I try to implement a bitwise filter using MYSQL (with udf if needed)
The filter is something like a AND but I want to use the mask to build a new bit string... Let me explain you with a sample :
Suppose I have a table with blob storing 8 bit streams:
Then I have a mask to apply to get the bits from data when mask value is 1
And so get the following expected results:
Is there a way to optimize the filtering, without looping on each bit of "mask" to get the corresponding value in "data" row...
CLARIFICATION
I've just taken 8 bits for the post, but it's more like 256 bytes
for Joe : To clarify the exemple, the mask 00101011 is interpreted as : get the bit value from data field at position 3,5,7,8, if you read the mask from left to right, enumerated from bit 1 to bit 8... Hope this clarification is "clear"...
You can use bitwise operators in MySQL:
http://dev.mysql.com/doc/refman/5.0/en/bit-functions.html
Example:
SELECT (data1 & b'00101011') as output1 FROM ......
Quick test:
SELECT (b'10110110' & b'00101011') as output1
This does a bitwise AND
with the binary pattern of the mask you specified.
See the above link for more toys.
The only way I know of doing what you want is something like
SELECT ((data >> 2) & 8) | ((data >> 1) & 4) | (data & 3) FROM ...
Obviously, you'll have to construct the expression based on your mask; it's not very difficult to do, just a bit tedious — you basically need to loop over the bits in the mask, something like this:
var mask = 0b00101011; var parts = new Array(); var shift = 0; var unshift = 0; while (mask > 0) { while ((mask & 1) == 0) { shift = shift + 1; mask = mask >> 1; } submask = 0; while ((mask & 1) == 1) { submask = submask + (1 << unshift); unshift = unshift + 1; mask = mask >> 1; } parts.push( "((data >> " + shift + ") & " + submask + ")" ); } var expr = parts.join( " | " ); console.log(expr);
The example code above is in JavaScript, so you can run it as a snippet here and get:
((data >> 0) & 3) | ((data >> 1) & 4) | ((data >> 2) & 8)
logged to the console, but it should be pretty easy to port to other languages.
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