  Various blockchain systems and schemes have been proposed since Bitcoin was
first introduced by Nakamoto Satoshi as a distributed ledger. However,
blockchains usually face criticisms, particularly on environmental concerns as
their ``proof-of-work'' based mining process usually consumes a considerable
amount of energy which hardly makes any useful contributions to the real world.
Therefore, the concept of ``proof-of-useful-work'' (PoUW) is proposed to
connect blockchain with practical application domain problems so the
computation power consumed in the mining process can be spent on useful
activities, such as solving optimization problems or training machine learning
models. This paper introduces HDCoin, a blockchain-based framework for an
emerging machine learning scheme: the brain-inspired hyperdimensional computing
(HDC). We formulate the model development of HDC as a problem that can be used
in blockchain mining. Specifically, we define the PoUW under the HDC scenario
and develop the entire mining process of HDCoin. During mining, miners are
competing to obtain the highest test accuracy on a given dataset. The winner
also has its model recorded in the blockchain and are available for the public
as a trustworthy HDC model. In addition, we also quantitatively examine the
performance of mining under different HDC configurations to illustrate the
adaptive mining difficulty.
