Function first_chain_of_xof 

pub const fn first_chain_of_xof(block: usize) -> i32

First chain that reads XOF block block.

Chain c reads XOF bytes 18c..18c+18. The answer is the smallest c whose last byte falls in or after block block, that is 18c + 18 > 64 · block. Chain c issues at T + c, so this is also the offset by which the block must be in the window.