def METHOD_NAME(freq1, freq2, delta_p, delta_s):
    '''
    FIR lowpass filter length estimator.
    '''
    df = abs(freq2 - freq1)
    ddp = math.log10(delta_p)
    dds = math.log10(delta_s)
    a1 = 5.309e-3
    a2 = 7.114e-2
    a3 = -4.761e-1
    a4 = -2.66e-3
    a5 = -5.941e-1
    a6 = -4.278e-1
    b1 = 11.01217
    b2 = 0.5124401
    t1 = a1 * ddp * ddp
    t2 = a2 * ddp
    t3 = a4 * ddp * ddp
    t4 = a5 * ddp
    dinf = ((t1 + t2 + a3) * dds) + (t3 + t4 + a6)
    ff = b1 + b2 * (ddp - dds)
    n = dinf / df - ff * df + 1
    return n