Annotation of embedaddon/php/ext/sqlite/libsqlite/src/encode.c, revision 1.1.1.1
1.1 misho 1: /*
2: ** 2002 April 25
3: **
4: ** The author disclaims copyright to this source code. In place of
5: ** a legal notice, here is a blessing:
6: **
7: ** May you do good and not evil.
8: ** May you find forgiveness for yourself and forgive others.
9: ** May you share freely, never taking more than you give.
10: **
11: *************************************************************************
12: ** This file contains helper routines used to translate binary data into
13: ** a null-terminated string (suitable for use in SQLite) and back again.
14: ** These are convenience routines for use by people who want to store binary
15: ** data in an SQLite database. The code in this file is not used by any other
16: ** part of the SQLite library.
17: **
18: ** $Id: encode.c 225725 2006-12-24 20:50:02Z iliaa $
19: */
20: #include <string.h>
21: #include <assert.h>
22:
23: /*
24: ** How This Encoder Works
25: **
26: ** The output is allowed to contain any character except 0x27 (') and
27: ** 0x00. This is accomplished by using an escape character to encode
28: ** 0x27 and 0x00 as a two-byte sequence. The escape character is always
29: ** 0x01. An 0x00 is encoded as the two byte sequence 0x01 0x01. The
30: ** 0x27 character is encoded as the two byte sequence 0x01 0x28. Finally,
31: ** the escape character itself is encoded as the two-character sequence
32: ** 0x01 0x02.
33: **
34: ** To summarize, the encoder works by using an escape sequences as follows:
35: **
36: ** 0x00 -> 0x01 0x01
37: ** 0x01 -> 0x01 0x02
38: ** 0x27 -> 0x01 0x28
39: **
40: ** If that were all the encoder did, it would work, but in certain cases
41: ** it could double the size of the encoded string. For example, to
42: ** encode a string of 100 0x27 characters would require 100 instances of
43: ** the 0x01 0x03 escape sequence resulting in a 200-character output.
44: ** We would prefer to keep the size of the encoded string smaller than
45: ** this.
46: **
47: ** To minimize the encoding size, we first add a fixed offset value to each
48: ** byte in the sequence. The addition is modulo 256. (That is to say, if
49: ** the sum of the original character value and the offset exceeds 256, then
50: ** the higher order bits are truncated.) The offset is chosen to minimize
51: ** the number of characters in the string that need to be escaped. For
52: ** example, in the case above where the string was composed of 100 0x27
53: ** characters, the offset might be 0x01. Each of the 0x27 characters would
54: ** then be converted into an 0x28 character which would not need to be
55: ** escaped at all and so the 100 character input string would be converted
56: ** into just 100 characters of output. Actually 101 characters of output -
57: ** we have to record the offset used as the first byte in the sequence so
58: ** that the string can be decoded. Since the offset value is stored as
59: ** part of the output string and the output string is not allowed to contain
60: ** characters 0x00 or 0x27, the offset cannot be 0x00 or 0x27.
61: **
62: ** Here, then, are the encoding steps:
63: **
64: ** (1) Choose an offset value and make it the first character of
65: ** output.
66: **
67: ** (2) Copy each input character into the output buffer, one by
68: ** one, adding the offset value as you copy.
69: **
70: ** (3) If the value of an input character plus offset is 0x00, replace
71: ** that one character by the two-character sequence 0x01 0x01.
72: ** If the sum is 0x01, replace it with 0x01 0x02. If the sum
73: ** is 0x27, replace it with 0x01 0x03.
74: **
75: ** (4) Put a 0x00 terminator at the end of the output.
76: **
77: ** Decoding is obvious:
78: **
79: ** (5) Copy encoded characters except the first into the decode
80: ** buffer. Set the first encoded character aside for use as
81: ** the offset in step 7 below.
82: **
83: ** (6) Convert each 0x01 0x01 sequence into a single character 0x00.
84: ** Convert 0x01 0x02 into 0x01. Convert 0x01 0x28 into 0x27.
85: **
86: ** (7) Subtract the offset value that was the first character of
87: ** the encoded buffer from all characters in the output buffer.
88: **
89: ** The only tricky part is step (1) - how to compute an offset value to
90: ** minimize the size of the output buffer. This is accomplished by testing
91: ** all offset values and picking the one that results in the fewest number
92: ** of escapes. To do that, we first scan the entire input and count the
93: ** number of occurances of each character value in the input. Suppose
94: ** the number of 0x00 characters is N(0), the number of occurances of 0x01
95: ** is N(1), and so forth up to the number of occurances of 0xff is N(255).
96: ** An offset of 0 is not allowed so we don't have to test it. The number
97: ** of escapes required for an offset of 1 is N(1)+N(2)+N(40). The number
98: ** of escapes required for an offset of 2 is N(2)+N(3)+N(41). And so forth.
99: ** In this way we find the offset that gives the minimum number of escapes,
100: ** and thus minimizes the length of the output string.
101: */
102:
103: /*
104: ** Encode a binary buffer "in" of size n bytes so that it contains
105: ** no instances of characters '\'' or '\000'. The output is
106: ** null-terminated and can be used as a string value in an INSERT
107: ** or UPDATE statement. Use sqlite_decode_binary() to convert the
108: ** string back into its original binary.
