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- <H2>2.2.6. Dictionary Filters</H2>
- <P>
- A <SPAN CLASS='term'>dictionary filter</SPAN> is a Filter which performs text substitution in the following manner. It maintains a collection of pairs of strings whose first components are words and whose second components represent replacement text — I'll call such a collection a <SPAN CLASS='term'>dictionary</SPAN>, and refer to the pairs it contains as <SPAN CLASS='term'>definitions</SPAN>. When a dictionary filter encounters a word which appears as the first component of a definition, it forwards the replacement text instead of the original word. Other words, whitespace and punctuation are forwarded unchanged.
- </P>
- <P>
- The basic algorithm is as follows: You examine characters one at a time, appending them to a string which I'll call the <SPAN CLASS='term'>current word</SPAN>. When you encounter a non-alphabetic character, you consult the dictionary to determine whether the current word appears as the first component of a definition. If it does, you forward the replacement text followed by the non-alphabetic character. Otherwise, you forward the current word followed by the non-alphabetic character. When the end-of-stream is reached, you consult the dictionary again and forward either the curent word or its replacement, as appropriate.
- </P>
- <P>
- In the following sections, I'll express this algorithm as a <A HREF="../classes/stdio_filter.html"><CODE>stdio_filter</CODE></A>, an <A HREF="../concepts/input_filter.html">InputFilter</A> and an <A HREF="../concepts/output_filter.html">OutputFilter</A>. The source code can be found in the header <A HREF="../../example/dictionary_filter.hpp"><CODE><libs/iostreams/example/dictionary_filter.hpp></CODE></A>.
- </P>
- <A NAME="dictionary"></A>
- <H4><CODE>dictionary</CODE></H4>
- <P>You can represent a dictionary using the following class:</P>
- <PRE class="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal"><map></SPAN>
- <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal"><string></SPAN>
- <SPAN CLASS='keyword'>namespace</SPAN> boost { <SPAN CLASS='keyword'>namespace</SPAN> iostreams { <SPAN CLASS='keyword'>namespace</SPAN> example {
- <SPAN CLASS="keyword">class</SPAN> dictionary {
- <SPAN CLASS="keyword">public</SPAN>:
- <SPAN CLASS="keyword">void</SPAN> add(std::string key, <SPAN CLASS="keyword">const</SPAN> std::string& value);
- <SPAN CLASS="keyword">void</SPAN> replace(std::string& key);
- <SPAN CLASS='comment'>/* ... */</SPAN>
- };
- } } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>
- <P>
- The member function <CODE>add</CODE> converts <CODE>key</CODE> to lower case and adds the pair <CODE>key</CODE>, <CODE>value</CODE> to the dictionary. The member function <CODE>replace</CODE> searches for a definition whose first component is equal to the result of converting <CODE>key</CODE> to lower case. If it finds such a definition, it assigns the replacement text to <CODE>key</CODE>, adjusting the case of the first character to match the case of the first character of <CODE>key</CODE>. Otherwise, it does nothing.
- </P>
- <A NAME="dictionary_stdio_filter"></A>
- <H4><CODE>dictionary_stdio_filter</CODE></H4>
- <P>You can express a dictionary filter as a <A HREF="../classes/stdio_filter.html"><CODE>stdio_filter</CODE></A> as follows:</P>
- <PRE class="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS="literal"><cstdio></SPAN> <SPAN CLASS="comment">// EOF</SPAN>
- <SPAN CLASS="preprocessor">#include</SPAN> <SPAN CLASS="literal"><iostream></SPAN> <SPAN CLASS="comment">// cin, cout</SPAN>
- <SPAN CLASS="preprocessor">#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/filter/stdio.hpp"><SPAN CLASS="literal"><boost/iostreams/filter/stdio.hpp></SPAN></A>
- <SPAN CLASS='keyword'>namespace</SPAN> boost { <SPAN CLASS='keyword'>namespace</SPAN> iostreams { <SPAN CLASS='keyword'>namespace</SPAN> example {
- <SPAN CLASS="keyword">class</SPAN> dictionary_stdio_filter : <SPAN CLASS="keyword"><SPAN CLASS="keyword"><SPAN CLASS="keyword">public</SPAN></SPAN></SPAN> stdio_filter {
- <SPAN CLASS="keyword">public</SPAN>:
- dictionary_stdio_filter(dictionary& d) : dictionary_(d) { }
- <SPAN CLASS="keyword">private</SPAN>:
- <SPAN CLASS="keyword">void</SPAN> do_filter()
- {
- <SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> std;
- <SPAN CLASS="keyword">while</SPAN> (<SPAN CLASS="keyword">true</SPAN>) {
- <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c = std::cin.get();
- <SPAN CLASS="keyword">if</SPAN> (c == <SPAN CLASS="numeric_literal">EOF</SPAN> || !std::isalpha((<SPAN CLASS="keyword">unsigned</SPAN> <SPAN CLASS="keyword">char</SPAN>) c)) {
