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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 not used by any other
-** part of the SQLite library.
-**
-** $Id$
-*/
-#include <string.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 0x03. 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 0x03
-**
-** 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 0x03 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.
-*/
-int sqlite_encode_binary(const unsigned char *in, int n, unsigned char *out){
- int i, j, e, m;
- int cnt[256];
- if( n<=0 ){
- 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;
- }
- }
- 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;
- 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, 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, nOut;
- 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;
- }
- nOut = sqlite_encode_binary(in, n, out);
- if( nOut!=strlen(out) ){
- printf(" ERROR return value is %d instead of %d\n", nOut, strlen(out));
- 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");
- }
-}
-#endif /* ENCODER_TEST */
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