2 files changed, 104 insertions, 1 deletions

diff --git a/js/main.ts b/js/main.ts
index ad9e0e99d..cb27690b5 100644
--- a/js/main.ts
+++ b/js/main.ts
@@ -9,7 +9,9 @@ import { assert, log } from "./util";
 import * as os from "./os";
 import { args } from "./deno";
 import { replLoop } from "./repl";
+import { xevalMain, XevalFunc } from "./xeval";
 import { setVersions } from "./version";
+import { window } from "./window";
 import { setLocation } from "./location";
 
 // builtin modules
@@ -43,7 +45,9 @@ export default function denoMain(name?: string): void {
   log("args", args);
   Object.freeze(args);
 
-  if (!mainModule) {
+  if (window["_xevalWrapper"] !== undefined) {
+    xevalMain(window["_xevalWrapper"] as XevalFunc, startResMsg.xevalDelim());
+  } else if (!mainModule) {
     replLoop();
   }
 }
diff --git a/js/xeval.ts b/js/xeval.ts
new file mode 100644
index 000000000..f769a2ead
--- /dev/null
+++ b/js/xeval.ts
@@ -0,0 +1,99 @@
+import { Buffer } from "./buffer";
+import { stdin } from "./files";
+import { TextEncoder, TextDecoder } from "./text_encoding";
+import { Reader } from "./io";
+
+export type XevalFunc = (v: string) => void;
+
+async function writeAll(buffer: Buffer, arr: Uint8Array): Promise<void> {
+  let bytesWritten = 0;
+  while (bytesWritten < arr.length) {
+    try {
+      const nwritten = await buffer.write(arr.subarray(bytesWritten));
+      bytesWritten += nwritten;
+    } catch {
+      return;
+    }
+  }
+}
+
+// TODO(kevinkassimo): Move this utility to deno_std.
+// Import from there once doable.
+// Read from reader until EOF and emit string chunks separated
+// by the given delimiter.
+async function* chunks(
+  reader: Reader,
+  delim: string
+): AsyncIterableIterator<string> {
+  const inputBuffer = new Buffer();
+  const inspectArr = new Uint8Array(1024);
+  const encoder = new TextEncoder();
+  const decoder = new TextDecoder();
+  // Avoid unicode problems
+  const delimArr = encoder.encode(delim);
+
+  // Record how far we have gone with delimiter matching.
+  let nextMatchIndex = 0;
+  while (true) {
+    const rr = await reader.read(inspectArr);
+    if (rr.nread < 0) {
+      // Silently fail.
+      break;
+    }
+    const sliceRead = inspectArr.subarray(0, rr.nread);
+    // Remember how far we have scanned through inspectArr.
+    let nextSliceStartIndex = 0;
+    for (let i = 0; i < sliceRead.length; i++) {
+      if (sliceRead[i] == delimArr[nextMatchIndex]) {
+        // One byte matches with delimiter, move 1 step forward.
+        nextMatchIndex++;
+      } else {
+        // Match delimiter failed. Start from beginning.
+        nextMatchIndex = 0;
+      }
+      // A complete match is found.
+      if (nextMatchIndex === delimArr.length) {
+        nextMatchIndex = 0; // Reset delim match index.
+        const sliceToJoin = sliceRead.subarray(nextSliceStartIndex, i + 1);
+        // Record where to start next chunk when a subsequent match is found.
+        nextSliceStartIndex = i + 1;
+        // Write slice to buffer before processing, since potentially
+        // part of the delimiter is stored in the buffer.
+        await writeAll(inputBuffer, sliceToJoin);
+
+        let readyBytes = inputBuffer.bytes();
+        inputBuffer.reset();
+        // Remove delimiter from buffer bytes.
+        readyBytes = readyBytes.subarray(
+          0,
+          readyBytes.length - delimArr.length
+        );
+        let readyChunk = decoder.decode(readyBytes);
+        yield readyChunk;
+      }
+    }
+    // Write all unprocessed chunk to buffer for future inspection.
+    await writeAll(inputBuffer, sliceRead.subarray(nextSliceStartIndex));
+    if (rr.eof) {
+      // Flush the remainder unprocessed chunk.
+      const lastChunk = inputBuffer.toString();
+      yield lastChunk;
+      break;
+    }
+  }
+}
+
+export async function xevalMain(
+  xevalFunc: XevalFunc,
+  delim_: string | null
+): Promise<void> {
+  if (!delim_) {
+    delim_ = "\n";
+  }
+  for await (const chunk of chunks(stdin, delim_)) {
+    // Ignore empty chunks.
+    if (chunk.length > 0) {
+      xevalFunc(chunk);
+    }
+  }
+}