#!/usr/bin/env bash

set -eu

NEW_REV=
PREVIOUS_REV="$(git rev-parse @~)"
OUTPUT_FILE=/dev/stdout
FLAKE_OUTPUTS=()
NIX_BUILD_ARGS=()

print_err() {
    printf "%s\n" "$1" >&2
}

sanitize_output() {
    if [ "$OUTPUT_FILE" != "/dev/stdout" ]; then
        sed 's/\x1b\[[0-9;]*m//g'
    else
        cat
    fi
}

add_shell() {
    local SYSTEM
    if [ $# -gt 0 ] && [ -n "$1" ]; then
        SYSTEM="$1"
    else
        SYSTEM="$(nix eval --raw --impure --expr 'builtins.currentSystem')"
    fi
    # Use 'inputDerivation' attribute to make sure that it is build-able
    FLAKE_OUTPUTS+=("devShells.$SYSTEM.default.inputDerivation")
}

add_host() {
    FLAKE_OUTPUTS+=("nixosConfigurations.$1.config.system.build.toplevel")
}

usage() {
    print_err "Usage: $0 [option]... [-- [nix build option]...]"
    print_err ""
    print_err "  -h, --help"
    print_err "    print this usage screen and exit"
    print_err "  -f, --flake-output"
    print_err "    specify which flake output's closures should be diffed,"
    print_err "    can be used multiple times to specify multiple outputs"
    print_err "  -o, --output"
    print_err "    specify where to output the closure diffs,"
    print_err "    defaults to printing to standard output"
    print_err "  -n, --new-rev"
    print_err "    which git revision should be considered the 'new' state,"
    print_err "    defaults to current state"
    print_err "  -p, --previous-rev"
    print_err "    which git revision should be considered the 'previous' state,"
    print_err "    defaults to HEAD~"
    print_err "  --host [name]"
    print_err "    specify the name of a NixOS output configuration whose"
    print_err "    closure should be diffed, can be used multiple times"
    print_err "    if no host name is given, defaults to current hostname"
    print_err "  --shell [system]"
    print_err "    specify a specific system's devShell output whose closure"
    print_err "    should be diffed, can be used multiple times"
    print_err "    if no system is given, defaults to current system"
    print_err ""
    print_err "when no flake outputs are specified, automatically queries for"
    print_err "all NixOS configurations, and devShell for current system"
}

is_option() {
    [ $# -gt 0 ] && [[ $1 =~ ^(-.|--.*)$ ]]
}

parse_args() {
    while [ $# -gt 0 ]; do
        opt="$1"
        shift

        case "$opt" in
            -h|--help)
                usage
                exit
                ;;
            -f|--flake-output)
                FLAKE_OUTPUTS+=("$1")
                shift
                ;;
            -o|--output)
                OUTPUT_FILE="$1"
                shift
                ;;
            -n|--new-rev)
                NEW_REV="$(git rev-parse "$1")"
                shift
                ;;
            -p|--previous-rev)
                PREVIOUS_REV="$(git rev-parse "$1")"
                shift
                ;;
            --host)
                if [ $# -gt 0 ] && ! is_option "$1"; then
                    add_host "$1"
                    shift
                else
                    add_host "$(hostname)"
                fi
                ;;
            --shell)
                if [ $# -gt 0 ] && ! is_option "$1"; then
                    add_shell "$1"
                    shift
                else
                    add_shell
                fi
                ;;
            --)
                NIX_BUILD_ARGS=("$@")
                break
                ;;
            *)
                print_err "Unknown argument '$opt'"
                usage
                exit 1
                ;;
        esac
    done
}

list_nixos_configurations() {
    nix eval '.#nixosConfigurations' \
        --apply 'attrs: with builtins; concatStringsSep "\n" (attrNames attrs)' \
        --raw
}

diff_output() {
    local PREV NEW;
    PREV="$(mktemp --dry-run)"
    NEW="$(mktemp --dry-run)"

    nix build "${NIX_BUILD_ARGS[@]}" ".?rev=${PREVIOUS_REV}#$1" -o "$PREV"
    nix build "${NIX_BUILD_ARGS[@]}" ".${NEW_REV:+?rev=$NEW_REV}#$1" -o "$NEW"

    {
        # shellcheck disable=SC2016
        printf 'Closure diff for `%s`:\n```\n' "$1"
        nix store diff-closures "$PREV" "$NEW" | sanitize_output
        printf '```\n\n'
    } >> "$OUTPUT_FILE"
}

parse_args "$@"

if [ "${#FLAKE_OUTPUTS[@]}" -eq 0 ]; then
    for host in $(list_nixos_configurations); do
        add_host "$host"
    done
    add_shell
fi

for out in "${FLAKE_OUTPUTS[@]}"; do
    diff_output "$out"
done