erlware_commons/src/ec_plists.erl

% @author Stephen Marsh
% @copyright 2007 Stephen Marsh freeyourmind ++ [$@|gmail.com]
% @doc plists is a drop-in replacement for module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>,
% making most list operations parallel. It can operate on each element in
% parallel, for IO-bound operations, on sublists in parallel, for
% taking advantage of multi-core machines with CPU-bound operations, and
% across erlang nodes, for parallizing inside a cluster. It handles
% errors and node failures. It can be configured, tuned, and tweaked to
% get optimal performance while minimizing overhead.
%
% Almost all the functions are
% identical to equivalent functions in lists, returning exactly the same
% result, and having both a form with an identical syntax that operates on
% each element in parallel and a form which takes an optional "malt",
% a specification for how to parallize the operation.
%
% fold is the one exception, parallel fold is different from linear fold.
% This module also include a simple mapreduce implementation, and the
% function runmany. All the other functions are implemented with runmany,
% which is as a generalization of parallel list operations.
%
% == Malts ==
% A malt specifies how to break a list into sublists, and can optionally
% specify a timeout, which nodes to run on, and how many processes to start
% per node.
%
% Malt = MaltComponent | [MaltComponent]<br/>
% MaltComponent = SubListSize::integer() | {processes, integer()} |
% {processes, schedulers} |
% {timeout, Milliseconds::integer()} | {nodes, [NodeSpec]}<br/>
% NodeSpec = Node::atom() | {Node::atom(), NumProcesses::integer()} |
% {Node::atom(), schedulers}
%
% An integer can be given to specify the exact size for
% sublists. 1 is a good choice for IO-bound operations and when
% the operation on each list element is expensive. Larger numbers
% minimize overhead and are faster for cheap operations.
%
% If the integer is omitted, and
% you have specified a {processes, X}, the list is
% split into X sublists. This is only
% useful when the time to process each element is close to identical and you
% know exactly how many lines of execution are available to you.
%
% If neither of the above applies, the sublist size defaults to 1.
%
% You can use {processes, X} to have the list processed
% by X processes on the local machine. A good choice for X is the number of
% lines of execution (cores) the machine provides. This can be done
% automatically with {processes, schedulers}, which sets
% the number of processes to the number of schedulers in the erlang virtual
% machine (probably equal to the number of cores).
%
% {timeout, Milliseconds} specifies a timeout. This is a timeout for the entire
% operation, both operating on the sublists and combining the results.
% exit(timeout) is evaluated if the timeout is exceeded.
%
% {nodes, NodeList} specifies that the operation should be done across nodes.
% Every element of NodeList is of the form {NodeName, NumProcesses} or
% NodeName, which means the same as {NodeName, 1}. plists runs
% NumProcesses processes on NodeName concurrently. A good choice for
% NumProcesses is the number of lines of execution (cores) a node provides
% plus one. This ensures the node is completely busy even when
% fetching a new sublist. This can be done automatically with
% {NodeName, schedulers}, in which case
% plists uses a cached value if it has one, and otherwise finds the number of
% schedulers in the remote node and adds one. This will ensure at least one
% busy process per core (assuming the node has a scheduler for each core).
%
% plists is able to recover if a node goes down.
% If all nodes go down, exit(allnodescrashed) is evaluated.
%
% Any of the above may be used as a malt, or may be combined into a list.
% {nodes, NodeList} and {processes, X} may not be combined.
%
% === Examples ===
% % start a process for each element (1-element sublists)<br/>
% 1
%
% % start a process for each ten elements (10-element sublists)<br/>
% 10
%
% % split the list into two sublists and process in two processes<br/>
% {processes, 2}
%
% % split the list into X sublists and process in X processes,<br/>
% % where X is the number of cores in the machine<br/>
% {processes, schedulers}
%
% % split the list into 10-element sublists and process in two processes<br/>
% [10, {processes, 2}]
%
% % timeout after one second. Assumes that a process should be started<br/>
% % for each element.<br/>
% {timeout, 1000}
%
% % Runs 3 processes at a time on apple@desktop,
% and 2 on orange@laptop<br/>
% % This is the best way to utilize all the CPU-power of a dual-core<br/>
% % desktop and a single-core laptop. Assumes that the list should be<br/>
% % split into 1-element sublists.<br/>
% {nodes, [{apple@desktop, 3}, {orange@laptop, 2}]}
%
% Like above, but makes plists figure out how many processes to use.
% {nodes, [{apple@desktop, schedulers}, {orange@laptop, schedulers}]}
%
% % Gives apple and orange three seconds to process the list as<br/>
% % 100-element sublists.<br/>
% [100, {timeout, 3000}, {nodes, [{apple@desktop, 3}, {orange@laptop, 2}]}]
%
% === Aside: Why Malt? ===
% I needed a word for this concept, so maybe my subconsciousness gave me one by
% making me misspell multiply. Maybe it is an acronym for Malt is A List
% Tearing Specification. Maybe it is a beer metaphor, suggesting that code
% only runs in parallel if bribed with spirits. It's jargon, learn it
% or you can't be part of the in-group.
%
% == Messages and Errors ==
% plists assures that no extraneous messages are left in or will later
% enter the message queue. This is guaranteed even in the event of an error.
%
% Errors in spawned processes are caught and propagated to the calling
% process. If you invoke
%
% plists:map(fun (X) -> 1/X end, [1, 2, 3, 0]).
%
% you get a badarith error, exactly like when you use lists:map.
%
% plists uses monitors to watch the processes it spawns. It is not a good idea
% to invoke plists when you are already monitoring processes. If one of them
% does a non-normal exit, plists receives the 'DOWN' message believing it to be
% from one of its own processes. The error propagation system goes into
% effect, which results in the error occuring in the calling process.
%
% == License ==
% The MIT License
%
% Copyright (c) 2007 Stephen Marsh
%
% Permission is hereby granted, free of charge, to any person obtaining a copy
% of this software and associated documentation files (the "Software"), to deal
% in the Software without restriction, including without limitation the rights
% to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
% copies of the Software, and to permit persons to whom the Software is
% furnished to do so, subject to the following conditions:
%
% The above copyright notice and this permission notice shall be included in
% all copies or substantial portions of the Software.
%
% THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
% IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
% FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
% AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
% LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
% OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
% THE SOFTWARE.


