Format allowing elo range queries and average elo output

[Sopel97]

consider a new format that allows elo ranges

• mostly db_beta but reduce count|offset to 6 bytes (6 bytes may be too little considering that we lose some stayistical properies by adding elo to the mix. We need to find a different place to borrow bits from)
• use 2 bytes to store elo rounded (down?) to a given number (say 64, or 100) - the smaller number the more space required
• have data sorted by elo to so we can easly merge similar entries
• when querying just filter the entries with not matching elo range
• query request gets a property "min_elo" and "max_elo" (left inclusive, right exclusive?)
• in query response have average elo (weighted average by count) for each bin

[Sopel97]

Revision 1. Have a constant elo granularity, possibly stored in a file if later setting it on creation is to be allowed.

Entry:

• 11B pos hash
• 1B elo divided by elo divisor (divisor minimum 20 to allow good range)
• The rest like in db_beta

On querying total elo is computed and divided by total count to get an average. Total elo should always fit in 64 bit int.

Add a field to the query "average_elo" to specify whether average elo should be returned

[Sopel97]

For now just a format that allows average elo.

• everything as in db_beta
• additional 8 byte field with total elo (assuming elos <4000 it will work with up to 2^52 positions)

[Sopel97]

Instead of collecting elo and average elo, which is meaningless, collect elo difference (while elo - black elo). "average_elo_difference" In the future maybe also have the previous one

[Sopel97]

for games without elos specified assume elo_difference==0

[Sopel97]

consider dividing elo difference by 10 (properly round) and storing the total in 4 bytes. Assuming +-400 average elo difference for a position it would have to be encountered 50 million times to overflow.

This gives us 4 bytes of padding that we could use for something else.

[Sopel97]

If nothing better comes up:

• first 16B like in db_beta
• 4B count
• 4B total elo difference / 10
• 4B first game index
• 4B last game index Total of 32 bytes per entry.

Effectively limits max number of games stored to 2^32, but it's a reasonable and a high limit. Requires indirection when reading game data like in db_alpha.

[Sopel97]

Store elo diff losslessly, use one additional byte taken from hash. Store 5 bytes of elo diff and 3 bytes of part of hash in one uint64, elo diff in most significant bits. struct D here https://godbolt.org/z/xnnAAh

edit. https://godbolt.org/z/SX5noP