feat: add collection helpers and email validation

node/collections.js

@@ -0,0 +1,16 @@
+export function groupBy(items, keyFn) {
+  return items.reduce((acc, item) => {
+    const key = keyFn(item);
+    (acc[key] ||= []).push(item);
+    return acc;
+  }, {});
+}
+
+export function partition(items, predicate) {
+  const pass = [];
+  const fail = [];
+  for (const item of items) {
+    (predicate(item) ? pass : fail).push(item);
+  }
+  return [pass, fail];
+}

python/emails.py

@@ -0,0 +1,14 @@
+import re
+
+EMAIL_RE = re.compile(r"^[^@\s]+@[^@\s]+\.[^@\s]+$")
+
+
+def is_valid_email(value):
+    return bool(EMAIL_RE.match(value))
+
+
+def normalize_email(value):
+    if not is_valid_email(value):
+        raise ValueError("invalid email address")
+    local, domain = value.split("@", 1)
+    return f"{local}@{domain.lower()}"