#!/usr/bin/env bash
set -euo pipefail

## Configuration

readonly libexec_dir="${0%/*}"
readonly format_dir=$libexec_dir/formats

configuration=${NIXOS_CONFIG:-$libexec_dir/configuration.nix}
flake_uri=
flake_attr=
format_path=
target_system=
cores=
run=
nix_args=(
  "$libexec_dir/nixos-generate.nix"
)
has_outlink=false
nix_build_args=()

## Functions

showUsage() {
  cat <<USAGE
Usage: $0 [options]

Options:

* --help: shows this help
* -c, --configuration PATH:
    select the nixos configuration to build. Default: $configuration
* --flake URI:
    selects the nixos configuration to build, using flake uri like "~/dotfiles#my-config"
* -f, --format NAME: select one of the pre-determined formats
* --format-path PATH: pass a custom format
* --list: list the available built-in formats
* --run: runs the configuration in a VM
         only works for the "vm" and "vm-nogui" formats
* --show-trace: show more detailed nix evaluation location information
* --system: specify the target system (eg: x86_64-linux)
* -o, --out-link: specify the outlink location for nix-build
* --cores : to control the maximum amount of parallelism. (see nix-build documentation)
* --option : Passed to nix-build (see nix-build documentation).
* --special-arg : Passed to the "specialArgs" variable
* -I KEY=VALUE: Add a key to the Nix expression search path.
USAGE
}

listFormats() {
  for format in "$format_dir"/*.nix; do
    basename "$format" ".nix"
  done
}

abort() {
  echo "aborted: $*" >&2
  exit 1
}

## Main ##

while [[ $# -gt 0 ]]; do
  case "$1" in
    -c | --configuration)
      configuration=$2
      shift
      ;;
    --flake)
      # Note: The reason I'm using awk over cut is because cutting with an
      # out-of-bounds field will return the last in-bound field instead of empty
      # string.
      flake="$(echo "$2" | awk -F'#' '{ print $1; }')"

      flake_uri="$(nix flake metadata --json -- "$flake" | jq -r .url)"
      flake_attr="$(echo "$2" | awk -F'#' '{ print $2; }')"
      shift
      ;;
    --cores)
      cores=$2
      shift
      ;;
    --disk-size)
      nix_build_args+=("--argstr" "diskSize" "$2")
      shift
      ;;
    --special-arg)
      nix_build_args+=("--argstr" "$2" "$3")
      shift 2
      ;;
    --option)
      nix_build_args+=("$1" "$2" "$3")
      shift 2
      ;;
    -f | --format)
      format_path=$format_dir/$2.nix
      shift
      ;;
    --format-path)
      format_path=$2
      shift
      ;;
    --help)
      showUsage
      exit
      ;;
    --list)
      listFormats
      exit
      ;;
    --run)
      run=1
      # default to the VM format
      if [[ -z $format_path ]]; then
        format_path=$format_dir/vm.nix
      fi
      ;;
    --show-trace)
      nix_args+=(--show-trace)
      ;;
    --system)
      target_system=$2
      shift
      ;;
    -I)
      nix_args+=(-I "$2")
      shift
      ;;
    -o | --out-link)
      nix_build_args+=(--out-link "$2")
      has_outlink=true
      shift
      ;;
    *)
      abort "unknown option $1"
      ;;
  esac
  shift
done

if ! $has_outlink; then
  nix_build_args+=(--no-link)
fi

if [[ -z $format_path ]]; then
  abort "missing format. use --help for more details"
fi

if [[ ! -f $format_path ]]; then
  abort "format file not found: $format_path"
fi

nix_args+=(--argstr formatConfig "$(realpath "$format_path")")

if [[ -z "$flake_uri" ]]; then
  nix_args+=(
    -I "nixos-config=$configuration"
  )
else
  nix_args+=(
    --argstr flakeUri "$flake_uri"
    --argstr flakeAttr "${flake_attr:-"$(hostname)"}"
  )
fi

if [[ -n $target_system ]]; then
  nix_args+=(--argstr system "$target_system")
fi

if [[ -n $cores ]]; then
  nix_args+=(--cores "$cores")
fi

formatAttr=$(nix-instantiate "${nix_args[@]}" --eval --json -A config.formatAttr | jq -r .)
fileExtension=$(nix-instantiate "${nix_args[@]}" --eval --json -A config.fileExtension | jq -r .)

out=$(nix build --extra-experimental-features nix-command -L --print-out-paths --file "${nix_args[@]}" "${nix_build_args[@]}" "config.system.build.$formatAttr")

if [[ -z $run ]]; then
  # show the first file, ignoring nix-support
  find "$out" -name "*$fileExtension" -xtype f -print -quit
else
  runner=$(find "$out"/bin -type l -print -quit)
  exec "$runner"
fi