#!/usr/bin/env python3
"""
Webcam motion presence detector using frame differencing.
Exit codes: 0 = motion detected, 1 = no motion, 2 = camera error (busy/unavailable)
Usage: presence_detect.py [camera_id]
"""
import sys
import time
import cv2
import numpy as np

FRAMES_TO_CHECK = 8
DIFFS_NEEDED = 2        # require motion in at least N consecutive-frame diffs
PIXEL_DELTA_THRESHOLD = 18   # per-pixel grayscale delta to count as "changed"
MOTION_AREA_RATIO = 0.008    # fraction of pixels that must change to call it motion
BLUR_KSIZE = (21, 21)        # Gaussian blur kernel to suppress sensor noise
# Spacing between the frames we diff. Without it the frames are grabbed
# back-to-back within a few milliseconds, so (a) slow, small movements barely
# differ between adjacent frames and (b) reading off the ffmpeg v4l2loopback
# mirror can hand back the SAME buffered frame twice → a zero delta even while
# you're moving. Spacing the grabs out gives slow motion a real temporal
# baseline and guarantees each compared frame is a fresh one.
INTER_FRAME_DELAY = 0.06     # seconds between grabs (~0.5s total observation window)


def detect(camera_id: int) -> int:
    cap = cv2.VideoCapture(camera_id)
    if not cap.isOpened():
        return 2

    motion_diffs = 0
    prev_gray = None
    try:
        for i in range(FRAMES_TO_CHECK):
            if i > 0:
                time.sleep(INTER_FRAME_DELAY)
            ok, frame = cap.read()
            if not ok:
                continue
            gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
            gray = cv2.GaussianBlur(gray, BLUR_KSIZE, 0)

            if prev_gray is not None:
                delta = cv2.absdiff(prev_gray, gray)
                changed = np.count_nonzero(delta > PIXEL_DELTA_THRESHOLD)
                if changed / delta.size >= MOTION_AREA_RATIO:
                    motion_diffs += 1

            prev_gray = gray
    finally:
        cap.release()

    return 0 if motion_diffs >= DIFFS_NEEDED else 1


if __name__ == "__main__":
    camera_id = int(sys.argv[1]) if len(sys.argv) > 1 else 0
    sys.exit(detect(camera_id))