2023 Day 9 Part 1

2023/day09.nim

@@ -0,0 +1,59 @@
+import std/algorithm
+import std/enumerate
+import std/math
+import std/sequtils
+import std/sets
+import std/strscans
+import std/strutils
+import std/sugar
+import std/tables
+
+proc dbg[T](v: T): T = (echo(v); v)
+
+type History = seq[int]
+
+func parseHistory(line: string): History =
+  line.strip.splitWhitespace.map(parseInt)
+
+func allZero(h: History): bool =
+  h.allIt(it == 0)
+
+func diff(h: History): History =
+  for (i, j) in zip(h[0..^1], h[1..^1]):
+    result.add(j - i)
+
+func deduce(h: History): seq[History] =
+  result.add h
+
+  var h = h
+  while not h.allZero:
+    h = h.diff
+    result.add h
+
+type Histories = seq[History]
+
+func parseHistories(text: string): Histories =
+  text.strip.splitLines.map(parseHistory)
+
+func extrapolate(hs: var Histories): seq[int] =
+  assert(hs[^1][^1] == 0)
+
+  var row = len(hs) - 1
+  result.add 0
+  hs[row].add 0
+
+  while row > 0:
+    let inferred = hs[row-1][^1] + hs[row][^1]
+    result.add inferred
+    hs[row-1].add inferred
+    dec row
+
+proc part1(): int =
+  let text = readFile("input/day09.txt")
+  var histories = parseHistories(text)
+
+  for h in histories:
+    var hh = deduce(h)
+    result.inc extrapolate(hh)[^1]
+
+echo part1()