Optimize max_line_len (#1109)

The old max_line_len code fell into a surprising slowdown where repeatedly inserting chars in the middle of a string falls into O(n^2) behavior. Surprising because V8 already implements advanced data structures for string slicing and joining (`SlicedString` and `ConsString`). It seems like this should be easily handled by the two.

Anyways, we avoid this by slowing building a committed string (which is unlikely to be changed). Whenever we insert, we commit the left hand side, and can avoid slicing it in the future.

In testing a 5mb file (done by repeating a program several times), I dropped the simple parse+output case from 5min to 5sec.

lib/output.js

@@ -176,6 +176,48 @@ function is_some_comments(comment) {
     );
 }
 
+class Rope {
+    constructor() {
+        this.committed = "";
+        this.current = "";
+    }
+
+    append(str) {
+        this.current += str;
+    }
+
+    insertAt(char, index) {
+        const { committed, current } = this;
+        if (index < committed.length) {
+            this.committed = committed.slice(0, index) + char + committed.slice(index);
+        } else if (index === committed.length) {
+            this.committed += char;
+        } else {
+            index -= committed.length;
+            this.committed += current.slice(0, index) + char;
+            this.current = current.slice(index);
+        }
+    }
+
+    charAt(index) {
+        const { committed } = this;
+        if (index < committed.length) return committed[index];
+        return this.current[index - committed.length];
+    }
+
+    curLength() {
+        return this.current.length;
+    }
+
+    length() {
+        return this.committed.length + this.current.length;
+    }
+
+    toString() {
+        return this.committed + this.current;
+    }
+}
+
 function OutputStream(options) {
 
     var readonly = !options;
@@ -240,7 +282,7 @@ function OutputStream(options) {
     var current_col = 0;
     var current_line = 1;
     var current_pos = 0;
-    var OUTPUT = "";
+    var OUTPUT = new Rope();
     let printed_comments = new Set();
 
     var to_utf8 = options.ascii_only ? function(str, identifier) {
@@ -371,19 +413,18 @@ function OutputStream(options) {
     var ensure_line_len = options.max_line_len ? function() {
         if (current_col > options.max_line_len) {
             if (might_add_newline) {
-                var left = OUTPUT.slice(0, might_add_newline);
-                var right = OUTPUT.slice(might_add_newline);
+                OUTPUT.insertAt("\n", might_add_newline);
+                const curLength = OUTPUT.curLength();
                 if (mappings) {
-                    var delta = right.length - current_col;
+                    var delta = curLength - current_col;
                     mappings.forEach(function(mapping) {
                         mapping.line++;
                         mapping.col += delta;
                     });
                 }
-                OUTPUT = left + "\n" + right;
                 current_line++;
                 current_pos++;
-                current_col = right.length;
+                current_col = curLength;
             }
         }
         if (might_add_newline) {
@@ -417,13 +458,13 @@ function OutputStream(options) {
 
             if (prev === ":" && ch === "}" || (!ch || !";}".includes(ch)) && prev !== ";") {
                 if (options.semicolons || requireSemicolonChars.has(ch)) {
-                    OUTPUT += ";";
+                    OUTPUT.append(";");
                     current_col++;
                     current_pos++;
                 } else {
                     ensure_line_len();
                     if (current_col > 0) {
-                        OUTPUT += "\n";
+                        OUTPUT.append("\n");
                         current_pos++;
                         current_line++;
                         current_col = 0;
@@ -447,7 +488,7 @@ function OutputStream(options) {
                 || (ch == "/" && ch == prev)
                 || ((ch == "+" || ch == "-") && ch == last)
             ) {
-                OUTPUT += " ";
+                OUTPUT.append(" ");
                 current_col++;
                 current_pos++;
             }
@@ -465,7 +506,7 @@ function OutputStream(options) {
             if (!might_add_newline) do_add_mapping();
         }
 
-        OUTPUT += str;
+        OUTPUT.append(str);
         has_parens = str[str.length - 1] == "(";
         current_pos += str.length;
         var a = str.split(/\r?\n/), n = a.length - 1;
@@ -505,15 +546,15 @@ function OutputStream(options) {
 
     var newline = options.beautify ? function() {
         if (newline_insert < 0) return print("\n");
-        if (OUTPUT[newline_insert] != "\n") {
-            OUTPUT = OUTPUT.slice(0, newline_insert) + "\n" + OUTPUT.slice(newline_insert);
+        if (OUTPUT.charAt(newline_insert) != "\n") {
+            OUTPUT.insertAt("\n", newline_insert);
             current_pos++;
             current_line++;
         }
         newline_insert++;
     } : options.max_line_len ? function() {
         ensure_line_len();
-        might_add_newline = OUTPUT.length;
+        might_add_newline = OUTPUT.length();
     } : noop;
 
     var semicolon = options.beautify ? function() {
@@ -579,13 +620,14 @@ function OutputStream(options) {
         if (might_add_newline) {
             ensure_line_len();
         }
-        return OUTPUT;
+        return OUTPUT.toString();
     }
 
     function has_nlb() {
-        let n = OUTPUT.length - 1;
+        const output = OUTPUT.toString();
+        let n = output.length - 1;
         while (n >= 0) {
-            const code = OUTPUT.charCodeAt(n);
+            const code = output.charCodeAt(n);
             if (code === CODE_LINE_BREAK) {
                 return true;
             }
@@ -722,7 +764,7 @@ function OutputStream(options) {
             !/comment[134]/.test(c.type)
         ))) return;
         printed_comments.add(comments);
-        var insert = OUTPUT.length;
+        var insert = OUTPUT.length();
         comments.filter(comment_filter, node).forEach(function(c, i) {
             if (printed_comments.has(c)) return;
             printed_comments.add(c);
@@ -751,7 +793,7 @@ function OutputStream(options) {
                 need_space = true;
             }
         });
-        if (OUTPUT.length > insert) newline_insert = insert;
+        if (OUTPUT.length() > insert) newline_insert = insert;
     }
 
     var stack = [];
@@ -781,7 +823,7 @@ function OutputStream(options) {
             var encoded = encode_string(str, quote);
             if (escape_directive === true && !encoded.includes("\\")) {
                 // Insert semicolons to break directive prologue
-                if (!EXPECT_DIRECTIVE.test(OUTPUT)) {
+                if (!EXPECT_DIRECTIVE.test(OUTPUT.toString())) {
                     force_semicolon();
                 }
                 force_semicolon();