"""Traverse Ordering (two-curve variant).

Two smooth tangled curves cross each other multiple times (perimeter-anchored
turtle walks, same generator style as line_intersections). Labeled marks
(A, B, C, ...) are placed on BOTH curves at well-spaced arc-length positions,
avoiding crossings and close-strand regions. Exactly one curve additionally
has a green 'S' marker at one of its endpoints. The task: starting from S,
follow that curve to its other endpoint and list the labels of marks on the
S-curve in the order visited. Marks on the other curve are distractors.
"""
from __future__ import annotations

import argparse
import json
import math
import os
import random
import string
import sys
from collections import defaultdict
from pathlib import Path
from typing import Dict, List, Tuple

import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt
import numpy as np
from tqdm import tqdm


LINE_COLOR = "#2f2f2f"
MARK_COLORS = [
    "#e63946", "#457b9d", "#2a9d8f", "#e9c46a", "#f4a261",
    "#264653", "#6a4c93", "#1982c4", "#8ac926", "#ff595e",
]
S_COLOR = "#1f9d3f"


# ── Perimeter-anchored turtle curve (from line_intersections) ──────

def _perimeter_point(s: float, width: int, height: int, margin: int
                     ) -> Tuple[np.ndarray, np.ndarray]:
    side = int(s) % 4
    t = s - int(s)
    if side == 0:
        x = margin + (width - 2 * margin) * t
        y = margin
        inward = np.array([0.0, 1.0])
    elif side == 1:
        x = width - margin
        y = margin + (height - 2 * margin) * t
        inward = np.array([-1.0, 0.0])
    elif side == 2:
        x = width - margin - (width - 2 * margin) * t
        y = height - margin
        inward = np.array([0.0, -1.0])
    else:
        x = margin
        y = height - margin - (height - 2 * margin) * t
        inward = np.array([1.0, 0.0])
    return np.array([x, y]), inward


def sample_perimeter_anchors(
    rng: random.Random,
    num_anchors: int,
    width: int,
    height: int,
    margin: int,
    min_gap: float | None = None,
    jitter_deg: float = 10.0,
    max_attempts: int = 300,
) -> List[Tuple[np.ndarray, np.ndarray]]:
    if min_gap is None:
        min_gap = (4.0 / num_anchors) * 0.5
    for _ in range(max_attempts):
        positions = sorted(rng.uniform(0.0, 4.0) for _ in range(num_anchors))
        ok = True
        for i in range(num_anchors):
            gap = (positions[(i + 1) % num_anchors] - positions[i]) % 4.0
            if gap < min_gap:
                ok = False
                break
        if ok:
            anchors = []
            for s in positions:
                pt, inward = _perimeter_point(s, width, height, margin)
                ang = math.atan2(inward[1], inward[0])
                ang += math.radians(rng.uniform(-jitter_deg, jitter_deg))
                inward = np.array([math.cos(ang), math.sin(ang)])
                anchors.append((pt, inward))
            return anchors
    return None


def _angle_diff(current: float, target: float) -> float:
    d = target - current
    return (d + math.pi) % (2 * math.pi) - math.pi


def build_anchored_curve(
    rng: random.Random,
    width: int,
    height: int,
    start_pos: np.ndarray,
    start_inward: np.ndarray,
    end_pos: np.ndarray,
    end_inward: np.ndarray,
    leader_length: float = 150.0,
    step_size: float = 3.3,
    max_turn: float = 0.028,
    drift_rate: float = 0.007,
    num_waypoints: int = 2,
    interior_margin: int = 260,
    max_steps: int = 1800,
) -> np.ndarray:
    pts: List[np.ndarray] = [start_pos.copy()]
    x, y = float(start_pos[0]), float(start_pos[1])
    theta = math.atan2(start_inward[1], start_inward[0])

    raw_wps: List[np.ndarray] = []
    for _ in range(num_waypoints):
        wx = rng.uniform(interior_margin, width - interior_margin)
        wy = rng.uniform(interior_margin, height - interior_margin)
        raw_wps.append(np.array([wx, wy]))
    direction = end_pos - start_pos
    dir_norm = direction / (np.linalg.norm(direction) + 1e-9)
    raw_wps.sort(key=lambda p: float(np.dot(p - start_pos, dir_norm)))

    funnel = end_pos + end_inward * 400.0
    lookahead = end_pos + (-end_inward) * 240.0
    waypoints: List[np.ndarray] = raw_wps + [funnel, lookahead]

    exit_theta = math.atan2(-end_inward[1], -end_inward[0])
    entry_steps = max(2, int(leader_length / step_size))

    wp_idx = 0
    drift = 0.0
    perp = np.array([-end_inward[1], end_inward[0]])

