From 85f86147ee2137cfbfb100406e7934c35f400385 Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <code@jdkaplan.dev>
Date: Fri, 31 Jul 2026 13:13:57 -0400
Subject: [PATCH] 2017 Day 10 Part 2

---
 2017/day10.py | 69 +++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 50 insertions(+), 19 deletions(-)

diff --git a/2017/day10.py b/2017/day10.py
index 95c973c..fb35e88 100644
--- a/2017/day10.py
+++ b/2017/day10.py
@@ -1,31 +1,62 @@
+from functools import reduce
+
 with open("input/day10.txt") as f:
     size, input = 256, f.read().strip()
 
-lengths = [int(i) for i in input.split(",")]
 
-ring = list(range(size))
-pos = 0
-skip = 0
+def part1(size, input):
+    lengths = [int(i) for i in input.split(",")]
 
-for length in lengths:
-    assert length <= len(ring), "overlap undefined"
+    ring = list(range(size))
+    _, _, ring = hash_round(0, 0, ring, lengths)
 
-    tail = min(length, len(ring) - pos)
-    head = max(length - tail, 0)
+    return ring[0] * ring[1]
 
-    a = pos + tail
-    b = 0 + head
 
-    prefix = ring[b:pos]
-    inside = ring[pos:a] + ring[:b]
-    suffix = ring[a:]
+def hash_round(pos, skip, ring, lengths):
+    for length in lengths:
+        assert length <= len(ring), "overlap undefined"
 
-    assert length == len(inside)
+        tail = min(length, len(ring) - pos)
+        head = max(length - tail, 0)
 
-    twisted = inside[::-1]
+        a = pos + tail
+        b = 0 + head
 
-    ring = twisted[tail:] + prefix + twisted[:tail] + suffix
-    pos = (pos + length + skip) % len(ring)
-    skip += 1
+        prefix = ring[b:pos]
+        inside = ring[pos:a] + ring[:b]
+        suffix = ring[a:]
 
-print(ring[0] * ring[1])
+        assert length == len(inside)
+
+        twisted = inside[::-1]
+
+        ring = twisted[tail:] + prefix + twisted[:tail] + suffix
+        pos = (pos + length + skip) % len(ring)
+        skip += 1
+
+    return pos, skip, ring
+
+
+def part2(size, input):
+    lengths = [ord(c) for c in input] + [17, 31, 73, 47, 23]
+
+    pos, skip = 0, 0
+    ring = list(range(size))
+    for _ in range(64):
+        pos, skip, ring = hash_round(pos, skip, ring, lengths)
+
+    dense = [0] * 16
+    for i in range(16):
+        start = 16 * i
+        block = ring[start : start + 16]
+        dense[i] = reduce(int.__xor__, block)
+
+    out = ""
+    for d in dense:
+        out += f"{d:02x}"
+    return out
+
+
+print(part1(size, input))
+print(part2(size, input))