- dictionary_.replace(current_word_);
- cout.write( current_word_.data(),
- <SPAN CLASS="keyword">static_cast</SPAN><streamsize>(current_word_.size()) );
- current_word_.erase();
- <SPAN CLASS="keyword">if</SPAN> (c == <SPAN CLASS="numeric_literal">EOF</SPAN>)
- break;
- cout.put(c);
- } <SPAN CLASS="keyword">else</SPAN> {
- current_word_ += c;
- }
- }
- }
- dictionary& dictionary_;
- std::string current_word_;
- };
- } } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>
- <P>
- The implementation of <CODE>do_filter</CODE> simply loops, reading characters from <CODE>std::cin</CODE> and <CODE>appending</CODE> them to the member variable <CODE>current_word_</CODE> until a non-alphabetic character or end-of-stream indication is encountered. When this occurs it uses its dictionary, stored in the member variable <CODE>dictionary_</CODE>, to replace the current word if necessary. Finally, it writes the current word, followed by the non-alphabetic character, if any, to <CODE>std::cout</CODE>.
- </P>
- <A NAME="dictionary_input_filter"></A>
- <H4><CODE>dictionary_input_filter</CODE></H4>
- <P>You can express a dictionary filter as an <A HREF="../concepts/input_filter.html">InputFilter</A> as follows:</P>
- <PRE class="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/char_traits.hpp"><SPAN CLASS="literal"><boost/iostreams/char_traits.hpp></SPAN></A> <SPAN CLASS="comment">// EOF, WOULD_BLOCK</SPAN>
- <SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/concepts.hpp"><SPAN CLASS="literal"><boost/iostreams/concepts.hpp></SPAN></A> <SPAN CLASS="comment">// input_filter</SPAN>
- <SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/operations.hpp"><SPAN CLASS="literal"><boost/iostreams/operations.hpp></SPAN></A> <SPAN CLASS="comment">// get</SPAN>
- <SPAN CLASS='keyword'>namespace</SPAN> boost { <SPAN CLASS='keyword'>namespace</SPAN> iostreams { <SPAN CLASS='keyword'>namespace</SPAN> example {
- <SPAN CLASS="keyword">class</SPAN> dictionary_input_filter : <SPAN CLASS="keyword"><SPAN CLASS="keyword"><SPAN CLASS="keyword">public</SPAN></SPAN></SPAN> input_filter {
- <SPAN CLASS="keyword">public</SPAN>:
- dictionary_input_filter(dictionary& d)
- : dictionary_(d), off_(std::string::npos), eof_(<SPAN CLASS="keyword">false</SPAN>)
- { }
- <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
- <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> get(Source& src);
- <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
- <SPAN CLASS="keyword">void</SPAN> close(Source&);
- <SPAN CLASS="keyword">private</SPAN>:
- dictionary& dictionary_;
- std::string current_word_;
- std::string::size_type off_;
- <SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> eof_;
- };
- } } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>
- <P>The function <CODE>get</CODE> is implemented as follows:</P>
- <PRE class="broken_ie"> <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
- <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> get(Source& src)
- {
- <SPAN CLASS='comment'>// Handle unfinished business.</SPAN>
- <SPAN CLASS="keyword">if</SPAN> (off_ != std::string::npos && off_ < current_word_.size())
- <SPAN CLASS="keyword">return</SPAN> current_word_[off_++];
- <SPAN CLASS="keyword">if</SPAN> (off_ == current_word_.size()) {
- current_word_.erase();
- off_ = std::string::npos;
- }
- <SPAN CLASS="keyword">if</SPAN> (eof_)
- <SPAN CLASS="keyword">return</SPAN> <SPAN CLASS="numeric_literal">EOF</SPAN>;
- <SPAN CLASS='comment'>// Compute curent word.</SPAN>
- <SPAN CLASS="keyword">while</SPAN> (<SPAN CLASS="keyword">true</SPAN>) {
- <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c;
- <SPAN CLASS="keyword">if</SPAN> ((c = iostreams::get(src)) == WOULD_BLOCK)
- <SPAN CLASS="keyword">return</SPAN> WOULD_BLOCK;
- <SPAN CLASS="keyword">if</SPAN> (c == <SPAN CLASS="numeric_literal">EOF</SPAN> || !std::isalpha((<SPAN CLASS="keyword">unsigned</SPAN> <SPAN CLASS="keyword">char</SPAN>) c)) {
- dictionary_.replace(current_word_);
- off_ = 0;
- <SPAN CLASS="keyword">if</SPAN> (c == <SPAN CLASS="numeric_literal">EOF</SPAN>)
- eof_ = <SPAN CLASS="keyword">true</SPAN>;
- <SPAN CLASS="keyword">else</SPAN>
- current_word_ += c;
- break;
- } <SPAN CLASS="keyword">else</SPAN> {
- current_word_ += c;
- }
- }
- <SPAN CLASS="keyword">return</SPAN> this->get(src); <SPAN CLASS='comment'>// Note: current_word_ is not empty.</SPAN>
- }</PRE>
- <P>
- You first check to see whether there are any characters which remain from a previous invocation of <CODE>get</CODE>. If so, you update some book keeping information and return the first such character.
