time_spirals.py

#!/usr/bin/env python3
"""Time Spirals -- Program 4 from "Marvel Super Heroes Computer Fun, Book One".

Faithful recreation of the type-in BASIC listing: a turtle draws an expanding
square spiral from screen center, erases it, then redraws it forever in random
characters. Cursor-addressed (no scrolling). Runs until you interrupt with
Ctrl-C -- "interrupt the program when you feel you've traveled enough."

    python time_spirals.py
"""
import math
import os
import random
import sys
import time

# 900-Line setup (Commodore 64 profile) + line 970: BC=33, CS=95
SW, SH = 40, 24
BC, CS = 33, 95
CELLS_PER_FRAME = 8
FRAME_DELAY = 0.03

GREEN = "\033[32m"
RESET = "\033[0m"
CLEAR = "\033[2J\033[H"


def enable_ansi():
    if os.name == "nt":
        os.system("")


def build_path():
    """Turtle path from lines 140-310: a fixed (VT,HT) sequence. PW ends at 1
    each pass (SH-2 even), so the path repeats and the space-erase aligns."""
    path = []
    XO, YO = SW // 2, SH // 2          # line 140
    PW = 1                             # line 130
    for I in range(1, SH - 1):         # line 150: FOR I=1 TO SH-2
        PW = -1 if PW == 1 else 1      # lines 160-170
        P2 = -PW                       # line 180
        XN = XO - I * PW               # line 190
        PX = XO
        while (PX <= XN) if P2 > 0 else (PX >= XN):   # 200-230
            path.append((YO, PX))
            PX += P2
        XO = XN                        # line 240
        YN = YO - I * PW               # line 250
        PY = YO
        while (PY <= YN) if P2 > 0 else (PY >= YN):   # 260-290
            path.append((PY, XO))
            PY += P2
        YO = YN                        # line 300
    return path


def glyph(code):
    return chr(code) if 33 <= code <= 126 else "#"


def random_char():                     # lines 930 + 340
    rd = random.randint(1, CS)
    return glyph(rd + BC)


def next_char(prev):                   # lines 320-340
    return " " if prev != " " else random_char()


def plot(vt, ht, ch):                  # GOSUB 910 + PRINT CH$
    if 1 <= vt <= SH and 1 <= ht <= SW:
        sys.stdout.write("\033[%d;%dH%s" % (vt, ht, ch))


def play(passes=None, delay=FRAME_DELAY):
    enable_ansi()
    path = build_path()
    sys.stdout.write(CLEAR + GREEN)
    ch = "*"
    done = 0
    try:
        while passes is None or done < passes:
            for n, (vt, ht) in enumerate(path):     # one full pass (lines 150-310)
                plot(vt, ht, ch)
                if n % CELLS_PER_FRAME == 0:
                    sys.stdout.flush()
                    time.sleep(delay)
            sys.stdout.flush()
            ch = next_char(ch)                       # lines 320-340
            done += 1
    except KeyboardInterrupt:
        pass
    sys.stdout.write("\033[%d;1H" % SH + RESET + "\n")
    sys.stdout.flush()


def main():
    play()


if __name__ == "__main__":
    main()