From c6cceec6e934fd89d2b46461b351ccfd17a2db09 Mon Sep 17 00:00:00 2001
From: Marcel <marcel@familienarchiv>
Date: Mon, 25 May 2026 13:21:39 +0200
Subject: [PATCH] feat(normalizer): Easter computus

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
---
 tools/import-normalizer/dates.py            | 21 +++++++++++++++++++++
 tools/import-normalizer/tests/test_dates.py |  9 +++++++++
 2 files changed, 30 insertions(+)
 create mode 100644 tools/import-normalizer/dates.py
 create mode 100644 tools/import-normalizer/tests/test_dates.py

diff --git a/tools/import-normalizer/dates.py b/tools/import-normalizer/dates.py
new file mode 100644
index 00000000..4df42f9f
--- /dev/null
+++ b/tools/import-normalizer/dates.py
@@ -0,0 +1,21 @@
+"""Tolerant historical date parsing for the family archive."""
+import datetime
+
+
+def easter(year: int) -> datetime.date:
+    """Easter Sunday (Gregorian) via the Anonymous Gregorian / Butcher algorithm."""
+    a = year % 19
+    b = year // 100
+    c = year % 100
+    d = b // 4
+    e = b % 4
+    f = (b + 8) // 25
+    g = (b - f + 1) // 3
+    h = (19 * a + b - d - g + 15) % 30
+    i = c // 4
+    k = c % 4
+    l = (32 + 2 * e + 2 * i - h - k) % 7
+    m = (a + 11 * h + 22 * l) // 451
+    month = (h + l - 7 * m + 114) // 31
+    day = ((h + l - 7 * m + 114) % 31) + 1
+    return datetime.date(year, month, day)
diff --git a/tools/import-normalizer/tests/test_dates.py b/tools/import-normalizer/tests/test_dates.py
new file mode 100644
index 00000000..d46df4d0
--- /dev/null
+++ b/tools/import-normalizer/tests/test_dates.py
@@ -0,0 +1,9 @@
+import datetime
+import dates
+
+def test_easter_known_years():
+    # Anonymous Gregorian algorithm — verified against published tables
+    assert dates.easter(2024) == datetime.date(2024, 3, 31)
+    assert dates.easter(2000) == datetime.date(2000, 4, 23)
+    assert dates.easter(1922) == datetime.date(1922, 4, 16)
+    assert dates.easter(1888) == datetime.date(1888, 4, 1)