Objects Should Stand Alone

In pettycoin, blocks contain shards, and shards contain transactions. These are represented by struct block, struct block_shard and then the transactions (or hashes, or NULL).

But I chose early on to save some space in struct block_shard like so:

/* Only transactions we've proven are in block go in here! */
struct block_shard {
	/* Which shard is this? */
	u16 shardnum;
	/* How many transactions do we have?  Faster than counting NULLs */
	u8 txcount;
	/* How many transaction hashes do we have? */
	u8 hashcount;

	/* If we don't know all hashes, we store array of proofs here. */
	struct protocol_proof **proof;

	/* Bits to discriminate the union: 0 = txp, 1 == hash */
	BITMAP_DECLARE(txp_or_hash, 255);

	union txp_or_hash u[ /* block->shard_nums[shardnum] */ ];
};

There are referred to by an array of pointers in struct block:

struct block {
	...
	const u8 *shard_nums;
	...
	/* Transactions: may not be fully populated. */
	struct block_shard **shard;
};

These are allocated lazily, so all of block->shard[] will be NULL to start with. This means we tend to hand around block + shardnum rather than a shard, so we can allocate on demand. We also can’t tell by looking at a shard whether all transactions are known or not: the struct block contains the size of the shard.

So some places took a block and a shard, some took a block and a shard number, and the result was confusion (and at least on bug). I decided that shards would always be fully populated in a block, and they would have a size field, so they were independent objects. The result was quite a nice cleanup.