Every player's results, kept on disk, and the boards made from them.

{:ok, ledger} = Cauldron2D.Ledger.start_link(path: Cauldron2D.Paths.data(:my_game, "ledger.dets"), metrics: [kills: {:sum, :kills}, ratio: {:ratio, :kills, :deaths}, wins: {:count, :won?}])
Cauldron2D.Ledger.record(ledger, "Pit", %{id: "alice", name: "alice", kills: 3, deaths: 1, won?: true})
Cauldron2D.Ledger.board(ledger, :week, :kills, arena: "Pit")

A result is a map with the player's account as :id (their :name when there is no id), the name they played under as :name, and whatever fields the metrics read. A result whose id is {:robot, n} is not kept. Cauldron2D.Ledger.Recorder takes them from a world's events.

Metrics

The game declares its metrics as a keyword list, each one of:

• {:sum, field} — the field summed over the rounds
• {:max, field} — the field's largest value
• {:count, field} — how many rounds had the field true
• {:ratio, field, by} — the sum of field over the sum of by, to two places; the sum of field alone when by sums to zero
• a function of the rounds returning a number

A board is the accounts of a period — :day (since midnight UTC), :week (the last seven days) or :all — ranked by one metric, each with every metric's value, the rounds counted and the name last played under, twenty at most. Boards are computed from the table and kept for ten seconds.

The file

The file belongs to one operating system process at a time, held through Cauldron2D.Ledger.Lock. Starting a ledger on a held file fails with {:error, {:held, pid}}, or with held: :idle starts one that keeps nothing: its boards are empty, record/4 and forget/2 answer {:error, {:held, pid}}, and access/1 says {:held, pid}. A DETS failure stops the ledger, and a restart reopens the file repaired.