Huffman Codes
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1 Huffman Codes Using Binary Files
description
Huffman Codes. Using Binary Files. Getting Started. Last class we extended a program to create a Huffman code and permit the user to encode and decode messages. We will use that program as our starting point today: - PowerPoint PPT Presentation
Transcript of Huffman Codes
1
Huffman Codes
Using Binary Files
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Getting Started
Last class we extended a program to create a Huffman code and permit the user to encode and decode messages.
We will use that program as our starting point today:
http://www.cse.usf.edu/~turnerr/Data_Structures/Downloads/2011_04_13_Huffman_Codes_with_Binary_IO/
File Huffman_Code_with_Associative_Map.zip
Download, extract, built, and run.
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Program in Action
Widen window to 100.
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Binary Output
Huffman codes are useful in real life only when we output the coded message as binary.
Let's modify do_encode to output a file. Start with a text file.
ASCII 1's and 0's Modify to write a binary file.
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main.cpp
Add at top of main.cpp:
#include <fstream>
Modified version of do_encode: http://www.cse.usf.edu/~turnerr/Data_Structures/
Downloads/2011_04_13_Huffman_Codes_with_Binary_IO/do_encode.cpp.txt
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Modifications to do_encode()void do_encode(void)
{
string msg;
string output_filename;
ofstream outfile;
string junk;
while (!outfile.good())
{
cout << "File name for output? ";
cin >> output_filename;
getline(cin,junk); // Skip newline char
outfile.open(output_filename.c_str());
if (!outfile.good())
{
cout << "Failed to open output file\n";
cout << "Please try again\n";
}
}
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Modifications to do_encode()
cout << "\n\nEnter message to encode\n";
getline(cin, msg);
for (size_t i = 0; i < msg.length(); ++i)
{
char next_char = tolower(msg[i]);
string code = huffman_tree.Encode_Char(next_char);
cout << code;
outfile << code;
}
cout << endl << endl;
outfile << endl << endl;
outfile.close();
cout << "File " << output_filename << " written\n";
}
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Clean Up Output
Comment out statements that output the tree and the code.
In main():int main(void)
{
cout << "This is the Huffman Code Program" << endl;
build_huffman_tree();
//huffman_tree.Display_List();
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In Huffman_Tree.cpp
void Huffman_Tree::Make_Decode_Tree(void)
{
node_list.sort();
//cout << "\nSorted list:\n";
//Display_List();
...
//cout << endl << "The Huffman Tree" << endl;
//Display_Decode_Tree(&decode_tree_root, 0);
//cout << endl << "The Code: " << endl;
//Display_Code(&decode_tree_root, "");
}
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Program in Action
Examine c:\out.txt
The Output File
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Invalid Characters
What should we do with characters that are not in the code?
Encode_Char() returns a zero length string.
Detect the error in do_encode(). Tell user about the error. Skip the invalid character in output.
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main.cpp
In do_encode()for (size_t i = 0; i < msg.length(); ++i)
{
char next_char = tolower(msg[i]);
string code = huffman_tree.Encode_Char(next_char);
if (code.size() == 0)
{
cout << endl << "Invalid character in input to do_encode: "
<< next_char << endl;
continue;
}
cout << code;
outfile << code << " ";
}
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Program Running
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The Output File
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Binary File I/O
Issues with binary files. Hardware architecture dependencies. Code is typically not portable. Output is by byte, not by bit For Huffman coding we need variable
length bit strings. Must know number of bits.
Encapsulate code to do binary file I/O in classes.
Provide relatively simple interface to the rest of the program.
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Binary Output File Class
Bit Count
BitsClient Classes
Binary Output File Class
Buffer
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Binary Input File Class
Bit Count
BitsClient Classes
Binary Input File Class
Buffer
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Binary File Classes
Binary_File
is_open
buffer
next_bit_position
filename
BUFFER_SIZE
FIRST_BIT_POSITION
+ Is_Open
Binary_Output_File
-fstream
+ Output_Bit_String
+ Close
- Write_Buffer
Binary_Input_File
-fstream
+ Get_Next_Bit
+ Close
-Read_Buffer
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Binary File I/O
Download http://www.cse.usf.edu/~turnerr/Data_Structures/
Downloads/2011_04_13_Binary_File_IO/ File Binary_File_IO_Classes.zip
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Binary File IO Classes
Copy into project folder and add to project.
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Add Binary File IO Files to Project
Build project.