    for step_count in range(max_steps):
        sd = ((x - end_pos[0]) * end_inward[0]
              + (y - end_pos[1]) * end_inward[1])
        lat = abs((x - end_pos[0]) * perp[0] + (y - end_pos[1]) * perp[1])

        if sd <= 0.0:
            break

        if step_count < entry_steps:
            turn = 0.0
        elif sd < leader_length and lat < 40.0:
            head_err = _angle_diff(theta, exit_theta)
            turn = max(-max_turn, min(max_turn, head_err))
        else:
            is_final = (wp_idx == len(waypoints) - 1)
            if is_final:
                wx, wy = waypoints[-1]
                pull = 0.18
            else:
                wx, wy = waypoints[wp_idx]
                steer_dist = math.hypot(wx - x, wy - y)
                if steer_dist < 140.0:
                    wp_idx += 1
                    continue
                pull = 0.09

            drift += rng.gauss(0, drift_rate)
            drift = max(-max_turn * 0.7, min(max_turn * 0.7, drift))
            turn = drift + rng.gauss(0, max_turn * 0.12)
            target_ang = math.atan2(wy - y, wx - x)
            steer = _angle_diff(theta, target_ang)
            turn += steer * pull
            turn = max(-max_turn, min(max_turn, turn))

        theta += turn
        x += step_size * math.cos(theta)
        y += step_size * math.sin(theta)
        pts.append(np.array([x, y]))

    return np.array(pts)


# ── Intersection detection ─────────────────────────────────────────

def _segments_cross(p0, p1, q0, q1) -> bool:
    eps = 1e-8
    o1 = (p1[0]-p0[0])*(q0[1]-p0[1]) - (p1[1]-p0[1])*(q0[0]-p0[0])
    o2 = (p1[0]-p0[0])*(q1[1]-p0[1]) - (p1[1]-p0[1])*(q1[0]-p0[0])
    o3 = (q1[0]-q0[0])*(p0[1]-q0[1]) - (q1[1]-q0[1])*(p0[0]-q0[0])
    o4 = (q1[0]-q0[0])*(p1[1]-q0[1]) - (q1[1]-q0[1])*(p1[0]-q0[0])
    return ((o1 > eps and o2 < -eps) or (o1 < -eps and o2 > eps)) and \
           ((o3 > eps and o4 < -eps) or (o3 < -eps and o4 > eps))


def _crossing_angle(p0, p1, q0, q1) -> float:
    d1 = p1 - p0
    d2 = q1 - q0
    cos_a = np.dot(d1, d2) / (np.linalg.norm(d1) * np.linalg.norm(d2) + 1e-12)
    return math.degrees(math.acos(min(1.0, abs(cos_a))))


def _find_crossings(
    poly_a: np.ndarray,
    poly_b: np.ndarray,
    same_curve: bool = False,
    min_seg_gap: int = 10,
):
    na = len(poly_a) - 1
    nb = len(poly_b) - 1

    a_min_x = np.minimum(poly_a[:-1, 0], poly_a[1:, 0])
    a_max_x = np.maximum(poly_a[:-1, 0], poly_a[1:, 0])
    a_min_y = np.minimum(poly_a[:-1, 1], poly_a[1:, 1])
    a_max_y = np.maximum(poly_a[:-1, 1], poly_a[1:, 1])
    b_min_x = np.minimum(poly_b[:-1, 0], poly_b[1:, 0])
    b_max_x = np.maximum(poly_b[:-1, 0], poly_b[1:, 0])
    b_min_y = np.minimum(poly_b[:-1, 1], poly_b[1:, 1])
    b_max_y = np.maximum(poly_b[:-1, 1], poly_b[1:, 1])

    all_min_x = np.concatenate([a_min_x, b_min_x])
    all_max_x = np.concatenate([a_max_x, b_max_x])
    cell_size = max(np.median(all_max_x - all_min_x), 1.0) * 3

    grid_a = defaultdict(list)
    for i in range(na):
        cx0 = int(a_min_x[i] / cell_size); cx1 = int(a_max_x[i] / cell_size)
        cy0 = int(a_min_y[i] / cell_size); cy1 = int(a_max_y[i] / cell_size)
        for gx in range(cx0, cx1 + 1):
            for gy in range(cy0, cy1 + 1):
                grid_a[(gx, gy)].append(i)

    grid_b = defaultdict(list)
    for j in range(nb):
        cx0 = int(b_min_x[j] / cell_size); cx1 = int(b_max_x[j] / cell_size)
        cy0 = int(b_min_y[j] / cell_size); cy1 = int(b_max_y[j] / cell_size)
        for gx in range(cx0, cx1 + 1):
            for gy in range(cy0, cy1 + 1):
                grid_b[(gx, gy)].append(j)

