How is the POW hash calculated?

Question

I read up on the fields in Ethereum blocks, but I was unable to find exactly how the POW hash is calculated. I would assume that it is an RLP encoded value of the previous block hash, ommer hashes, nonce, state trie root hash etc.

Can anyone show me the exact calculation in terms of ETHASH? You don't have to show how ETHASH works, just what the input to ETHASH is.

By PoW hash, I mean the block hash that must be lower than the target threshold (which is reciprocal to the target difficulty).

Answer 1

The main Hashimoto mining loop is defined in the following Python pseudo-code (found here):

def mine(full_size, dataset, header, difficulty):
    # zero-pad target to compare with hash on the same digit
    target = zpad(encode_int(2**256 // difficulty), 64)[::-1]
    from random import randint
    nonce = randint(0, 2**64)
    while hashimoto_full(full_size, dataset, header, nonce) > target:
        nonce = (nonce + 1) % 2**64
    return nonce

I would assume that it is an RLP-encoded value of the previous block hash, ommer hashes, nonce, state trie root hash etc.

The Hashimoto function requires the truncated RPL-encoded header of the current block being mined, which itself contains the parentHash (H_p in the Yellow Paper), together with the mining nonce. The truncated header doesn't contain the mixHash (H_m) or header nonce (H_n).

Here's the code:

def hashimoto(header, nonce, full_size, dataset_lookup):
    n = full_size / HASH_BYTES
    w = MIX_BYTES // WORD_BYTES
    mixhashes = MIX_BYTES / HASH_BYTES
    # combine header+nonce into a 64 byte seed
    s = sha3_512(header + nonce[::-1])
    # start the mix with replicated s
    mix = []
    for _ in range(MIX_BYTES / HASH_BYTES):
        mix.extend(s)
    # mix in random dataset nodes
    for i in range(ACCESSES):
        p = fnv(i ^ s[0], mix[i % w]) % (n // mixhashes) * mixhashes
        newdata = []
        for j in range(MIX_BYTES / HASH_BYTES):
            newdata.extend(dataset_lookup(p + j))
        mix = map(fnv, mix, newdata)
    # compress mix
    cmix = []
    for i in range(0, len(mix), 4):
        cmix.append(fnv(fnv(fnv(mix[i], mix[i+1]), mix[i+2]), mix[i+3]))
    return {
        "mix digest": serialize_hash(cmix),
        "result": serialize_hash(sha3_256(s+cmix))
    }