Day13

2020/src/day13.nim

@@ -1,3 +1,35 @@
-import streams
-proc part1*(s: Stream): int = 9
-proc part2*(s: Stream): int = 9
+import streams, sequtils, strutils
+
+proc part1*(s: Stream): int =
+  let schedule = toSeq(s.lines)
+  let departure = schedule[0].parseInt
+  var ss = schedule[1].split(",").filterIt(it != "x" and it != "").map(parseInt)
+  var mn = (0, departure*2)
+  for n in ss:
+    let q = n * ((departure div n) + 1)
+    if q < mn[1]:
+      mn = (n, q)
+  mn[0] * (mn[1] - departure)
+
+import tables
+proc part2*(s: Stream): int =
+  var ss = toSeq(s.lines)[1].split(",").filterIt(it != "")
+  var bb = initTable[int, int]()
+  for n in 0..<ss.len:
+    if ss[n] == "x": continue
+    bb[n] = ss[n].parseInt
+  let incr = bb[0]
+  var i = 0
+  while true:
+    # TODO: this takes too long, need to learn the math?
+    for j in 1..<ss.len:
+      if bb.contains j:
+        let d = i+j
+        let b = bb[j]
+        let nn = b * (d div b)
+        # echo ("checking", i, j, b, d, nn)
+        if d != nn: break
+        # echo ("good", j)
+        if j == ss.len-1: return i
+      else: continue
+    inc i, incr