BlockUtils conversion does not work both ways about core-js HOT 3 CLOSED

I modelled the difficulty according to the bitcoin specs (see https://en.bitcoin.it/wiki/Difficulty and https://en.bitcoin.it/wiki/Target). The minimum difficulty is 1, which corresponds to the max target Policy.BLOCK_TARGET_MAX (compact 0x1f00ffff). The maximum difficulty is Policy.BLOCK_TARGET_MAX (target 1, compact 0x01010000).

Current differences to Bitcoin:

• We currently use a MAX_TARGET of compact 0x1f00ffff (bitcoin 0x1d00ffff), i.e. difficulty 1 is 16 leading zero bits (bitcoin 32). Otherwise, our wimpy browser miner needs like 3 days to mine a block at the lowest difficulty (which really sucks for testing).
• In some cases, the Bitcoin client uses a target of 0 (difficulty infinity). We enforce a minimum target of 1.

from core-js.

The behaviour difficulty !== BlockUtils.compactToDifficulty(BlockUtils.difficultyToCompact(difficulty)) for some difficulties is due to the fact the the compact representation is lossy. The compact representation (or nBits) encodes a 256 bit target value in 32 bits and is passed along in the block headers, so we want it to be small.

When computing BlockUtils.difficultyToCompact(difficulty), we first compute the target value that is difficulty times harder than the Policy.BLOCK_TARGET_MAX: target = Policy.BLOCK_TARGET_MAX / difficulty. For difficulty = 250 this gives a target value of 10623d70a3d70a000000000000000000000000000000000000000000000. The compact representation of this target is 1e010623, which loses a lot of precision. Expanding this to a target again gives a truncated target value of 10623000000000000000000000000000000000000000000000000000000, which corresponds to difficulty = 250.00312933077026.

We only pass around compact target representations, i.e. all clients should work with truncated targets. The behaviour seems indeed unexpected, but in the end, I guess that's how it's supposed to work.

from core-js.

Makes sense. I adjusted the test to now only rely on nBits instead of the difficulty.

from core-js.