# How Lighthouses Work 6, Exercise 1: planning work around the tide
TIDE_PERIOD_H = 12.42   # assumed twice-daily tide cycle
EXPOSED_H = 4.0         # the Bell Rock article: covered ~20 h a day, so ~4 h exposed
STONES = 2500           # the Bell Rock article: about 2500 granite stones

def season_hours(days, usable_share):
    return days * EXPOSED_H * usable_share

for share in (0.25, 0.5, 0.75):
    h = season_hours(100, share)
    print("100-day season at", share, "usable:", h, "h")
    print("   seasons for 2500 stones at 3 stones/h:", round(STONES / 3 / h, 2))

slip_h = 2 * TIDE_PERIOD_H - 24
print("slip per day:", round(slip_h * 60, 1), "min")
start = 6.0   # a low-water window opens at 06:00 on day 0
for day in range(0, 8):
    t = (start + day * slip_h) % 24
    print("day", day, ": window opens at", "%02d:%02d" % (int(t), round((t % 1) * 60)))
print("days until the same clock time returns:", round(24 / slip_h, 1))

# Output:
# 100-day season at 0.25 usable: 100.0 h
#    seasons for 2500 stones at 3 stones/h: 8.33
# 100-day season at 0.5 usable: 200.0 h
#    seasons for 2500 stones at 3 stones/h: 4.17
# 100-day season at 0.75 usable: 300.0 h
#    seasons for 2500 stones at 3 stones/h: 2.78
# slip per day: 50.4 min
# day 0 : window opens at 06:00
# day 1 : window opens at 06:50
# day 2 : window opens at 07:41
# day 3 : window opens at 08:31
# day 4 : window opens at 09:22
# day 5 : window opens at 10:12
# day 6 : window opens at 11:02
# day 7 : window opens at 11:53
# days until the same clock time returns: 28.6
#
# With 4 hours of exposure a day, a 100-day season gives 100, 200 or 300 working hours at 25, 50 or 75 per cent usable, so 2,500 stones at 3 stones an hour (833 hours) need 8.3, 4.2 or 2.8 such seasons. The usable shares and the laying rate are my assumptions; the sources give neither, and the 2,500 figure is the Bell Rock article's single "about 2500". The tide slips by about 50 minutes a day (2 x 12.42 h - 24 h, with 12.42 h an assumed typical tide cycle), so a 06:00 window opens at 11:53 a week later and the clock time repeats after about 28.6 days. That drift is why a tide-limited job cannot be scheduled by the clock: the working hours move around the day and the night. All values come from the model, not from records of the building.