    checked = set()
    worst_angle = 180.0
    details: List[Dict] = []
    for cell_key in grid_a:
        if cell_key not in grid_b:
            continue
        for i in grid_a[cell_key]:
            for j in grid_b[cell_key]:
                if same_curve:
                    ii, jj = min(i, j), max(i, j)
                    if jj - ii < min_seg_gap:
                        continue
                    key = (ii, jj)
                else:
                    key = (i, j)
                if key in checked:
                    continue
                checked.add(key)
                if a_max_x[i] < b_min_x[j] or b_max_x[j] < a_min_x[i] or \
                   a_max_y[i] < b_min_y[j] or b_max_y[j] < a_min_y[i]:
                    continue
                if same_curve:
                    si, sj = key
                    p0, p1 = poly_a[si], poly_a[si + 1]
                    q0, q1 = poly_b[sj], poly_b[sj + 1]
                else:
                    si, sj = i, j
                    p0, p1 = poly_a[si], poly_a[si + 1]
                    q0, q1 = poly_b[sj], poly_b[sj + 1]
                if not _segments_cross(p0, p1, q0, q1):
                    continue
                d1 = p1 - p0
                d2 = q1 - q0
                denom = d1[0] * d2[1] - d1[1] * d2[0]
                if abs(denom) < 1e-12:
                    continue
                ti = ((q0[0] - p0[0]) * d2[1] - (q0[1] - p0[1]) * d2[0]) / denom
                tj = ((q0[0] - p0[0]) * d1[1] - (q0[1] - p0[1]) * d1[0]) / denom
                px = float(p0[0] + ti * d1[0])
                py = float(p0[1] + ti * d1[1])
                angle = _crossing_angle(p0, p1, q0, q1)
                worst_angle = min(worst_angle, angle)
                details.append({
                    "i": int(si), "j": int(sj),
                    "ti": float(ti), "tj": float(tj),
                    "px": px, "py": py, "angle": float(angle),
                })
    return len(details), worst_angle, details


def _details_to_points(details: List[Dict]) -> np.ndarray:
    if not details:
        return np.zeros((0, 2))
    return np.array([[d["px"], d["py"]] for d in details])


def _curves_too_close(
    poly_a: np.ndarray,
    poly_b: np.ndarray,
    same_curve: bool = False,
    min_dist: float = 7.0,
    sample_step: int = 3,
    self_index_gap: int = 50,
    known_crossings: np.ndarray | None = None,
    crossing_exclude_radius: float = 30.0,
) -> bool:
    nb = len(poly_b) - 1
    b_pts = poly_b[:-1]
    b_vecs = poly_b[1:] - poly_b[:-1]
    b_lens_sq = np.maximum((b_vecs ** 2).sum(axis=1), 1e-12)
    has_crossings = known_crossings is not None and len(known_crossings) > 0
    for idx in range(0, len(poly_a), sample_step):
        px, py = poly_a[idx]
        p = np.array([px, py])
        dp = p - b_pts
        t = (dp * b_vecs).sum(axis=1) / b_lens_sq
        t = np.clip(t, 0.0, 1.0)
        proj = b_pts + t[:, None] * b_vecs
        dists = np.sqrt(((p - proj) ** 2).sum(axis=1))
        if same_curve:
            mask = np.abs(np.arange(nb) - idx) < self_index_gap
            dists[mask] = 9999.0
        if dists.min() < min_dist:
            if has_crossings:
                cross_dists = np.sqrt(((p - known_crossings) ** 2).sum(axis=1))
                if cross_dists.min() < crossing_exclude_radius:
                    continue
            return True
    return False


# ── Mark placement ─────────────────────────────────────────────────

def place_marks_on_curve(
    rng: random.Random,
    polyline: np.ndarray,
    other_polyline: np.ndarray,
    num_marks: int,
    avoid_points: List[np.ndarray],
    existing_marks: List[Tuple[np.ndarray, int]] | None = None,
    min_arc_frac: float = 0.10,
    min_pixel_to_crossing: float = 70.0,
    min_pixel_to_other_strand: float = 36.0,
    min_pixel_between_marks: float = 90.0,
    endpoint_arc_margin_frac: float = 0.08,
    max_iter: int = 1200,
    extra_polylines: List[np.ndarray] | None = None,
) -> List[int]:
    """Place `num_marks` indices along `polyline`, avoiding crossing points,
    regions where the OTHER curve passes close, and existing marks on either
    curve. Endpoints are reserved (for the S marker).
    """
    n = len(polyline)
    diffs = np.diff(polyline, axis=0)
    seg_lens = np.sqrt((diffs ** 2).sum(axis=1))
    arc = np.concatenate([[0.0], np.cumsum(seg_lens)])
    total_len = arc[-1]
    min_arc_dist = total_len * min_arc_frac
    margin_arc = total_len * endpoint_arc_margin_frac

