Add caching for printing (#2259)

* Add caching for printing

For printing the last argument expansion, we need to print the same node a bit differently with a flag. It's not easy to re-print just the node that is different so we end up printing all the sub-tree twice. Since it is often nested, it means that we run into an exponential complexity.

I finally found a simple solution: we can use a Map to store the nodes and their printed values. It is really cheap to do so (I can't notice a time difference with or without on normal code) and allows us to stop going through the two sub-trees in the bad case.

Fixes #1250

* Update printer.js

src/printer.js

@@ -4657,26 +4657,41 @@ function isObjectType(n) {
 function printAstToDoc(ast, options, addAlignmentSize) {
   addAlignmentSize = addAlignmentSize || 0;
 
+  const cache = new Map();
+
   function printGenerically(path, args) {
     const node = path.getValue();
+
+    const shouldCache = node && typeof node === "object" && args === undefined;
+    if (shouldCache && cache.has(node)) {
+      return cache.get(node);
+    }
+
     const parent = path.getParentNode(0);
     // We let JSXElement print its comments itself because it adds () around
     // UnionTypeAnnotation has to align the child without the comments
+    let res;
     if (
       (node && node.type === "JSXElement") ||
       (parent &&
         (parent.type === "UnionTypeAnnotation" ||
           parent.type === "TSUnionType"))
     ) {
-      return genericPrint(path, options, printGenerically, args);
+      res = genericPrint(path, options, printGenerically, args);
+    } else {
+      res = comments.printComments(
+        path,
+        p => genericPrint(p, options, printGenerically, args),
+        options,
+        args && args.needsSemi
+      );
     }
 
-    return comments.printComments(
-      path,
-      p => genericPrint(p, options, printGenerically, args),
-      options,
-      args && args.needsSemi
-    );
+    if (shouldCache) {
+      cache.set(node, res);
+    }
+
+    return res;
   }
 
   let doc = printGenerically(new FastPath(ast));

tests/performance/__snapshots__/jsfmt.spec.js.snap

@@ -10,7 +10,17 @@ someObject.someFunction().then(function() {
             return someObject.someFunction().then(function() {
               return someObject.someFunction().then(function() {
                 return someObject.someFunction().then(function() {
-                  anotherFunction();
+                  return someObject.someFunction().then(function() {
+                    return someObject.someFunction().then(function() {
+                      return someObject.someFunction().then(function() {
+                        return someObject.someFunction().then(function() {
+                          return someObject.someFunction().then(function() {
+                            anotherFunction();
+                          });
+                        });
+                      });
+                    });
+                  });
                 });
               });
             });
@@ -30,7 +40,17 @@ someObject.someFunction().then(function() {
             return someObject.someFunction().then(function() {
               return someObject.someFunction().then(function() {
                 return someObject.someFunction().then(function() {
-                  anotherFunction();
+                  return someObject.someFunction().then(function() {
+                    return someObject.someFunction().then(function() {
+                      return someObject.someFunction().then(function() {
+                        return someObject.someFunction().then(function() {
+                          return someObject.someFunction().then(function() {
+                            anotherFunction();
+                          });
+                        });
+                      });
+                    });
+                  });
                 });
               });
             });

tests/performance/nested.js

@@ -7,7 +7,17 @@ someObject.someFunction().then(function() {
             return someObject.someFunction().then(function() {
               return someObject.someFunction().then(function() {
                 return someObject.someFunction().then(function() {
-                  anotherFunction();
+                  return someObject.someFunction().then(function() {
+                    return someObject.someFunction().then(function() {
+                      return someObject.someFunction().then(function() {
+                        return someObject.someFunction().then(function() {
+                          return someObject.someFunction().then(function() {
+                            anotherFunction();
+                          });
+                        });
+                      });
+                    });
+                  });
                 });
               });
             });