-module(plists).
-export([all/2, all/3, any/2, any/3, filter/2, filter/3,
fold/3, fold/4, fold/5, foreach/2, foreach/3, map/2, map/3,
partition/2, partition/3, sort/1, sort/2, sort/3,
usort/1, usort/2, usort/3, mapreduce/2, mapreduce/3, mapreduce/5,
runmany/3, runmany/4]).

% Everything here is defined in terms of runmany.
% The following methods are convient interfaces to runmany.

% @doc Same semantics as in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>.
% @spec (Fun, List) -> bool()
all(Fun, List) ->
    all(Fun, List, 1).

% @doc Same semantics as in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>.
% @spec (Fun, List, Malt) -> bool()
all(Fun, List, Malt) ->
    try runmany(fun (L) ->
                        B = lists:all(Fun, L),
                        if B ->
                                nil;
                           true ->
                                exit(notall)
                        end
                end,
                fun (_A1, _A2) ->
                        nil
                end,
                List, Malt) of
        _ ->
            true
    catch exit:notall ->
                  false
    end.

% @doc Same semantics as in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>.
% @spec (Fun, List) -> bool()
any(Fun, List) ->
    any(Fun, List, 1).

% @doc Same semantics as in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>.
% @spec (Fun, List, Malt) -> bool()
any(Fun, List, Malt) ->
    try runmany(fun (L) ->
                        B = lists:any(Fun, L),
                        if B ->
                                exit(any);
                           true ->
                                nil
                        end
                end,
                fun (_A1, _A2) ->
                        nil
                end,
                List, Malt) of
        _ ->
            false
    catch exit:any ->
            true
    end.