    # Precompute other curve's segments for fast distance queries.
    op = other_polyline
    o_pts = op[:-1]
    o_vecs = op[1:] - op[:-1]
    o_lens_sq = np.maximum((o_vecs ** 2).sum(axis=1), 1e-12)

    def dist_to_other(pt: np.ndarray) -> float:
        dp = pt - o_pts
        t = (dp * o_vecs).sum(axis=1) / o_lens_sq
        t = np.clip(t, 0.0, 1.0)
        proj = o_pts + t[:, None] * o_vecs
        return float(np.sqrt(((pt - proj) ** 2).sum(axis=1)).min())

    extra_bundles = []
    for ep in (extra_polylines or []):
        e_pts = ep[:-1]
        e_vecs = ep[1:] - ep[:-1]
        e_lens_sq = np.maximum((e_vecs ** 2).sum(axis=1), 1e-12)
        extra_bundles.append((e_pts, e_vecs, e_lens_sq))

    def dist_to_any_extra(pt: np.ndarray) -> float:
        if not extra_bundles:
            return 1e9
        best = 1e9
        for e_pts, e_vecs, e_lens_sq in extra_bundles:
            dp = pt - e_pts
            t = (dp * e_vecs).sum(axis=1) / e_lens_sq
            t = np.clip(t, 0.0, 1.0)
            proj = e_pts + t[:, None] * e_vecs
            d = float(np.sqrt(((pt - proj) ** 2).sum(axis=1)).min())
            if d < best:
                best = d
        return best

    def too_near_avoid(pt: np.ndarray) -> bool:
        for cp in avoid_points:
            if np.sqrt(((pt - cp) ** 2).sum()) < min_pixel_to_crossing:
                return True
        return False

    existing = existing_marks or []
    mark_indices: List[int] = []
    for _ in range(max_iter):
        if len(mark_indices) == num_marks:
            break
        t = rng.uniform(margin_arc, total_len - margin_arc)
        idx = int(np.searchsorted(arc, t))
        idx = max(1, min(n - 2, idx))
        pt = polyline[idx]
        if too_near_avoid(pt):
            continue
        if dist_to_other(pt) < min_pixel_to_other_strand:
            continue
        if dist_to_any_extra(pt) < min_pixel_to_other_strand:
            continue
        if any(abs(arc[idx] - arc[m]) < min_arc_dist for m in mark_indices):
            continue
        if any(np.sqrt(((pt - polyline[m]) ** 2).sum()) < min_pixel_between_marks
               for m in mark_indices):
            continue
        if any(np.sqrt(((pt - ep) ** 2).sum()) < min_pixel_between_marks
               for ep, _ in existing):
            continue
        mark_indices.append(idx)

    mark_indices.sort()
    return mark_indices


def label_anchor_for_mark(
    polyline: np.ndarray,
    other_polyline: np.ndarray,
    mark_idx: int,
    width: int,
    height: int,
    offset: float = 48.0,
    tangent_span: int = 6,
) -> Tuple[float, float]:
    n = len(polyline)
    lo = max(0, mark_idx - tangent_span)
    hi = min(n - 1, mark_idx + tangent_span)
    tan = polyline[hi] - polyline[lo]
    tnorm = math.hypot(float(tan[0]), float(tan[1]))
    if tnorm < 1e-8:
        ux, uy = 0.0, -1.0
    else:
        ux = float(tan[0]) / tnorm
        uy = float(tan[1]) / tnorm
    perps = [(-uy, ux), (uy, -ux)]
    pt = polyline[mark_idx]
    best = None
    best_score = -1e9
    for px, py in perps:
        cx = float(pt[0]) + px * offset
        cy = float(pt[1]) + py * offset
        score = 0.0
        d_self = float(np.sqrt(((polyline - np.array([cx, cy])) ** 2).sum(axis=1)).min())
        d_other = float(np.sqrt(((other_polyline - np.array([cx, cy])) ** 2).sum(axis=1)).min())
        score += min(d_self, d_other)
        if 30 < cx < width - 30 and 30 < cy < height - 30:
            score += 200.0
        if score > best_score:
            best_score = score
            best = (cx, cy)
    cx, cy = best
    return (
        float(np.clip(cx, 34, width - 34)),
        float(np.clip(cy, 34, height - 34)),
    )


