Solve 2025 day 9 pt 2

Flood fill was too expensive, so I tried solving it with ray casting.

I believe it is the 2nd or 3rd time during 11 runs where it has been
requested.

wip

wip

2025-python/output/__init__.py

@@ -1,5 +1,6 @@
 import re
 from math import inf
+from PIL import Image
 
 # Directions/Adjacents for 2D matrices, in the order UP, RIGHT, DOWN, LEFT
 D = [
@@ -90,15 +91,47 @@ def vdbg(seen, mask=("#", "."), M=None):
         r = max(r, x)
         b = max(b, y)
         l = min(l, x)
-    H = b - t + 1
-    W = r - l + 1
-    osr = t
-    osc = l
+    H = b + 1
+    W = r + 1
+    osr = 0 #t
+    osc = 0 #l
     C, Z = mask
     def _m(r, c):
         return M[r][c] if M else Z
+    O = []
     for r in range(H):
-        print("".join([C if (r + osr, c + osc) in seen else _m(r, c) for c in range(W)]))
+        O.append("".join([C if (r + osr, c + osc) in seen else _m(r, c) for c in range(W)]))
+    print("\n".join(O))
+
+
+def svg(seen):
+    """Print-debug visited positions of a matrix"""
+    t = inf
+    l = inf
+    b = 0
+    r = 0
+    rects = []
+    for y, x in seen:
+        t = min(t, y)
+        r = max(r, x)
+        b = max(b, y)
+        l = min(l, x)
+    H = b - t + 1
+    W = r - l + 1
+    print(t, r, b, l)
+    im = Image.new(mode="RGB", size=(W,H), color=(255,255,255))
+    for y, x in seen:
+        im.putpixel((x-l, y-t), (0, 0, 0, 255))
+    im.save("aoc.png")
+    
+    for y, x in seen:
+        rects.append(f"<rect x='{x-l}' y='{y-t}' width='1' fill='black' stroke='black' stroke-width='10' height='1' />")
+    with open("svg.svg", "w") as f:
+        f.write(f"""
+                <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 {W} {H}" width="{W}" height="{H}">
+                    {"".join(rects)}
+                </svg>    
+                """.strip())
 
 
 def vvdbg(seen, h, w):

2025-python/output/day_09.py

@@ -1,3 +1,4 @@
+from math import inf
 from itertools import combinations
 
 from output import ints
@@ -7,13 +8,21 @@ def solve(data):
     p1 = 0
     p2 = 0
     A = [tuple(ints(pos)) for pos in data.splitlines()]
-
+    T = inf
+    R = 0
+    B = 0
+    L = inf
+    for r, c in A:
+        T = min(r, T)
+        R = max(c, R)
+        B = max(r, B)
+        L = min(c, L)
     S = A[0]
     V = set()
-    B = set()
-
+    W = set()
     while True:
         V.add(S)
+        W.add(S)
         y, x = S
         he = [(r, c) for r, c in A if r == y and (r, c) not in V]
         ve = [(r, c) for r, c in A if c == x and (r, c) not in V]
@@ -28,61 +37,41 @@ def solve(data):
         y2, x2 = E
         for r in range(min(y, y2), max(y, y2) + 1):
             for c in range(min(x, x2), max(x, x2) + 1):
-                B.add((r, c))
+                W.add((r, c))
         if E == A[0]:
             break
         S = E
-    # find out a better start for Flood fill by visualizing
-    Y = 1841
-    X = 53478
-    # or, find a way to floodfill without BFS
-    # end
-    y, x = Y, X
-    Q = [(y, x)]
-    V = V | B
-    while Q:
-        yx = Q.pop()
-        if yx in V:
-            continue
-        V.add(yx)
-        y, x = yx
-        for dy, dx in D:
-            Q.append((dy + y, dx + x))
+    V = V | W
     for a, b in combinations(A, r=2):
         y1, x1 = a
         y2, x2 = b
         x = abs(x1 - x2) + 1
         y = abs(y1 - y2) + 1
         p1 = max(p1, x * y)
-        if (
-            len(
-                set(
-                    [
-                        (min(y1, y2), min(x1, x2)),
-                        (min(y1, y2), max(x1, x2)),
-                        (max(y1, y2), min(x1, x2)),
-                        (max(y1, y2), max(x1, x2)),
-                    ]
-                )
-                - V
-            )
-            == 0
-        ):
+        if _within(W, a, b, T, R, B, L):
             p2 = max(p2, max(p2, x * y))
     return p1, p2
 
 
-if __name__ == "__main__":
-    inp = """
-7,1
-11,1
-11,7
-9,7
-9,5
-2,5
-2,3
-7,3    """.strip()
+def _within(W, a, b, T, R, B, L):
+    y1, x1 = a
+    y2, x2 = b
+    for r in range(min(y1, y2) + 1, max(y1, y2)):
+        for c in range(min(x1, x2) + 1, max(x1, x2)):
+            if (r, c) in W:
+                continue
+            for s, e in [(T, r), (r, B)]:
+                z = sum((nr, c) in W for nr in range(s, e))
+                if z and z % 2 == 0:
+                    return False
+            for s, e in [(L, c), (c, R)]:
+                z = sum((r, nc) in W for nc in range(s, e))
+                if z and z % 2 == 0:
+                    return False
+    return True
 
+
+if __name__ == "__main__":
     with open("./input/09.txt", "r") as f:
         inp = f.read().strip()