Add xeval

Co-authored-by: Nayeem Rahman <muhammed.9939@gmail.com>

xeval/mod.ts

@@ -0,0 +1,145 @@
+import { parse } from "../flags/mod.ts";
+const { Buffer, EOF, args, exit, stdin, writeAll } = Deno;
+type Reader = Deno.Reader;
+
+/* eslint-disable-next-line max-len */
+// See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AsyncFunction.
+const AsyncFunction = Object.getPrototypeOf(async function(): Promise<void> {})
+  .constructor;
+
+function showHelp(): void {
+  console.log(`Deno xeval
+
+USAGE:
+  deno -A https://deno.land/std/xeval/mod.ts [OPTIONS] <code>
+
+OPTIONS:
+  -d, --delim <delim>       Set delimiter, defaults to newline
+  -I, --replvar <replvar>   Set variable name to be used in eval, defaults to $
+
+ARGS:
+  <code>
+`);
+}
+
+export type XevalFunc = (v: string) => void;
+
+export interface XevalOptions {
+  delimiter?: string;
+}
+
+const DEFAULT_DELIMITER = "\n";
+
+// TODO(kevinkassimo): Move this utility somewhere public in 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) {
+    let result = await reader.read(inspectArr);
+    let rr = result === EOF ? 0 : result;
+    if (rr < 0) {
+      // Silently fail.
+      break;
+    }
+    const sliceRead = inspectArr.subarray(0, rr);
+    // 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 (result === EOF) {
+      // Flush the remainder unprocessed chunk.
+      const lastChunk = inputBuffer.toString();
+      yield lastChunk;
+      break;
+    }
+  }
+}
+
+export async function xeval(
+  reader: Reader,
+  xevalFunc: XevalFunc,
+  { delimiter = DEFAULT_DELIMITER }: XevalOptions = {}
+): Promise<void> {
+  for await (const chunk of chunks(reader, delimiter)) {
+    // Ignore empty chunks.
+    if (chunk.length > 0) {
+      await xevalFunc(chunk);
+    }
+  }
+}
+
+async function main(): Promise<void> {
+  const parsedArgs = parse(args.slice(1), {
+    boolean: ["help"],
+    string: ["delim", "replvar"],
+    alias: {
+      delim: ["d"],
+      replvar: ["I"],
+      help: ["h"]
+    }
+  });
+
+  if (parsedArgs.help || parsedArgs._.length != 1) {
+    return showHelp();
+  }
+
+  const delimiter = parsedArgs.delim || DEFAULT_DELIMITER;
+  const replVar = parsedArgs.replvar || "$";
+  const code = parsedArgs._[0];
+
+  // new AsyncFunction()'s error message for this particular case isn't great.
+  if (!replVar.match(/^[_$A-z][_$A-z0-9]*$/)) {
+    console.error(`Bad replvar identifier: "${replVar}"`);
+    exit(1);
+  }
+
+  const xEvalFunc = new AsyncFunction(replVar, code);
+
+  await xeval(stdin, xEvalFunc, { delimiter });
+}
+
+if (import.meta.main) {
+  main();
+}