# ── Instance sampling ──────────────────────────────────────────────

def sample_instance(
    rng: random.Random,
    width: int,
    height: int,
    min_marks_total: int = 4,
    max_marks_total: int = 7,
    min_crossings: int = 7,
    max_crossings: int = 25,
    min_pixel_between_marks: float = 90.0,
    s_endpoint_excl_px: float = 90.0,
    perimeter_margin: int = 70,
    interior_margin: int = 260,
    max_attempts: int = 2500,
    num_waypoints: int = 5,
    s_marks_override: int | None = None,
    o_marks_override: int | None = None,
    num_extra_distractor_curves: int = 0,
    extra_distractor_max_attempts: int = 80,
) -> Dict | None:
    labels_pool = list(string.ascii_uppercase)
    # reserve S
    labels_pool = [l for l in labels_pool if l != "S"]

    for _ in range(max_attempts):
        anchors = sample_perimeter_anchors(
            rng, 4, width, height, margin=perimeter_margin,
        )
        if anchors is None:
            continue

        # Pair anchor i with anchor i+2 (opposite-side pairing).
        pairs = [(anchors[0], anchors[2]), (anchors[1], anchors[3])]
        rng.shuffle(pairs)

        polylines = []
        ok = True
        for (sp, sd), (ep, ed) in pairs:
            poly = build_anchored_curve(
                rng, width, height,
                start_pos=sp, start_inward=sd,
                end_pos=ep, end_inward=ed,
                leader_length=150.0,
                step_size=rng.uniform(3.0, 3.8),
                max_turn=rng.uniform(0.034, 0.044),
                drift_rate=rng.uniform(0.010, 0.016),
                num_waypoints=num_waypoints,
                interior_margin=interior_margin,
                max_steps=3200,
            )
            if len(poly) < 50:
                ok = False
                break
            polylines.append(poly)
        if not ok:
            continue

        # Crossings between the two curves (no self-crossings desired, but allow).
        pair_count, pair_min_angle, pair_details = _find_crossings(
            polylines[0], polylines[1]
        )
        self_counts = []
        self_details_list = []
        self_min_angle = 180.0
        for p in polylines:
            sc, sa, sd_ = _find_crossings(p, p, same_curve=True)
            self_counts.append(sc)
            self_details_list.append(sd_)
            self_min_angle = min(self_min_angle, sa)

        total_cross = pair_count + sum(self_counts)
        if pair_count < min_crossings or pair_count > max_crossings:
            continue
        # Keep self-crossings modest so traversal order stays clear.
        if sum(self_counts) > 3:
            continue

        min_angle = min(pair_min_angle, self_min_angle)
        if min_angle < 30.0:
            continue

        # Hygiene: no close strands outside crossing regions.
        all_cross_pts = _details_to_points(pair_details)
        # 1% of canvas short side keeps strands visibly separated at any
        # resolution while staying tractable for the sampler.
        min_dist_px = 0.01 * float(min(width, height))
        if _curves_too_close(polylines[0], polylines[1],
                             min_dist=min_dist_px,
                             known_crossings=all_cross_pts):
            continue
        too_close_self = False
        for i, p in enumerate(polylines):
            if _curves_too_close(p, p, same_curve=True,
                                 min_dist=min_dist_px,
                                 known_crossings=_details_to_points(self_details_list[i])):
                too_close_self = True
                break
        if too_close_self:
            continue

        # Extra unlabeled distractor curves (difficulty-driven).
        extra_polylines: List[np.ndarray] = []
        extra_pair_details_all: List[Dict] = []
        extra_self_details_all: List[Dict] = []
        extra_ok = True
        for _extra_idx in range(num_extra_distractor_curves):
            placed = False
            for _attempt in range(extra_distractor_max_attempts):
                extra_anchors = sample_perimeter_anchors(
                    rng, 2, width, height, margin=perimeter_margin,
                )
                if extra_anchors is None:
                    continue
                (esp, esd), (eep, eed) = extra_anchors[0], extra_anchors[1]
                epoly = build_anchored_curve(
                    rng, width, height,
                    start_pos=esp, start_inward=esd,
                    end_pos=eep, end_inward=eed,
                    leader_length=150.0,
                    step_size=rng.uniform(3.0, 3.8),
                    max_turn=rng.uniform(0.034, 0.044),
                    drift_rate=rng.uniform(0.010, 0.016),
                    num_waypoints=num_waypoints,
                    interior_margin=interior_margin,
                    max_steps=3200,
                )
                if len(epoly) < 50:
                    continue
                # Self-crossing hygiene for the extra curve.
                esc, esa, esd_det = _find_crossings(epoly, epoly, same_curve=True)
                if esc > 3 or esa < 30.0:
                    continue
                if _curves_too_close(epoly, epoly, same_curve=True,
                                     min_dist=min_dist_px,
                                     known_crossings=_details_to_points(esd_det)):
                    continue
                # Cross-curve hygiene vs all existing polylines.
                all_existing = polylines + extra_polylines
                bad = False
                candidate_pair_details: List[Dict] = []
                for existing_poly in all_existing:
                    ec, ea, ed_det = _find_crossings(epoly, existing_poly)
                    if ea < 30.0:
                        bad = True
                        break
                    if _curves_too_close(epoly, existing_poly,
                                         min_dist=min_dist_px,
                                         known_crossings=_details_to_points(ed_det)):
                        bad = True
                        break
                    candidate_pair_details.extend(ed_det)
                if bad:
                    continue
                extra_polylines.append(epoly)
                extra_pair_details_all.extend(candidate_pair_details)
                extra_self_details_all.extend(esd_det)
                placed = True
                break
            if not placed:
                extra_ok = False
                break
        if not extra_ok:
            continue

