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DEFINITIONS
This source file includes following definitions.
- sqlite_encode_binary
- sqlite_decode_binary
- main
/*
** 2002 April 25
**
** The author disclaims copyright to this source code. In place of
** a legal notice, here is a blessing:
**
** May you do good and not evil.
** May you find forgiveness for yourself and forgive others.
** May you share freely, never taking more than you give.
**
*************************************************************************
** This file contains helper routines used to translate binary data into
** a null-terminated string (suitable for use in SQLite) and back again.
** These are convenience routines for use by people who want to store binary
** data in an SQLite database. The code in this file is not used by any other
** part of the SQLite library.
**
** $Id: encode.c,v 1.5.4.1.2.1 2006/12/24 20:50:02 iliaa Exp $
*/
#include <string.h>
#include <assert.h>
/*
** How This Encoder Works
**
** The output is allowed to contain any character except 0x27 (') and
** 0x00. This is accomplished by using an escape character to encode
** 0x27 and 0x00 as a two-byte sequence. The escape character is always
** 0x01. An 0x00 is encoded as the two byte sequence 0x01 0x01. The
** 0x27 character is encoded as the two byte sequence 0x01 0x28. Finally,
** the escape character itself is encoded as the two-character sequence
** 0x01 0x02.
**
** To summarize, the encoder works by using an escape sequences as follows:
**
** 0x00 -> 0x01 0x01
** 0x01 -> 0x01 0x02
** 0x27 -> 0x01 0x28
**
** If that were all the encoder did, it would work, but in certain cases
** it could double the size of the encoded string. For example, to
** encode a string of 100 0x27 characters would require 100 instances of
** the 0x01 0x03 escape sequence resulting in a 200-character output.
** We would prefer to keep the size of the encoded string smaller than
** this.
**
** To minimize the encoding size, we first add a fixed offset value to each
** byte in the sequence. The addition is modulo 256. (That is to say, if
** the sum of the original character value and the offset exceeds 256, then
** the higher order bits are truncated.) The offset is chosen to minimize
** the number of characters in the string that need to be escaped. For
** example, in the case above where the string was composed of 100 0x27
** characters, the offset might be 0x01. Each of the 0x27 characters would
** then be converted into an 0x28 character which would not need to be
** escaped at all and so the 100 character input string would be converted
** into just 100 characters of output. Actually 101 characters of output -
** we have to record the offset used as the first byte in the sequence so
** that the string can be decoded. Since the offset value is stored as
** part of the output string and the output string is not allowed to contain
** characters 0x00 or 0x27, the offset cannot be 0x00 or 0x27.
**
** Here, then, are the encoding steps:
**
** (1) Choose an offset value and make it the first character of
** output.
**
** (2) Copy each input character into the output buffer, one by
** one, adding the offset value as you copy.
**
** (3) If the value of an input character plus offset is 0x00, replace
** that one character by the two-character sequence 0x01 0x01.
** If the sum is 0x01, replace it with 0x01 0x02. If the sum
** is 0x27, replace it with 0x01 0x03.
**
** (4) Put a 0x00 terminator at the end of the output.
**
** Decoding is obvious:
**
** (5) Copy encoded characters except the first into the decode
** buffer. Set the first encoded character aside for use as
** the offset in step 7 below.
**
** (6) Convert each 0x01 0x01 sequence into a single character 0x00.
** Convert 0x01 0x02 into 0x01. Convert 0x01 0x28 into 0x27.
**
** (7) Subtract the offset value that was the first character of
** the encoded buffer from all characters in the output buffer.
**
** The only tricky part is step (1) - how to compute an offset value to
** minimize the size of the output buffer. This is accomplished by testing
** all offset values and picking the one that results in the fewest number
** of escapes. To do that, we first scan the entire input and count the
** number of occurances of each character value in the input. Suppose
** the number of 0x00 characters is N(0), the number of occurances of 0x01
** is N(1), and so forth up to the number of occurances of 0xff is N(255).
** An offset of 0 is not allowed so we don't have to test it. The number
** of escapes required for an offset of 1 is N(1)+N(2)+N(40). The number
** of escapes required for an offset of 2 is N(2)+N(3)+N(41). And so forth.
** In this way we find the offset that gives the minimum number of escapes,
** and thus minimizes the length of the output string.
*/