% @doc Same semantics as in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>.
% @spec (Fun, List) -> list()
filter(Fun, List) ->
    filter(Fun, List, 1).

% @doc Same semantics as in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>.
% @spec (Fun, List, Malt) -> list()
filter(Fun, List, Malt) ->
    runmany(fun (L) ->
                    lists:filter(Fun, L)
            end,
            {reverse, fun (A1, A2) ->
                    A1 ++ A2
            end},
            List, Malt).

% Note that with parallel fold there is not foldl and foldr,
% instead just one fold that can fuse Accumlators.

% @doc Like below, but assumes 1 as the Malt. This function is almost useless,
% and is intended only to aid converting code from using lists to plists.
% @spec (Fun, InitAcc, List) -> term()
fold(Fun, InitAcc, List) ->
    fold(Fun, Fun, InitAcc, List, 1).

% @doc Like below, but uses the Fun as the Fuse by default.
% @spec (Fun, InitAcc, List, Malt) -> term()
fold(Fun, InitAcc, List, Malt) ->
    fold(Fun, Fun, InitAcc, List, Malt).

% @doc fold is more complex when made parallel. There is no foldl and foldr,
% accumulators aren't passed in any defined order.
% The list is split into sublists which are folded together. Fun is
% identical to the function passed to lists:fold[lr], it takes
% (an element, and the accumulator) and returns -> a new accumulator.
% It is used for the initial stage of folding sublists. Fuse fuses together
% the results, it takes (Results1, Result2) and returns -> a new result.
% By default sublists are fused left to right, each result of a fuse being
% fed into the first element of the next fuse. The result of the last fuse
% is the result.
%
% Fusing may also run in parallel using a recursive algorithm,
% by specifying the fuse as {recursive, Fuse}. See
% the discussion in {@link runmany/4}.
%
% Malt is the malt for the initial folding of sublists, and for the
% possible recursive fuse.
% @spec (Fun, Fuse, InitAcc, List, Malt) -> term()
fold(Fun, Fuse, InitAcc, List, Malt) ->
    Fun2 = fun (L) -> lists:foldl(Fun, InitAcc, L) end,
    runmany(Fun2, Fuse, List, Malt).

% @doc Similiar to foreach in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>
% except it makes no guarantee about the order it processes list elements.
% @spec (Fun, List) -> void()
foreach(Fun, List) ->
    foreach(Fun, List, 1).

% @doc Similiar to foreach in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>
% except it makes no guarantee about the order it processes list elements.
% @spec (Fun, List, Malt) -> void()
foreach(Fun, List, Malt) ->
    runmany(fun (L) ->
                    lists:foreach(Fun, L)
            end,
            fun (_A1, _A2) ->
                    ok
            end,
            List, Malt).

% @doc Same semantics as in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>.
% @spec (Fun, List) -> list()
map(Fun, List) ->
    map(Fun, List, 1).

% @doc Same semantics as in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>.
% @spec (Fun, List, Malt) -> list()
map(Fun, List, Malt) ->
    runmany(fun (L) ->
                    lists:map(Fun, L)
            end,
            {reverse, fun (A1, A2) ->
                    A1 ++ A2
            end},
            List, Malt).

% @doc Same semantics as in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>.
% @spec (Fun, List) -> {list(), list()}
partition(Fun, List) ->
    partition(Fun, List, 1).

% @doc Same semantics as in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>.
% @spec (Fun, List, Malt) -> {list(), list()}
partition(Fun, List, Malt) ->
    runmany(fun (L) ->
                    lists:partition(Fun, L)
            end,
            {reverse, fun ({True1, False1}, {True2, False2}) ->
                    {True1 ++ True2, False1 ++ False2}
            end},
            List, Malt).

% SORTMALT needs to be tuned
-define(SORTMALT, 100).

% @doc Same semantics as in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>.
% @spec (List) -> list()
sort(List) ->
    sort(fun (A, B) ->
                 A =< B
         end,
         List).

