How to : Adaptive huffman coding
Adaptive huffman coding
LZH implementation in Pascal
Contributor: DOUGLAS WEBB
Unit LZH;
{$A+,B-,D-,E-,F-,I+,L-,N-,O-,R-,S-,V-}
(*
LZHUF.C English version 1.0
Based on Japanese version 29-NOV-1988
LZSS coded by Haruhiko OKUMURA
Adaptive Huffman Coding coded by Haruyasu YOSHIZAKI
Edited and translated to English by Kenji RIKITAKE
Translated from C to Turbo Pascal by Douglas Webb 2/18/91
Update and bug correction of TP version 4/29/91 (Sorry!!)
*)
{
This Unit allows the user to commpress data using a combination of
LZSS Compression and adaptive Huffman coding, or conversely to deCompress
data that was previously Compressed by this Unit.
There are a number of options as to where the data being Compressed/
deCompressed is coming from/going to.
In fact it requires that you pass the "LZHPack" Procedure 2 procedural
parameter of Type 'GetProcType' and 'PutProcType' (declared below) which
will accept 3 parameters and act in every way like a 'BlockRead'/'BlockWrite'
Procedure call. Your 'GetProcType' Procedure should return the data
to be Compressed, and Your 'PutProcType' Procedure should do something with
the Compressed data (ie., put it in a File). In Case you need to know (and
you do if you want to deCompress this data again) the number of Bytes in the
Compressed data (original, not Compressed size) is returned in 'Bytes_Written'.
GetBytesProc = Procedure(Var DTA; NBytes:Word; Var Bytes_Got : Word);
DTA is the start of a memory location where the inFormation returned should
be. NBytes is the number of Bytes requested. The actual number of Bytes
returned must be passed in Bytes_Got (if there is no more data then 0
should be returned).
PutBytesProc = Procedure(Var DTA; NBytes:Word; Var Bytes_Got : Word);
As above except instead of asking For data the Procedure is dumping out
Compressed data, do somthing With it.
"LZHUnPack" is basically the same thing in reverse. It requires
procedural parameters of Type 'PutProcType'/'GetProcType' which
will act as above. 'GetProcType' must retrieve data Compressed using
"LZHPack" (above) and feed it to the unpacking routine as requested.
'PutProcType' must accept the deCompressed data and do something
withit. You must also pass in the original size of the deCompressed data,
failure to do so will have adverse results.
Don't Forget that as procedural parameters the 'GetProcType'/'PutProcType'
Procedures must be Compiled in the 'F+' state to avoid a catastrophe.
}
{ note: All the large data structures For these routines are allocated when
needed from the heap, and deallocated when finished. So when not in use
memory requirements are minimal. However, this Unit Uses about 34K of
heap space, and 400 Bytes of stack when in use. }
Interface
Type
PutBytesProc = Procedure(Var DTA; NBytes : Word; Var Bytes_Put : Word);
GetBytesProc = Procedure(Var DTA; NBytes : Word; Var Bytes_Got : Word);
Procedure LZHPack(Var Bytes_Written : LongInt;
GetBytes : GetBytesProc;
PutBytes : PutBytesProc);
Procedure LZHUnpack(TextSize : LongInt;
GetBytes : GetBytesProc;
PutBytes : PutBytesProc);
Implementation
Const
Exit_OK = 0;
Exit_FAILED = 1;
{ LZSS Parameters }
N = 4096; { Size of String buffer }
F = 60; { Size of look-ahead buffer }
THRESHOLD = 2;
NUL = N; { end of tree's node }
{ Huffman coding parameters }
N_Char = (256 - THRESHOLD + F);
{ Character code (:= 0..N_Char-1) }
T = (N_Char * 2 - 1); { Size of table }
R = (T - 1); { root position }
{ update when cumulative frequency }
{ reaches to this value }
MAX_FREQ = $8000;
{
Tables For encoding/decoding upper 6 bits of
sliding dictionary Pointer
}
{ encoder table }
p_len : Array[0..63] of Byte =
($03, $04, $04, $04, $05, $05, $05, $05,
$05, $05, $05, $05, $06, $06, $06, $06,
$06, $06, $06, $06, $06, $06, $06, $06,
$07, $07, $07, $07, $07, $07, $07, $07,
$07, $07, $07, $07, $07, $07, $07, $07,
$07, $07, $07, $07, $07, $07, $07, $07,
$08, $08, $08, $08, $08, $08, $08, $08,
$08, $08, $08, $08, $08, $08, $08, $08);
p_code : Array[0..63] of Byte =
($00, $20, $30, $40, $50, $58, $60, $68,
$70, $78, $80, $88, $90, $94, $98, $9C,
$A0, $A4, $A8, $AC, $B0, $B4, $B8, $BC,
$C0, $C2, $C4, $C6, $C8, $CA, $CC, $CE,
$D0, $D2, $D4, $D6, $D8, $DA, $DC, $DE,
$E0, $E2, $E4, $E6, $E8, $EA, $EC, $EE,
$F0, $F1, $F2, $F3, $F4, $F5, $F6, $F7,
$F8, $F9, $FA, $FB, $FC, $FD, $FE, $FF);
{ decoder table }
d_code : Array[0..255] of Byte =
($00, $00, $00, $00, $00, $00, $00, $00,
$00, $00, $00, $00, $00, $00, $00, $00,
$00, $00, $00, $00, $00, $00, $00, $00,
$00, $00, $00, $00, $00, $00, $00, $00,
$01, $01, $01,..