from typing import Iterable, Union

import numpy as np


def convert(
    from_value: Union[float, Iterable],
    from_unit: str,
    gamma: float = 42.576e6,
    to_unit: str = str(),
) -> Union[float, Iterable]:
    """
    Converts gradient amplitude or slew rate from unit `from_unit` to unit `to_unit` with gyromagnetic ratio `gamma`.

    Parameters
    ----------
    from_value : float
        Gradient amplitude or slew rate to convert from.
    from_unit : str
        Unit of gradient amplitude or slew rate to convert from.
    to_unit : str, default=''
        Unit of gradient amplitude or slew rate to convert to.
    gamma : float, default=42.576e6
        Gyromagnetic ratio. Default is 42.576e6, for Hydrogen.

    Returns
    -------
    out : float
        Converted gradient amplitude or slew rate.

    Raises
    ------
    ValueError
        If an invalid `from_unit` is passed. Must be one of 'Hz/m', 'mT/m', or 'rad/ms/mm'.
        If an invalid `to_unit` is passed. Must be one of 'Hz/m/s', 'mT/m/ms', 'T/m/s', 'rad/ms/mm/ms'.
    """
    valid_grad_units = ["Hz/m", "mT/m", "rad/ms/mm"]
    valid_slew_units = ["Hz/m/s", "mT/m/ms", "T/m/s", "rad/ms/mm/ms"]
    valid_units = valid_grad_units + valid_slew_units

    if from_unit not in valid_units:
        raise ValueError(
            "Invalid from_unit. Must be one of 'Hz/m', 'mT/m', or 'rad/ms/mm' for gradients;"
            "or must be one of 'Hz/m/s', 'mT/m/ms', 'T/m/s', 'rad/ms/mm/ms' for slew rate."
        )

    if to_unit != "" and to_unit not in valid_units:
        raise ValueError(
            "Invalid to_unit. Must be one of 'Hz/m/s', 'mT/m/ms', 'T/m/s', 'rad/ms/mm/ms' for gradients;"
            "or must be one of 'Hz/m/s', 'mT/m/ms', 'T/m/s', 'rad/ms/mm/ms' for slew rate.."
        )

    if to_unit == "":
        if from_unit in valid_grad_units:
            to_unit = valid_grad_units[0]
        elif from_unit in valid_slew_units:
            to_unit = valid_slew_units[0]

    # Convert to standard units
    # Grad units
    if from_unit == "Hz/m":
        standard = from_value
    elif from_unit == "mT/m":
        standard = from_value * 1e-3 * gamma
    elif from_unit == "rad/ms/mm":
        standard = from_value * 1e6 / (2 * np.pi)
    # Slew units
    elif from_unit == "Hz/m/s":
        standard = from_value
    elif from_unit == "mT/m/ms" or from_unit == "T/m/s":
        standard = from_value * gamma
    elif from_unit == "rad/ms/mm/ms":
        standard = from_value * 1e9 / (2 * np.pi)

    # Convert from standard units
    # Grad units
    if to_unit == "Hz/m":
        out = standard
    elif to_unit == "mT/m":
        out = 1e3 * standard / gamma
    elif to_unit == "rad/ms/mm":
        out = standard * 2 * np.pi * 1e-6
    # Slew units
    elif to_unit == "Hz/m/s":
        out = standard
    elif to_unit == "mT/m/ms" or to_unit == "T/m/s":
        out = standard / gamma
    elif to_unit == "rad/ms/mm/ms":
        out = standard * 2 * np.pi * 1e-9

    return out