% @doc Same semantics as in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>.
% @spec (Fun, List) -> list()
sort(Fun, List) ->
    sort(Fun, List, ?SORTMALT).

% @doc This version lets you specify your own malt for sort.
%
% sort splits the list into sublists and sorts them, and it merges the
% sorted lists together. These are done in parallel. Each sublist is
% sorted in a seperate process, and each merging of results is done in a
% seperate process. Malt defaults to 100, causing the list to be split into
% 100-element sublists.
% @spec (Fun, List, Malt) -> list()
sort(Fun, List, Malt) ->
    Fun2 = fun (L) ->
            lists:sort(Fun, L)
    end,
    Fuse = fun (A1, A2) ->
            lists:merge(Fun, A1, A2)
    end,
    runmany(Fun2, {recursive, Fuse}, List, Malt).

% @doc Same semantics as in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>.
% @spec (List) -> list()
usort(List) ->
    usort(fun (A, B) ->
                 A =< B
         end,
         List).

% @doc Same semantics as in module
% <a href="http://www.erlang.org/doc/man/lists.html">lists</a>.
% @spec (Fun, List) -> list()
usort(Fun, List) ->
    usort(Fun, List, ?SORTMALT).

% @doc This version lets you specify your own malt for usort.
%
% usort splits the list into sublists and sorts them, and it merges the
% sorted lists together. These are done in parallel. Each sublist is
% sorted in a seperate process, and each merging of results is done in a
% seperate process. Malt defaults to 100, causing the list to be split into
% 100-element sublists.
%
% usort removes duplicate elments while it sorts.
% @spec (Fun, List, Malt) -> list()
usort(Fun, List, Malt) ->
    Fun2 = fun (L) ->
            lists:usort(Fun, L)
    end,
    Fuse = fun (A1, A2) ->
            lists:umerge(Fun, A1, A2)
    end,
    runmany(Fun2, {recursive, Fuse}, List, Malt).

% @doc Like below, assumes default MapMalt of 1.
% @spec (MapFunc, List) -> Dict
% MapFunc = (term()) -> DeepListOfKeyValuePairs
% DeepListOfKeyValuePairs = [DeepListOfKeyValuePairs] | {Key, Value}
mapreduce(MapFunc, List) ->
    mapreduce(MapFunc, List, 1).

% Like below, but uses a default reducer that collects all
% {Key, Value} pairs into a
% <a href="http://www.erlang.org/doc/man/dict.html">dict</a>,
% with values {Key, [Value1, Value2...]}.
% This dict is returned as the result.
mapreduce(MapFunc, List, MapMalt) ->
    mapreduce(MapFunc, List, dict:new(), fun add_key/3, MapMalt).

% @doc This is a very basic mapreduce. You won't write a Google-rivaling
% search engine with it. It has no equivalent in lists. Each
% element in the list is run through the MapFunc, which produces either
% a {Key, Value} pair, or a lists of key value pairs, or a list of lists of
% key value pairs...etc. A reducer process runs in parallel with the mapping
% processes, collecting the key value pairs. It starts with a state given by
% InitState, and for each {Key, Value} pair that it receives it invokes
% ReduceFunc(OldState, Key, Value) to compute its new state. mapreduce returns
% the reducer's final state.
%
% MapMalt is the malt for the mapping operation, with a default value of 1,
% meaning each element of the list is mapped by a seperate process.
%
% mapreduce requires OTP R11B, or it may leave monitoring messages in the
% message queue.
% @spec (MapFunc, List, InitState, ReduceFunc, MapMalt) -> Dict
% MapFunc = (term()) -> DeepListOfKeyValuePairs
% DeepListOfKeyValuePairs = [DeepListOfKeyValuePairs] | {Key, Value}
% ReduceFunc = (OldState::term(), Key::term(), Value::term() -> NewState::term()
mapreduce(MapFunc, List, InitState, ReduceFunc, MapMalt) ->
    Parent = self(),
    {Reducer, ReducerRef} =
        erlang:spawn_monitor(fun () ->
                                     reducer(Parent, 0, InitState, ReduceFunc)
                             end),
    MapFunc2 = fun (L) ->
                       Reducer ! lists:map(MapFunc, L),
                       1
               end,
    SentMessages = try runmany(MapFunc2, fun (A, B) -> A+B end, List, MapMalt)
                   catch
                       exit:Reason ->
                           erlang:demonitor(ReducerRef, [flush]),
                           Reducer ! die,
                           exit(Reason)
                   end,
    Reducer ! {mappers, done, SentMessages},
    Results = receive
                  {Reducer, Results2} ->
                      Results2;
                  {'DOWN', _, _, Reducer, Reason2} ->
                      exit(Reason2)
              end,
    receive
        {'DOWN', _, _, Reducer, normal} ->
            nil
    end,
    Results.

