mapreduce_sequential.erl

% Copyright (c) 2011-2012, Tom Van Cutsem, Vrije Universiteit Brussel
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% A sequential implementation of the Google MapReduce pattern
-module(mapreduce_sequential).
-export([mapreduce/3]).
-import(lists, [foreach/2]).

%% Input = List[{K1, V1}]
%% Map(K1, V1) -> returns a List[{K2,V2}]
%% Reduce(K2, List[V2]) -> returns a List[V2]
%% Returns a Map[K2,List[V2]]
mapreduce(Input, Map, Reduce) ->
  %% Map Phase
  IntermediateLists = lists:map(fun ({K1,V1}) -> Map(K1,V1) end, Input),
  IntermediatePairs = lists:flatten(IntermediateLists),
  IntermediateMap = lists:foldl(
    fun({K2,V2}, Dict) ->
      dict:append(K2,V2,Dict)
    end, dict:new(), IntermediatePairs),
  
  %% Reduce Phase
  dict:map(fun(K2, ListOfV2) ->
             Reduce(K2, ListOfV2)
           end, IntermediateMap).