import numpy as np
from pyti import catch_errors
from pyti.function_helper import fill_for_noncomputable_vals


def relative_strength_index(data, period):
    """
    Relative Strength Index.

    Formula:
    RSI = 100 - (100 / 1 + (prevGain/prevLoss))
    """
    catch_errors.check_for_period_error(data, period)

    period = int(period)
    changes = map(
        lambda data_tup:
        data_tup[1] - data_tup[0],
        zip(data[::1], data[1::1])
        )

    filtered_gain = map(lambda val: val < 0, changes)
    gains = map(
        lambda idx:
        0 if filtered_gain[idx] is True else changes[idx],
        range(0, len(filtered_gain))
        )

    filtered_loss = map(lambda val: val > 0, changes)
    losses = map(
        lambda idx:
        0 if filtered_loss[idx] is True else abs(changes[idx]),
        range(0, len(filtered_loss))
        )

    avg_gain = np.mean(gains[:period])
    avg_loss = np.mean(losses[:period])

    rsi = []
    if avg_loss == 0:
        rsi.append(100)
    else:
        rs = avg_gain / avg_loss
        rsi.append(100 - (100 / (1 + rs)))

    for idx in range(1, len(data) - period):
        avg_gain = ((avg_gain * (period - 1) +
                    gains[idx + (period - 1)]) / period)
        avg_loss = ((avg_loss * (period - 1) +
                    losses[idx + (period - 1)]) / period)

        if avg_loss == 0:
            rsi.append(100)
        else:
            rs = avg_gain / avg_loss
            rsi.append(100 - (100 / (1 + rs)))

    rsi = fill_for_noncomputable_vals(data, rsi)

    return rsi
