"""Lossless character boundaries constrained by an actual token counter."""
from collections.abc import Callable


def split_to_fit(text: str, fits: Callable[[str], bool]) -> list[str]:
    """Keep every character; use natural boundaries when they fit near the end."""
    pieces = []
    while text:
        if fits(text):
            pieces.append(text)
            break
        low, high = 0, len(text)
        while low < high:
            middle = (low + high + 1) // 2
            if fits(text[:middle]):
                low = middle
            else:
                high = middle - 1
        if low == 0:
            raise ValueError('Token budget cannot contain one Unicode character.')
        # Avoid breaking a word unless its length makes that unavoidable.
        boundary = max(text.rfind('\n', 0, low), text.rfind(' ', 0, low)) + 1
        cut = boundary if boundary >= low * 0.7 else low
        while cut and not fits(text[:cut]):
            cut -= 1
        if not cut:
            raise ValueError('No valid token boundary.')
        pieces.append(text[:cut])
        text = text[cut:]
    return pieces