from .math_round_float import round_float

POW10 = [1.0, 10.0, 100.0, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9, 1e10, 1e11, 1e12]


def format_decimal(value: float, decimals: int, trim_zeros: bool, group_separator: str) -> str:
    """A float as decimal text, never in exponent form and never "-0".

    str(), String() and Rust's Display disagree about floats (1e-07, 1e-7,
    0.0000001), so the number is rounded once, turned into a whole count of
    10^-decimals units, and the digits of that integer are laid out by hand.
    """
    rounded = round_float(value, decimals)
    scaled = abs(rounded) * POW10[decimals]
    if scaled >= 1e15:
        raise ValueError("value is too large to format at %d decimal places, received %r" % (decimals, value))
    units = int(round_float(scaled, 0))
    digits = str(units).rjust(decimals + 1, "0")
    whole = digits[: len(digits) - decimals]
    fraction = digits[len(digits) - decimals:]
    if trim_zeros:
        fraction = fraction.rstrip("0")
    grouped = ""
    for i, ch in enumerate(whole):
        if i > 0 and (len(whole) - i) % 3 == 0:
            grouped += group_separator
        grouped += ch
    body = grouped + "." + fraction if fraction else grouped
    return "-" + body if units > 0 and rounded < 0 else body