Day Nine

2 files changed, 1029 insertions, 0 deletions

diff --git a/2020/input/nine.txt b/2020/input/nine.txt
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diff --git a/2020/nine.nim b/2020/nine.nim
new file mode 100644
index 0000000..8b15391
--- /dev/null
+++ b/2020/nine.nim
@@ -0,0 +1,29 @@
+# Day Nine: Encoding Error
+import os, strutils, sequtils
+
+let input: string = paramStr(1)
+let xmas: seq[int] = map(split(strip(readFile(input)), '\n'), parseInt)
+const preamble: int = 25
+var encryption, weakness, smallest: int
+
+for i in preamble ..< len(xmas):
+  var valid: bool
+  for j in 0 .. preamble:
+    for k in 0 .. preamble:
+      if xmas[i - k] + xmas[i - j] == xmas[i]:
+        valid = true
+  if not valid:
+    encryption = xmas[i]
+for largest in 0 ..< len(xmas):
+  var sum: int = 0
+  for val in smallest .. largest:
+    inc(sum, xmas[val])
+  while sum > encryption:
+    dec(sum, xmas[smallest])
+    inc(smallest)
+  if sum == encryption:
+    weakness = xmas[smallest] + xmas[largest]
+    break
+
+echo encryption
+echo weakness