Strip whitespace in linear time

lib/whitespace-control.js

@@ -41,7 +41,9 @@ WhitespaceControl.prototype.Program = function (program) {
         // If we are on a standalone node, save the indent info for partials
         if (current.type === 'PartialStatement') {
           // Pull out the whitespace from the final line
-          current.indent = /([ \t]+$)/.exec(body[i - 1].original)[1];
+          current.indent = /(?:^|[^ \t])([ \t]+)$/.exec(
+            body[i - 1].original
+          )[1];
         }
       }
     }
@@ -172,9 +174,13 @@ function isPrevWhitespace(body, i, isRoot) {
   }
 
   if (prev.type === 'ContentStatement') {
-    return (sibling || !isRoot ? /\r?\n\s*?$/ : /(^|\r?\n)\s*?$/).test(
-      prev.original
-    );
+    // Matches when the trailing whitespace contains a newline (or, for the
+    // root's first node, is the whole string). See omitLeft for the anchoring.
+    return (
+      sibling || !isRoot
+        ? /(?:^|\S)[^\S\n]*\n\s*$/
+        : /^\s*$|(?:^|\S)[^\S\n]*\n\s*$/
+    ).test(prev.original);
   }
 }
 function isNextWhitespace(body, i, isRoot) {
@@ -239,7 +245,13 @@ function omitLeft(body, i, multiple) {
 
   // We omit the last node if it's whitespace only and not preceded by a non-content node.
   let original = current.value;
-  current.value = current.value.replace(multiple ? /\s+$/ : /[ \t]+$/, '');
+  // The match must start after a non-whitespace character (or at the start):
+  // a plain /\s+$/ is retried at every position of a whitespace run, which
+  // takes quadratic time on long runs.
+  current.value = current.value.replace(
+    multiple ? /(^|\S)\s+$/ : /(^|[^ \t])[ \t]+$/,
+    '$1'
+  );
   current.leftStripped = current.value !== original;
   return current.leftStripped;
 }

spec/ast.js

@@ -47,6 +47,35 @@ describe('ast', function () {
         equals(ast.body[1].program.body[0].value, ' \nfoo\n ');
       });
     });
+
+    describe('performance', function () {
+      // Each template below takes several seconds to parse, and so exceeds the
+      // test timeout, if finding trailing whitespace is not linear in its length.
+      let spaces = ' '.repeat(100000),
+        newlines = '\n'.repeat(100000);
+
+      it('should strip whitespace in linear time', function () {
+        let ast = parse(spaces + 'x' + spaces + '{{~foo}}');
+
+        equals(ast.body[0].value, spaces + 'x');
+      });
+
+      it('should detect standalone lines in linear time', function () {
+        let ast = parse(newlines + 'x{{foo}}');
+        equals(ast.body[0].value, newlines + 'x');
+
+        ast = parse('{{foo}}' + newlines + 'x{{foo}}');
+        equals(ast.body[1].value, newlines + 'x');
+      });
+
+      it('should find partial indentation in linear time', function () {
+        let ast = parse(spaces + 'x\n  {{> dude}}\n');
+
+        equals(ast.body[0].value, spaces + 'x\n');
+        equals(ast.body[1].indent, '  ');
+        equals(ast.body[2].value, '');
+      });
+    });
   });
 
   describe('node details', function () {