flake.nix

# adapted from:
# - https://litchipi.github.io/nix/2023/01/12/build-jekyll-blog-with-nix.html
# - https://technogothic.net/pages/JekyllOnNix/
{
  description = "Build the blog";

  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";

  outputs = inputs: with inputs; let
    system = "x86_64-linux";
    pkgs = import nixpkgs { inherit system; };

    # Specify gems and dependencies here
    jekyll-version = "~> 4.2.2";

    inc_gems = {
      webrick = "~> 1.7";
    };

    inc_plugins = {
      jekyll-scholar = "~> 7.0";
    };

    # Generation of the Gemfile
    generate_gemfile = let
      gems = builtins.concatStringsSep "\n" (pkgs.lib.attrsets.mapAttrsToList (name: version:
        "gem \"${name}\", \"${version}\""
      ) inc_gems);

      plugins = builtins.concatStringsSep "\n" (pkgs.lib.attrsets.mapAttrsToList (name: version:
        "  gem \"${name}\", \"${version}\""
      ) inc_plugins);
    in
      ''
        # This file is autogenerated by the flake.nix file, do not edit
        source "https://rubygems.org"

        # Gems dependencies to be installed
        ${gems}
        # HACK to keep sass-embedded working
        gem "jekyll", "${jekyll-version}", group: :jekyll_plugins

        # Github Pages plugins
        group :jekyll_plugins do
        ${plugins}
        end
      '';

    # The build environment
    env = pkgs.bundlerEnv {
      name = "blog";
      ruby = pkgs.ruby;
      gemdir = ./.;
    };

    # Utility to run a script easily in the flakes app
    simple_script = name: add_deps: text: let
      exec = pkgs.writeShellApplication {
        inherit name text;
        runtimeInputs = with pkgs; [
          gnumake
          gcc
          bundler
        ] ++ add_deps;
      };
    in {
      type = "app";
      program = "${exec}/bin/${name}";
    };

  in {
    apps.${system} = {
      # nix run -> serves the website locally
      default = simple_script "serve_blog" [] ''
        echo "Bundler env: ${env}"
        ${env}/bin/bundler exec -- jekyll serve --trace
      '';

      # nix run .#build -> builds the website locally
      build = simple_script "build_blog" [] ''
        echo "Bundler env: ${env}"
        ${env}/bin/bundler exec -- jekyll build --trace
      '';

      # nix run .#generate -> Re-generate the gemfile, lockfile, build environment and gemset.nix
      #   To use only if added a dependency, bumped a version, etc ...
      generate = simple_script "generate_blog_env" [ pkgs.bundix ] ''
        set -e

        rm -f gemset.nix Gemfile Gemfile.lock .bundle/config
        cat << EOF > Gemfile
        ${generate_gemfile}
        EOF

        export BUNDLE_PATH=vendor
        export BUNDLE_CACHE_ALL=true
        export BUNDLE_NO_INSTALL=true
        export BUNDLE_FORCE_RUBY_PLATFORM=true

        bundler update
        bundler lock
        bundler package
        bundix --magic
        rm -rf vendor
      '';
    };
  };
}