day[01]: solution for Part 1 and Part 2

init, Codex-generated scaffolding

.direnv/flake-profile

···
+flake-profile-20-link

.direnv/flake-profile-20-link

···
+/nix/store/qd65s0dwvzj0w0lv3qm6zci2sx8qn7d4-nix-shell-env

.envrc

···
+use flake

.gitignore

···
+build/
+.cache/

Justfile

···
+set shell := ["bash", "-eu", "-o", "pipefail", "-c"]
+
+default_day := `date +'%d'`
+
+default: sync build
+
+# Run this whenever you change build files. This generates the latest build config.
+sync:
+  meson setup --reconfigure build/
+
+# Build everything Meson knows about.
+build:
+  meson compile -C build/
+
+# Run tests for a specific day. Defaults to today's day-of-month.
+test day=default_day: sync
+  day=$(printf '%02d' "{{day}}"); meson test -C build "day${day}"
+
+# Run a day's solution on the "real" data set.
+run day=default_day: sync
+  day=$(printf '%02d' "{{day}}"); meson compile -C build "run-day${day}"
+
+# Clean the build folder. Useful... sometimes.
+clean:
+  meson setup --wipe build/

README.md

···
+# Advent of Code 2025 (C++)
+
+A Meson + Just playground tailored for day-by-day AoC work. Each day is its own executable that knows where to find its data without manual path plumbing.
+
+## Workflow
+
+```sh
+nix develop     # optional if tooling is installed already; brings in clang/meson/just
+just            # runs sync + build
+just test 5     # run tests for day 05
+just run        # run today's solution on "real" data
+```
+
+`just test`/`run` default to today's day-of-month; pass an explicit day to override. Meson owns the build/run logic; Just only wires day selection and delegates.
+
+## Layout
+
+- `src/<day>/solution.cxx` — main for each day (`src/00/solution.cxx` is a stub).
+- `data/<day>/` — input directory for that day.
+- `DATA_FOLDER` — compile-time string macro injected by Meson pointing at `data/<day>/`.
+
+Add new days by creating a `src/<day>/solution.cxx` (use two-digit day numbers) and a matching `data/<day>/` folder. Re-run `just sync` after adding new days so Meson picks them up.

compile_commands.json

···
+builddir/compile_commands.json

data/00/input.txt

···
+sample line 1
+sample line 2
+another line for testing access to DATA_FOLDER

data/01/input.txt

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+L27

data/01/test.txt

···
+L68
+L30
+R48
+L5
+R60
+L55
+L1
+L99
+R14
+L82

flake.lock

···
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1763464769,
+        "narHash": "sha256-AJHrsT7VoeQzErpBRlLJM1SODcaayp0joAoEA35yiwM=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "6f374686605df381de8541c072038472a5ea2e2d",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

···
+{
+  description = "Flake-based development environment for C++.";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable"; # unstable Nixpkgs
+
+  outputs =
+    { self, ... }@inputs:
+
+    let
+      supportedSystems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
+      forEachSupportedSystem =
+        f:
+        inputs.nixpkgs.lib.genAttrs supportedSystems (
+          system:
+          f {
+            pkgs = import inputs.nixpkgs { inherit system; };
+          }
+        );
+    in
+    {
+      devShells = forEachSupportedSystem (
+        { pkgs }:
+        {
+          default =
+            pkgs.mkShell.override
+              {
+                # Override stdenv in order to change compiler:
+                stdenv = pkgs.clangStdenv;
+                # Comment the line above if you want GCC
+              }
+              {
+                packages =
+                  with pkgs;
+                  [
+                    clang-tools # IMPORTANT - it **always** MUST stay right above clang
+                    clang
+
+                    codespell
+                    cppcheck
+                    doxygen
+                    gtest
+                    lcov
+                    vcpkg
+                    vcpkg-tool
+
+                    # BUILD
+                    pkg-config
+                    meson
+                    ninja
+                    just
+
+                    # LIBRARIES
+                    libcxx
+
+                    # DEBUGGERS
+                    lldb
+                  ]
+                  ++ (if system == "aarch64-darwin" then [ ] else [ gdb ]);
+
+                  env = {
+                    # Tell clangd to use the real clang wrapper that *knows* about include paths.
+                    CLANGD_FLAGS = "--query-driver=${pkgs.clang}/bin/clang*";
+
+                    # Ensure Meson picks clang instead of GCC.
+                    CC = "${pkgs.clang}/bin/clang";
+                    CXX = "${pkgs.clang}/bin/clang++";
+                  };
+              };
+        }
+      );
+
+      # Feel free to delete this once the project is initialized.
+      templates.default = {
+        path = ./.;
+        description = "Meson-based C++ project template";
+      };
+    };
+}

meson.build

···
+project(
+  'aoc25',
+  'cpp',
+  default_options : [
+    'warning_level=3',
+    'cpp_std=c++23',
+  ],
+)
+
+subdir('src')

