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

VERBOSE=false
SHOW_TRACE_OPT=""

# Parse options
if [[ "$1" == "-v" || "$1" == "--verbose" ]]; then
  VERBOSE=true
  SHOW_TRACE_OPT="--show-trace"
  shift # Remove the verbose flag from arguments
fi

# Check if hostname argument is provided
if [ "$#" -ne 1 ]; then
  echo "Usage: $0 [-v|--verbose] <hostname>" >&2
  exit 1
fi

HOSTNAME="$1"

# Check if 'nix-instantiate' command is available
if ! command -v nix-instantiate > /dev/null; then
  echo "ERROR: 'nix-instantiate' command not found. Please ensure Nix is installed and in your PATH." >&2
  exit 1
fi

# Determine the absolute directory where the script itself is located
SCRIPT_DIR=$(dirname "$(readlink -f "$0")")

# Construct the absolute path to the host's configuration file
# and resolve it to a canonical path
CONFIG_PATH=$(readlink -f "$SCRIPT_DIR/../hosts/$HOSTNAME/configuration.nix")

# Verify that the CONFIG_PATH exists and is a regular file
if [ ! -f "$CONFIG_PATH" ]; then
  echo "ERROR: Configuration file not found at '$CONFIG_PATH' for host '$HOSTNAME'." >&2
  exit 1
fi

# Check for host-specific channel file
CHANNEL_PATH="$SCRIPT_DIR/../hosts/$HOSTNAME/channel"
CHANNEL_OPT=""

if [ -f "$CHANNEL_PATH" ]; then
  CHANNEL_URL=$(cat "$CHANNEL_PATH")
  # Append /nixexprs.tar.xz to get the actual tarball URL
  TARBALL_URL="${CHANNEL_URL}/nixexprs.tar.xz"
  echo "INFO: Using channel '$TARBALL_URL' from '$CHANNEL_PATH'."
  CHANNEL_OPT="-I nixpkgs=$TARBALL_URL"
else
  echo "WARNING: No channel file found at '$CHANNEL_PATH'. Using system default." >&2
fi

echo "INFO: Attempting dry-build for host '$HOSTNAME' using configuration '$CONFIG_PATH'..."
if [ "$VERBOSE" = true ]; then
  echo "INFO: Verbose mode enabled, --show-trace will be used."
fi

# Execute nix-instantiate to evaluate the configuration
# nix-instantiate fetches fresh tarballs and catches all evaluation errors
# unlike nixos-rebuild which may use cached results
NIX_OUTPUT_ERR=$(nix-instantiate $SHOW_TRACE_OPT $CHANNEL_OPT -I nixos-config="$CONFIG_PATH" '<nixpkgs/nixos>' -A system 2>&1)
NIX_EXIT_STATUS=$?

# Check the exit status
if [ "$NIX_EXIT_STATUS" -eq 0 ]; then
  echo "INFO: Dry-build for host '$HOSTNAME' completed successfully."
  if [ "$VERBOSE" = true ]; then
    echo "Output from nix-instantiate:"
    echo "$NIX_OUTPUT_ERR"
  fi
  exit 0
else
  echo "ERROR: Dry-build for host '$HOSTNAME' failed. 'nix-instantiate' exited with status $NIX_EXIT_STATUS." >&2
  echo "Output from nix-instantiate:" >&2
  echo "$NIX_OUTPUT_ERR" >&2
  exit "$NIX_EXIT_STATUS"
fi