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

# Check for hostname argument
if [[ $# -ne 1 ]]; then
  echo "Usage: $0 <nixos-hostname>"
  exit 1
fi

HOST="$1"
REPO_PATH="git+file://$PWD"

CURRENT_COMMIT=$(git rev-parse HEAD)
PREV_COMMIT=$(git rev-parse HEAD^)

# Check if the host exists at the current commit
if ! nix eval --quiet --expr "
  let
    flake = builtins.getFlake \"${REPO_PATH}?rev=${CURRENT_COMMIT}\";
  in
    builtins.hasAttr \"${HOST}\" flake.nixosConfigurations
"; then
  echo "Error: Host '${HOST}' not found in nixosConfigurations at HEAD."
  exit 1
fi

# Now check for derivation change
RESULT=$(nix eval --raw --expr "
  let
    flake = builtins.getFlake;
    prev = flake \"${REPO_PATH}?rev=${PREV_COMMIT}\";
    curr = flake \"${REPO_PATH}?rev=${CURRENT_COMMIT}\";
  in
    if prev.nixosConfigurations.\"${HOST}\".config.system.build.toplevel !=
       curr.nixosConfigurations.\"${HOST}\".config.system.build.toplevel
    then \"changed\" else \"unchanged\"
")

# Output result
if [[ $RESULT == "changed" ]]; then
  echo "System derivation changed for host '${HOST}' — deploy!"
else
  echo "No change for host '${HOST}' — skip deployment."
fi