reducer(Parent, NumReceived, State, Func) ->
    receive
        die ->
            nil;
        {mappers, done, NumReceived} ->
            Parent ! {self (), State};
        Keys  ->
            reducer(Parent, NumReceived + 1, each_key(State, Func, Keys), Func)
    end.

each_key(State, Func, {Key, Value}) ->
    Func(State, Key, Value);
each_key(State, Func, [List|Keys]) ->
    each_key(each_key(State, Func, List), Func, Keys);
each_key(State, _, []) ->
    State.

add_key(Dict, Key, Value) ->
    case dict:is_key(Key, Dict) of
        true ->
            dict:append(Key, Value, Dict);
        false ->
            dict:store(Key, [Value], Dict)
    end.

% @doc Like below, but assumes a Malt of 1,
% meaning each element of the list is processed by a seperate process.
% @spec (Fun, Fuse, List) -> term()
runmany(Fun, Fuse, List) ->
    runmany(Fun, Fuse, List, 1).

% Begin internal stuff (though runmany/4 is exported).

% @doc All of the other functions are implemented with runmany. runmany
% takes a List, splits it into sublists, and starts processes to operate on
% each sublist, all done according to Malt. Each process passes its sublist
% into Fun and sends the result back.
%
% The results are then fused together to get the final result. There are two
% ways this can operate, lineraly and recursively. If Fuse is a function,
% a fuse is done linearly left-to-right on the sublists, the results
% of processing the first and second sublists being passed to Fuse, then
% the result of the first fuse and processing the third sublits, and so on. If
% Fuse is {reverse, FuseFunc}, then a fuse is done right-to-left, the results
% of processing the second-to-last and last sublists being passed to FuseFunc,
% then the results of processing the third-to-last sublist and
% the results of the first fuse, and and so forth.
% Both methods preserve the original order of the lists elements.
%
% To do a recursive fuse, pass Fuse as {recursive, FuseFunc}.
% The recursive fuse makes no guarantee about the order the results of
% sublists, or the results of fuses are passed to FuseFunc. It
% continues fusing pairs of results until it is down to one.
%
% Recursive fuse is down in parallel with processing the sublists, and a
% process is spawned to fuse each pair of results. It is a parallized
% algorithm. Linear fuse is done after all results of processing sublists
% have been collected, and can only run in a single process.
%
% Even if you pass {recursive, FuseFunc}, a recursive fuse is only done if
% the malt contains {nodes, NodeList} or {processes, X}. If this is not the
% case, a linear fuse is done.
% @spec (Fun, Fuse, List, Malt) -> term()
% Fun = (list()) -> term()
% Fuse = FuseFunc | {recursive, FuseFunc}
% FuseFunc = (term(), term()) -> term()
runmany(Fun, Fuse, List, Malt) when is_list(Malt) ->
    runmany(Fun, Fuse, List, local, no_split, Malt);
runmany(Fun, Fuse, List, Malt) ->
    runmany(Fun, Fuse, List, [Malt]).

