"""Extract causal events (collisions, starts/stops of motion) from a trajectory.

This is what makes question generation *grounded*: every question/answer is
derived from events that verifiably happened in the recorded physics rollout.
"""

import numpy as np

from . import config


def extract_events(states, collisions, wall_hits, n):
    """Args:
        states: (T, n, 4) per-frame (x, y, vx, vy)
        collisions: [(t, i, j)] substep-level contact log
        wall_hits: [(t, i, axis, side)]
        n: number of objects

    Returns dict with:
        collisions:  [{frame, objects:[i,j]}] grouped collision events
        starts:      {obj_idx: [frames]}  times each object starts moving
        stops:       {obj_idx: [frames]}
        initially_moving: [obj_idx]
        moving:      (T, n) bool mask
        speeds:      (T, n)
    """
    T = states.shape[0]
    speeds = np.linalg.norm(states[:, :, 2:], axis=-1)          # (T, n)
    moving = speeds > config.MOVE_THRESH

    # ---- group the contact log into collision events ----
    grouped = []
    for (t, i, j) in sorted(collisions, key=lambda c: c[0]):
        if grouped and grouped[-1]["objects"] == [i, j] and \
                t - grouped[-1]["end"] <= config.MIN_EVENT_GAP_FRAMES:
            grouped[-1]["end"] = t
        else:
            grouped.append({"frame": t, "end": t, "objects": [i, j]})
    collision_events = [{"frame": g["frame"], "objects": g["objects"]} for g in grouped]

    # ---- start / stop of motion per object ----
    starts, stops, initially_moving = {}, {}, []
    min_run = config.MIN_RUN_FRAMES
    for o in range(n):
        m = moving[:, o]
        sts, sps = [], []
        t = 0
        while t < T:
            if m[t]:
                t2 = t
                while t2 < T and m[t2]:
                    t2 += 1
                if t2 - t >= min_run:
                    if t == 0:
                        initially_moving.append(o)
                    else:
                        sts.append(t)
                    sps.append(t2)          # frame where it (first) stops
                t = t2
            else:
                t += 1
        starts[o] = sts
        stops[o] = [s for s in sps if s < T - 1]   # ignore end-of-video "stop"

    return {
        "collisions": collision_events,
        "starts": starts,
        "stops": stops,
        "initially_moving": sorted(initially_moving),
        "moving": moving,
        "speeds": speeds,
        "wall_hits": wall_hits,
    }