nix: nix-install nixpkgs-install
nix-update: nix-update nixpkgs-update

nix-install:
	if [ "$$(nix --version)" != "nix (Nix) $$(cat .lib/nix/version)" ]; then \
		sh .lib/nix/install; \
	fi

nix-update: .lib/nix/install .lib/nix/install.asc
.PHONY: .lib/nix/install .lib/nix/install.asc
NIX_GPGKEY:=B541D55301270E0BCF15CA5D8170B4726D7198DE
.lib/nix/install .lib/nix/install.asc:
	# DOC: https://nixos.org/nix/download.html
	cd .lib/nix && curl -OO https://nixos.org/releases/nix/nix-$$(cat .lib/nix/version)/{install,install.asc}
	gpg2 --recv-keys $(NIX_GPGKEY)
	gpg2 --verify .lib/nix/install.asc

nixpkgs-update: .lib/nix/nixpkgs.nix
.PHONY: .lib/nix/nixpkgs.nix
.lib/nix/nixpkgs.nix:
	# NOTE: this pins down the specific nixpkgs revision
	# currently used by the last Nix channel of the user;
	# it should be run from time to time.
	# DOC: https://nixos.wiki/wiki/How_to_fetch_Nixpkgs_with_an_empty_NIX_PATH
	set -x; \
	channel=$$(nix-channel --list | cut -f 1 -d ' ' | tail -1) && \
	rev=$$(cat ~/.nix-defexpr/channels/$$channel/.git-revision) && \
	sha256=$$(nix-prefetch-url --unpack https://github.com/NixOS/nixpkgs/archive/$$rev.tar.gz) && \
	echo >$@ "builtins.fetchTarball {url=\"https://github.com/NixOS/nixpkgs/archive/$$rev.tar.gz\"; sha256=\"$$sha256\";}"
nixpkgs-clean:
	# Remove dependencies no longer set in .lib/nix/shell.nix
	rm -rf .cache/nix/gcroots
	# To free disk space, you can now run something like:
	#    nix-store --gc

#cachix-update:
#	#! nix-shell -i bash https://github.com/NixOS/nixpkgs/tarball/db557aab7b690f5e0e3348459f2e4dc8fd0d9298 -A cachix
#	# NOTE: build the shell environment and exit on completion.
#	nix-shell .lib/nix/shell.nix --command exit
#	# NOTE: get a list of all the derivations used
#	# in the shell environment and push them to the cache.
#	nix-store -qR --include-outputs $(nix-instantiate .lib/nix/shell.nix) | cachix push $1