From 422a378ec06d1c06909a342efa6964cc828e425e Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Mon, 4 Dec 2023 23:08:03 -0800
Subject: [PATCH] 2023 Day 5 Part 2

---
 2023/day05.nim | 91 ++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 88 insertions(+), 3 deletions(-)

diff --git a/2023/day05.nim b/2023/day05.nim
index 0a275e3..31f48da 100644
--- a/2023/day05.nim
+++ b/2023/day05.nim
@@ -44,8 +44,8 @@ func lookup(m: Map, src: int): int =
       return remap.lookup(src)
   src
 
-type Almanac = tuple
-  seeds: HashSet[int]
+type Almanac = object
+  seeds: seq[int]
   mapsBySrc: Table[string, Map]
   mapsByDst: Table[string, Map]
 
@@ -57,7 +57,7 @@ func parseAlmanac(s: string): Almanac =
     raise newException(ValueError, sections[0])
 
   for num in seeds.strip.splitWhitespace:
-    result.seeds.incl num.parseInt
+    result.seeds.add num.parseInt
 
   for section in sections[1..^1]:
     let map = parseMap(section)
@@ -85,4 +85,89 @@ proc part1(): int =
 
   locations.min
 
+type Range = tuple
+  lo: int
+  hi: int
+
+func newRange(start: int, length: int): Range =
+  (start, start + length - 1)
+
+func empty(r: Range): bool =
+  r.lo > r.hi
+
+func intersect(a: Range, b: Range): Range =
+  (max(a.lo, b.lo), min(a.hi, b.hi))
+
+proc lookup(r: Remap, src: Range): Range =
+  let delta = r.dst_start - r.src_start
+  (src.lo + delta, src.hi + delta)
+
+proc lookup(m: Map, src: Range): seq[Range] =
+  var unmapped = @[src]
+
+  for remap in m.remaps:
+    var still_unmapped: seq[Range]
+    for src in unmapped:
+      let rr = newRange(remap.src_start, remap.length)
+      if src.intersect(rr).empty:
+        still_unmapped.add src
+        continue
+
+      let dn = src.lo
+      let lo = max(src.lo, rr.lo)
+      let hi = min(src.hi, rr.hi)
+      let up = src.hi
+
+      let prefix = (dn, lo - 1)
+      let inside = (lo, hi)
+      let suffix = (hi + 1, up)
+
+      if not prefix.empty:
+        still_unmapped.add prefix
+
+      if not suffix.empty:
+        still_unmapped.add suffix
+
+      if not inside.empty:
+        result.add remap.lookup(inside)
+
+    unmapped = still_unmapped
+
+  for src in unmapped:
+    result.add src
+
+proc lookup(m: Map, srcs: seq[Range]): seq[Range] =
+  for src in srcs:
+    result.add m.lookup(src)
+
+proc lookup(a: Almanac, dst: string, seeds: seq[Range]): seq[Range] =
+  var map = a.mapsBySrc["seed"]
+  var src = seeds
+
+  while true:
+    src = map.lookup(src)
+    if map.dst == dst:
+      return src
+
+    map = a.mapsBySrc[map.dst]
+
+proc part2(): int =
+  let text = readFile("input/day05.txt")
+  let almanac = parseAlmanac(text)
+
+  var ranges: seq[Range]
+
+  for i in 0 ..< floorDiv(almanac.seeds.len, 2):
+    let start = almanac.seeds[2*i]
+    let len = almanac.seeds[2*i + 1]
+    ranges.add newRange(start, len)
+
+  let locations = collect(newSeq):
+    for locs in almanac.lookup("location", ranges):
+      if not locs.empty:
+        locs.lo
+
+  locations.min
+
 echo part1()
+echo part2()