Day Thirteen

2 files changed, 31 insertions, 0 deletions

diff --git a/2020/input/thirteen.txt b/2020/input/thirteen.txt
new file mode 100644
index 0000000..ca9d2bb
--- /dev/null
+++ b/2020/input/thirteen.txt
@@ -0,0 +1,2 @@
+1015292
+19,x,x,x,x,x,x,x,x,41,x,x,x,x,x,x,x,x,x,743,x,x,x,x,x,x,x,x,x,x,x,x,13,17,x,x,x,x,x,x,x,x,x,x,x,x,x,x,29,x,643,x,x,x,x,x,37,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,23
diff --git a/2020/thirteen.nim b/2020/thirteen.nim
new file mode 100644
index 0000000..d01e681
--- /dev/null
+++ b/2020/thirteen.nim
@@ -0,0 +1,29 @@
+# Day Thirteen: Shuttle Search
+import os, strutils, sequtils
+
+let input: string = paramStr(1)
+let arrival: int = parseInt(splitLines(readFile(input))[0])
+let buses: seq[int] = map(split(splitLines(readFile(input))[1], ','),
+  func (bus: string): int =
+    if bus == "x":
+      0
+    else:
+      parseInt(bus))
+
+var earliest: tuple[id, wait: int] = (999, 999)
+for bus in buses:
+  if bus > 0:
+    if bus - (arrival mod bus) < earliest.wait:
+      earliest = (bus, bus - (arrival mod bus))
+echo earliest.id * earliest.wait
+
+# blatantly stolen code - what on earth is a chinese remainder theorem?
+# (c: lizthegrey)
+var timestamp: int = 0
+var step: int = 1
+for i, bus in buses:
+  if bus != 0:
+    while (timestamp+i) mod bus != 0:
+      timestamp += step
+    step *= bus
+echo timestamp