1 files changed, 181 insertions, 0 deletions
diff --git a/std/examples/xeval.ts b/std/examples/xeval.ts
new file mode 100644
index 000000000..75c864745
--- /dev/null
+++ b/std/examples/xeval.ts
@@ -0,0 +1,181 @@
+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;
+
+/* eslint-disable max-len */
+const HELP_MSG = `xeval
+
+Eval a script on lines from stdin.
+Read from standard input and eval code on each whitespace-delimited
+string chunks.
+
+Print all the usernames in /etc/passwd:
+  cat /etc/passwd | deno -A https://deno.land/std/examples/xeval.ts -- "a = $.split(':'); if (a) console.log(a[0])"
+
+A complicated way to print the current git branch:
+  git branch | deno -A https://deno.land/std/examples/xeval.ts -- -I 'line' "if (line.startsWith('*')) console.log(line.slice(2))"
+
+Demonstrates breaking the input up by space delimiter instead of by lines:
+  cat LICENSE | deno -A https://deno.land/std/examples/xeval.ts -- -d " " "if ($ === 'MIT') console.log('MIT licensed')",
+
+USAGE:
+  deno -A https://deno.land/std/examples/xeval.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>`;
+/* eslint-enable max-len */
+
+export type XevalFunc = (v: string) => void;
+
+export interface XevalOptions {
+  delimiter?: string;
+}
+
+const DEFAULT_DELIMITER = "\n";
+
+// Generate longest proper prefix which is also suffix array.
+function createLPS(pat: Uint8Array): Uint8Array {
+  const lps = new Uint8Array(pat.length);
+  lps[0] = 0;
+  let prefixEnd = 0;
+  let i = 1;
+  while (i < lps.length) {
+    if (pat[i] == pat[prefixEnd]) {
+      prefixEnd++;
+      lps[i] = prefixEnd;
+      i++;
+    } else if (prefixEnd === 0) {
+      lps[i] = 0;
+      i++;
+    } else {
+      prefixEnd = pat[prefixEnd - 1];
+    }
+  }
+  return lps;
+}
+
+// Read from reader until EOF and emit string chunks separated
+// by the given delimiter.
+async function* chunks(
+  reader: Reader,
+  delim: string
+): AsyncIterableIterator<string> {
+  const encoder = new TextEncoder();
+  const decoder = new TextDecoder();
+  // Avoid unicode problems
+  const delimArr = encoder.encode(delim);
+  const delimLen = delimArr.length;
+  const delimLPS = createLPS(delimArr);
+
+  let inputBuffer = new Buffer();
+  const inspectArr = new Uint8Array(Math.max(1024, delimLen + 1));
+
+  // Modified KMP
+  let inspectIndex = 0;
+  let matchIndex = 0;
+  while (true) {
+    const result = await reader.read(inspectArr);
+    if (result === EOF) {
+      // Yield last chunk.
+      const lastChunk = inputBuffer.toString();
+      yield lastChunk;
+      return;
+    }
+    if ((result as number) < 0) {
+      // Discard all remaining and silently fail.
+      return;
+    }
+    const sliceRead = inspectArr.subarray(0, result as number);
+    await writeAll(inputBuffer, sliceRead);
+
+    let sliceToProcess = inputBuffer.bytes();
+    while (inspectIndex < sliceToProcess.length) {
+      if (sliceToProcess[inspectIndex] === delimArr[matchIndex]) {
+        inspectIndex++;
+        matchIndex++;
+        if (matchIndex === delimLen) {
+          // Full match
+          const matchEnd = inspectIndex - delimLen;
+          const readyBytes = sliceToProcess.subarray(0, matchEnd);
+          // Copy
+          const pendingBytes = sliceToProcess.slice(inspectIndex);
+          const readyChunk = decoder.decode(readyBytes);
+          yield readyChunk;
+          // Reset match, different from KMP.
+          sliceToProcess = pendingBytes;
+          inspectIndex = 0;
+          matchIndex = 0;
+        }
+      } else {
+        if (matchIndex === 0) {
+          inspectIndex++;
+        } else {
+          matchIndex = delimLPS[matchIndex - 1];
+        }
+      }
+    }
+    // Keep inspectIndex and matchIndex.
+    inputBuffer = new Buffer(sliceToProcess);
+  }
+}
+
+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"]
+    },
+    default: {
+      delim: DEFAULT_DELIMITER,
+      replvar: "$"
+    }
+  });
+  if (parsedArgs._.length != 1) {
+    console.error(HELP_MSG);
+    exit(1);
+  }
+  if (parsedArgs.help) {
+    return console.log(HELP_MSG);
+  }
+
+  const delimiter = parsedArgs.delim;
+  const replVar = parsedArgs.replvar;
+  const code = parsedArgs._[0];
+
+  // new AsyncFunction()'s error message for this particular case isn't great.
+  if (!replVar.match(/^[_$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();
+}