def get_filter_candidate(x: int, c: int) -> int:
    h2  = hash160_pubkey(x)
    val = int.from_bytes(h2, 'big')
    return val >> (160 - c)

def prefilter_scan(start: int, end: int, target: str, c: int):
    t1  = get_filter_target(target, c)
    ops = 0
    for x in range(start, end+1):
        if get_filter_candidate(x, c) != t1:
            continue
        ops += 1
        if derive_address(x) == target:
            return x, ops
    return None, ops
[/quote]

You have an error in your script. You do H160 and continue before you increment ops. So the heavy op is not accounted always.

Also - if the key isn't found, the method simply fails.

I also believe that the actual HEAVY operation that actually ever matters is having a public key of the private key - because that is the actual step where you go from the scalar domain into the discrete log injective-only domain.