Add day7 problem + solution

day7/p2.nim

@@ -0,0 +1,38 @@
+import os
+import std/[strutils, sequtils]
+
+proc main() =
+  # the idea here is to find a position from all possible
+  # crab positions at which the sum of all fuel consumption
+  # is the minimum. So we find the total fuel consumption for
+  # all possible crab positions, then pick the minimum one
+  let
+    fileName = paramStr(1)
+    f = open(fileName, fmRead)
+
+  var crabs = f.readLine().split(",").mapIt(it.strip.parseInt)
+
+  f.close()
+
+  let
+    maxVal = max(crabs)
+
+  var
+    minSum = int.high
+    minPos = -1
+
+  for pos in 0 .. maxVal:
+    var sum = 0
+    for crab in crabs:
+      let N = abs(crab - pos)
+      # sum of 1 + 2 + 3 + ... N is N(N+1)/2
+      sum += int((N*(N+1))/2)
+    if sum < minSum:
+      minSum = sum
+      minPos = pos
+
+  # sum of 1 + 2 + 3 + ... N is N(N+1)/2
+  let fuel = crabs.mapIt(int(abs(it-minPos)*(abs(it-minPos)+1)/2)).foldl(a+b)
+  echo "Min fuel: ", fuel
+
+main()