/*
** Encode a binary buffer "in" of size n bytes so that it contains
** no instances of characters '\'' or '\000'. The output is
** null-terminated and can be used as a string value in an INSERT
** or UPDATE statement. Use sqlite_decode_binary() to convert the
** string back into its original binary.
**
** The result is written into a preallocated output buffer "out".
** "out" must be able to hold at least 2 +(257*n)/254 bytes.
** In other words, the output will be expanded by as much as 3
** bytes for every 254 bytes of input plus 2 bytes of fixed overhead.
** (This is approximately 2 + 1.0118*n or about a 1.2% size increase.)
**
** The return value is the number of characters in the encoded
** string, excluding the "\000" terminator.
**
** If out==NULL then no output is generated but the routine still returns
** the number of characters that would have been generated if out had
** not been NULL.
*/
int sqlite_encode_binary(const unsigned char *in, int n, unsigned char *out){
int i, j, e, m;
unsigned char x;
int cnt[256];
if( n<=0 ){
if( out ){
out[0] = 'x';
out[1] = 0;
}
return 1;
}
memset(cnt, 0, sizeof(cnt));
for(i=n-1; i>=0; i--){ cnt[in[i]]++; }
m = n;
for(i=1; i<256; i++){
int sum;
if( i=='\'' ) continue;
sum = cnt[i] + cnt[(i+1)&0xff] + cnt[(i+'\'')&0xff];
if( sum<m ){
m = sum;
e = i;
if( m==0 ) break;
}
}
if( out==0 ){
return n+m+1;
}
out[0] = e;
j = 1;
for(i=0; i<n; i++){
x = in[i] - e;
if( x==0 || x==1 || x=='\''){
out[j++] = 1;
x++;
}
out[j++] = x;
}
out[j] = 0;
assert( j==n+m+1 );
return j;
}
/*
** Decode the string "in" into binary data and write it into "out".
** This routine reverses the encoding created by sqlite_encode_binary().
** The output will always be a few bytes less than the input. The number
** of bytes of output is returned. If the input is not a well-formed
** encoding, -1 is returned.
**
** The "in" and "out" parameters may point to the same buffer in order
** to decode a string in place.
*/
int sqlite_decode_binary(const unsigned char *in, unsigned char *out){
int i, e;
unsigned char c;
e = *(in++);
if (e == 0) {
return 0;
}
i = 0;
while( (c = *(in++))!=0 ){
if (c == 1) {
c = *(in++) - 1;
}
out[i++] = c + e;
}
return i;
}
#ifdef ENCODER_TEST
#include <stdio.h>
/*
** The subroutines above are not tested by the usual test suite. To test
** these routines, compile just this one file with a -DENCODER_TEST=1 option
** and run the result.
*/
int main(int argc, char **argv){
int i, j, n, m, nOut, nByteIn, nByteOut;
unsigned char in[30000];
unsigned char out[33000];
nByteIn = nByteOut = 0;
for(i=0; i<sizeof(in); i++){
printf("Test %d: ", i+1);
n = rand() % (i+1);
if( i%100==0 ){
int k;
for(j=k=0; j<n; j++){
/* if( k==0 || k=='\'' ) k++; */
in[j] = k;
k = (k+1)&0xff;
}
}else{
for(j=0; j<n; j++) in[j] = rand() & 0xff;
}
nByteIn += n;
nOut = sqlite_encode_binary(in, n, out);
nByteOut += nOut;
if( nOut!=strlen(out) ){
printf(" ERROR return value is %d instead of %d\n", nOut, strlen(out));
exit(1);
}
if( nOut!=sqlite_encode_binary(in, n, 0) ){
printf(" ERROR actual output size disagrees with predicted size\n");
exit(1);
}
m = (256*n + 1262)/253;
printf("size %d->%d (max %d)", n, strlen(out)+1, m);
if( strlen(out)+1>m ){
printf(" ERROR output too big\n");
exit(1);
}
for(j=0; out[j]; j++){
if( out[j]=='\'' ){
printf(" ERROR contains (')\n");
exit(1);
}
}
j = sqlite_decode_binary(out, out);
if( j!=n ){
printf(" ERROR decode size %d\n", j);
exit(1);
}
if( memcmp(in, out, n)!=0 ){
printf(" ERROR decode mismatch\n");
exit(1);
}
printf(" OK\n");
}
fprintf(stderr,"Finished. Total encoding: %d->%d bytes\n",
nByteIn, nByteOut);
fprintf(stderr,"Avg size increase: %.3f%%\n",
(nByteOut-nByteIn)*100.0/(double)nByteIn);
}
#endif /* ENCODER_TEST */