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# wordcount.j
#
# Implementation of parallelized "word-count" of a text, inspired by the
# Hadoop WordCount example. Uses @spawn and fetch() to parallelize
# the "map" task. Reduce is currently done single-threaded.
#
# To run in parallel on a string stored in variable `text`:
# julia -p <N>
# julia> @everywhere load("<julia_dir>/examples/wordcount.j")
# julia> ...(define text)...
# julia> counts=parallel_wordcount(text)
#
# Or to run on a group of files, writing results to an output file:
# julia -p <N>
# julia> @everywhere load("<julia_dir/examples/wordcount.j")
# julia> wordcount_files("/tmp/output.txt", "/tmp/input1.txt","/tmp/input2.txt",...)
# "Map" function.
# Takes a string. Returns a Dict with the number of times each word
# appears in that string.
function wordcount(text)
words=split(text,(' ','\n','\t','-','.',',',':',';'),false)
counts=Dict()
for w = words
counts[w]=get(counts,w,0)+1
end
return counts
end
# "Reduce" function.
# Takes a collection of Dicts in the format returned by wordcount()
# Returns a Dict in which words that appear in multiple inputs
# have their totals added together.
function wcreduce(wcs)
counts=Dict()
for c = wcs
for (k,v)=c
counts[k] = get(counts,k,0)+v
end
end
return counts
end
# Splits input string into nprocs() equal-sized chunks (last one rounds up),
# and @spawns wordcount() for each chunk to run in parallel. Then fetch()s
# results and performs wcreduce().
function parallel_wordcount(text)
lines=split(text,'\n',false)
np=nprocs()
unitsize=ceil(length(lines)/np)
wcounts={}
rrefs={}
# spawn procs
for i=1:np
first=unitsize*(i-1)+1
last=unitsize*i
if last>length(lines)
last=length(lines)
end
subtext=join(lines[int(first):int(last)],"\n")
push(rrefs, @spawn wordcount( subtext ) )
end
# fetch results
while length(rrefs)>0
push(wcounts,fetch(pop(rrefs)))
end
# reduce
count=wcreduce(wcounts)
return count
end
# Takes the name of a result file, and a list of input file names.
# Combines the contents of all files, then performs a parallel_wordcount
# on the resulting string. Writes the results to result_file.
function wordcount_files(result_file,input_file_names...)
text=""
for f = input_file_names
fh=open(f)
text=join( {text,readall(fh)}, "\n" )
close(fh)
end
wc=parallel_wordcount(text)
fo=open(result_file,"w")
for (k,v) = wc
with_output_stream(fo,println,k,"=",v)
end
end
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