Day 3

day_03/solve.nim

@@ -0,0 +1,43 @@
+import std/[strutils, sugar, options]
+
+let priorityReference = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
+
+const input = readFile("day_03/input.txt")
+
+proc priority(c: char): int {.inline.} =
+  priorityReference.find(c) + 1
+
+proc first[T](s: openArray[T], test: T -> bool): Option[T] =
+  for item in s:
+    if item.test:
+      return some(item)
+  return none[T]()
+
+iterator groups(s: string): seq[string] =
+  let lines = s.splitLines()
+  var i = 0
+  while i < lines.len:
+    yield lines[i..i+2]
+    i += 3
+
+# Find the item type that appears in both compartments of each rucksack.
+# What is the sum of the priorities of those item types?
+proc solvePart1() =
+  var sum = 0
+  for runsack in input.splitLines:
+    let compartment1 = runsack.substr(0, int(runsack.len / 2) - 1)
+    let compartment2 = runsack.substr(int(runsack.len / 2), runsack.len - 1)
+    sum += compartment1.first(c => compartment2.contains(c)).map(c => c.priority).get(0)
+  echo "Sum of misaligned items: " & $sum
+
+# Find the item type that corresponds to the badges of each three-Elf group.
+# What is the sum of the priorities of those item types?
+proc solvePart2() =
+  var sum = 0
+  for group in groups(input):
+    sum += group[0].first(c => group[1].contains(c) and group[2].contains(c)).map(c => c.priority).get(0)
+  echo "Sum of group badges: " & $sum
+
+if isMainModule:
+  solvePart1()
+  solvePart2()