Add a hash to the stamp

The hash doesn't need to be cryptographic, so we'll use a simple
non-cryptographic hash that we can be assured is always available.

Signed-off-by: David A. Wheeler <[email protected]>

Author: david-a-wheeler

docs/labs/checker.js

@@ -227,17 +227,48 @@ function findIndexes(form) {
     }
 }
 
+/** Compute cyrb53 non-cryptographic hash */
+// cyrb53 (c) 2018 bryc (github.com/bryc). License: Public domain. Attribution appreciated.
+// A fast and simple 64-bit (or 53-bit) string hash function with decent collision resistance.
+// Largely inspired by MurmurHash2/3, but with a focus on speed/simplicity.
+// See https://stackoverflow.com/questions/7616461/generate-a-hash-from-string-in-javascript/52171480#52171480
+// https://github.com/bryc/code/blob/master/jshash/experimental/cyrb53.js
+// https://gist.github.com/jlevy/c246006675becc446360a798e2b2d781
+const cyrb64 = (str, seed = 0) => {
+  let h1 = 0xdeadbeef ^ seed, h2 = 0x41c6ce57 ^ seed;
+  for(let i = 0, ch; i < str.length; i++) {
+    ch = str.charCodeAt(i);
+    h1 = Math.imul(h1 ^ ch, 2654435761);
+    h2 = Math.imul(h2 ^ ch, 1597334677);
+  }
+  h1  = Math.imul(h1 ^ (h1 >>> 16), 2246822507);
+  h1 ^= Math.imul(h2 ^ (h2 >>> 13), 3266489909);
+  h2  = Math.imul(h2 ^ (h2 >>> 16), 2246822507);
+  h2 ^= Math.imul(h1 ^ (h1 >>> 13), 3266489909);
+  // For a single 53-bit numeric return value we could return
+  // 4294967296 * (2097151 & h2) + (h1 >>> 0);
+  // but we instead return the full 64-bit value:
+  return [h2>>>0, h1>>>0];
+};
+
+// An improved, *insecure* 64-bit hash that's short, fast, and has no dependencies.
+// Output is always 14 characters.
+// https://gist.github.com/jlevy/c246006675becc446360a798e2b2d781
+const cyrb64Hash =  (str, seed = 0) => {
+  const [h2, h1] = cyrb64(str, seed);
+  return h2.toString(36).padStart(7, '0') + h1.toString(36).padStart(7, '0');
+}
+
+/** Create a stamp to indicate completion. */
 function makeStamp() {
     let timeStamp = (new Date()).toISOString();
     let uuid = crypto.randomUUID();
     let resultBeginning = `${timeStamp} ${uuid}`;
-    return resultBeginning;
-    // TODO: Add hash. JavaScript has such a function
-    // in the browser, but it's only available in secure contexts and
-    // returns a promise, so it's not clear we should use it.
-    // let digestHex = digestMessage(resultBeginning);
-    // let digestEnding = digestHex.slice(-8);
-    // return `${resultBeginning} ${digestEnding}`;
+    // Browsers have a SHA-256 cryptographic hash available, but *only*
+    // when they're in a "secure state". We don't need the hash to be
+    // cryptographic, since the data is clearly in view. Use a simple one.
+    let hash = cyrb64Hash(resultBeginning);
+    return `${resultBeginning} ${hash}`;
 }
 
 /**