demo_grep.erl

% Copyright (c) 2011-2012, Tom Van Cutsem, Vrije Universiteit Brussel
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% Grep (text search) example using the basic parallel MapReduce
% implementation:
-module(demo_grep).
-export([grep/2]).
-import(mapreduce, [mapreduce/3]).

%% Grep
grep(DirName, Word) ->
  NumberedFiles = demo_inverted_index:list_numbered_files(DirName),
  Index = mapreduce(NumberedFiles,
                    make_filter_mapper(Word), fun remove_duplicates/3),
  dict:find(Word, Index).

% this function is used as a Map function
make_filter_mapper(MatchWord) ->
  fun (_Index, FileName, Emit) ->
    {ok, [Words]} = file:consult(FileName),
    lists:foreach(fun (Word) ->
      case MatchWord == Word of
        true -> Emit(Word, FileName);
        false -> false
      end
    end, Words)
  end.
  
% this function is used as a Reduce function
remove_duplicates(Word, FileNames, Emit) ->
  UniqueFiles = sets:to_list(sets:from_list(FileNames)),
  lists:foreach(fun (FileName) -> Emit(Word, FileName) end, UniqueFiles).