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255 lines
8.8 KiB
C
255 lines
8.8 KiB
C
/*
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** 2002 April 25
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**
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** The author disclaims copyright to this source code. In place of
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** a legal notice, here is a blessing:
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**
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** May you do good and not evil.
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** May you find forgiveness for yourself and forgive others.
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** May you share freely, never taking more than you give.
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**
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*************************************************************************
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** This file contains helper routines used to translate binary data into
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** a null-terminated string (suitable for use in SQLite) and back again.
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** These are convenience routines for use by people who want to store binary
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** data in an SQLite database. The code in this file is not used by any other
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** part of the SQLite library.
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**
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** $Id: encode.c,v 1.1.1.1 2004/08/08 15:03:57 matt Exp $
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*/
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#include <string.h>
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#include <assert.h>
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/*
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** How This Encoder Works
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**
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** The output is allowed to contain any character except 0x27 (') and
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** 0x00. This is accomplished by using an escape character to encode
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** 0x27 and 0x00 as a two-byte sequence. The escape character is always
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** 0x01. An 0x00 is encoded as the two byte sequence 0x01 0x01. The
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** 0x27 character is encoded as the two byte sequence 0x01 0x28. Finally,
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** the escape character itself is encoded as the two-character sequence
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** 0x01 0x02.
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**
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** To summarize, the encoder works by using an escape sequences as follows:
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**
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** 0x00 -> 0x01 0x01
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** 0x01 -> 0x01 0x02
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** 0x27 -> 0x01 0x28
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**
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** If that were all the encoder did, it would work, but in certain cases
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** it could double the size of the encoded string. For example, to
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** encode a string of 100 0x27 characters would require 100 instances of
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** the 0x01 0x03 escape sequence resulting in a 200-character output.
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** We would prefer to keep the size of the encoded string smaller than
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** this.
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**
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** To minimize the encoding size, we first add a fixed offset value to each
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** byte in the sequence. The addition is modulo 256. (That is to say, if
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** the sum of the original character value and the offset exceeds 256, then
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** the higher order bits are truncated.) The offset is chosen to minimize
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** the number of characters in the string that need to be escaped. For
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** example, in the case above where the string was composed of 100 0x27
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** characters, the offset might be 0x01. Each of the 0x27 characters would
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** then be converted into an 0x28 character which would not need to be
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** escaped at all and so the 100 character input string would be converted
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** into just 100 characters of output. Actually 101 characters of output -
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** we have to record the offset used as the first byte in the sequence so
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** that the string can be decoded. Since the offset value is stored as
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** part of the output string and the output string is not allowed to contain
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** characters 0x00 or 0x27, the offset cannot be 0x00 or 0x27.
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**
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** Here, then, are the encoding steps:
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**
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** (1) Choose an offset value and make it the first character of
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** output.
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**
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** (2) Copy each input character into the output buffer, one by
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** one, adding the offset value as you copy.
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**
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** (3) If the value of an input character plus offset is 0x00, replace
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** that one character by the two-character sequence 0x01 0x01.
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** If the sum is 0x01, replace it with 0x01 0x02. If the sum
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** is 0x27, replace it with 0x01 0x03.
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**
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** (4) Put a 0x00 terminator at the end of the output.
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**
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** Decoding is obvious:
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**
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** (5) Copy encoded characters except the first into the decode
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** buffer. Set the first encoded character aside for use as
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** the offset in step 7 below.
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**
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** (6) Convert each 0x01 0x01 sequence into a single character 0x00.
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** Convert 0x01 0x02 into 0x01. Convert 0x01 0x28 into 0x27.
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**
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** (7) Subtract the offset value that was the first character of
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** the encoded buffer from all characters in the output buffer.
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**
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** The only tricky part is step (1) - how to compute an offset value to
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** minimize the size of the output buffer. This is accomplished by testing
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** all offset values and picking the one that results in the fewest number
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** of escapes. To do that, we first scan the entire input and count the
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** number of occurances of each character value in the input. Suppose
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** the number of 0x00 characters is N(0), the number of occurances of 0x01
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** is N(1), and so forth up to the number of occurances of 0xff is N(255).
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** An offset of 0 is not allowed so we don't have to test it. The number
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** of escapes required for an offset of 1 is N(1)+N(2)+N(40). The number
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** of escapes required for an offset of 2 is N(2)+N(3)+N(41). And so forth.
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** In this way we find the offset that gives the minimum number of escapes,
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** and thus minimizes the length of the output string.
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*/
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/*
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** Encode a binary buffer "in" of size n bytes so that it contains
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** no instances of characters '\'' or '\000'. The output is
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** null-terminated and can be used as a string value in an INSERT
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** or UPDATE statement. Use sqlite_decode_binary() to convert the
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** string back into its original binary.
