| | """Run-time calculation of offset into Python string structure |
| | |
| | Does a scan to find the digits of pi in a string structure |
| | in order to produce an offset that can be used to produce |
| | data-pointers from Python strings. |
| | |
| | Porting note: |
| | |
| | Currently this uses id( str a ) to get the base address |
| | of the Python string. Python implementations where id( a ) |
| | is *not* the memory address of the string will not work! |
| | """ |
| | import ctypes |
| | from OpenGL._bytes import bytes,unicode |
| | PI_DIGITS = '31415926535897931' |
| |
|
| | def calculateOffset( ): |
| | """Calculates the data-pointer offset for strings |
| | |
| | This does a sequential scan for 100 bytes from the id |
| | of a string to find special data-value stored in the |
| | string (the digits of PI). It produces a dataPointer |
| | function which adds that offset to the id of the |
| | passed strings. |
| | """ |
| | finalOffset = None |
| | a = PI_DIGITS |
| | |
| | initial = id(a) |
| | targetType = ctypes.POINTER( ctypes.c_char ) |
| | for offset in range( 100 ): |
| | vector = ctypes.cast( initial+offset,targetType ) |
| | allMatched = True |
| | for index,digit in enumerate( a ): |
| | if vector[index] != digit: |
| | allMatched = False |
| | break |
| | if allMatched: |
| | finalOffset = offset |
| | break |
| | if finalOffset is not None: |
| | def dataPointer( data ): |
| | """Return the data-pointer from the array using calculated offset |
| | |
| | data -- a Python string |
| | |
| | Returns the raw data-pointer to the internal buffer of the passed string |
| | """ |
| | if not isinstance( data, bytes ): |
| | raise TypeError( |
| | """This function can only handle Python strings! Got %s"""%( |
| | type(data), |
| | ) |
| | ) |
| | return id(data) + finalOffset |
| | |
| | dataPointer.offset = finalOffset |
| | return dataPointer |
| | raise RuntimeError( |
| | """Unable to determine dataPointer offset for strings!""" |
| | ) |
| |
|
| | dataPointer = calculateOffset() |
| |
|
| | if __name__ == "__main__": |
| | a = 'this' |
| | print((id(a), dataPointer( a ), dataPointer(a) - id(a))) |
| | |
| |
|
