%% -*- mode: erlang; tab-width: 4; indent-tabs-mode: 1; st-rulers: [70] -*-
%% vim: ts=4 sw=4 ft=erlang noet
-module(crc_bench).

%% API
-export([bench/2]).
-export([bench/3]).
-export([compare/3]).

%% Records
-record(stat, {
	acc = 0 :: non_neg_integer(),
	min = 0 :: non_neg_integer(),
	max = 0 :: non_neg_integer()
}).

%% Types
-type arguments_list(Type) :: [Type].
-type arguments_function(Type) ::
	fun(() -> arguments_list(Type)).
-type arguments(Type) :: arguments_function(Type) | arguments_list(Type).
-type arguments() :: arguments(term()).

-export_type([arguments/1]).
-export_type([arguments/0]).

-type metric() :: #{
	acc := non_neg_integer(),
	avg := float(),
	min := non_neg_integer(),
	max := non_neg_integer()
}.

-export_type([metric/0]).

-type stats() :: #{
	reds := metric(),
	time := metric()
}.

-export_type([stats/0]).

%%====================================================================
%% API
%%====================================================================

-spec bench(function(), arguments()) -> stats().
bench(Function, Arguments) ->
	bench(Function, Arguments, 1).

-spec bench(function(), arguments(), non_neg_integer()) -> stats().
bench(Function, Arguments, N)
		when is_function(Function)
		andalso (is_list(Arguments) orelse is_function(Arguments, 0))
		andalso (is_integer(N) andalso N > 0) ->
	{Time, Reds} = bench_loop(N, erlang:self(), #stat{}, #stat{}, Function, Arguments),
	#{
		time => stat_final(Time, N),
		reds => stat_final(Reds, N)
	}.

-spec compare([{atom(), function()}], arguments(), non_neg_integer()) -> [{atom(), stats()}].
compare(Groups, Arguments, N)
		when is_list(Groups)
		andalso (is_list(Arguments) orelse is_function(Arguments, 0))
		andalso (is_integer(N) andalso N > 0) ->
	ResolvedArguments = resolve(Arguments),
	[begin
		{Label, bench(Function, ResolvedArguments, N)}
	end || {Label, Function} <- Groups, is_atom(Label) andalso is_function(Function)].

%%%-------------------------------------------------------------------
%%% Internal functions
%%%-------------------------------------------------------------------

%% @private
bench_loop(0, _Self, Time, Reds, _Function, _Arguments) ->
	{Time, Reds};
bench_loop(I, Self, Time0, Reds0, Function, Arguments) ->
	Args = resolve(Arguments),
	T1 = erlang:monotonic_time(),
	{reductions, R1} = erlang:process_info(Self, reductions),
	_ = erlang:apply(Function, Args),
	{reductions, R2} = erlang:process_info(Self, reductions),
	T2 = erlang:monotonic_time(),
	Time1 = stat_update(Time0, T2 - T1),
	Reds1 = stat_update(Reds0, R2 - R1),
	bench_loop(I - 1, Self, Time1, Reds1, Function, Arguments).

%% @private
resolve(Arguments) when is_function(Arguments, 0) ->
	resolve(Arguments());
resolve(Arguments) when is_list(Arguments) ->
	Arguments.

%% @private
stat_final(#stat{ acc = Acc, min = Min, max = Max }, N) ->
	#{
		acc => Acc,
		avg => (Acc / N),
		min => Min,
		max => Max
	}.

%% @private
stat_update(Stat = #stat{ acc = Acc, min = Min, max = Max }, Val) ->
	Stat#stat{
		acc = Acc + Val,
		min =
			case Val < Min of
				_ when Min =:= 0 ->
					Val;
				true ->
					Val;
				false ->
					Min
			end,
		max =
			case Val > Max of
				true ->
					Val;
				false ->
					Max
			end
	}.