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Binary_File.h#pragma once
#include <string>
using std::string;
class Binary_File
{
public:
Binary_File(const string& Filename);
virtual void Close() = 0;
bool Is_Open() const {return is_open;};
protected:
static const int BUFFER_SIZE = 1024; // Size in bytes
static const int FIRST_BIT_POSITION = 8*sizeof(size_t);
union Buffer
{
char bits[BUFFER_SIZE];
size_t bit_count;
};
void Reset_Buffer(void);
const string filename;
bool is_open;
Buffer buffer;
size_t next_bit_position;
};
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Binary_File.cpp
#include "Binary_File.h"
Binary_File::Binary_File(const string& Filename) :
is_open(false), filename(Filename)
{
Reset_Buffer();
}
void Binary_File::Reset_Buffer(void)
{
for (int i = 0; i < BUFFER_SIZE; ++i)
{
buffer.bits[i] = 0;
}
next_bit_position = FIRST_BIT_POSITION;
}
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Binary_Output_File.h
#pragma once
#include <iostream>
#include <fstream>
#include <string>
#include "Binary_File.h"
using std::string;
class Binary_Output_File : public Binary_File
{
public:
Binary_Output_File(const string& filename);
void Output(const string& bit_string);
void Close();
private:
std::fstream outfile;
void Write_Buffer();
};
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Binary_Output_File.cpp
#include <cassert>
#include <cmath>
#include "Binary_Output_File.h"
using namespace std;
Binary_Output_File::Binary_Output_File(const string& filename) : Binary_File(filename)
{
outfile.open(filename.c_str(), ios::out | ios::binary );
if (outfile.fail())
{
string err_msg("Error opening output file ");
err_msg += filename;
throw err_msg;
}
Reset_Buffer();
is_open = true;
}
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Binary_Output_File.cpp
void Binary_Output_File::Write_Buffer()
{
assert (is_open);
if (next_bit_position == FIRST_BIT_POSITION)
{
return;
}
buffer.bit_count = next_bit_position - FIRST_BIT_POSITION;
size_t nr_bytes = (size_t) ceil(next_bit_position / 8.0);
outfile.write( buffer.bits, nr_bytes);
Reset_Buffer();
}
Binary_Output_File.cppvoid Binary_Output_File::Output(const string& bit_string)
{
assert(is_open);
for (size_t i = 0; i < bit_string.size(); ++i)
{
if (bit_string[i] == '1')
{
size_t byte_position = next_bit_position / 8;
size_t bit_position_within_byte = next_bit_position % 8;
buffer.bits[byte_position] |= (0x80 >> bit_position_within_byte);
}
else
{
assert(bit_string[i] == '0');
}
++next_bit_position;
if (next_bit_position == BUFFER_SIZE*8)
{
Write_Buffer();
}
}
}
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Binary_Output_File.cpp
void Binary_Output_File::Close()
{
Write_Buffer();
outfile.close();
is_open = false;
}
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Using Binary File IO
Now let's modify do_encode() to write a binary file.
Add at top of main.cpp:#include "Binary_Output_File.h"
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do_encode()
void do_encode(void)
{
string msg;
string output_filename;
Binary_Output_File* outfile;
string junk;
while (true)
{
cout << "File name for output? ";
cin >> output_filename;
getline(cin, junk); // Skip newline char
try
{
outfile = new Binary_Output_File(output_filename);
break;
}
catch (const string& msg)
{
cout << msg << endl;
}
}
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do_encode()
cout << "Enter message to encode\n";
getline(cin, msg);
for (size_t i = 0; i < msg.length(); ++i)
{
char next_char = tolower(msg[i]);
string code = huffman_tree.Encode_Char(next_char); if (code.size() == 0)
{
cout << endl << "Invalid character in input to do_encode: "
<< next_char << endl;
continue;
}
cout << code;
outfile->Output(code);
}
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do_encode()
cout << endl << endl;
outfile->Close();
delete(outfile);
cout << "File " << output_filename << " written\n";
}
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Some Test Data
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Program in Action
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c:\test.dat
Look at the output file in Visual Studio File > Open > File
Bit Count = 16 0000 0001 0010 0011
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Binary Input
Now let's modify do_decode() to read a binary input file rather than reading 1's and 0's from the keyboard.
Add at top of main.cpp:#include "Binary_Input_File.h"
http://www.cse.usf.edu/~turnerr/Data_Structures/Downloads/2011_04_13_Huffman_Codes_with_Binary_IO/do_decode.cpp.txt
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do_decode()void do_decode(void)
{
string msg;
string input_filename;
Binary_Input_File* infile;
string junk;
while (true)
{
cout << "File name for input? ";
cin >> input_filename;
getline(cin, junk); // Skip newline char
try
{
infile = new Binary_Input_File(input_filename);
break;
}
catch (const string& msg)
{
cout << msg << endl;
}
}
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do_decode()
string coded_message = "";
string original_message;
while (infile->Is_Open())
{
int next_bit = infile->Get_Next_Bit();
if (next_bit < 0) break;
if (next_bit == 0)
{
coded_message += "0";
}
else
{
coded_message += "1";
}
}
original_message = huffman_tree.Decode_Msg(coded_message);
cout << "Original message: " << original_message << endl;
cout << endl << endl;
}
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Reading a Binary File
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Another Example
