Adaptive Differential Pulse Code Modulation (ADPCM)
ADPCM is a very well-defined standard algorithm. The standard is expressed in exact detail and must be implemented exactly to run the test vectors. Some of these tests include inputting digital test bytes, processing them with the algorithm, then checking for proper bit-for-bit output.
The ADPCM algorithm generates a code difference between the current input sample and a predicted value from past output.