def calculate_iou(box1, box2):
    """Compute IoU between two boxes."""
    xA = max(box1[0], box2[0])
    yA = max(box1[1], box2[1])
    xB = min(box1[2], box2[2])
    yB = min(box1[3], box2[3])

    inter_area = max(0, xB - xA) * max(0, yB - yA)
    box1_area = (box1[2] - box1[0]) * (box1[3] - box1[1])
    box2_area = (box2[2] - box2[0]) * (box2[3] - box2[1])
    union_area = box1_area + box2_area - inter_area

    return inter_area / union_area if union_area > 0 else 0