 Faster Ethash check against difficulty

Faster Ethash check against difficulty - HackMD

Check against boundary using integer division

Ethereum Yellow Paper in section 11.5 defines PoW difficulty check as

n \leq \frac{2^{256}}{H_d}

where n is Ethash final hash and H_d is current block difficulty.

The \frac{2^{256}}{H_d} is called boundary. In practical implementations integer division is used to validate the check.

n \leq \left\lfloor\frac{2^{256}}{H_d}\right\rfloor

This is correct because of the following fact (proof):

x \in \mathbb{R}, n \in \mathbb{Z}: n \leq x \iff n \leq \left\lfloor x \right\rfloor

Check against difficulty using multiplication

We can avoid using big integer division (which is slow and complex to implement) by transforming the orignal check formula into:

nH_d \leq 2^{256}

Benefits

1. Integer division has been replaced with multiplication. The 256 x 256 → 512 multiplication is straight forward to implement.
2. Degenerated values of difficulty (0 and 1) do not require special handling.

Implementation

This has been implemented in ethash 0.8.0.

Side notes

1. The check nH_d \leq 2^{256} can be further decomposed into nH_d \lt 2^{256} \lor nH_d = 2^{256} where the first part is 256-bit multiplication overflow check and the second part is very unlikely (or even impossible considering the difficulty update formula).
2. The difficulty values on Ethereum Mainnet safely stay within 64-bit boundaries. Therefore, optimized path can be used for such values for both integer division and multiplication.
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