From 6c0caca1e16d449525e0cd796bf8b0efa8102b72 Mon Sep 17 00:00:00 2001
From: Alfred Melch <dev@melch.pro>
Date: Thu, 17 Dec 2020 14:00:54 +0100
Subject: [PATCH] Solve 2020/13

---
 2020/13/input        |  2 ++
 2020/13/solution1.py | 22 ++++++++++++
 2020/13/solution2.py | 82 ++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 106 insertions(+)
 create mode 100644 2020/13/input
 create mode 100644 2020/13/solution1.py
 create mode 100644 2020/13/solution2.py

diff --git a/2020/13/input b/2020/13/input
new file mode 100644
index 0000000..096e2cc
--- /dev/null
+++ b/2020/13/input
@@ -0,0 +1,2 @@
+1002618
+19,x,x,x,x,x,x,x,x,41,x,x,x,37,x,x,x,x,x,367,x,x,x,x,x,x,x,x,x,x,x,x,13,x,x,x,17,x,x,x,x,x,x,x,x,x,x,x,29,x,373,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,23
diff --git a/2020/13/solution1.py b/2020/13/solution1.py
new file mode 100644
index 0000000..c6edf97
--- /dev/null
+++ b/2020/13/solution1.py
@@ -0,0 +1,22 @@
+import re
+
+with open('input', 'r') as f:
+    arrival = int(f.readline())
+    buses = list(map(int, re.findall(r'[0-9]+', f.readline().strip())))
+
+earliest = None
+earliest_bus = None
+
+print(f'{arrival=}')
+
+for bus in buses:
+    wait = bus - arrival % bus
+    wait = 0 if wait == bus else wait
+    if earliest is None or wait < earliest:
+        earliest = wait
+        earliest_bus = bus
+    print(bus, wait)
+
+print(f'{earliest=} {earliest_bus=}')
+print('Solution')
+print(earliest * earliest_bus)
diff --git a/2020/13/solution2.py b/2020/13/solution2.py
new file mode 100644
index 0000000..deaf66f
--- /dev/null
+++ b/2020/13/solution2.py
@@ -0,0 +1,82 @@
+"""Solution to 2020/13 part2
+
+Declarations:
+t:                      timestamp
+bus_id, bus:            used as slope (a) in the linear function
+                        observation: bus_ids are distinct prime numbers
+offset:                 postion of the bus in the list
+                        t + offset = arrival of bus
+normalized_offset:      used so f(0) is a correct (non-negative) solution
+                        = -offset % bus_id
+                        used as intercept (b) in linear function
+bus_solution:           solutions for y in the scope of one bus.
+solution:	        smallest y that is a solution to all linear functions
+
+Functions:
+- The solutions per bus. linear (y=a*x+b)
+- t = bus_id * x + normalized_offset
+- x: natural numbers. xth solution (time of arrival - index)
+- y: solutions for t in the scope of one bus. t + delta = arrival of bus
+
+Combining two functions:
+- Given      y = x0 * a0 + b0
+- Given      y = x1 * a1 + b1
+- x0 * a0 + b0 = x1 * a1 + b1
+-           x1 = (x0 * a0 + b0 - b1) / a1
+- Remember: x0 and x1 have to be whole numbers
+- Find linear function x0 = n * a + b that describes the possible solutions
+- Let x0 be in linear function x0 = n * a + b
+- x1 = ((n * a + b) * a0 + b0 - b1) / a1
+- x1 = (n * a * a0 + a0 * b + b0 -b1) / a1
+- x1 = (n * a * a0) / a1 + (a0 * b + b0 - b1) / a1
+- For x1 to be a whole number:
+  - (n * a * a0) needs to be divisible by a1
+  - since a's prime factors are distinct from a1 set a = a1
+  - a0 * b + b0 - b1 needs to be divisible by a1
+  - find b by brute force (b = magic_number)
+- => x0 = n * a1 + magic_number
+- Apply to y = x0 * a0 + b0
+- y = (n * a1 + magic_number) * a0 + b0
+- y = n * (a0 * a1) + (a0 + b0 + magic_number)
+  - new a = a0 * a1
+  - new b = a0 + b0 + magic_number
+
+Apply this schema recursively (or iteratively) to all functions. Each interim
+result represents a formula for all possible solutions that includes the
+processed bus_ids. At last set n = 0 (in y = n * a + b) to receive the smallest
+solution (earliest timestamp).
+"""
+
+with open('input', 'r') as f:
+    f.readline()
+    buses = f.readline().strip().split(',')
+
+departures = []
+i = 0
+for offset, bus in enumerate(buses):
+    try:
+        bus = int(bus)
+        normalized_offset = -offset % bus
+        print(f'y = x{i} * {bus:>3} + {normalized_offset:>3}   ({offset=:>2})')
+        departures.append((bus, normalized_offset))
+        i += 1
+    except ValueError:
+        pass
+print()
+
+a = 1
+b = 0
+for a_cur, b_cur in departures:
+    magic_nr = None
+    i = 0
+    while magic_nr is None:
+        if (a * i + b - b_cur) % a_cur == 0:
+            magic_nr = i
+        i += 1
+    b = b + a * magic_nr
+    a = a * a_cur
+    print(f'y = n * {a} + {b} | {magic_nr=}')
+print()
+
+print('Solution:')
+print(b)