"""trimmed_mean — Arithmetic mean after trimming extreme percentiles."""

import math
import numpy as np


def trimmed_mean(values: list[float], trim: float = 0.05) -> float:
    """Return the trimmed arithmetic mean of values.

    Cuts the bottom `trim` and top `trim` percentiles before averaging.
    Returns math.nan for an empty list or when trimming removes all elements.

    Args:
        values: List of numeric values.
        trim: Fraction to cut from each tail (0 <= trim < 0.5).

    Returns:
        Trimmed mean as float, or math.nan if the list is empty.
    """
    if not values:
        return math.nan
    arr = np.array(values, dtype=float)
    lo = np.percentile(arr, trim * 100)
    hi = np.percentile(arr, (1 - trim) * 100)
    trimmed = arr[(arr >= lo) & (arr <= hi)]
    if len(trimmed) == 0:
        return math.nan
    return float(np.mean(trimmed))