From b7ee636c6fc96566b55d40bb9186edb513004fbe Mon Sep 17 00:00:00 2001
From: Huanyi Xie <74809554+xiehuanyi@users.noreply.github.com>
Date: Tue, 13 Oct 2026 11:34:34 +0300
Subject: [PATCH] Validate Finnish identity code birth dates

---
 src/validators/i18n/fi.py |  6 +++++-
 tests/i18n/test_fi.py     | 23 +++++++++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/src/validators/i18n/fi.py b/src/validators/i18n/fi.py
index 12fb281b..0ccda256 100644
--- a/src/validators/i18n/fi.py
+++ b/src/validators/i18n/fi.py
@@ -1,6 +1,7 @@
 """Finland."""
 
 # standard
+from datetime import date
 from functools import lru_cache
 import re
 
@@ -22,7 +23,7 @@ def _ssn_pattern(ssn_check_marks: str):
         (?P<date>(0[1-9]|[1-2]\d|3[01])
         (0[1-9]|1[012])
         (\d{{2}}))
-        [ABCDEFYXWVU+-]
+        (?P<century>[ABCDEFYXWVU+-])
         (?P<serial>(\d{{3}}))
         (?P<checksum>[{check_marks}])$""".format(check_marks=ssn_check_marks),
         re.VERBOSE,
@@ -95,6 +96,9 @@ def fi_ssn(value: str, /, *, allow_temporal_ssn: bool = True):
     if not (result := re.fullmatch(_ssn_pattern(ssn_check_marks), value)):
         return False
     gd = result.groupdict()
+    century = 1800 if gd["century"] == "+" else 2000 if gd["century"] in "ABCDEF" else 1900
+    birth_date = gd["date"]
+    date(century + int(birth_date[4:]), int(birth_date[2:4]), int(birth_date[:2]))
     checksum = int(gd["date"] + gd["serial"])
     return (
         int(gd["serial"]) >= 2
diff --git a/tests/i18n/test_fi.py b/tests/i18n/test_fi.py
index 64f479cf..08daef4f 100644
--- a/tests/i18n/test_fi.py
+++ b/tests/i18n/test_fi.py
@@ -79,3 +79,26 @@ def test_returns_failed_validation_on_invalid_ssn(value: str):
 def test_returns_failed_validation_on_temporal_ssn_when_not_allowed():
     """Test returns failed validation on temporal-ssn when not allowed."""
     assert isinstance(fi_ssn("010190-900P", allow_temporal_ssn=False), ValidationError)
+
+
+@pytest.mark.parametrize(
+    ("birth_date", "century", "valid"),
+    [
+        ("310299", "-", False),
+        ("310499", "-", False),
+        ("290201", "A", False),
+        ("290200", "+", False),
+        ("290200", "-", False),
+        ("290200", "Y", False),
+        ("290200", "A", True),
+        ("290200", "F", True),
+        ("290204", "-", True),
+        ("290204", "U", True),
+    ],
+)
+def test_ssn_validates_birth_date_in_its_century(birth_date: str, century: str, valid: bool):
+    """Reject impossible birth dates even when the control character is correct."""
+    serial = "010"
+    checksum = "0123456789ABCDEFHJKLMNPRSTUVWXY"[int(birth_date + serial) % 31]
+    value = birth_date + century + serial + checksum
+    assert bool(fi_ssn(value)) is valid