#!/usr/bin/env bash
set -efu

declare file attribute nix_options special_args
eval "$(jq -r '@sh "attribute=\(.attribute) file=\(.file) nix_options=\(.nix_options) special_args=\(.special_args)"')"
if [ "${nix_options}" != '{"options":{}}' ]; then
  options=$(echo "${nix_options}" | jq -r '.options | to_entries | map("--option \(.key) \(.value)") | join(" ")')
else
  options=""
fi
if [[ ${special_args-} == "{}" ]]; then
  # no special arguments, proceed as normal
  if [[ -n ${file-} ]] && [[ -e ${file-} ]]; then
    # shellcheck disable=SC2086
    out=$(nix build --no-link --json $options -f "$file" "$attribute")
  else
    # shellcheck disable=SC2086
    out=$(nix build --no-link --json ${options} "$attribute")
  fi
else
  if [[ ${file-} != 'null' ]]; then
    echo "special_args are currently only supported when using flakes!" >&2
    exit 1
  fi
  # pass the args in a pure fashion by extending the original config
  rest="$(echo "${attribute}" | cut -d "#" -f 2)"
  # e.g. config_path=nixosConfigurations.aarch64-linux.myconfig
  config_path="${rest%.config.*}"
  # e.g. config_attribute=config.system.build.toplevel
  config_attribute="config.${rest#*.config.}"

  # grab flake nar from error message
  flake_rel="$(echo "${attribute}" | cut -d "#" -f 1)"
  # e.g. flake_rel="."
  flake_dir="$(readlink -f "${flake_rel}")"
  flake_path="${flake_dir}/flake.nix"
  flake_json="$(nix flake prefetch "${flake_dir}" --json)"
  flake_nar="$(echo "$flake_json" | jq -r '.hash')"
  store_path="$(echo "${flake_json}" | jq -r '.storePath')"
  # while we have a store path now, for a repo this reflects its root level,
  # so search for the largest child segment yielding a match in that store dir.
  iter_path="${flake_path}"
  while [[ ${iter_path} != "/" ]]; do
    parent="$(dirname "${iter_path}")"
    child_segment="${flake_path//$parent/}"
    if [[ -f "${store_path}${child_segment}" ]]; then
      target_segment="${child_segment}"
    fi
    iter_path="${parent}"
  done
  # substitute variables into the template
  nix_expr="(builtins.getFlake ''file://${flake_dir}?dir=${target_segment//\/flake.nix/}&narHash=${flake_nar}'').${config_path}.extendModules { specialArgs = builtins.fromJSON ''${special_args}''; }"
  # inject `special_args` into nixos config's `specialArgs`
  # shellcheck disable=SC2086
  out=$(nix build --no-link --json ${options} --expr "${nix_expr}" "${config_attribute}")
fi
printf '%s' "$out" | jq -c '.[].outputs'