src/00/solution.cxx

···
+#include <cstdlib>
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+#include <print>
+#include <string_view>
+#include <string>
+
+// DATA_FOLDER is injected by Meson so solutions do not have to care about paths.
+#ifndef DATA_FOLDER
+#error "DATA_FOLDER is not defined. Meson should inject it via cpp_args."
+#endif
+
+int main(int argc, char **argv) {
+  std::string_view mode = "real";
+  if (const char *env_mode = std::getenv("AOC_MODE")) {
+    mode = env_mode;
+  }
+  if (argc > 1) {
+    mode = argv[1];
+  }
+
+  std::println("Day 00 placeholder");
+  std::println("Mode: {}", mode);
+  std::println("Data folder: {}", DATA_FOLDER);
+
+  const std::filesystem::path data_root{DATA_FOLDER};
+  const auto input_path = data_root / "input.txt";
+
+  std::ifstream input{input_path};
+  if (!input.is_open()) {
+    std::println(std::cerr, "Failed to open {}", input_path.string());
+    return EXIT_FAILURE;
+  }
+
+  std::println("Contents of {}:", input_path.string());
+  for (std::string line; std::getline(input, line);) {
+    std::println("  {}", line);
+  }
+
+  return EXIT_SUCCESS;
+}

src/01/solution.cxx

···
+#include "common/istest.h"
+#include <algorithm>
+#include <cstdlib>
+#include <filesystem>
+#include <fstream>
+#include <print>
+#include <ranges>
+#include <string>
+#include <string_view>
+
+#ifndef DATA_FOLDER
+#error "DATA_FOLDER is not defined. Meson should inject it via cpp_args."
+#endif // !DATA_FOLDER
+
+const std::filesystem::path data_root{DATA_FOLDER};
+
+struct Rotary {
+  int value = 50;
+};
+
+// @return how many times the rotary crossed 0, but *not* the times it stopped
+// at 0
+int rotate(Rotary &rotary, int value) {
+  int result = rotary.value + value;
+  int crossed_0 = 0;
+
+  int begin = std::min(result, rotary.value); // looks modern, but it's really
+  int end = std::max(result, rotary.value);   // hideous
+  auto range = std::ranges::views::iota(begin + 1, end) |
+               std::ranges::views::filter([](int x) { return x % 100 == 0; });
+
+  for (int _ : range) { // Kotlin .forEach would have looked so much better
+    crossed_0 += 1;
+  }
+
+  while (result < 0) {
+    result += 100;
+  }
+  while (result > 99) {
+    result -= 100;
+  }
+  rotary.value = result;
+
+  return crossed_0;
+}
+
+int main() {
+  // scaffolding
+  std::string data_path{};
+  if (is_test()) {
+    data_path = data_root / "test.txt";
+  } else {
+    data_path = data_root / "input.txt";
+  }
+
+  std::ifstream input{data_path};
+  if (!input.is_open()) {
+    std::println("Can't open the file.");
+    return 1;
+  }
+  // end scaffolding
+
+  Rotary rotary{};
+  int password_part_1 = 0;
+  int password_part_2 = 0;
+
+  int times_crossed_0 = 0;
+
+  // horrendous code below
+
+  for (std::string line; std::getline(input, line);) {
+    std::string_view rotation = std::string_view(line.data(), 1);
+    std::string_view number_sv = std::string_view(line.data() + 1);
+
+    auto number = std::stoi(number_sv.data());
+
+    if (rotation == "L") {
+      number = -number;
+    }
+
+    // ridiculous - a function has both side effect and a return value
+    // this shit reminds me of out parameter convention so much...
+    times_crossed_0 += rotate(rotary, number);
+
+    if (rotary.value == 0) {
+      password_part_1 += 1;
+    }
+  }
+
+  password_part_2 = password_part_1 + times_crossed_0;
+
+  std::println("Eureka! {} / {}", password_part_1, password_part_2);
+
+  input.close();
+  return 0;
+}

src/common/istest.cxx

···
+#include <cstdlib>
+#include <string_view>
+
+bool is_test() {
+  std::string_view mode = "real";
+  if (const char *env_mode = std::getenv("AOC_MODE")) {
+    mode = env_mode;
+  }
+
+  return mode == "test";
+}

src/common/istest.h

···
+bool is_test();

src/meson.build

···
+common_inc = include_directories('common')
+common_lib = static_library(
+  'aoc_common',
+  sources: files('common/istest.cxx'),
+  include_directories: common_inc,
+)
+
+day_list_cmd = run_command(
+  'python3',
+  '-c',
+  '''
+import os, sys
+root = sys.argv[1]
+for name in sorted(os.listdir(root)):
+    if name == 'common':
+        continue
+    path = os.path.join(root, name)
+    if os.path.isdir(path) and os.path.isfile(os.path.join(path, 'solution.cxx')):
+        print(name)
+''',
+  meson.current_source_dir(),
+  check: true,
+)
+
+day_dirs_raw = day_list_cmd.stdout().strip()
+day_dirs = []
+if day_dirs_raw != ''
+  day_dirs = day_dirs_raw.split('\n')
+endif
+
+foreach day : day_dirs
+  day_src = join_paths(day, 'solution.cxx')
+  day_data = join_paths(meson.project_source_root(), 'data', day)
+
+  day_exe = executable(
+    'day' + day,
+    sources: day_src,
+    include_directories: [common_inc],
+    link_with: common_lib,
+    cpp_args: ['-DDATA_FOLDER="@0@"'.format(day_data)],
+  )
+
+  test(
+    'day' + day,
+    day_exe,
+    args: ['--mode=test'],
+    env: {
+      'DATA_FOLDER': day_data,
+      'AOC_MODE': 'test',
+    },
+  )
+
+  run_target(
+    'run-day' + day,
+    command: day_exe,
+    env: {
+      'DATA_FOLDER': day_data,
+      'AOC_MODE': 'real',
+    },
+  )
+endforeach