Document Nix solution

2025/day01/README.md

@@ -0,0 +1,17 @@
+# Day 01: Nix
+
+Nix is a functional language made for the Nix package manager. As 
+
+To run the solution program, start the `nix` repl with the solution program and call the `solve`
+function with the path to the input file.
+
+```console
+$ nix repl --option max-call-depth 10000 --file solve.nix
+nix-repl> solve ./sample.txt
+```
+
+Some observations:
+
+- The `max-call-depth` needs to be bumped to at least 10k for the main input files, otherwise you
+  hit the limit in the recursion.
+- The standard library is lacking several basics but most of those you can build yourself

2025/day01/solve.nix

@@ -4,28 +4,34 @@
             data = builtins.readFile input_file;
             lines = builtins.filter (s: s != "" && builtins.isString s) (builtins.split "\n" data);
 
+            # Basic maths because the standard library doesn't have it
             mod = a: b: a - (a / b) * b;
+            abs = n: if n < 0 then -n else n;
 
-            recurse = list: position:
+            recurse = list: position: score1: score2:
                 let
                     first = builtins.head list;
-                    # This is not guaranteed to work but it does
+                    # This is not guaranteed to work but it's good enough
                     num_part = builtins.substring 1 999 first;
                     num = builtins.fromJSON num_part;
-                    next = if (builtins.substring 0 1 first) == "R"
-                        then
-                            position + num
-                        else
-                            position - num;
-
-                    new_position = mod ((mod next 100) + 100) 100; 
+                    delta = mod (if (builtins.substring 0 1 first) == "R" then num else -num) 100;
+                    next = mod (position + delta + 100) 100;
                     
-                    score = if new_position == 0 then 1 else 0;
-                in
-                    if list == [] then
+                    score = if next == 0 then 1 else 0;
+                    circles = num / 100;
+
+                    passed = if position == 0 then
                         0
                     else
-                        score + (recurse (builtins.tail list) new_position);
+                        if delta < 0 && delta + position <= 0 || delta > 0 && delta + position >= 100 then
+                            1
+                        else
+                            0;
+                in
+                    if list == [] then
+                        [score1 score2]
+                    else
+                        recurse (builtins.tail list) next (score + score1) (score2 + passed + circles);
         in
-            recurse lines 50;
+            recurse lines 50 0 0;
 }