        # Collect points to avoid for mark placement: all crossings.
        avoid_pts: List[np.ndarray] = [np.array([d["px"], d["py"]])
                                       for d in pair_details]
        for sd_ in self_details_list:
            avoid_pts.extend(np.array([d["px"], d["py"]]) for d in sd_)
        for d_ in extra_pair_details_all:
            avoid_pts.append(np.array([d_["px"], d_["py"]]))
        for d_ in extra_self_details_all:
            avoid_pts.append(np.array([d_["px"], d_["py"]]))

        # Pick S-curve and which endpoint is S.
        s_curve = rng.randint(0, 1)
        s_at_start = rng.random() < 0.5
        other_curve = 1 - s_curve

        # Decide mark counts. Default: 10 total, 5 on S-curve, 5 on the
        # distractor curve. If the caller narrows the range we split roughly
        # evenly, keeping at least 2 on each curve.
        total_marks = rng.randint(min_marks_total, max_marks_total)
        s_marks = total_marks // 2
        s_marks = max(2, min(s_marks, total_marks - 2))
        o_marks = total_marks - s_marks

        s_poly = polylines[s_curve]
        o_poly = polylines[other_curve]

        s_indices = place_marks_on_curve(
            rng, s_poly, o_poly, s_marks, avoid_points=avoid_pts,
            min_pixel_between_marks=min_pixel_between_marks,
            extra_polylines=extra_polylines,
        )
        if len(s_indices) < s_marks:
            continue

        # Existing marks (pixel-space) to avoid from other curve's perspective.
        existing_for_other = [(s_poly[i], i) for i in s_indices]
        o_indices = place_marks_on_curve(
            rng, o_poly, s_poly, o_marks, avoid_points=avoid_pts,
            existing_marks=existing_for_other,
            min_pixel_between_marks=min_pixel_between_marks,
            extra_polylines=extra_polylines,
        )
        if len(o_indices) < o_marks:
            continue

        # Also avoid marks too close to the S endpoint on s_poly.
        s_endpoint_pt = s_poly[0] if s_at_start else s_poly[-1]
        all_mark_pts = [s_poly[i] for i in s_indices] + [o_poly[i] for i in o_indices]
        if any(np.sqrt(((s_endpoint_pt - p) ** 2).sum()) < s_endpoint_excl_px
               for p in all_mark_pts):
            continue

        # Assign labels. Pool of letters sized to total_marks, shuffled.
        pool = labels_pool[:total_marks]
        rng.shuffle(pool)
        s_labels = pool[:s_marks]
        o_labels = pool[s_marks:s_marks + o_marks]

        # Determine traversal order on the S-curve.
        # s_indices is ascending in polyline order. If s_at_start, forward;
        # else reverse.
        order_pairs = list(zip(s_indices, s_labels))
        if not s_at_start:
            order_pairs = order_pairs[::-1]
        answer_sequence = [lab for _, lab in order_pairs]
        answer = ", ".join(answer_sequence)

        # Build mark info.
        marks_info: List[Dict] = []
        for idx, lab in zip(s_indices, s_labels):
            lx, ly = label_anchor_for_mark(s_poly, o_poly, idx, width, height)
            marks_info.append({
                "label": lab,
                "curve": s_curve,
                "on_s_curve": True,
                "polyline_index": int(idx),
                "x": round(float(s_poly[idx, 0]), 2),
                "y": round(float(s_poly[idx, 1]), 2),
                "label_x": round(lx, 2),
                "label_y": round(ly, 2),
            })
        for idx, lab in zip(o_indices, o_labels):
            lx, ly = label_anchor_for_mark(o_poly, s_poly, idx, width, height)
            marks_info.append({
                "label": lab,
                "curve": other_curve,
                "on_s_curve": False,
                "polyline_index": int(idx),
                "x": round(float(o_poly[idx, 0]), 2),
                "y": round(float(o_poly[idx, 1]), 2),
                "label_x": round(lx, 2),
                "label_y": round(ly, 2),
            })