runmany(Fun, Fuse, List, Nodes, no_split, [MaltTerm|Malt]) when is_integer(MaltTerm) ->
    runmany(Fun, Fuse, List, Nodes, MaltTerm, Malt);
% run a process for each scheduler
runmany(Fun, Fuse, List, local, Split, [{processes, schedulers}|Malt]) ->
    S = erlang:system_info(schedulers),
    runmany(Fun, Fuse, List, local, Split, [{processes, S}|Malt]);
% Split the list into X sublists, where X is the number of processes
runmany(Fun, Fuse, List, local, no_split, [{processes, X}|_]=Malt) ->
    L = length(List),
    case L rem X of
        0 ->
            runmany(Fun, Fuse, List, local, L div X, Malt);
        _ ->
            runmany(Fun, Fuse, List, local, L div X + 1, Malt)
    end;
% run X process on local machine
runmany(Fun, Fuse, List, local, Split, [{processes, X}|Malt]) ->
    Nodes = lists:duplicate(X, node()),
    runmany(Fun, Fuse, List, Nodes, Split, Malt);
runmany(Fun, Fuse, List, Nodes, Split, [{timeout, X}|Malt]) ->
    Parent = self(),
    Timer = spawn(fun () ->
                          receive
                              stoptimer ->
                                  Parent ! {timerstopped, self()}
                          after X ->
                                  Parent ! {timerrang, self()},
                                  receive
                                      stoptimer ->
                                          Parent ! {timerstopped, self()}
                                  end
                          end
                  end),
    Ans = try runmany(Fun, Fuse, List, Nodes, Split, Malt)
          catch
              % we really just want the after block, the syntax
              % makes this catch necessary.
              willneverhappen ->
                  nil
          after
            Timer ! stoptimer,
            cleanup_timer(Timer)
          end,
    Ans;
runmany(Fun, Fuse, List, local, Split, [{nodes, NodeList}|Malt]) ->
    Nodes = lists:foldl(fun ({Node, schedulers}, A) ->
                                X = schedulers_on_node(Node) + 1,
                                lists:reverse(lists:duplicate(X, Node), A);
                            ({Node, X}, A) ->
                                lists:reverse(lists:duplicate(X, Node), A);
                            (Node, A) ->
                                [Node|A]
                        end,
                        [], NodeList),
    runmany(Fun, Fuse, List, Nodes, Split, Malt);
% local recursive fuse, for when we weren't invoked with {processes, X}
% or {nodes, NodeList}. Degenerates recursive fuse into linear fuse.
runmany(Fun, {recursive, Fuse}, List, local, Split, []) ->
    runmany(Fun, Fuse, List, local, Split, []);
% by default, operate on each element seperately
runmany(Fun, Fuse, List, Nodes, no_split, []) ->
    runmany(Fun, Fuse, List, Nodes, 1, []);
runmany(Fun, Fuse, List, local, Split, []) ->
    List2 = splitmany(List, Split),
    local_runmany(Fun, Fuse, List2);
runmany(Fun, Fuse, List, Nodes, Split, []) ->
    List2 = splitmany(List, Split),
    cluster_runmany(Fun, Fuse, List2, Nodes).

cleanup_timer(Timer) ->
    receive
        {timerrang, Timer} ->
            cleanup_timer(Timer);
        {timerstopped, Timer} ->
            nil
    end.

schedulers_on_node(Node) ->
    case get(plists_schedulers_on_nodes) of
        undefined ->
            X = determine_schedulers(Node),
            put(plists_schedulers_on_nodes,
                dict:store(Node, X, dict:new())),
            X;
        Dict ->
            case dict:is_key(Node, Dict) of
                true ->
                    dict:fetch(Node, Dict);
                false ->
                    X = determine_schedulers(Node),
                    put(plists_schedulers_on_nodes,
                        dict:store(Node, X, Dict)),
                    X
            end
    end.

determine_schedulers(Node) ->
    Parent = self(),
    Child = spawn(Node, fun () ->
                  Parent ! {self(), erlang:system_info(schedulers)}
          end),
    erlang:monitor(process, Child),
    receive
        {Child, X} ->
            receive
                {'DOWN', _, _, Child, _Reason} ->
                    nil
            end,
            X;
        {'DOWN', _, _, Child, Reason} when Reason =/= normal ->
            0
    end.

