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294 lines
9.1 KiB
C++
294 lines
9.1 KiB
C++
/*
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* @copyright
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* Copyright (C) 2009-2013, Intel Corporation
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* All rights reserved.
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*
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* @copyright
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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*
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* * Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* * Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in
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* the documentation and/or other materials provided with the
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* distribution.
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* * Neither the name of Intel Corporation nor the names of its
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* contributors may be used to endorse or promote products derived
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* from this software without specific prior written permission.
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*
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* @copyright
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* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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* HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
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* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
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* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
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* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
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* WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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* POSSIBILITY OF SUCH DAMAGE.
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*
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*/
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/*
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* reducer_ostream.h
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*
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* Purpose: Hyper-object to write to 'std::ostream's
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*
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* Classes: reducer_ostream
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*
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* Description:
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* ============
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* Output streams ('std::ostream's) are a convenient means of writing text to
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* files, the user console, or sockets. In a serial program, text is written
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* to an ostream in a specific, logical order. For example, computing while
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* traversing a data structure and printing them to an 'ostream' will result
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* in the values being printed in the order of traversal. In a parallel
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* version of the same program, however, different parts of the data structure
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* may be traversed in a different order, resulting in a non-deterministic
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* ordering of the stream. Worse, multiple strands may write to the same
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* stream simultaneously, resulting in a data race. Replacing the
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* 'std::ostream' with a 'cilk::reducer_ostream' will solve both problems: Data
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* will appeaer in the stream in the same order as it would for the serial
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* program, and there will be no races (no locks) on the common stream.
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*
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* Usage Example:
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* ==============
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* Assume we wish to traverse an array of objects, performing an operation on
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* each object and writing the result to a file. Without a reducer_ostream,
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* we have a race on the 'output' file stream:
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*..
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* void compute(std::ostream& os, double x)
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* {
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* // Perform some significant computation and print the result:
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* os << std::asin(x);
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* }
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*
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* int test()
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* {
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* const std::size_t ARRAY_SIZE = 1000000;
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* extern double myArray[ARRAY_SIZE];
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*
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* std::ofstream output("output.txt");
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* cilk_for (std::size_t i = 0; i < ARRAY_SIZE; ++i)
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* {
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* compute(output, myArray[i]);
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* }
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*
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* return 0;
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* }
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*..
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* The race is solved by using a reducer_ostream to proxy the 'output' file:
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*..
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* void compute(cilk::reducer_ostream& os, double x)
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* {
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* // Perform some significant computation and print the result:
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* *os << std::asin(x);
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* }
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*
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* int test()
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* {
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* const std::size_t ARRAY_SIZE = 1000000;
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* extern double myArray[ARRAY_SIZE];
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*
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* std::ofstream output("output.txt");
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* cilk::reducer_ostream hyper_output(output);
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* cilk_for (std::size_t i = 0; i < ARRAY_SIZE; ++i)
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* {
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* compute(hyper_output, myArray[i]);
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* }
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*
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* return 0;
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* }
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*..
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*
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* Limitations:
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* ============
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* There are two possible values for the formatting flags immediately after a
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* 'cilk_spawn' statement: they may either have the value that was set by the
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* spawn function, or they may have default values. Because of
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* non-determinism in the processor scheduling, there is no way to determine
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* which it will be. Similarly, the formatting flags after a 'cilk_sync' may
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* or may not have the same value as before the sync. Therefore, one must use
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* a disciplined coding style to avoid formatting errors. There are two
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* approaches to mitigating the problem: The first is to eliminate the
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* difference between the two possible outcomes by ensuring that the spawned
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* function always returns the flags to their initial state:
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*..
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* void compute(cilk::reducer_ostream& os, double x)
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* {
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* // Perform some significant computation and print the result:
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* int saveprec = os.precision(5);
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* os << std::asin(x);
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* os.precision(saveprec);
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* }
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*..
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* The second approach is to write your streaming operations such that they
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* don't depend on the previous state of the formatting flags by setting any
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* important flags before every block of output:
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*..
