def common_prefix(text1, text2):
    """
    Determine the common prefix of two strings.

    Args:
      text1: First string.
      text2: Second string.

    Returns:
      The number of characters common to the start of each string.
    """
    if not text1 or not text2 or text1[0] != text2[0]:
        return 0
    pointermin = 0
    len_text1 = len(text1)
    len_text2 = len(text2)
    pointermax = min(len_text1, len_text2)
    pointermid = pointermax
    pointerstart = 0
    while pointermin < pointermid:
        if text1[pointerstart:pointermid] == text2[pointerstart:pointermid]:
            pointermin = pointermid
            pointerstart = pointermin
        else:
            pointermax = pointermid
        pointermid = (pointermax - pointermin) // 2 + pointermin
    return pointermid
