Day 13

day13/input.txt

@@ -0,0 +1,2 @@
+1007153
+29,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,433,x,x,x,x,x,x,x,x,x,x,x,x,13,17,x,x,x,x,19,x,x,x,23,x,x,x,x,x,x,x,977,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,41

day13/part1.py

@@ -0,0 +1,7 @@
+lines = [x.strip() for x in open("input.txt")]
+
+timestamp = int(lines[0])
+busses = (int(x) for x in lines[1].split(',') if x != 'x')
+next_departures = ((x, timestamp // x * x + x) for x in busses)
+next_bus = min(next_departures, key=lambda x: x[1])
+print(next_bus[0] * (next_bus[1]-timestamp))

day13/part2.py

@@ -0,0 +1,30 @@
+import math
+
+def lcm(a, b):
+    return abs(a*b) // math.gcd(a, b)
+
+
+# an+b=xn+y -> n=(y-b)/(a-x)
+# -> cn+d
+def merge(a, b, x, y):
+    d1 = b
+    d2 = y
+    while d1 != d2:
+        if d1 < d2:
+            d1 += a * ((d2-d1-1) // a + 1)
+        else:
+            d2 += x * ((d1-d2-1) // x + 1)
+    d = d1
+    c = lcm(a, x)
+    return c, d
+
+
+lines = [x.strip() for x in open("input.txt")]
+busses = ((int(x), -idx) for idx, x in enumerate(lines[1].split(',')) if x != 'x')
+
+current = (1, 0)
+for bus in busses:
+    current = merge(*current, *bus)
+
+
+print(current[1])