class ConfigError(Exception):
    """Raised for a missing or wrong-typed config key, named by its full
    dotted path (e.g. "TempCtrl.Inner.Hold.kp") so the error points straight
    at the offending line in config.json instead of surfacing as a bare
    KeyError/TypeError from wherever the value first gets used."""


def require(config, path, type=None):
    node = config
    parts = path.split('.')
    for i, key in enumerate(parts):
        if not isinstance(node, dict) or key not in node:
            raise ConfigError("missing required config key: {}".format('.'.join(parts[:i + 1])))
        node = node[key]
    if type is not None and not isinstance(node, type):
        type_name = ' or '.join(t.__name__ for t in type) if isinstance(type, tuple) else type.__name__
        raise ConfigError("config key {} must be {}, got {!r}".format(path, type_name, node))
    return node


def validate_temp_ctrl(config):
    """Checked eagerly at startup so a missing/misnamed PID gain fails with
    a clear message here instead of a bare KeyError mid-__init__ or on the
    first process() tick (this bit us once already: config.json.sim's gain/
    Model nesting mismatch). Only the gains every pid_type actually reads
    unconditionally are required; anything with its own default elsewhere
    (Outer.y_hold_min, Inner.Hold.yi_max, the whole Thresholds section -
    see temp_controller_base.py/temp_controller_fsm.py's DEFAULT_THRESHOLDS)
    stays optional here too, on purpose."""
    require(config, 'TempCtrl', dict)
    require(config, 'TempCtrl.pid_type', str)
    for gain in ('kp', 'ki', 'kd', 'kt'):
        require(config, 'TempCtrl.Outer.{}'.format(gain), (int, float))
        for branch in ('Heat', 'Hold', 'Cool'):
            require(config, 'TempCtrl.Inner.{}.{}'.format(branch, gain), (int, float))