- </P>
- <P>
- The <CODE>while</CODE> loop is very similar to that of <A HREF="#dictionary_stdio_filter"><CODE>dictionary_stdio_filter::do_filter</CODE></A>: it reads characters from the <A HREF="../concepts/source.html">Source</A> <CODE>src</CODE>, appending them to <CODE>current_word_</CODE> until a non-alphabetic character, <CODE>EOF</CODE> or <CODE>WOULD_BLOCK</CODE> is encountered. The value <CODE>WOULD_BLOCK</CODE> is passed on to the caller. In the remaining cases, the dictionary is consulted to determine the appropriate replacement text.
- </P>
- <P>Finally, <CODE>get</CODE> is called recursively to return the first character of the current word.</P>
- <P>As usual, the function <CODE>close</CODE> resets the Filter's state:</P>
- <PRE class="broken_ie"> <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Source>
- <SPAN CLASS="keyword">void</SPAN> close(Source&)
- {
- current_word_.erase();
- off_ = std::string::npos;
- eof_ = <SPAN CLASS="keyword">false</SPAN>;
- }</PRE>
- <A NAME="dictionary_output_filter"></A>
- <H4><CODE>dictionary_output_filter</CODE></H4>
- <P>You can express a dictionary filter as an <A HREF="../concepts/output_filter.html">OutputFilter</A> as follows:</P>
- <PRE class="broken_ie"><SPAN CLASS='preprocessor'>#include</SPAN> <SPAN CLASS='literal'><algorithm></SPAN> <SPAN CLASS='comment'>// swap</SPAN>
- <SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/concepts.hpp"><SPAN CLASS="literal"><boost/iostreams/concepts.hpp></SPAN></A> <SPAN CLASS="comment">// output_filter</SPAN>
- <SPAN CLASS='preprocessor'>#include</SPAN> <A CLASS="header" HREF="../../../../boost/iostreams/operations.hpp"><SPAN CLASS="literal"><boost/iostreams/operations.hpp></SPAN></A> <SPAN CLASS="comment">// write</SPAN>
- <SPAN CLASS='keyword'>namespace</SPAN> boost { <SPAN CLASS='keyword'>namespace</SPAN> iostreams { <SPAN CLASS='keyword'>namespace</SPAN> example {
- <SPAN CLASS="keyword">class</SPAN> dictionary_output_filter : <SPAN CLASS="keyword"><SPAN CLASS="keyword"><SPAN CLASS="keyword">public</SPAN></SPAN></SPAN> output_filter {
- <SPAN CLASS="keyword">public</SPAN>:
- <SPAN CLASS="keyword">typedef</SPAN> std::map<std::string, std::string> map_type;
- dictionary_output_filter(dictionary& d)
- : dictionary_(d), off_(std::string::npos)
- { }
- <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
- <SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put(Sink& dest, <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c);
- <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
- <SPAN CLASS="keyword">void</SPAN> close(Sink& dest);
- <SPAN CLASS="keyword">private</SPAN>:
- <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
- <SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> write_current_word(Sink& dest);
- dictionary& dictionary_;
- std::string current_word_;
- std::string::size_type off_;
- };
- } } } <SPAN CLASS="comment">// End namespace boost::iostreams:example</SPAN></PRE>
- <P>Let's look first at the helper function <CODE>write_current_word</CODE>:</P>
- <PRE class="broken_ie"> <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
- <SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> write_current_word(Sink& dest)
- {
- <SPAN CLASS="keyword">using</SPAN> <SPAN CLASS="keyword">namespace</SPAN> std;
- streamsize amt = <SPAN CLASS="keyword">static_cast</SPAN><streamsize>(current_word_.size() - off_);
- streamsize result =
- iostreams::write(dest, current_word_.data() + off_, amt);
- <SPAN CLASS="keyword">if</SPAN> (result == amt) {
- current_word_.erase();
- off_ = string::npos;
- <SPAN CLASS="keyword">return</SPAN> <SPAN CLASS="keyword">true</SPAN>;
- } <SPAN CLASS="keyword">else</SPAN> {
- off_ += <SPAN CLASS="keyword">static_cast</SPAN><string::size_type>(result);
- <SPAN CLASS="keyword">return</SPAN> <SPAN CLASS="keyword">false</SPAN>;
- }
- }</PRE>
- <P>
- This function attempts to write <CODE>current_word_</CODE>, beginning at the offset <CODE>off_</CODE>, to the provided <A HREF="../concepts/sink.html">Sink</A>. If the entire sequence is successfully written, <CODE>current_word_</CODE> is cleared and the function returns <CODE>true</CODE>. Otherwise the member variable <CODE>off_</CODE> is updated to point to the first unwritten character and the function fails.
