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/*
** 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 used by any other
** part of the SQLite library.
**
** $Id: encode.c,v 1.2 2002/04/25 23:06:47 drh Exp $
*/
/*
** 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 (256*n + 1262)/253 bytes.
** In other words, the output will be expanded by as much as 3
** bytes for every 253 bytes of input plus 2 bytes of fixed overhead.
** (This is approximately 2 + 1.019*n or about a 2% size increase.)
*/
void sqlite_encode_binary(const unsigned char *in, int n, unsigned char *out){
int i, j, e, m;
int cnt[256];
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;
}
}
out[0] = e;
j = 1;
for(i=0; i<n; i++){
int c = (in[i] - e)&0xff;
if( c==0 ){
out[j++] = 1;
out[j++] = 1;
}else if( c==1 ){
out[j++] = 1;
out[j++] = 2;
}else if( c=='\'' ){
out[j++] = 1;
out[j++] = 3;
}else{
out[j++] = c;
}
}
out[j++] = 0;
}
/*
** Decode the string "in" into binary data and write it into "out".
** This routine reverses the encoded 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, c, e;
e = *(in++);
i = 0;
while( (c = *(in++))!=0 ){
if( c==1 ){
c = *(in++);
if( c==1 ){
c = 0;
}else if( c==2 ){
c = 1;
}else if( c==3 ){
c = '\'';
}else{
return -1;
}
}
out[i++] = (c + e)&0xff;
}
return i;
}
#ifdef ENCODER_TEST
/*
** 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;
unsigned char in[30000];
unsigned char out[33000];
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;
}
sqlite_encode_binary(in, n, out);
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");
}
}
#endif /* ENCODER_TEST */
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