Solve day 2 part 2

Author: fwcd

day02/derivation.nix

@@ -18,7 +18,7 @@
         echo "Usage: \$0 <path to input>"
         exit 1
       fi
-      exec nix-instantiate --eval --strict --arg inputPath "\"\$(${coreutils.outPath}/bin/realpath \$1)\"" "\$(dirname "\$0")/../share/day02.nix"
+      exec nix-instantiate --eval --show-trace --strict --arg inputPath "\"\$(${coreutils.outPath}/bin/realpath \$1)\"" "\$(dirname "\$0")/../share/day02.nix"
       EOF
 
       chmod +x $out/bin/day02

day02/src/day02.nix

@@ -1,16 +1,21 @@
 { inputPath, lib ? import <nixpkgs/lib> }:
-  let inherit (builtins) length readFile;
-      inherit (lib.lists) all count filter head tail zipListsWith;
+  let inherit (builtins) head tail length readFile;
+      inherit (lib.lists) all any count filter range zipListsWith;
       inherit (lib.strings) splitString toInt stringLength;
+      inherit (lib.trivial) flip;
   in
   let abs        = x: if x >= 0 then x else -x;
       compose    = f: g: x: f (g x);
+      dropNth    = n: xs: if n == 0 then tail xs else [(head xs)] ++ dropNth (n - 1) (tail xs);
+      removeOne  = r: map (flip dropNth r) (range 0 (length r - 1));
       input      = readFile inputPath;
       isNonEmpty = s: stringLength s > 0;
       lines      = filter isNonEmpty (splitString "\n" input);
       reports    = map (compose (map toInt) (splitString " ")) lines;
-      isSafe     = r: let diffs = zipListsWith (x: y: x - y) r (tail r);
+      isSafe1    = r: let diffs = zipListsWith (x: y: x - y) r (tail r);
                       in (all (x: x > 0) diffs || all (x: x < 0) diffs)
                          && all (x: x >= 1 && x <= 3) (map abs diffs);
-      part1      = count isSafe reports;
-  in "Part 1: " + toString part1
+      isSafe2    = compose (any isSafe1) removeOne;
+      part1      = count isSafe1 reports;
+      part2      = count isSafe2 reports;
+  in "Part 1: " + toString part1 + ", Part 2: " + toString part2