Day 9

Author: Vic-Nas

aoc/2025/09/09.vn.py

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+# Advent of code Day 09
+# https://adventofcode.com/2025/day/9
+# 09/12/2025
+   
+E = 10 ** -3 
+    
+class Line:
+    def __init__(self, p1, p2):
+        x1, y1 = p1
+        x2, y2 = p2
+        if x1 == x2:
+            self.a, self.b, self.c = 1, 0, x1
+        else:
+            a = (y1-y2)/(x1-x2)
+            b = (x2*y1-x1*y2)/(x2-x1)
+            self.a, self.b, self.c = a, -1, -b
+        self.line = self
+            
+    def has(self, p):
+        if not p: return False
+        x, y = p
+        return abs(x*self.a+y*self.b-self.c) <= E
+    
+    def inter(self, other):
+        a, b, p = self.a, self.b, self.c
+        c, d, q = other.a, other.b, other.c
+        
+        det = a*d-b*c
+        if abs(det) <= E: return None
+        a, b, c, d = d, -b, -c, a
+        x = a*p+b*q
+        y = c*p+d*q
+        return (x/det, y/det)
+    
+
+class Ray:
+    def __init__(self, line, condition):
+        self.line = line
+        self.condition = condition
+    
+    def has(self, p):
+        if not p: return False
+        if not self.line.has(p): return False
+        return self.condition(p)
+    
+    def inter(self, ray):
+        inter = self.line.inter(ray.line)
+        return self.has(inter) and ray.has(inter)
+    
+with open("input.txt") as file:
+    inp = list(map(lambda s: tuple(map(int, s.strip().split(","))), file.readlines()))
+
+from tqdm.auto import tqdm
+from functools import cache
+from itertools import combinations
+
+
+xs = sorted(set(x for x, y in inp))
+ys = sorted(set(y for x, y in inp))
+
+original = inp
+inp = [(xs.index(x), ys.index(y)) for x, y in inp]
+
+edges = []
+for i, (x1, y1) in enumerate(inp):
+    x2, y2 = inp[(i+1) % len(inp)]
+    
+    line = Line((x1, y1), (x2, y2))
+    if x1 == x2:
+        def hits(p, Y1=y1, Y2=y2):
+            _, y = p
+            return min(Y1, Y2) <= y <= max(Y1, Y2)
+    elif y1 == y2:
+        def hits(p, X1=x1, X2=x2):
+            x, _ = p
+            return min(X1, X2) <= x <= max(X1, X2)
+    
+    edges.append(Ray(line, hits))
+
+@cache
+def inside(p):
+    # Evil ray algorithm
+    # To be inside, the four rays must cross
+    # something
+    if p in inp: return True
+    x, y = p
+    
+    rays = [
+        Ray(Line(p, (x-10, y)), lambda p1, X=x: p1[0] <= X),
+        Ray(Line(p, (x, y+10)), lambda p1, Y=y: p1[1] >= Y),
+        Ray(Line(p, (x+10, y)), lambda p1, X=x: p1[0] >= X),
+        Ray(Line(p, (x, y-10)), lambda p1, Y=y: p1[1] <= Y)
+    ]
+    
+    hits = [0, 0, 0, 0]
+    for edge in edges:
+        for i, ray in enumerate(rays):
+            if ray.inter(edge):
+                hits[i] = 1
+        if sum(hits) == 4: return True
+    return sum(hits) == 4
+
+res1, res2 = 0, 0
+
+for (x1, y1), (x2, y2) in tqdm(list(combinations(inp, 2))):
+    # area computed with real coords
+    area = (abs(xs[x2] - xs[x1]) + 1) * (abs(ys[y2] - ys[y1]) + 1)
+    res1 = max(res1, area)
+    
+    # Checks on reduced coords
+    allIn = True
+    for x in range(min(x1, x2), max(x1, x2) + 1):
+        for y in range(min(y1, y2), max(y1, y2) + 1):
+            if not inside((x, y)):
+                allIn = False
+                break
+        if not allIn:
+            break
+    
+    if allIn:
+        res2 = max(res2, area)
+
+print(res1, res2)