which can be useful for diffing etc.
+  String _getUnusedName(String proposedName, Set<String> usedNames) {
+    // Try single-character names with characters that occur in the
+    // input.
+    for (int i = 0; i < proposedName.length; i++) {
+      String candidate = proposedName[i];
+      int code = candidate.charCodeAt(0);
+      if (code < $A) continue;
+      if (code > $z) continue;
+      if (code > $Z && code < $a) continue;
+      if (!usedNames.contains(candidate)) return candidate;
+    }
+
+    int hash = _calculateHash(proposedName);
+    // Avoid very small hashes that won't try many names.
+    hash = hash < 1000 ? hash * 314159 : hash;  // Yes, it's prime.
+
+    // Try other n-character names based on the hash.  We try one to three
+    // character identifiers.  For each length we try around 10 different names
+    // in a predictable order determined by the proposed name.  This is in order
+    // to make the renamer stable: small changes in the input should nornally
+    // result in relatively small changes in the output.
+    for (var n = 1; n <= 3; n++) {
+      int h = hash;
+      while (h > 10) {
+        var codes = <int>[_letterNumber(h)];
+        int h2 = h ~/ ALPHABET_CHARACTERS;
+        for (var i = 1; i < n; i++) {
+          codes.add(_alphaNumericNumber(h2));
+          h2 ~/= ALPHANUMERIC_CHARACTERS;
+        }
+        final candidate = new String.fromCharCodes(codes);
+        if (!usedNames.contains(candidate) && !jsReserved.contains(candidate)) {
+          return candidate;
+        }
+        // Try again with a slightly different hash.  After around 10 turns
+        // around this loop h is zero and we try a longer name.
+        h ~/= 7;
+      }
+    }
+
+    // If we can't find a hash based name in the three-letter space, then base
+    // the name on a letter and a counter.
+    var startLetter = new String.fromCharCodes([_letterNumber(hash)]);
+    var i = 0;
+    while (usedNames.contains("$startLetter$i")) {
+      i++;
+    }
+    return "$startLetter$i";
+  }
+
+  int _calculateHash(String name) {
+    int h = 0;
+    for (int i = 0; i < name.length; i++) {
+      h += name.charCodeAt(i);
+      h &= 0xffffffff;
+      h += h << 10;
+      h &= 0xffffffff;
+      h ^= h >> 6;
+      h &= 0xffffffff;
+    }
+    return h;
+  }
+
+  int _letterNumber(int x) {
+    if (x >= ALPHABET_CHARACTERS) x %= ALPHABET_CHARACTERS;
+    if (x < 26) return $a + x;
+    return $A + x - 26;
+  }
+
+  int _alphaNumericNumber(int x) {
+    if (x >= ALPHANUMERIC_CHARACTERS) x %= ALPHANUMERIC_CHARACTERS;
+    if (x < 26) return $a + x;
+    if (x < 52) return $A + x - 26;
+    return $0 + x - 52;
+  }
+
+}