add ulid timestamp extraction script

static/js/ulid.js

@@ -0,0 +1,32 @@
+let canonicalBase32 = "0123456789ABCDEFGHJKMNPQRSTVWXYZ";
+let canonicalBase32Rev = {};
+for (let i = 0; i < canonicalBase32.length; ++i) {
+    canonicalBase32Rev[canonicalBase32.charAt(i)] = i;
+}
+
+// Works on BigInt to support numbers larger than (1 << 31).
+function decodeCanonicalBase32(string) {
+    let result = 0n;
+    let cursor = 0b1n;
+    for (let i = string.length - 1; i >= 0; --i) {
+        result += cursor * BigInt(canonicalBase32Rev[string[i]]);
+        cursor <<= 5n;
+    }
+    return result;
+}
+
+export function getTimestamp(ulid) {
+    return new Date(Number(decodeCanonicalBase32(ulid.substring(0, 10))));
+}
+
+export function isCanonicalUlid(id) {
+    if (id.length != 26) {
+        return false;
+    }
+    for (let i = 0; i < id.length; ++i) {
+        if (!canonicalBase32.includes(id.charAt(i))) {
+            return false;
+        }
+    }
+    return true;
+}

template/tree.hbs

@@ -24,6 +24,7 @@
 
     <script>const TREEHOUSE_SITE = `{{ config.site }}`;</script>
     <script type="module" src="{{ config.site }}/navmap.js"></script>
+    <script type="module" src="{{ config.site }}/static/js/ulid.js"></script>
     <script type="module" src="{{ config.site }}/static/js/usability.js"></script>
     <script type="module" src="{{ config.site }}/static/js/tree.js"></script>
     <script type="module" src="{{ config.site }}/static/js/emoji.js"></script>