- </P>
- <P>Using <CODE>write_current_word</CODE> you can implement <CODE>put</CODE> as follows:</P>
- <PRE class="broken_ie"> <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
- <SPAN CLASS="keyword"><SPAN CLASS="keyword">bool</SPAN></SPAN> put(Sink& dest, <SPAN CLASS="keyword"><SPAN CLASS="keyword">int</SPAN></SPAN> c)
- {
- <SPAN CLASS="keyword">if</SPAN> (off_ != std::string::npos && !write_current_word(dest))
- <SPAN CLASS="keyword">return</SPAN> <SPAN CLASS="keyword">false</SPAN>;
- <SPAN CLASS="keyword">if</SPAN> (!std::isalpha((<SPAN CLASS="keyword">unsigned</SPAN> <SPAN CLASS="keyword">char</SPAN>) c)) {
- dictionary_.replace(current_word_);
- off_ = 0;
- }
- current_word_ += c;
- <SPAN CLASS="keyword">return</SPAN> <SPAN CLASS="keyword">true</SPAN>;
- }</PRE>
- <P>
- As in the implementation of <A HREF="#dictionary_input_filter"><CODE>dictionary_input_filter::get</CODE></A>, you first check to see whether there are any characters from a previous invocation of <CODE>put</CODE> which remain to be written. If so, you attempt to write these characters using <CODE>write_current_word</CODE>. If successful, you next examine the given character <CODE>c</CODE>. If it is a non-alphabetic character, you consult the dictionary to determine the appropriate replacement text. In any case, you append <CODE>c</CODE> to <CODE>current_word_</CODE> and return <CODE>true</CODE>.
- </P>
- <P>The function <CODE>close</CODE> has more work to do in this case than simply reseting the Filter's state. Unless the last character of the unfiltered sequence happened to be a non-alphabetic character, the contents of current_word_ will not yet have been written:</P>
- <PRE class="broken_ie"> <SPAN CLASS="keyword">template</SPAN><<SPAN CLASS="keyword">typename</SPAN> Sink>
- void close(Sink& dest)
- {
- <SPAN CLASS='comment'>// Reset current_word_ and off_, saving old values.</SPAN>
- std::string current_word;
- std::string::size_type off = <SPAN CLASS='numeric_literal'>0</SPAN>;
- current_word.swap(current_word_);
- std::swap(off, off_);
- <SPAN CLASS='comment'>// Write remaining characters to dest.</SPAN>
- <SPAN CLASS="keyword">if</SPAN> (off == std::string::npos) {
- dictionary_.replace(current_word);
- off = <SPAN CLASS='numeric_literal'>0</SPAN>;
- }
- <SPAN CLASS="keyword">if</SPAN> (!current_word.empty())
- iostreams::write(
- dest,
- current_word.data() + off,
- <SPAN CLASS="keyword">static_cast</SPAN><std::streamsize>(current_word.size() - off)
- );
- }</PRE>
- <P>Note that you may assume that the template argument is a <A HREF="../concepts/blocking.html">Blocking</A> <A HREF="../concepts/sink.html">Sink</A>, and that you must reset the values of <CODE>current_word_</CODE> and <CODE>off_</CODE> before calling <A HREF="../functions/write.html"><CODE>write</CODE></A>, in case <A HREF="../functions/write.html"><CODE>write</CODE></A> throws an exception.</P>
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