109: **
110: ** The result is written into a preallocated output buffer "out".
111: ** "out" must be able to hold at least 2 +(257*n)/254 bytes.
112: ** In other words, the output will be expanded by as much as 3
113: ** bytes for every 254 bytes of input plus 2 bytes of fixed overhead.
114: ** (This is approximately 2 + 1.0118*n or about a 1.2% size increase.)
115: **
116: ** The return value is the number of characters in the encoded
117: ** string, excluding the "\000" terminator.
118: **
119: ** If out==NULL then no output is generated but the routine still returns
120: ** the number of characters that would have been generated if out had
121: ** not been NULL.
122: */
123: int sqlite_encode_binary(const unsigned char *in, int n, unsigned char *out){
124: int i, j, e, m;
125: unsigned char x;
126: int cnt[256];
127: if( n<=0 ){
128: if( out ){
129: out[0] = 'x';
130: out[1] = 0;
131: }
132: return 1;
133: }
134: memset(cnt, 0, sizeof(cnt));
135: for(i=n-1; i>=0; i--){ cnt[in[i]]++; }
136: m = n;
137: for(i=1; i<256; i++){
138: int sum;
139: if( i=='\'' ) continue;
140: sum = cnt[i] + cnt[(i+1)&0xff] + cnt[(i+'\'')&0xff];
141: if( sum<m ){
142: m = sum;
143: e = i;
144: if( m==0 ) break;
145: }
146: }
147: if( out==0 ){
148: return n+m+1;
149: }
150: out[0] = e;
151: j = 1;
152: for(i=0; i<n; i++){
153: x = in[i] - e;
154: if( x==0 || x==1 || x=='\''){
155: out[j++] = 1;
156: x++;
157: }
158: out[j++] = x;
159: }
160: out[j] = 0;
161: assert( j==n+m+1 );
162: return j;
163: }
164:
165: /*
166: ** Decode the string "in" into binary data and write it into "out".
167: ** This routine reverses the encoding created by sqlite_encode_binary().
168: ** The output will always be a few bytes less than the input. The number
169: ** of bytes of output is returned. If the input is not a well-formed
170: ** encoding, -1 is returned.
171: **
172: ** The "in" and "out" parameters may point to the same buffer in order
173: ** to decode a string in place.
174: */
175: int sqlite_decode_binary(const unsigned char *in, unsigned char *out){
176: int i, e;
177: unsigned char c;
178: e = *(in++);
179: if (e == 0) {
180: return 0;
181: }
182: i = 0;
183: while( (c = *(in++))!=0 ){
184: if (c == 1) {
185: c = *(in++) - 1;
186: }
187: out[i++] = c + e;
188: }
189: return i;
190: }
191:
192: #ifdef ENCODER_TEST
193: #include <stdio.h>
194: /*
195: ** The subroutines above are not tested by the usual test suite. To test
196: ** these routines, compile just this one file with a -DENCODER_TEST=1 option
197: ** and run the result.
198: */
199: int main(int argc, char **argv){
200: int i, j, n, m, nOut, nByteIn, nByteOut;
201: unsigned char in[30000];
202: unsigned char out[33000];
203:
204: nByteIn = nByteOut = 0;
205: for(i=0; i<sizeof(in); i++){
206: printf("Test %d: ", i+1);
207: n = rand() % (i+1);
208: if( i%100==0 ){
209: int k;
210: for(j=k=0; j<n; j++){
211: /* if( k==0 || k=='\'' ) k++; */
212: in[j] = k;
213: k = (k+1)&0xff;
214: }
215: }else{
216: for(j=0; j<n; j++) in[j] = rand() & 0xff;
217: }
218: nByteIn += n;
219: nOut = sqlite_encode_binary(in, n, out);
220: nByteOut += nOut;
221: if( nOut!=strlen(out) ){
222: printf(" ERROR return value is %d instead of %d\n", nOut, strlen(out));
223: exit(1);
224: }
225: if( nOut!=sqlite_encode_binary(in, n, 0) ){
226: printf(" ERROR actual output size disagrees with predicted size\n");
227: exit(1);
228: }
229: m = (256*n + 1262)/253;
230: printf("size %d->%d (max %d)", n, strlen(out)+1, m);
231: if( strlen(out)+1>m ){
232: printf(" ERROR output too big\n");
233: exit(1);
234: }
235: for(j=0; out[j]; j++){
236: if( out[j]=='\'' ){
237: printf(" ERROR contains (')\n");
238: exit(1);
239: }
240: }
241: j = sqlite_decode_binary(out, out);
242: if( j!=n ){
243: printf(" ERROR decode size %d\n", j);
244: exit(1);
245: }
246: if( memcmp(in, out, n)!=0 ){
247: printf(" ERROR decode mismatch\n");
248: exit(1);
249: }
250: printf(" OK\n");
251: }
252: fprintf(stderr,"Finished. Total encoding: %d->%d bytes\n",
253: nByteIn, nByteOut);
254: fprintf(stderr,"Avg size increase: %.3f%%\n",
255: (nByteOut-nByteIn)*100.0/(double)nByteIn);
256: }
257: #endif /* ENCODER_TEST */
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