% local runmany, for when we weren't invoked with {processes, X}
% or {nodes, NodeList}. Every sublist is processed in parallel.
local_runmany(Fun, Fuse, List) ->
    Parent = self (),
    Pids = lists:map(fun (L) ->
                             F = fun () ->
                                         Parent !
                                             {self (), Fun(L)}
                                 end,
                             {Pid, _} = erlang:spawn_monitor(F),
                             Pid
                     end,
                     List),
    Answers = try lists:map(fun receivefrom/1, Pids)
              catch throw:Message ->
                      {BadPid, Reason} = Message,
                      handle_error(BadPid, Reason, Pids)
              end,
    lists:foreach(fun (Pid) ->
                          normal_cleanup(Pid)
                  end, Pids),
    fuse(Fuse, Answers).

receivefrom(Pid) ->
    receive
        {Pid, R} ->
            R;
        {'DOWN', _, _, BadPid, Reason} when Reason =/= normal ->
            throw({BadPid, Reason});
        {timerrang, _} ->
            throw({nil, timeout})
    end.

% Convert List into [{Number, Sublist}]
cluster_runmany(Fun, Fuse, List, Nodes) ->
    {List2, _} = lists:foldl(fun (X, {L, Count}) ->
                                {[{Count, X}|L], Count+1}
                        end,
                        {[], 0}, List),
    cluster_runmany(Fun, Fuse, List2, Nodes, [], []).

% Add a pair of results into the TaskList as a fusing task
cluster_runmany(Fun, {recursive, Fuse}, [], Nodes, Running,
                [{_, R1}, {_, R2}|Results]) ->
    cluster_runmany(Fun, {recursive, Fuse}, [{fuse, R1, R2}], Nodes,
                    Running, Results);
% recursive fuse done, return result
cluster_runmany(_, {recursive, _Fuse}, [], _Nodes, [], [{_, Result}]) ->
    Result;
% edge case where we are asked to do nothing
cluster_runmany(_, {recursive, _Fuse}, [], _Nodes, [], []) ->
    [];
% We're done, now we just have to [linear] fuse the results
cluster_runmany(_, Fuse, [], _Nodes, [], Results) ->
    fuse(Fuse, lists:map(fun ({_, R}) -> R end,
                         lists:sort(fun ({A, _}, {B, _}) ->
                                            A =< B
                                    end,
                                    lists:reverse(Results))));
% We have a ready node and a sublist or fuse to be processed, so we start
% a new process
cluster_runmany(Fun, Fuse, [Task|TaskList], [N|Nodes], Running, Results) ->
    Parent = self(),
    case Task of
        {Num, L2} ->
            Fun2 = fun () ->
                           Parent ! {self(), Num, Fun(L2)}
                   end;
        {fuse, R1, R2} ->
            {recursive, FuseFunc} = Fuse,
            Fun2 = fun () ->
                           Parent ! {self(), fuse, FuseFunc(R1, R2)}
                   end
    end,
    Fun3 = fun () ->
                   try Fun2()
                   catch
                         exit:siblingdied ->
                           ok;
                         exit:Reason ->
                           Parent ! {self(), error, Reason};
                         error:R ->
                           Parent ! {self(), error, {R, erlang:get_stacktrace()}};
                         throw:R ->
                           Parent ! {self(), error, {{nocatch, R}, erlang:get_stacktrace()}}
                         end
           end,
    Pid = spawn(N, Fun3),
    erlang:monitor(process, Pid),
    cluster_runmany(Fun, Fuse, TaskList, Nodes, [{Pid, N, Task}|Running], Results);
% We can't start a new process, but can watch over already running ones
cluster_runmany(Fun, Fuse, TaskList, Nodes, Running, Results) when length(Running) > 0 ->
    receive
        {_Pid, error, Reason} ->
            RunningPids = lists:map(fun ({Pid, _, _}) ->
                                            Pid
                                    end,
                                    Running),
            handle_error(junkvalue, Reason, RunningPids);
        {Pid, Num, Result} ->
            % throw out the exit message, Reason should be
            % normal, noproc, or noconnection
            receive {'DOWN', _, _, Pid, _Reason} ->
                    nil
            end,
            {Running2, FinishedNode, _} = delete_running(Pid, Running, []),
            cluster_runmany(Fun, Fuse, TaskList,
                            [FinishedNode|Nodes], Running2, [{Num, Result}|Results]);
        {timerrang, _} ->
            RunningPids = lists:map(fun ({Pid, _, _}) ->
                                            Pid
                                    end,
                                    Running),
            handle_error(nil, timeout, RunningPids);
        % node failure
        {'DOWN', _, _, Pid, noconnection} ->
            {Running2, _DeadNode, Task} = delete_running(Pid, Running, []),
            cluster_runmany(Fun, Fuse, [Task|TaskList], Nodes,
                            Running2, Results);
        % could a noproc exit message come before the message from
        % the process? we are assuming it can't.
        % this clause is unlikely to get invoked due to cluster_runmany's
        % spawned processes. It will still catch errors in mapreduce's
        % reduce process, however.
        {'DOWN', _, _, BadPid, Reason} when Reason =/= normal ->
            RunningPids = lists:map(fun ({Pid, _, _}) ->
                                            Pid
                                    end,
                                    Running),
            handle_error(BadPid, Reason, RunningPids)
    end;
% We have data, but no nodes either available or occupied
cluster_runmany(_, _, [_Non|_Empty], []=_Nodes, []=_Running, _) ->
    exit(allnodescrashed).