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**
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** The result is written into a preallocated output buffer "out".
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** "out" must be able to hold at least 2 +(257*n)/254 bytes.
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** In other words, the output will be expanded by as much as 3
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** bytes for every 254 bytes of input plus 2 bytes of fixed overhead.
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** (This is approximately 2 + 1.0118*n or about a 1.2% size increase.)
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**
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** The return value is the number of characters in the encoded
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** string, excluding the "\000" terminator.
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**
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** If out==NULL then no output is generated but the routine still returns
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** the number of characters that would have been generated if out had
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** not been NULL.
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*/
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int sqlite_encode_binary(const unsigned char *in, int n, unsigned char *out){
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int i, j, e, m;
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unsigned char x;
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int cnt[256];
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if( n<=0 ){
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if( out ){
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out[0] = 'x';
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out[1] = 0;
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}
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return 1;
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}
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memset(cnt, 0, sizeof(cnt));
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for(i=n-1; i>=0; i--){ cnt[in[i]]++; }
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m = n;
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for(i=1; i<256; i++){
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int sum;
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if( i=='\'' ) continue;
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sum = cnt[i] + cnt[(i+1)&0xff] + cnt[(i+'\'')&0xff];
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if( sum<m ){
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m = sum;
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e = i;
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if( m==0 ) break;
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}
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}
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if( out==0 ){
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return n+m+1;
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}
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out[0] = e;
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j = 1;
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for(i=0; i<n; i++){
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x = in[i] - e;
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if( x==0 || x==1 || x=='\''){
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out[j++] = 1;
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x++;
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}
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out[j++] = x;
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}
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out[j] = 0;
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assert( j==n+m+1 );
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return j;
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}
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/*
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** Decode the string "in" into binary data and write it into "out".
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** This routine reverses the encoding created by sqlite_encode_binary().
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** The output will always be a few bytes less than the input. The number
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** of bytes of output is returned. If the input is not a well-formed
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** encoding, -1 is returned.
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**
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** The "in" and "out" parameters may point to the same buffer in order
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** to decode a string in place.
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*/
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int sqlite_decode_binary(const unsigned char *in, unsigned char *out){
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int i, e;
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unsigned char c;
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e = *(in++);
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i = 0;
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while( (c = *(in++))!=0 ){
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if( c==1 ){
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c = *(in++) - 1;
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}
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out[i++] = c + e;
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}
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return i;
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}
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#ifdef ENCODER_TEST
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#include <stdio.h>
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/*
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** The subroutines above are not tested by the usual test suite. To test
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** these routines, compile just this one file with a -DENCODER_TEST=1 option
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** and run the result.
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*/
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int main(int argc, char **argv){
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int i, j, n, m, nOut, nByteIn, nByteOut;
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unsigned char in[30000];
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unsigned char out[33000];
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nByteIn = nByteOut = 0;
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for(i=0; i<sizeof(in); i++){
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printf("Test %d: ", i+1);
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n = rand() % (i+1);
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if( i%100==0 ){
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int k;
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for(j=k=0; j<n; j++){
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/* if( k==0 || k=='\'' ) k++; */
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in[j] = k;
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k = (k+1)&0xff;
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}
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}else{
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for(j=0; j<n; j++) in[j] = rand() & 0xff;
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}
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nByteIn += n;
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nOut = sqlite_encode_binary(in, n, out);
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nByteOut += nOut;
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if( nOut!=strlen(out) ){
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printf(" ERROR return value is %d instead of %d\n", nOut, strlen(out));
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exit(1);
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}
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if( nOut!=sqlite_encode_binary(in, n, 0) ){
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printf(" ERROR actual output size disagrees with predicted size\n");
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exit(1);
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}
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m = (256*n + 1262)/253;
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printf("size %d->%d (max %d)", n, strlen(out)+1, m);
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if( strlen(out)+1>m ){
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printf(" ERROR output too big\n");
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exit(1);
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}
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for(j=0; out[j]; j++){
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if( out[j]=='\'' ){
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printf(" ERROR contains (')\n");
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exit(1);
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}
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}
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j = sqlite_decode_binary(out, out);
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if( j!=n ){
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printf(" ERROR decode size %d\n", j);
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exit(1);
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}
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if( memcmp(in, out, n)!=0 ){
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printf(" ERROR decode mismatch\n");
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exit(1);
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}
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printf(" OK\n");
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}
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fprintf(stderr,"Finished. Total encoding: %d->%d bytes\n",
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nByteIn, nByteOut);
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fprintf(stderr,"Avg size increase: %.3f%%\n",
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(nByteOut-nByteIn)*100.0/(double)nByteIn);
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}
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#endif /* ENCODER_TEST */
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