From b89dc656c2242f2686ee3d03fca6d164cc92a5f3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Hu=C3=A1ng=20J=C3=B9nli=C3=A0ng?= <i@jhuang.me>
Date: Thu, 29 Aug 2019 15:45:27 -0400
Subject: [PATCH] refactor: remove lookahead usage from babylon core

---
 packages/babel-parser/src/parser/statement.js | 14 +++++-----
 packages/babel-parser/src/parser/util.js      | 26 ++++++++++++++++---
 2 files changed, 28 insertions(+), 12 deletions(-)

diff --git a/packages/babel-parser/src/parser/statement.js b/packages/babel-parser/src/parser/statement.js
index a761e6dfc77b..2facc7108622 100644
--- a/packages/babel-parser/src/parser/statement.js
+++ b/packages/babel-parser/src/parser/statement.js
@@ -1738,10 +1738,10 @@ export default class StatementParser extends ExpressionParser {
   maybeParseExportDeclaration(node: N.Node): boolean {
     if (this.shouldParseExportDeclaration()) {
       if (this.isContextual("async")) {
-        const next = this.lookahead();
+        const next = this.nextTokenStart();
 
         // export async;
-        if (next.type !== tt._function) {
+        if (!this.isUnparsedContextual(next, "function")) {
           this.unexpected(next.start, `Unexpected token, expected "function"`);
         }
       }
@@ -1760,9 +1760,7 @@ export default class StatementParser extends ExpressionParser {
     const next = this.nextTokenStart();
     return (
       !lineBreak.test(this.input.slice(this.state.pos, next)) &&
-      this.input.slice(next, next + 8) === "function" &&
-      (next + 8 === this.length ||
-        !isIdentifierChar(this.input.charCodeAt(next + 8)))
+      this.isUnparsedContextual(next, "function")
     );
   }
 
@@ -1824,10 +1822,10 @@ export default class StatementParser extends ExpressionParser {
       return false;
     }
 
-    const lookahead = this.lookahead();
+    const next = this.nextTokenStart();
     return (
-      lookahead.type === tt.comma ||
-      (lookahead.type === tt.name && lookahead.value === "from")
+      this.input.charCodeAt(next) === charCodes.comma ||
+      this.isUnparsedContextual(next, "from")
     );
   }
 
diff --git a/packages/babel-parser/src/parser/util.js b/packages/babel-parser/src/parser/util.js
index bd6139a2480a..678bb6b18379 100644
--- a/packages/babel-parser/src/parser/util.js
+++ b/packages/babel-parser/src/parser/util.js
@@ -4,6 +4,8 @@ import { types as tt, type TokenType } from "../tokenizer/types";
 import Tokenizer from "../tokenizer";
 import type { Node } from "../types";
 import { lineBreak, skipWhiteSpace } from "../util/whitespace";
+import { isIdentifierChar } from "../util/identifier";
+import * as charCodes from "charcodes";
 
 const literal = /^('|")((?:\\?.)*?)\1/;
 
@@ -26,8 +28,15 @@ export default class UtilParser extends Tokenizer {
   }
 
   isLookaheadRelational(op: "<" | ">"): boolean {
-    const l = this.lookahead();
-    return l.type === tt.relational && l.value === op;
+    const next = this.nextTokenStart();
+    if (this.input.charAt(next) === op) {
+      if (next + 1 === this.input.length) {
+        return true;
+      }
+      const afterNext = this.input.charCodeAt(next + 1);
+      return afterNext !== op.charCodeAt(0) && afterNext !== charCodes.equalsTo;
+    }
+    return false;
   }
 
   // TODO
@@ -60,9 +69,18 @@ export default class UtilParser extends Tokenizer {
     );
   }
 
+  isUnparsedContextual(nameStart: number, name: string): boolean {
+    const nameEnd = nameStart + name.length;
+    return (
+      this.input.slice(nameStart, nameEnd) === name &&
+      (nameEnd === this.input.length ||
+        !isIdentifierChar(this.input.charCodeAt(nameEnd)))
+    );
+  }
+
   isLookaheadContextual(name: string): boolean {
-    const l = this.lookahead();
-    return l.type === tt.name && l.value === name;
+    const next = this.nextTokenStart();
+    return this.isUnparsedContextual(next, name);
   }
 
   // Consumes contextual keyword if possible.