        # S marker info.
        s_pt = s_poly[0] if s_at_start else s_poly[-1]
        # Label offset for S: away from curve tangent.
        s_tan_idx = 10 if s_at_start else len(s_poly) - 11
        s_idx_for_anchor = 0 if s_at_start else len(s_poly) - 1
        slx, sly = label_anchor_for_mark(s_poly, o_poly, s_idx_for_anchor,
                                         width, height, offset=52.0,
                                         tangent_span=10)

        all_labels_display = sorted(s_labels + o_labels)
        question = (
            "The image shows two smooth curves that cross each other. "
            f"Labeled points ({', '.join(all_labels_display)}) are marked on "
            "both curves. A single green 'S' marks the start endpoint of one "
            "of the curves. Starting from S, follow THAT curve (the one S "
            "lies on) all the way to its other endpoint, and list the labels "
            "of the marked points you visit in order, separated by commas "
            "(for example: B, D, A). Ignore any labeled points that lie on "
            "the OTHER curve. Provide your final answer enclosed in "
            "<answer>...</answer> tags."
        )

        all_polylines = polylines + extra_polylines
        return {
            "width": width,
            "height": height,
            "polylines": all_polylines,
            "num_extra_distractor_curves": int(len(extra_polylines)),
            "s_curve": int(s_curve),
            "s_at_start": bool(s_at_start),
            "s_point": {
                "x": round(float(s_pt[0]), 2),
                "y": round(float(s_pt[1]), 2),
                "label_x": round(float(slx), 2),
                "label_y": round(float(sly), 2),
            },
            "marks": marks_info,
            "num_marks_total": total_marks,
            "num_marks_on_s": s_marks,
            "num_pair_crossings": int(pair_count),
            "num_self_crossings": int(sum(self_counts)),
            "question": question,
            "answer": answer,
        }
    return None


# ── Rendering ──────────────────────────────────────────────────────

def render_instance(out_path: Path, record: Dict, noise_seed: int) -> None:
    width = int(record["width"])
    height = int(record["height"])
    polylines = record["polylines"]
    marks = record["marks"]
    s_point = record["s_point"]

    fig = plt.figure(figsize=(width / 100, height / 100), dpi=100)
    ax = fig.add_axes([0, 0, 1, 1])
    ax.set_xlim(0, width)
    ax.set_ylim(height, 0)
    ax.axis("off")
    ax.set_facecolor("#f3efe8")

    nrng = np.random.default_rng(noise_seed)
    noise = nrng.normal(0.0, 1.0, size=(height, width))
    noise = (noise - noise.min()) / max(noise.max() - noise.min(), 1e-6)
    ax.imshow(noise, cmap="Greys", alpha=0.06, extent=(0, width, height, 0),
              interpolation="bilinear")

    for i, poly in enumerate(polylines):
        ax.plot(poly[:, 0], poly[:, 1],
                color=LINE_COLOR, linewidth=2.8,
                solid_capstyle="round", solid_joinstyle="round",
                zorder=2.0 + i * 0.05)

    # Subtle dots at all four endpoints.
    for poly in polylines:
        for ep in (poly[0], poly[-1]):
            ax.scatter([ep[0]], [ep[1]], s=30,
                       facecolors="#f3efe8", edgecolors="#999",
                       linewidths=1.0, zorder=3.2)

    # Marks with offset labels.
    for i, mark in enumerate(marks):
        x, y = mark["x"], mark["y"]
        lx = mark.get("label_x", x)
        ly = mark.get("label_y", y - 22)
        color = MARK_COLORS[i % len(MARK_COLORS)]
        ax.plot([x, lx], [y, ly],
                color="#9b7b56", linewidth=1.0, alpha=0.7,
                linestyle=(0, (2.0, 3.2)), zorder=3.8)
        ax.scatter([x], [y], s=120, facecolors=color, edgecolors="white",
                   linewidths=1.5, zorder=4.0)
        ax.text(lx, ly, mark["label"],
                fontsize=18, fontweight="bold", color=color,
                ha="center", va="center", zorder=5,
                bbox=dict(facecolor="#f3efe8", edgecolor=color,
                          boxstyle="round,pad=0.25", alpha=0.95, linewidth=1.2))

