| """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))) |
| |
|
|
