#!/usr/bin/env bash
set -e

# --print: avoid dependency on environment
optPrint=
if [ "$1" == "--print" ]; then
	optPrint=true
	shift
fi

if [ "$#" != 1 ] && [ "$#" != 2 ]; then
	cat <<-EOF
	Usage: $0 [--print] commit-spec [commit-spec]
	    You need to be in a git-controlled nixpkgs tree.
	    The current state of the tree will be used if the second commit is missing.
	EOF
	exit 1
fi

# A slightly hacky way to get the config.
parallel="$(echo 'config.rebuild-amount.parallel or false' | nix-repl . 2>/dev/null \
			| grep -v '^\(nix-repl.*\)\?$' | tail -n 1 || true)"

echo "Estimating rebuild amount by counting changed Hydra jobs."

toRemove=()

cleanup() {
	rm -rf "${toRemove[@]}"
}
trap cleanup EXIT SIGINT SIGQUIT ERR

MKTEMP='mktemp --tmpdir nix-rebuild-amount-XXXXXXXX'

nixexpr() {
	cat <<-EONIX
		let
		  lib = import $1/lib;
		  hydraJobs = import $1/pkgs/top-level/release.nix
		    # Compromise: accuracy vs. resources needed for evaluation.
		    { supportedSystems = cfg.systems or [ "x86_64-linux" "x86_64-darwin" ]; };
		  cfg = (import $1 {}).config.rebuild-amount or {};

		  recurseIntoAttrs = attrs: attrs // { recurseForDerivations = true; };

		  # hydraJobs leaves recurseForDerivations as empty attrmaps;
		  # that would break nix-env and we also need to recurse everywhere.
		  tweak = lib.mapAttrs
		    (name: val:
		      if name == "recurseForDerivations" then true
		      else if lib.isAttrs val && val.type or null != "derivation"
		              then recurseIntoAttrs (tweak val)
		      else val
		    );

		  # Some of these contain explicit references to platform(s) we want to avoid;
		  # some even (transitively) depend on ~/.nixpkgs/config.nix (!)
		  blacklist = [
		    "tarball" "metrics" "manual"
		    "darwin-tested" "unstable" "stdenvBootstrapTools"
		    "moduleSystem" "lib-tests" # these just confuse the output
		  ];

		in
		  tweak (builtins.removeAttrs hydraJobs blacklist)
	EONIX
}

# Output packages in tree $2 that weren't in $1.
# Changing the output hash or name is taken as a change.
# Extra nix-env parameters can be in $3
newPkgs() {
	# We use files instead of pipes, as running multiple nix-env processes
	# could eat too much memory for a standard 4GiB machine.
	local -a list
	for i in 1 2; do
		local l="$($MKTEMP)"
		list[$i]="$l"
		toRemove+=("$l")

		local expr="$($MKTEMP)"
		toRemove+=("$expr")
		nixexpr "${!i}" > "$expr"

		nix-env -f "$expr" -qaP --no-name --out-path --show-trace $3 \
			| sort > "${list[$i]}" &

		if [ "$parallel" != "true" ]; then
			wait
		fi
	done

	wait
	comm -13 "${list[@]}"
}

# Prepare nixpkgs trees.
declare -a tree
for i in 1 2; do
	if [ -n "${!i}" ]; then # use the given commit
		dir="$($MKTEMP -d)"
		tree[$i]="$dir"
		toRemove+=("$dir")

		git clone --shared --no-checkout --quiet . "${tree[$i]}"
		(cd "${tree[$i]}" && git checkout --quiet "${!i}")
	else #use the current tree
		tree[$i]="$(pwd)"
	fi
done

newlist="$($MKTEMP)"
toRemove+=("$newlist")
# Notes:
#	- the evaluation is done on x86_64-linux, like on Hydra.
#	- using $newlist file so that newPkgs() isn't in a sub-shell (because of toRemove)
newPkgs "${tree[1]}" "${tree[2]}" '--argstr system "x86_64-linux"' > "$newlist"

# Hacky: keep only the last word of each attribute path and sort.
sed -n 's/\([^. ]*\.\)*\([^. ]*\) .*$/\2/p' < "$newlist" \
	| sort | uniq -c

if [ -n "$optPrint" ]; then
	echo
	cat "$newlist"
fi