import re

_MESSAGE_ID_PATTERN = re.compile(r"<[^>]+>")


def normalize_message_id(value: str | None) -> str | None:
    if value is None:
        return None
    cleaned = value.strip()
    if not cleaned:
        return None
    if not cleaned.startswith("<"):
        cleaned = f"<{cleaned.strip('<>')}>"
    return cleaned.lower()


def parse_message_id_list(header_value: str | None) -> list[str]:
    if not header_value:
        return []
    folded = re.sub(r"\s+", " ", header_value.strip())
    seen: set[str] = set()
    ordered: list[str] = []
    for match in _MESSAGE_ID_PATTERN.findall(folded):
        normalized = normalize_message_id(match)
        if normalized and normalized not in seen:
            seen.add(normalized)
            ordered.append(normalized)
    if not ordered:
        single = normalize_message_id(folded)
        if single:
            ordered.append(single)
    return ordered


def first_message_id(header_value: str | None) -> str | None:
    values = parse_message_id_list(header_value)
    return values[0] if values else None


def build_references_header(references: list[str], parent_message_id: str) -> str:
    chain: list[str] = []
    seen: set[str] = set()
    for value in references:
        normalized = normalize_message_id(value)
        if normalized and normalized not in seen:
            seen.add(normalized)
            chain.append(normalized)
    parent = normalize_message_id(parent_message_id)
    if parent and parent not in seen:
        chain.append(parent)
    return " ".join(chain)