# tk_inward_cache_demo.py

# tk_inward_cache_demo.py

import tkinter as tk

WW, HH = 600, 600
UNIT = 10

root = tk.Tk()
root.geometry('+0+0')
canvas = tk.Canvas(root, width=WW, height=HH)
canvas.pack()

for y in range(0, HH, UNIT):
    for x in range(0, WW, UNIT):
        canvas.create_rectangle(x, y, x + UNIT, y + UNIT, outline='gray')

cache = []
def plot_unit(x, y):
    canvas.create_rectangle(x * UNIT, y * UNIT, (x + 1) * UNIT, (y + 1) * UNIT, fill='red', outline='')
    cache.append((x, y))
    root.update()

def test():
    bb = (WW + 1) // UNIT
    i = 0
    j = bb
    while j:
        for x in range(i, j):
            plot_unit(x, j - 1)
            plot_unit(x, bb - j)
        i += 1
        j -= 1
        for y in range(i, j):
            plot_unit(j, y)
            plot_unit(bb - j - 1, y)

test()