    # S marker on the S-curve endpoint — green, larger.
    sx, sy = s_point["x"], s_point["y"]
    slx, sly = s_point["label_x"], s_point["label_y"]
    ax.plot([sx, slx], [sy, sly],
            color=S_COLOR, linewidth=1.2, alpha=0.8,
            linestyle=(0, (2.0, 3.2)), zorder=4.2)
    ax.scatter([sx], [sy], s=180, facecolors=S_COLOR, edgecolors="white",
               linewidths=2.0, zorder=5.0)
    ax.text(slx, sly, "S",
            fontsize=20, fontweight="bold", color=S_COLOR,
            ha="center", va="center", zorder=6,
            bbox=dict(facecolor="#f3efe8", edgecolor=S_COLOR,
                      boxstyle="round,pad=0.3", alpha=0.95, linewidth=1.5))

    fig.savefig(out_path, dpi=100, bbox_inches="tight", pad_inches=0)
    plt.close(fig)


# ── Main ───────────────────────────────────────────────────────────

def _polyline_to_list(p: np.ndarray) -> List[List[float]]:
    return [[round(float(x), 2), round(float(y), 2)] for x, y in p]


def main():
    parser = argparse.ArgumentParser()
    parser.add_argument("--output-root", type=Path, required=True)
    parser.add_argument("--count", type=int, default=6)
    parser.add_argument("--seed", type=int, default=42)
    parser.add_argument("--width", type=int, default=1024)
    parser.add_argument("--height", type=int, default=1024)
    parser.add_argument("--min-marks", type=int, default=10)
    parser.add_argument("--max-marks", type=int, default=10)
    parser.add_argument("--min-crossings", type=int, default=7)
    parser.add_argument("--max-crossings", type=int, default=25)
    parser.add_argument("--difficulty", type=int, default=5,
                        help="Integer difficulty >=0; scales crossings and marks.")
    parser.add_argument("--workers", type=int, default=8)
    args = parser.parse_args()

    d = max(0, int(args.difficulty))
    # Defaults (used whether difficulty override fires or not).
    min_pixel_between_marks = 90.0
    s_endpoint_excl_px = 90.0
    num_waypoints = 5
    num_extra_distractor_curves = 0
    if d > 0:
        args.min_crossings = 4 + d
        args.max_crossings = 6 + 2 * d
        s_marks = 4 + d // 2
        o_marks = 4 + d // 2
        args.min_marks = s_marks + o_marks
        args.max_marks = s_marks + o_marks
        num_waypoints = 5 + d
        min_pixel_between_marks = float(max(55, 90 - 5 * d))
        s_endpoint_excl_px = float(max(50, 90 - 5 * d))
        num_extra_distractor_curves = d // 3

    # Canvas scaling with difficulty (sqrt of mark-density growth).
    N_d = 8 + 4 * d
    N_0 = 8
    s_scale = math.sqrt(max(1.0, N_d / N_0))
    args.width = int(round(args.width * s_scale))
    args.height = int(round(args.height * s_scale))

    out_root = args.output_root
    img_dir = out_root / "images"
    img_dir.mkdir(parents=True, exist_ok=True)

    rng = random.Random(args.seed)
    records = []

    sys.path.insert(0, str(Path(__file__).resolve().parents[3]))
    from _sample_pool import parallel_sample_records  # noqa: E402

    def _attempt(rng):
        return sample_instance(
            rng, args.width, args.height,
            min_marks_total=args.min_marks,
            max_marks_total=args.max_marks,
            min_crossings=args.min_crossings,
            max_crossings=args.max_crossings,
            min_pixel_between_marks=min_pixel_between_marks,
            s_endpoint_excl_px=s_endpoint_excl_px,
            num_waypoints=num_waypoints,
            num_extra_distractor_curves=num_extra_distractor_curves,
            max_attempts=50,
        )

    records_raw = parallel_sample_records(
        _attempt, count=args.count, workers=args.workers,
        seed_base=args.seed,
    )
    rng_render = random.Random(args.seed ^ 0xA5A5)
    for idx, record in enumerate(records_raw):
        name = f"traverse_ordering_{idx:05d}.png"
        ns = rng_render.randint(0, 10**9)
        render_instance(img_dir / name, record, noise_seed=ns)
        polylines = record.pop("polylines")
        record["polylines"] = [_polyline_to_list(p) for p in polylines]
        record["image"] = f"images/{name}"
        records.append(record)
    print(f"  {len(records)}/{args.count} valid samples (workers={args.workers})")

    with (out_root / "annotations.jsonl").open("w") as fh:
        for r in records:
            fh.write(json.dumps(r) + "\n")

    data_json = {
        "task": "traverse_ordering",
        "category": "sequential_traversal",
        "count": len(records),
        "items": [
            {"image": r["image"], "question": r["question"], "answer": r["answer"]}
            for r in records
        ],
    }
    (out_root / "data.json").write_text(json.dumps(data_json, indent=2))
    print(f"Saved {len(records)} items to {out_root}")


if __name__ == "__main__":
    main()