method: reverseByteOrder does not always work

[adafruit]

method: reverseByteOrder does not always work

[phrogger]

------------------------------------ CLIP --------------------------------------
#!/usr/bin/python

Demonstration of bug in Adafruit_I2C.reverseByteOrder()

from Adafruit_I2C import *

Initialize the class; address and bus number must be that of a valid I2C device

interface = Adafruit_I2C(0x18 , busnum=2)

for i in range( 0x03FD, 0x0404):
y = ((i << 8 & 0xFF00) | (i >> 8 & 0X00FF)) # Reverse 16 bit byte order
z = interface.reverseByteOrder(y)
print 'Value = %04X, Input = %04X, Result = %04X' % (i, y, z)

---------------------------------- END CLIP -------------------------------------

Resulting demonstration

Result should equal Value.

Input is the byte reversal of Value, and is correct by inspection

root@BBB1:~# python bugdemo.py
Value = 03FD, Input = FD03, Result = 03FD
Value = 03FE, Input = FE03, Result = 03FE
Value = 03FF, Input = FF03, Result = 03FF
Value = 0400, Input = 0004, Result = 0004 # <<<<<<<<<< FAIL
Value = 0401, Input = 0104, Result = 0401
Value = 0402, Input = 0204, Result = 0402
Value = 0403, Input = 0304, Result = 0403

---

Suggested fix:
I think it is impossible to make Adafruit_I2C.reverseByteOrder() work as written.
It is trying to determine the size of a value by detecting the presence of "ones"
to mark the first valid byte.
This fails when a leading valid byte is 0x00.

I suggest that Adafruit_I2C.reverseByteOrder() be replaced by two separate methods:

def reverseByteOrder16(self, data):
    "Reverses the byte order of a 16-bit value"
    val = ((data << 8 & 0xFF00) | (data >> 8 & 0X00FF))
    return val

def reverseByteOrder32(self, data):
    "Reverses the byte order of a 32-bit value"
    val = ((data << 24 & 0xFF000000) | (data << 8 & 0xFF0000) | (data >> 8 & 0xFF00) | (data >> 24 & 0xFF))
    return val

[ladyada]

Thank you for the Issue!
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