if test (count $argv) -ne 1
    echo "usage: deploy-to <host>" >&2
    return 2
end

set host $argv[1]

git rev-parse --show-toplevel >/dev/null 2>&1
or begin
    echo "deploy-to: run this from a git checkout" >&2
    return 2
end

set repository_root (git rev-parse --show-toplevel)
or return $status

set untracked_nix_files (git -C $repository_root ls-files --others --exclude-standard -- '*.nix' flake.nix flake.lock)
if test (count $untracked_nix_files) -gt 0
    echo "deploy-to: warning: untracked Nix files are not part of Git flake evaluation" >&2
    printf '  %s\n' $untracked_nix_files >&2
end

set installable "$repository_root#nixosConfigurations.$host.config.system.build.toplevel"

echo "deploy-to: building $installable" >&2
set system_path (nix build --no-link --print-out-paths $installable)
or begin
    echo "deploy-to: build failed for $host" >&2
    return $status
end

if test (count $system_path) -ne 1
    echo "deploy-to: expected one build output, got "(count $system_path) >&2
    return 1
end

set closure_paths (nix-store -qR $system_path)
or begin
    echo "deploy-to: closure query failed for $host" >&2
    return $status
end

echo "deploy-to: checking which closure paths $host is missing" >&2
set missing_paths (printf '%s\n' $closure_paths | ssh $host 'xargs -r nix-store --check-validity --print-invalid')
or begin
    echo "deploy-to: remote closure check failed for $host" >&2
    return $status
end

set remote_nar

if test (count $missing_paths) -gt 0
    set nar_file (mktemp --suffix=.nar)
    or return $status
    set remote_nar "/tmp/deploy-to-$host-"(date +%s)"-$fish_pid.nar"

    echo "deploy-to: exporting "(count $missing_paths)" missing closure paths for $system_path" >&2
    nix-store --export $missing_paths >$nar_file
    or begin
        set export_status $status
        rm -f $nar_file
        echo "deploy-to: closure export failed for $host" >&2
        return $export_status
    end

    echo "deploy-to: copying closure archive to $host:$remote_nar" >&2
    scp $nar_file $host:$remote_nar
    set scp_status $status
    rm -f $nar_file
    if test $scp_status -ne 0
        echo "deploy-to: closure archive copy failed for $host" >&2
        return $scp_status
    end

    echo "deploy-to: importing the closure, recording $system_path, and activating it on $host" >&2
else
    echo "deploy-to: $host already has the full closure for $system_path" >&2
    echo "deploy-to: recording $system_path and activating it on $host" >&2
end

set remote_script (
    string join '; ' \
        'system_path=$1' \
        'remote_nar=$2' \
        'if test -n "$remote_nar"; then nix-store --import < "$remote_nar"; rm -f "$remote_nar"; fi' \
        'nix-env --profile /nix/var/nix/profiles/system --set "$system_path"' \
        '"$system_path/bin/switch-to-configuration" switch'
)

ssh -t $host sudo sh -e -c (string escape -- $remote_script) sh (string escape -- $system_path $remote_nar)
or begin
    echo "deploy-to: deployment failed for $host" >&2
    return $status
end