A Python function that, given a lookup value, will find the index of the closest value in an equally-spaced list of numbers in constant time

closest_index_in_range.py

def closest_index_in_range(lower, upper, step, value):
    '''
    Find the index of the closest value to `value` in the range
    [lower, lower + step, ..., upper - step, upper] in constant time. `upper`
    must be greater than `lower`. If `value` is outside the range, return the
    corresponding boundary index (0 or the last index). When two values are
    equally close, the index of the smaller is returned.
    
    For example, `closest_index_in_range(-5, 5, 0.5, -4.8)` will return 0, since
    -4.8 is closest to -5.
    '''
    if value >= upper:
        return int((upper - lower)/step)
    elif value < lower:
        return 0

    value = value - lower
    upper = upper - lower
    lower = 0

    index = int(value//step + 1)

    if step*index - value >= value - step*(index - 1):
        index -= 1

    return index