Add day9 solution.

day9.cr

@@ -0,0 +1,37 @@
+require "advent"
+INPUT = input(2020, 9).lines.map(&.to_i64) 
+
+def is_sum(is, from, to, n)
+  if to <= from
+    return n == 0_i64
+  end
+  return is_sum(is, from, to-1, n) || is_sum(is, from, to-1, n-is[to-1])
+end
+
+def part1(input)
+  is = input
+  i = 25
+  loop do
+    return is[i] unless is_sum(is, i-25, i, is[i])
+    i += 1
+  end
+end
+
+def part2(input, i)
+  input.each_with_index do |e, j|
+    next if e == i
+    acc = i-e
+    k = j
+    while acc > 0
+      k += 1
+      acc -= input[k]
+    end
+    if acc == 0
+      min, max = input[j..k].minmax
+      return min+max
+    end
+  end
+end
+
+p1 = part1(INPUT.clone)
+puts part2(INPUT.clone, p1)