निश्चित पंक्तियों की दृश्य सीमाएँ

from math import isfinite, ceil

def row_window(count, row_height, offset, viewport, overscan=1):
    if type(count) is not int or count < 0 or type(overscan) is not int or overscan < 0:
        raise ValueError("rows.count")
    if not all(type(x) in (int, float) and isfinite(x) for x in (row_height, offset, viewport)) or row_height <= 0 or viewport < 0:
        raise ValueError("rows.geometry")
    start_units = max(0, offset) / row_height
    first = count if start_units >= count else int(start_units)
    if first == count or viewport == 0:
        return first, first
    span = viewport / row_height
    end = count if span >= count - start_units else min(count, ceil(start_units + span) + overscan)
    return max(0, first - overscan), end