Allow early termination to limit execution time with degenerate cases (…

…#365)

Co-authored-by: Tony Spataro <anthony.spataro@appfolio.com>

src/diff/base.js

@@ -30,6 +30,10 @@ Diff.prototype = {
     let newLen = newString.length, oldLen = oldString.length;
     let editLength = 1;
     let maxEditLength = newLen + oldLen;
+    if(options.maxEditLength) {
+      maxEditLength = Math.min(maxEditLength, options.maxEditLength);
+    }
+
     let bestPath = [{ newPos: -1, components: [] }];
 
     // Seed editLength = 0, i.e. the content starts with the same values
@@ -87,12 +91,11 @@ Diff.prototype = {
 
     // Performs the length of edit iteration. Is a bit fugly as this has to support the
     // sync and async mode which is never fun. Loops over execEditLength until a value
-    // is produced.
+    // is produced, or until the edit length exceeds options.maxEditLength (if given),
+    // in which case it will return undefined.
     if (callback) {
       (function exec() {
         setTimeout(function() {
-          // This should not happen, but we want to be safe.
-          /* istanbul ignore next */
           if (editLength > maxEditLength) {
             return callback();
           }

src/patch/create.js

@@ -9,6 +9,10 @@ export function structuredPatch(oldFileName, newFileName, oldStr, newStr, oldHea
   }
 
   const diff = diffLines(oldStr, newStr, options);
+  if(!diff) {
+    return;
+  }
+
   diff.push({value: '', lines: []}); // Append an empty value to make cleanup easier
 
   function contextLines(lines) {

test/diff/line.js

@@ -46,6 +46,24 @@ describe('diff/line', function() {
         '');
       expect(convertChangesToXML(diffResult)).to.equal('<del>line\n\nold value \n\nline</del>');
     });
+
+    describe('given options.maxEditLength', function() {
+      it('terminates early', function() {
+        const diffResult = diffLines(
+          'line\nold value\nline',
+          'line\nnew value\nline', { maxEditLength: 1 });
+        expect(diffResult).to.be.undefined;
+      });
+      it('terminates early - async', function(done) {
+        function callback(diffResult) {
+          expect(diffResult).to.be.undefined;
+          done();
+        }
+        diffLines(
+          'line\nold value\nline',
+          'line\nnew value\nline', { callback, maxEditLength: 1 });
+      });
+    });
   });
 
   // Trimmed Line Diff

test/patch/create.js

@@ -721,6 +721,19 @@ describe('patch/create', function() {
         }]
       });
     });
+
+    describe('given options.maxEditLength', function() {
+      const options = { maxEditLength: 1 };
+
+      it('terminates early', function() {
+        const res = structuredPatch(
+          'oldfile', 'newfile',
+          'line2\nline3\nline4\n', 'line2\nline3\nline5',
+          'header1', 'header2', options
+        );
+        expect(res).to.be.undefined;
+      });
+    });
   });
 
   describe('#formatPatch', function() {