From 272523b8b2cc448e03ca919dcdb7243ca1fe4c7a Mon Sep 17 00:00:00 2001
From: "Peter J. Holzer" <hjp@hjp.at>
Date: Sun, 13 Dec 2020 23:09:39 +0100
Subject: [PATCH] Wrong solution for day 13, part 2

This computes *a* solution, but not the smallest one.
---
 13/part2 | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)
 create mode 100755 13/part2

diff --git a/13/part2 b/13/part2
new file mode 100755
index 0000000..3d25a0b
--- /dev/null
+++ b/13/part2
@@ -0,0 +1,50 @@
+#!/usr/bin/python3
+
+def ext_euclid(a, b):
+    """
+    returns solutions x and y for equation ax + by = gcd(a, b)
+    """
+
+    (old_r, r) = (a, b)
+    (old_s, s) = (1, 0)
+    (old_t, t) = (0, 1)
+
+    while r:
+        quotient = old_r // r
+        (old_r, r) = (r, old_r - quotient * r)
+        (old_s, s) = (s, old_s - quotient * s)
+        (old_t, t) = (t, old_t - quotient * t)
+    return old_s, old_t, old_r
+
+n = []
+a = []
+with open("input") as fh:
+    now = int(fh.readline())
+    for i, x in enumerate(fh.readline().split(",")):
+        if x != "x":
+            x = int(x)
+            n.append(x)
+            a.append((x - i) % x)
+
+print(n)
+print(a)
+
+n1 = n.pop()
+n2 = n.pop()
+a1 = a.pop()
+a2 = a.pop()
+m1, m2, _ = ext_euclid(n1, n2)
+assert _ == 1
+x = a1 * m2 * n2 + a2 * m1 * n1
+print(x)
+while n:
+    n1 = n1 * n2
+    a1 = x
+
+    n2 = n.pop()
+    a2 = a.pop()
+
+    m1, m2, _ = ext_euclid(n1, n2)
+    assert _ == 1
+    x = a1 * m2 * n2 + a2 * m1 * n1
+    print(x)