Merge pull request #39 from e-neighborhood-watch/master

Replace String.join

notes/transformations.md

@@ -459,3 +459,13 @@ It's pretty common to put things in a tuple (or threeple) to start a case statem
 ```
 
 We could skip allocating the tuple though.
+
+# String Joining
+
+For joining (and concating) strings, Elm uses Javascript's Array join method after converting the list into a Javascript array.
+We can replace this implementation with a faster one that traverses the list and builds the string through concatenation instead.
+
+## Results Summary
+
+* Not included in elm-optimize-level-2 tool
+* The implementation used is similar to the improved String.join implementation [here](https://gitlab.com/e-neighborhood-watch/elm-string-benchmarks/#stringjoin) which sees some serious improvements over Elm's normal String.join.

src/transform.ts

@@ -23,6 +23,7 @@ import { createPassUnwrappedFunctionsTransformer } from './transforms/passUnwrap
 import { replaceVDomNode } from './transforms/adjustVirtualDom';
 import { inlineNumberToString } from './transforms/inlineNumberToString';
 import { replaceListFunctions } from './transforms/replaceListFunctions';
+import { replaceStringFunctions } from './transforms/replaceStringFunctions';
 import { reportFunctionStatusInBenchmarks, v8Debug } from './transforms/analyze';
 
 export type Options = {
@@ -80,6 +81,7 @@ export const transform = async (
   let inlineCtx: InlineContext | undefined;
   const transformations: any[] = removeDisabled([
     [transforms.replaceListFunctions, replaceListFunctions],
+    [transforms.replaceStringFunctions, replaceStringFunctions],
     
     [transforms.v8Analysis, v8Debug],
     [transforms.variantShapes, normalizeVariantShapes],

src/transforms/replaceStringFunctions.ts

@@ -0,0 +1,40 @@
+import ts, { isIdentifier } from 'typescript';
+import { ast } from './utils/create';
+
+const $elm$core$String$join = `
+var $elm$core$String$join = F2(function (sep, strs) {
+  if (!strs.b) {
+    return "";
+  }
+  var acc = strs.a;
+  strs = strs.b;
+
+  for (; strs.b; strs = strs.b) {
+    acc = acc + sep + strs.a;
+  }
+
+  return acc;
+};
+`;
+
+
+const replacements = {
+  $elm$core$String$join,
+};
+
+export const replaceStringFunctions: ts.TransformerFactory<ts.SourceFile> = (
+  context
+) => (sourceFile) => {
+  const visitor = (node: ts.Node): ts.VisitResult<ts.Node> => {
+    if (ts.isVariableStatement(node)) {
+      const name = node.declarationList.declarations[0]?.name;
+      if (isIdentifier(name) && name.text in replacements) {
+        const key = name.text as keyof typeof replacements;
+        return ast(replacements[key]);
+      }
+    }
+    return ts.visitEachChild(node, visitor, context);
+  };
+
+  return ts.visitNode(sourceFile, visitor);
+};

src/types.ts

@@ -35,6 +35,7 @@ export type Transforms = {
   objectUpdate: ObjectUpdate | false;
   unusedValues: boolean;
   replaceListFunctions: boolean;
+  replaceStringFunctions: boolean;
   v8Analysis: boolean;
 };
 
@@ -77,5 +78,6 @@ export const toolDefaults: Transforms = {
   objectUpdate: false,
   unusedValues: false,
   replaceListFunctions: false,
+  replaceStringFunctions: false,
   v8Analysis: false
 };