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* cilk_spawn compute(hyper_output, value);
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*
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* hyper_output->precision(2); // Don't depend on previous precision
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* *hyper_output << f();
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* *hyper_output << g();
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*..
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* Another concern is memory usage. A reducer_ostream will buffer as much text
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* as necessary to ensure that the order of output matches that of the serial
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* version of the program. If all spawn branches perform an equal amount of
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* output, then one can expect that half of the output before a sync will be
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* buffered in memory. This hyperobject is therefore not well suited for
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* serializing very large quantities of text output.
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*/
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#ifndef REDUCER_OSTREAM_H_INCLUDED
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#define REDUCER_OSTREAM_H_INCLUDED
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#include <cilk/reducer.h>
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#include <iostream>
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#include <sstream>
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namespace cilk {
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/**
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* @brief Class 'reducer_ostream' is the representation of a hyperobject for
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* output text streaming.
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*/
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class reducer_ostream
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{
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public:
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/// Internal representation of the per-strand view of the data for reducer_ostream
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class View: public std::ostream
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{
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public:
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/// Type of the std::stream reducer_ostream is based on
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typedef std::ostream Base;
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friend class reducer_ostream;
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View():
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std::ostream(0)
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{
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Base::rdbuf(&strbuf_);
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};
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private:
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void use_ostream (const std::ostream &os)
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{
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Base::rdbuf(os.rdbuf());
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Base::flags(os.flags()); // Copy formatting flags
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Base::setstate(os.rdstate()); // Copy error state
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}
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private:
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std::stringbuf strbuf_;
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};
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public:
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/// Definition of data view, operation, and identity for reducer_ostream
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struct Monoid: monoid_base< View >
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{
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static void reduce (View *left, View *right);
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};
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private:
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// Hyperobject to serve up views
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reducer<Monoid> imp_;
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// Methods that provide the API for the reducer
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public:
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// Construct an initial 'reducer_ostream' from an 'std::ostream'. The
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// specified 'os' stream is used as the eventual destination for all
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// text streamed to this hyperobject.
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explicit reducer_ostream(const std::ostream &os);
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// Return a modifiable reference to the underlying 'ostream' object.
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std::ostream& get_reference();
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/**
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* Append data from some type to the reducer_ostream
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*
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* @param v Value to be appended to the reducer_ostream
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*/
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template<typename T>
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std::ostream &
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operator<< (const T &v)
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{
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return imp_.view() << v;
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}
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/**
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* Append data from a std::ostream to the reducer_ostream
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*
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* @param _Pfn std::ostream to copy from
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*/
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std::ostream &
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operator<< (std::ostream &(*_Pfn)(std::ostream &))
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{
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View &v = imp_.view();
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return ((*_Pfn)(v));
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}
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reducer_ostream& operator*() { return *this; }
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reducer_ostream const& operator*() const { return *this; }
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reducer_ostream* operator->() { return this; }
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reducer_ostream const* operator->() const { return this; }
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};
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// -------------------------------------------
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// class reducer_ostream::Monoid
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// -------------------------------------------
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/**
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* Appends string from "right" reducer_basic_string onto the end of
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* the "left". When done, the "right" reducer_basic_string is empty.
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*/
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void
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reducer_ostream::Monoid::reduce(View *left, View *right)
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{
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left->operator<< (&right->strbuf_);
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}
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// --------------------------
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// class reducer_ostream
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// --------------------------
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/**
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* Construct a reducer_ostream which will write to the specified std::ostream
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*
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* @param os std::ostream to write to
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*/
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inline
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reducer_ostream::reducer_ostream(const std::ostream &os) :
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imp_()
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{
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View &v = imp_.view();
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v.use_ostream(os);
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}
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/**
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* Get a reference to the std::ostream
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*/
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inline
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std::ostream &
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reducer_ostream::get_reference()
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{
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View &v = imp_.view();
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return v;
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}
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} // namespace cilk
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#endif // REDUCER_OSTREAM_H_INCLUDED
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