demo_frequency.erl

% Copyright (c) 2011-2012, Tom Van Cutsem, Vrije Universiteit Brussel
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% Word frequency count example using the simple parallel MapReduce
% implementation:
-module(demo_frequency).
-export([freq/1]).
-import(mapreduce, [mapreduce/3]).

%% Word frequency count
freq(DirName) ->
  NumberedFiles = demo_inverted_index:list_numbered_files(DirName),
  mapreduce(NumberedFiles, fun enum_words/3, fun add_words/3).

% this function is used as a Map function
enum_words(_Index, FileName, Emit) ->
  {ok, [Words]} = file:consult(FileName),
  lists:foreach(fun (Word) -> Emit(Word, 1) end, Words).

% this function is used as a Reduce function
add_words(Word, Counts, Emit) ->
  Total = lists:foldl(fun (Count, Partial) -> Count + Partial end, 0, Counts),
  Emit(Word, Total).