"""Demonstrate the preprocessing strategy on the 9-vertex example.

Start: F = (0,3,6) at depth 1, no balanced surface switch exists
(F has no edge of "span 1" -- no edge with a single outer-cycle
vertex between the endpoints, hence no ear neighbour).

Step: perform the (unbalanced) surface switch on edge uv = 03, with
F' = (0,2,3) and third vertex x = 2; in Case (ii) the flip removes 03
and adds wx = 62.

Result: A = (0,2,6) at depth 1 has edge 02 at span 1, so the ear
(0,1,2) is now a balanced-switch target.
"""
import os
import math
import networkx as nx
import matplotlib.pyplot as plt
from matplotlib.patches import Polygon

OUT_DIR = os.path.join(os.path.dirname(os.path.abspath(__file__)), os.pardir)

n = 9
POS = {i: (math.cos(math.radians(90 - i * 360 / n)),
           math.sin(math.radians(90 - i * 360 / n))) for i in range(n)}
OUTER_EDGES = [(i, (i + 1) % n) for i in range(n)]
outer_set = {frozenset(e) for e in OUTER_EDGES}


def face_edges(f):
    return {frozenset((f[0], f[1])), frozenset((f[1], f[2])),
            frozenset((f[0], f[2]))}


def compute_depths(faces):
    D = nx.Graph()
    D.add_nodes_from(range(len(faces)))
    for i, fi in enumerate(faces):
        for j, fj in enumerate(faces):
            if i < j and face_edges(fi) & face_edges(fj):
                D.add_edge(i, j)
    B = [i for i, f in enumerate(faces)
         if len(face_edges(f) & outer_set) >= 1]
    return {i: min(nx.shortest_path_length(D, i, b) for b in B)
            for i in range(len(faces))}


CHORDS_BEFORE = [(0, 2), (0, 3), (3, 5), (3, 6), (0, 6), (6, 8)]
FACES_BEFORE = [
    (0, 1, 2), (0, 2, 3), (3, 4, 5), (3, 5, 6),
    (6, 7, 8), (6, 8, 0), (0, 3, 6),
]

# After non-balanced switch on edge 03: remove 03, add 26
CHORDS_AFTER = [c for c in CHORDS_BEFORE if set(c) != {0, 3}] + [(2, 6)]
FACES_AFTER = [
    (0, 1, 2), (3, 4, 5), (3, 5, 6),
    (6, 7, 8), (6, 8, 0),
    (0, 2, 6), (2, 3, 6),
]


def draw(ax, faces, chords, depth, title, highlight_edges=None,
         green_edges=None):
    palette = {0: '#86efac', 1: '#fde68a', 2: '#fca5a5'}
    edge_pal = {0: '#16a34a', 1: '#d97706', 2: '#dc2626'}
    for i, f in enumerate(faces):
        d = depth[i]
        poly = Polygon([POS[v] for v in f], closed=True,
                       facecolor=palette.get(d, '#ddd'),
                       edgecolor=edge_pal.get(d, '#333'),
                       linewidth=1.4, alpha=0.7, zorder=0)
        ax.add_patch(poly)
        cx = sum(POS[v][0] for v in f) / 3
        cy = sum(POS[v][1] for v in f) / 3
        ax.text(cx, cy, str(d), ha='center', va='center', fontsize=11,
                color=edge_pal.get(d, '#333'), fontweight='bold')
    for (a, b) in OUTER_EDGES + chords:
        color = '#333'; lw = 1.2
        if highlight_edges and ((a, b) in highlight_edges or
                                 (b, a) in highlight_edges):
            color = '#dc2626'; lw = 3.0
        if green_edges and ((a, b) in green_edges or
                            (b, a) in green_edges):
            color = '#16a34a'; lw = 3.0
        ax.plot([POS[a][0], POS[b][0]], [POS[a][1], POS[b][1]],
                color=color, linewidth=lw, zorder=1)
    for i, (x, y) in POS.items():
        ax.scatter([x], [y], s=270, c='#1f2937', edgecolors='black',
                   linewidths=1.0, zorder=2)
        ax.text(x, y, str(i), ha='center', va='center',
                fontsize=9, color='white', fontweight='bold', zorder=3)
    ax.set_aspect('equal'); ax.axis('off')
    ax.set_xlim(-1.3, 1.3); ax.set_ylim(-1.3, 1.3)
    ax.set_title(title, fontsize=11)


depth_before = compute_depths(FACES_BEFORE)
depth_after = compute_depths(FACES_AFTER)
print('BEFORE:')
for i, f in enumerate(FACES_BEFORE):
    print(f'  {f} -> depth {depth_before[i]}')
print('AFTER:')
for i, f in enumerate(FACES_AFTER):
    print(f'  {f} -> depth {depth_after[i]}')

fig, axes = plt.subplots(1, 2, figsize=(14, 7))
draw(axes[0], FACES_BEFORE, CHORDS_BEFORE, depth_before,
     'Before: F=(0,3,6) depth 1; spans (2,2,2) so no ear neighbour',
     highlight_edges=[(0, 3)])
draw(axes[1], FACES_AFTER, CHORDS_AFTER, depth_after,
     'After non-balanced switch 03->26: A=(0,2,6) depth 1; edge 02 has span 1',
     green_edges=[(2, 6)], highlight_edges=[(0, 2)])

fig.tight_layout()
out = os.path.join(OUT_DIR, 'fig_preprocessing.png')
fig.savefig(out, dpi=180, bbox_inches='tight')
plt.close(fig)
print(f'wrote {out}')