%% Copyright (c) 2017-Present Pivotal Software, Inc.  All rights reserved.
%%
%% This package, Looking Glass, is double-licensed under the Mozilla
%% Public License 1.1 ("MPL") and the Apache License version 2
%% ("ASL"). For the MPL, please see LICENSE-MPL-RabbitMQ. For the ASL,
%% please see LICENSE-APACHE2.
%%
%% This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND,
%% either express or implied. See the LICENSE file for specific language governing
%% rights and limitations of this software.
%%
%% If you have any questions regarding licensing, please contact us at
%% info@rabbitmq.com.

-module(lg_file_reader).

-export([fold/3]).
-export([foreach/2]).

-export([open/1]).
-export([read_event/1]).
-export([close/1]).

-record(state, {
    io_device :: file:io_device(),
    ctx :: lz4f:dctx(),
    buffer = <<>> :: binary(),
    offset = 0 :: non_neg_integer(),
    uncompressed_offset = 0 :: non_neg_integer()
}).

%% High level API.

fold(Fun, Acc, Filename) ->
    {ok, IoDevice} = open(Filename),
    Ctx = lz4f:create_decompression_context(),
    Ret = fold1(#state{io_device=IoDevice, ctx=Ctx}, Fun, Acc),
    ok = close(IoDevice),
    Ret.

fold1(State0, Fun, Acc0) ->
    case read_event(State0) of
        {ok, Event, State} ->
            Acc = Fun(Event, Acc0),
            fold1(State, Fun, Acc);
        eof ->
            {ok, Acc0};
        Error = {error, _, _} ->
            Error
    end.

foreach(Fun, Filename) ->
    {ok, IoDevice} = open(Filename),
    Ctx = lz4f:create_decompression_context(),
    Ret = foreach1(#state{io_device=IoDevice, ctx=Ctx}, Fun),
    ok = close(IoDevice),
    Ret.

foreach1(State0, Fun) ->
    case read_event(State0) of
        {ok, Event, State} ->
            _ = Fun(Event),
            foreach1(State, Fun);
        eof ->
            ok;
        Error = {error, _, _} ->
            Error
    end.

%% Low level API.

open(Filename) ->
    file:open(Filename, [read, binary]).

read_event(State=#state{buffer=Buffer}) ->
    case Buffer of
        <<Size:32, Bin:Size/binary, Rest/bits>> ->
            convert_event_body(State#state{buffer=Rest}, Bin);
        _ ->
            read_file(State)
    end.

read_file(State=#state{io_device=IoDevice, ctx=Ctx, buffer=Buffer, offset=Offset}) ->
    case file:read(IoDevice, 1000) of
        {ok, Data0} ->
            Data = iolist_to_binary(lz4f:decompress(Ctx, Data0)),
            read_event(State#state{buffer= <<Buffer/binary, Data/binary>>,
                offset=Offset + byte_size(Data0)});
        eof ->
            eof;
        {error, Reason} ->
            {error, Reason,
                'An error occurred while trying to read from the file.'}
    end.

convert_event_body(State=#state{offset=Offset, uncompressed_offset=UnOffset}, Bin) ->
    try binary_to_term(Bin) of
        Term ->
            {ok, Term, State#state{uncompressed_offset=UnOffset + byte_size(Bin)}}
    catch Class:Reason ->
        {error, {crash, Class, Reason, Offset, UnOffset},
            'The binary form of an event could not be decoded to an Erlang term.'}
    end.

close(IoDevice) ->
    file:close(IoDevice).