"""Rule-based field validators for document fraud detection.

Pure standard library. Every validator returns (ok: bool, reason: str).
Rules encode publicly documented validity constraints:
  - SSN structure per SSA randomization (area 001-899 excluding 666,
    group 01-99, serial 0001-9999; 000/666/9xx areas, 00 groups and
    0000 serials have never been issued)
  - ICAO Doc 9303 TD3 (passport) MRZ check digits, weights 7-3-1
"""
import datetime as _dt
import re

_SSN_RE = re.compile(r"^(\d{3})-(\d{2})-(\d{4})$")


def validate_ssn(value):
    if value is None:
        return False, "missing"
    m = _SSN_RE.match(value.strip())
    if not m:
        return False, "malformed"
    area, group, serial = (int(x) for x in m.groups())
    if area == 0 or area == 666 or 900 <= area <= 999:
        return False, "invalid_area"
    if group == 0:
        return False, "invalid_group"
    if serial == 0:
        return False, "invalid_serial"
    return True, "ok"


_DATE_FORMATS = ("%m/%d/%Y", "%Y-%m-%d", "%d.%m.%Y", "%Y%m%d")


def parse_date(value):
    for fmt in _DATE_FORMATS:
        try:
            return _dt.datetime.strptime(value.strip(), fmt).date()
        except (ValueError, AttributeError, TypeError):
            continue
    return None


def validate_date(value, must_be_past=False, reference=None):
    d = parse_date(value) if isinstance(value, str) else None
    if d is None:
        return False, "unparseable"
    ref = reference or _dt.date.today()
    if must_be_past and d > ref:
        return False, "future_date"
    return True, "ok"


_NAME_RE = re.compile(r"^[A-Z][A-Z' -]{0,63}$", re.IGNORECASE)


def validate_name(value):
    if not value or not value.strip():
        return False, "missing"
    if not _NAME_RE.match(value.strip()):
        return False, "bad_characters"
    return True, "ok"


_MRZ_ALPHABET = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"


def mrz_check_digit(field):
    total = 0
    for i, ch in enumerate(field):
        v = 0 if ch == "<" else _MRZ_ALPHABET.index(ch)
        total += v * (7, 3, 1)[i % 3]
    return str(total % 10)


def validate_mrz_line2(line):
    line = (line or "").strip()
    if len(line) != 44:
        return False, "bad_length"
    if not re.match(r"^[A-Z0-9<]{44}$", line):
        return False, "bad_characters"
    sex = line[20]
    if not (sex == "<" or sex in "MF"):
        return False, "bad_sex"
    for part, label in ((line[13:19], "dob"), (line[21:27], "exp")):
        if not re.match(r"^\d{6}$", part):
            return False, "bad_" + label + "_format"
        mm, dd = int(part[2:4]), int(part[4:6])
        if not (1 <= mm <= 12 and 1 <= dd <= 31):
            return False, "bad_" + label + "_value"
    composite = line[0:10] + line[13:20] + line[21:43]
    checks = (
        (9, line[0:9], "pno"),
        (19, line[13:19], "dob"),
        (27, line[21:27], "exp"),
        (42, line[28:42], "personal"),
        (43, composite, "composite"),
    )
    for pos, material, name in checks:
        if line[pos] != mrz_check_digit(material):
            return False, "check_digit_" + name
    return True, "ok"


def parse_date_any(value):
    return parse_date(value)


def validate_document(fields, reference=None):
    findings = []
    if "document_number" in fields:
        ok, why = validate_ssn(fields["document_number"])
        if not ok:
            findings.append("document_number:" + why)
    if "name" in fields:
        ok, why = validate_name(fields["name"])
        if not ok:
            findings.append("name:" + why)
    for key in ("date_of_birth", "issue_date", "expiry_date"):
        if key in fields:
            ok, why = validate_date(fields[key], reference=reference)
            if not ok:
                findings.append(key + ":" + why)
    ref = reference or _dt.date.today()
    dob = parse_date(fields.get("date_of_birth", ""))
    if dob and dob > ref:
        findings.append("dob_in_future")
    if dob and (ref - dob).days > 150 * 365:
        findings.append("implausible_age")
    issued = parse_date(fields.get("issue_date", ""))
    expiry = parse_date(fields.get("expiry_date", ""))
    if issued and expiry and expiry <= issued:
        findings.append("expiry_before_issue")
    if "mrz_line2" in fields:
        ok, why = validate_mrz_line2(fields["mrz_line2"])
        if not ok:
            findings.append("mrz:" + why)
    return findings