delete_running(Pid, [{Pid, Node, List}|Running], Acc) ->
    {Running ++ Acc, Node, List};
delete_running(Pid, [R|Running], Acc) ->
    delete_running(Pid, Running, [R|Acc]).

handle_error(BadPid, Reason, Pids) ->
    lists:foreach(fun (Pid) ->
                          exit(Pid, siblingdied)
                  end, Pids),
    lists:foreach(fun (Pid) ->
                          error_cleanup(Pid, BadPid)
                  end, Pids),
    exit(Reason).

error_cleanup(BadPid, BadPid) ->
    ok;
error_cleanup(Pid, BadPid) ->
    receive
        {Pid, _} ->
            error_cleanup(Pid, BadPid);
        {Pid, _, _} ->
            error_cleanup(Pid, BadPid);
        {'DOWN', _, _, Pid, _Reason} ->
            ok
    end.

normal_cleanup(Pid) ->
    receive
        {'DOWN', _, _, Pid, _Reason} ->
            ok
    end.

% edge case
fuse(_, []) ->
    [];
fuse({reverse, _}=Fuse, Results) ->
    [RL|ResultsR] = lists:reverse(Results),
    fuse(Fuse, ResultsR, RL);
fuse(Fuse, [R1|Results]) ->
    fuse(Fuse, Results, R1).

fuse({reverse, FuseFunc}=Fuse, [R2|Results], R1) ->
    fuse(Fuse, Results, FuseFunc(R2, R1));
fuse(Fuse, [R2|Results], R1) ->
    fuse(Fuse, Results, Fuse(R1, R2));
fuse(_, [], R) ->
    R.

% Splits a list into a list of sublists, each of size Size,
% except for the last element which is less if the original list
% could not be evenly divided into Size-sized lists.
splitmany(List, Size) ->
    splitmany(List, [], Size).

splitmany([], Acc, _) ->
    lists:reverse(Acc);
splitmany(List, Acc, Size) ->
    {Top, NList} = split(Size, List),
    splitmany(NList, [Top|Acc], Size).

% Like lists:split, except it splits a list smaller than its first
% parameter
split(Size, List) ->
    split(Size, List, []).

split(0, List, Acc) ->
    {lists:reverse(Acc), List};
split(Size, [H|List], Acc) ->
    split(Size - 1, List, [H|Acc]);
split(_, [], Acc) ->
    {lists:reverse(Acc), []}.