import ipaddress
import re


USERNAME_PATTERN = re.compile(r"^[A-Za-z0-9._-]+$")
SSH_USERNAME_PATTERN = re.compile(r"^[A-Za-z0-9._-]+$")
HOST_LABEL_PATTERN = re.compile(r"^[A-Za-z0-9-]{1,63}$")


def normalize_required_text(value: str, *, field_name: str) -> str:
    normalized = value.strip()
    if not normalized:
        raise ValueError(f"{field_name} must not be empty.")
    return normalized


def normalize_optional_text(value: str | None) -> str | None:
    if value is None:
        return None

    normalized = value.strip()
    return normalized or None


def validate_username(value: str) -> str:
    normalized = normalize_required_text(value, field_name="username")
    if not USERNAME_PATTERN.fullmatch(normalized):
        raise ValueError(
            "username may contain only letters, digits, dot, underscore, and dash."
        )
    return normalized


def validate_ssh_username(value: str | None) -> str | None:
    normalized = normalize_optional_text(value)
    if normalized is None:
        return None
    if not SSH_USERNAME_PATTERN.fullmatch(normalized):
        raise ValueError(
            "ssh_username may contain only letters, digits, dot, underscore, and dash."
        )
    return normalized


def validate_host(value: str) -> str:
    normalized = normalize_required_text(value, field_name="host")
    if any(fragment in normalized for fragment in ("://", "/", "@", " ")):
        raise ValueError("host must be a hostname or IP address without URL formatting.")

    try:
        ipaddress.ip_address(normalized)
        return normalized
    except ValueError:
        labels = normalized.split(".")
        if any(not label for label in labels):
            raise ValueError("host contains an empty hostname label.") from None
        if not all(HOST_LABEL_PATTERN.fullmatch(label) for label in labels):
            raise ValueError("host contains invalid hostname characters.") from None
        if any(label.startswith("-") or label.endswith("-") for label in labels):
            raise ValueError("host labels must not start or end with a dash.") from None
        return normalized


def validate_log_source(value: str | None) -> str | None:
    normalized = normalize_optional_text(value)
    if normalized is None:
        return None
    if not normalized.startswith("file:/"):
        raise ValueError("log_source must use the format 'file:/absolute/path'.")
    return normalized