@staticmethod
def _normalize(diffs):
    """Normalizes difficulty scores to 0.0-1.0 where 1.0 is hardest."""
    low = min(diffs.values())
    delta = max(diffs.values()) - low
    out = {}
    for text_id, score in diffs.items():
        out[text_id] = 1.0 - float(score - low) / delta
    return out
