# docs:
# - <https://nixos.wiki/wiki/Flakes>
# - <https://serokell.io/blog/practical-nix-flakes>

{
  inputs = {
    # nixpkgs.url = "nixpkgs/nixos-22.05";
    nixpkgs.url = "nixpkgs/nixos-unstable";
    mobile-nixos = {
      url = "github:nixos/mobile-nixos";
      flake = false;
    };
    home-manager = {
      url = "github:nix-community/home-manager/release-22.05";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    sops-nix.url = "github:Mic92/sops-nix";
    impermanence.url = "github:nix-community/impermanence";
  };

  outputs = { self, nixpkgs, mobile-nixos, home-manager, sops-nix, impermanence }:
  let
    patchedPkgs = system: nixpkgs.legacyPackages.${system}.applyPatches {
      name = "nixpkgs-patched-uninsane";
      src = nixpkgs;
      patches = import ./nixpatches/list.nix nixpkgs.legacyPackages.${system}.fetchpatch;
    };
    # return something which behaves like `pkgs`, for the provided system
    # `local` = architecture of builder. `target` = architecture of the system beying deployed to
    nixpkgsFor = local: target: import (patchedPkgs target) { crossSystem = target; localSystem = local; };
    # evaluate ONLY our overlay, for the provided system
    customPackagesFor = local: target: import ./pkgs/overlay.nix (nixpkgsFor local target) (nixpkgsFor local target);
    decl-machine = { name, local, target }:
    let
      nixosSystem = import ((patchedPkgs target) + "/nixos/lib/eval-config.nix");
    in (nixosSystem {
      # by default the local system is the same as the target, employing emulation when they differ
      system = target;
      specialArgs = { inherit mobile-nixos home-manager impermanence; };
      modules = [
        ./modules
        ./machines/${name}
        (import ./helpers/set-hostname.nix name)
        home-manager.nixosModule
        impermanence.nixosModule
        sops-nix.nixosModules.sops
        {
          nixpkgs.config.allowUnfree = true;
          nixpkgs.overlays = [
            (import "${mobile-nixos}/overlay/overlay.nix")
            (import ./pkgs/overlay.nix)
            (next: prev: {
              # non-emulated packages build *from* local *for* target.
              # for large packages like the linux kernel which are expensive to build under emulation,
              # the config can explicitly pull such packages from `pkgs.cross` to do more efficient cross-compilation.
              cross = (nixpkgsFor local target) // (customPackagesFor local target);
            })
          ];
        }
      ];
    });

    decl-bootable-machine = { name, local, target }: rec {
      nixosConfiguration = decl-machine { inherit name local target; };
      # this produces a EFI-bootable .img file (GPT with a /boot partition and a system (/ or /nix) partition).
      # after building this:
      #   - flash it to a bootable medium (SD card, flash drive, HDD)
      #   - resize the root partition (use cfdisk)
      #   - mount the part
      #      - chown root:nixbld <part>/nix/store
      #      - chown root:root -R <part>/nix/store/*
      #      - chown root:root -R <part>/persist  # if using impermanence
      #      - populate any important things (persist/, home/colin/.ssh, etc)
      #   - boot
      #   - if fs wasn't resized automatically, then `sudo btrfs filesystem resize max /`
      #   - checkout this flake into /etc/nixos AND UPDATE THE FS UUIDS.
      #   - `nixos-rebuild --flake './#<machine>' switch`
      img = nixosConfiguration.config.system.build.img;
    };
    machines.servo = decl-bootable-machine { name = "servo"; local = "aarch64-linux"; target = "aarch64-linux"; };
    machines.desko = decl-bootable-machine { name = "desko"; local = "x86_64-linux"; target = "x86_64-linux"; };
    machines.lappy = decl-bootable-machine { name = "lappy"; local = "x86_64-linux"; target = "x86_64-linux"; };
    machines.moby = decl-bootable-machine { name = "moby"; local = "aarch64-linux"; target = "aarch64-linux"; };
    # special cross-compiled variant, to speed up deploys from an x86 box to the arm target
    # note that these *do* produce different store paths, because the closure for the tools used to cross compile
    # v.s. emulate differ.
    # so deploying moby-cross and then moby incurs some rebuilding.
    machines.moby-cross = decl-bootable-machine { name = "moby"; local = "x86_64-linux"; target = "aarch64-linux"; };
    machines.rescue = decl-bootable-machine { name = "rescue"; local = "x86_64-linux"; target = "x86_64-linux"; };
  in {
    nixosConfigurations = builtins.mapAttrs (name: value: value.nixosConfiguration) machines;
    imgs = builtins.mapAttrs (name: value: value.img) machines;
    packages.x86_64-linux = customPackagesFor "x86_64-linux" "x86_64-linux";
    packages.aarch64-linux = customPackagesFor "aarch64-linux" "aarch64-linux";
  };
}