diff options
Diffstat (limited to 'sqlite-jdbc/src/main/java/SQLite/StringEncoder.java')
| -rw-r--r-- | sqlite-jdbc/src/main/java/SQLite/StringEncoder.java | 201 |
1 files changed, 201 insertions, 0 deletions
diff --git a/sqlite-jdbc/src/main/java/SQLite/StringEncoder.java b/sqlite-jdbc/src/main/java/SQLite/StringEncoder.java new file mode 100644 index 0000000..c2f20ad --- /dev/null +++ b/sqlite-jdbc/src/main/java/SQLite/StringEncoder.java @@ -0,0 +1,201 @@ +package SQLite; + +/** + * String encoder/decoder for SQLite. + * + * This module was kindly donated by Eric van der Maarel of Nedap N.V. + * + * This encoder was implemented based on an original idea from an anonymous + * author in the source code of the SQLite distribution. + * I feel obliged to provide a quote from the original C-source code: + * + * "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." + * + */ + +public class StringEncoder { + + /** + * Encodes the given byte array into a string that can be used by + * the SQLite database. The database cannot handle null (0x00) and + * the character '\'' (0x27). The encoding consists of escaping + * these characters with a reserved character (0x01). The escaping + * is applied after determining and applying a shift that minimizes + * the number of escapes required. + * With this encoding the data of original size n is increased to a + * maximum of 1+(n*257)/254. + * For sufficiently large n the overhead is thus less than 1.2%. + * @param a the byte array to be encoded. A null reference is handled as + * an empty array. + * @return the encoded bytes as a string. When an empty array is + * provided a string of length 1 is returned, the value of + * which is bogus. + * When decoded with this class' <code>decode</code> method + * a string of size 1 will return an empty byte array. + */ + + public static String encode(byte[] a) { + // check input + if (a == null || a.length == 0) { + // bogus shift, no data + return "x"; + } + // determine count + int[] cnt = new int[256]; + for (int i = 0 ; i < a.length; i++) { + cnt[a[i] & 0xff]++; + } + // determine shift for minimum number of escapes + int shift = 1; + int nEscapes = a.length; + for (int i = 1; i < 256; i++) { + if (i == '\'') { + continue; + } + int sum = cnt[i] + cnt[(i + 1) & 0xff] + cnt[(i + '\'') & 0xff]; + if (sum < nEscapes) { + nEscapes = sum; + shift = i; + if (nEscapes == 0) { + // cannot become smaller + break; + } + } + } + // construct encoded output + int outLen = a.length + nEscapes + 1; + StringBuffer out = new StringBuffer(outLen); + out.append((char)shift); + for (int i = 0; i < a.length; i++) { + // apply shift + char c = (char)((a[i] - shift)&0xff); + // insert escapes + if (c == 0) { // forbidden + out.append((char)1); + out.append((char)1); + } else if (c == 1) { // escape character + out.append((char)1); + out.append((char)2); + } else if (c == '\'') { // forbidden + out.append((char)1); + out.append((char)3); + } else { + out.append(c); + } + } + return out.toString(); + } + + /** + * Decodes the given string that is assumed to be a valid encoding + * of a byte array. Typically the given string is generated by + * this class' <code>encode</code> method. + * @param s the given string encoding. + * @return the byte array obtained from the decoding. + * @throws IllegalArgumentException when the string given is not + * a valid encoded string for this encoder. + */ + + public static byte[] decode(String s) { + char[] a = s.toCharArray(); + if (a.length > 2 && a[0] == 'X' && + a[1] == '\'' && a[a.length-1] == '\'') { + // SQLite3 BLOB syntax + byte[] result = new byte[(a.length-3)/2]; + for (int i = 2, k = 0; i < a.length - 1; i += 2, k++) { + byte tmp = (byte) (a[i] - '0'); + if (tmp > 15) { + tmp -= 0x20; + } + result[k] = (byte) (tmp << 4); + tmp = (byte) (a[i+1] - '0'); + if (tmp > 15) { + tmp -= 0x20; + } + result[k] |= tmp; + } + return result; + } + // first element is the shift + byte[] result = new byte[a.length-1]; + int i = 0; + int shift = s.charAt(i++); + int j = 0; + while (i < s.length()) { + int c; + if ((c = s.charAt(i++)) == 1) { // escape character found + if ((c = s.charAt(i++)) == 1) { + c = 0; + } else if (c == 2) { + c = 1; + } else if (c == 3) { + c = '\''; + } else { + throw new IllegalArgumentException( + "invalid string passed to decoder: " + j); + } + } + // do shift + result[j++] = (byte)((c + shift) & 0xff); + } + int outLen = j; + // provide array of correct length + if (result.length != outLen) { + result = byteCopy(result, 0, outLen, new byte[outLen]); + } + return result; + } + + /** + * Copies count elements from source, starting at element with + * index offset, to the given target. + * @param source the source. + * @param offset the offset. + * @param count the number of elements to be copied. + * @param target the target to be returned. + * @return the target being copied to. + */ + + private static byte[] byteCopy(byte[] source, int offset, + int count, byte[] target) { + for (int i = offset, j = 0; i < offset + count; i++, j++) { + target[j] = source[i]; + } + return target; + } + + + static final char[] xdigits = { + '0', '1', '2', '3', '4', '5', '6', '7', + '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' + }; + + /** + * Encodes the given byte array into SQLite3 blob notation, ie X'..' + * @param a the byte array to be encoded. A null reference is handled as + * an empty array. + * @return the encoded bytes as a string. + */ + + public static String encodeX(byte[] a) { + // check input + if (a == null || a.length == 0) { + return "X''"; + } + int outLen = a.length + 3; + StringBuffer out = new StringBuffer(outLen); + out.append('X'); + out.append('\''); + for (int i = 0; i < a.length; i++) { + out.append(xdigits[a[i] >> 4]); + out.append(xdigits[a[i] & 0x0F]); + } + out.append('\''); + return out.toString(); + } +} |