refactor compute_char_frequency() (#2152)

- minimise maintenance when updating AST
- maximise code sharing between `master` & `harmony`

lib/output.js

@@ -501,6 +501,7 @@ function OutputStream(options) {
             use_asm = prev_use_asm;
         }
     });
+    AST_Node.DEFMETHOD("_print", AST_Node.prototype.print);
 
     AST_Node.DEFMETHOD("print_to_string", function(options){
         var s = OutputStream(options);

lib/scope.js

@@ -361,7 +361,8 @@ AST_Function.DEFMETHOD("next_mangled", function(options, def){
 });
 
 AST_Symbol.DEFMETHOD("unmangleable", function(options){
-    return this.definition().unmangleable(options);
+    var def = this.definition();
+    return !def || def.unmangleable(options);
 });
 
 // labels are always mangleable
@@ -460,103 +461,69 @@ AST_Toplevel.DEFMETHOD("mangle_names", function(options){
 
 AST_Toplevel.DEFMETHOD("compute_char_frequency", function(options){
     options = this._default_mangler_options(options);
-    var tw = new TreeWalker(function(node){
+    try {
-        if (node instanceof AST_Constant)
+        AST_Node.prototype.print = function(stream, force_parens) {
-            base54.consider(node.print_to_string());
+            this._print(stream, force_parens);
-        else if (node instanceof AST_Return)
+            if (this instanceof AST_Symbol && !this.unmangleable(options)) {
-            base54.consider("return");
+                base54.consider(this.name, -1);
-        else if (node instanceof AST_Throw)
+            } else if (options.properties) {
-            base54.consider("throw");
+                if (this instanceof AST_Dot) {
-        else if (node instanceof AST_Continue)
+                    base54.consider(this.property, -1);
-            base54.consider("continue");
+                } else if (this instanceof AST_Sub) {
-        else if (node instanceof AST_Break)
+                    skip_string(this.property);
-            base54.consider("break");
+                }
-        else if (node instanceof AST_Debugger)
+            }
-            base54.consider("debugger");
+        };
-        else if (node instanceof AST_Directive)
+        base54.consider(this.print_to_string(), 1);
-            base54.consider(node.value);
+    } finally {
-        else if (node instanceof AST_While)
+        AST_Node.prototype.print = AST_Node.prototype._print;
-            base54.consider("while");
+    }
-        else if (node instanceof AST_Do)
-            base54.consider("do while");
-        else if (node instanceof AST_If) {
-            base54.consider("if");
-            if (node.alternative) base54.consider("else");
-        }
-        else if (node instanceof AST_Var)
-            base54.consider("var");
-        else if (node instanceof AST_Lambda)
-            base54.consider("function");
-        else if (node instanceof AST_For)
-            base54.consider("for");
-        else if (node instanceof AST_ForIn)
-            base54.consider("for in");
-        else if (node instanceof AST_Switch)
-            base54.consider("switch");
-        else if (node instanceof AST_Case)
-            base54.consider("case");
-        else if (node instanceof AST_Default)
-            base54.consider("default");
-        else if (node instanceof AST_With)
-            base54.consider("with");
-        else if (node instanceof AST_ObjectSetter)
-            base54.consider("set" + node.key);
-        else if (node instanceof AST_ObjectGetter)
-            base54.consider("get" + node.key);
-        else if (node instanceof AST_ObjectKeyVal)
-            base54.consider(node.key);
-        else if (node instanceof AST_New)
-            base54.consider("new");
-        else if (node instanceof AST_This)
-            base54.consider("this");
-        else if (node instanceof AST_Try)
-            base54.consider("try");
-        else if (node instanceof AST_Catch)
-            base54.consider("catch");
-        else if (node instanceof AST_Finally)
-            base54.consider("finally");
-        else if (node instanceof AST_Symbol && node.unmangleable(options))
-            base54.consider(node.name);
-        else if (node instanceof AST_Unary || node instanceof AST_Binary)
-            base54.consider(node.operator);
-        else if (node instanceof AST_Dot)
-            base54.consider(node.property);
-    });
-    this.walk(tw);
     base54.sort();
+
+    function skip_string(node) {
+        if (node instanceof AST_String) {
+            base54.consider(node.value, -1);
+        } else if (node instanceof AST_Conditional) {
+            skip_string(node.consequent);
+            skip_string(node.alternative);
+        } else if (node instanceof AST_Sequence) {
+            skip_string(node.expressions[node.expressions.length - 1]);
+        }
+    }
 });
 
 var base54 = (function() {
-    var string = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ$_0123456789";
+    var leading = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ$_".split("");
+    var digits = "0123456789".split("");
     var chars, frequency;
     function reset() {
         frequency = Object.create(null);
-        chars = string.split("").map(function(ch){ return ch.charCodeAt(0) });
+        leading.forEach(function(ch) {
-        chars.forEach(function(ch){ frequency[ch] = 0 });
+            frequency[ch] = 0;
+        });
+        digits.forEach(function(ch) {
+            frequency[ch] = 0;
+        });
     }
-    base54.consider = function(str){
+    base54.consider = function(str, delta) {
         for (var i = str.length; --i >= 0;) {
-            var code = str.charCodeAt(i);
+            frequency[str[i]] += delta;
-            if (code in frequency) ++frequency[code];
         }
     };
+    function compare(a, b) {
+        return frequency[b] - frequency[a];
+    }
     base54.sort = function() {
-        chars = mergeSort(chars, function(a, b){
+        chars = mergeSort(leading, compare).concat(mergeSort(digits, compare));
-            if (is_digit(a) && !is_digit(b)) return 1;
-            if (is_digit(b) && !is_digit(a)) return -1;
-            return frequency[b] - frequency[a];
-        });
     };
     base54.reset = reset;
     reset();
-    base54.get = function(){ return chars };
-    base54.freq = function(){ return frequency };
     function base54(num) {
         var ret = "", base = 54;
         num++;
         do {
             num--;
-            ret += String.fromCharCode(chars[num % base]);
+            ret += chars[num % base];
             num = Math.floor(num / base);
             base = 64;
         } while (num > 0);