Day 9 part 2 done

2021/nine.nim

@@ -1,4 +1,4 @@
-import ./common, std/[strutils, sequtils, strformat, sugar, sets, math]
+import ./common, std/[strutils, sequtils, strformat, sugar, sets, math, algorithm]
 
 proc toGrid(s: seq[string]): seq[seq[int]] =
   for l in s: result.add(l.mapIt(int(it)-int('0')))
@@ -14,11 +14,37 @@ proc p1(c: seq[seq[int]]): int =
       if y < h-1 and c[y+1][x] <= n: continue
       result += n + 1
 
+proc p2(c: seq[seq[int]]): int =
+  let h = c.len()
+  let w = c[0].len()
+  var crawledPoints = toHashSet[(int, int)]([])
+  var basins: seq[int] = @[]
+  for y,l in c.pairs():
+    for x,n in l.pairs():
+      if crawledPoints.contains((x, y)): continue
+      crawledPoints.incl((x, y))
+      if n >= 9: continue
+      var eligiblePoints = toHashSet[(int, int)]([(x, y)])
+      var basin = 0
+      while eligiblePoints.len() > 0:
+        var newEligiblePoints = toHashSet[(int, int)]([])
+        for (cx,cy) in eligiblePoints:
+          inc basin
+          for (nx,ny) in [(cx-1,cy),(cx+1,cy),(cx,cy-1),(cx,cy+1)]:
+            if crawledPoints.contains((nx,ny)): continue
+            if nx >= 0 and nx < w and ny >= 0 and ny < h and c[ny][nx] < 9:
+              newEligiblePoints.incl((nx,ny))
+              crawledPoints.incl((nx,ny))
+        eligiblePoints = newEligiblePoints
+      basins.add(basin)
+  basins.sort(system.cmp[int], SortOrder.Descending)
+  basins[0] * basins[1] * basins[2]
+
 doDay(9, (n) => n.loadInput().toGrid(),
   p1,
-  p1,
+  p2,
   """2199943210
 3987894921
 9856789892
 8767896789
-9899965678""".split('\n').toGrid(), 15, 15)
+9899965678""".split('\n').toGrid(), 15, 1134)