Block Structure and Headers

pub struct BlockHeader {
    pub version: i32,              // 4 bytes
    pub prev_block_hash: [u8; 32], // 32 bytes — links to parent
    pub merkle_root: [u8; 32],     // 32 bytes — commits to all tx in block
    pub time: u32,                  // 4 bytes — unix timestamp
    pub bits: u32,                  // 4 bytes — encoded difficulty target
    pub nonce: u32,                 // 4 bytes — miners adjust to find valid hash
}
// Total: 80 bytes

// Block hash = sha256(sha256(serialize(header)))
// Must be < target (decoded from bits) for the block to be valid.

pub fn block_hash(h: &BlockHeader) -> [u8; 32] {
    let mut buf = Vec::with_capacity(80);
    buf.extend(&h.version.to_le_bytes());
    buf.extend(&h.prev_block_hash);
    buf.extend(&h.merkle_root);
    buf.extend(&h.time.to_le_bytes());
    buf.extend(&h.bits.to_le_bytes());
    buf.extend(&h.nonce.to_le_bytes());
    sha256(&sha256(&buf))
}

// Merkle root commits to every transaction in the block — a 32-byte
// summary that lets light clients verify a tx is included with O(log n)
// hashes via a merkle proof.

// Bits encoding is compact: an 8-bit exponent + 24-bit mantissa.
// target = mantissa * 256^(exponent - 3)
// Block 100000 had bits 0x1b04864c -> target ~5.7e62 (very easy by today's standards)