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64 lines
2.3 KiB
Erlang
64 lines
2.3 KiB
Erlang
-module(log).
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-export([open/2, append/2, tip/1, replay/3, entries/1]).
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%% Per-actor activity log — the canonical record of everything an
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%% actor has emitted, in chronological order. Per design §15.2 this
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%% lives on disk as a JSONL segment file; v1 starts with an in-memory
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%% backend so the API and seq-number machinery can be locked down
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%% before the on-disk format is added (Step 3b).
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%%
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%% State shape (a property list):
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%% [{actor, ActorId}, {base, BasePath}, {seq, NextSeq}, {entries, [Act|...]}]
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%%
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%% `entries` stores activities in append order — i.e. oldest first.
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%% `seq` is the next sequence number that will be assigned by append.
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%% `base` is kept on the state for forward-compatibility with 3b
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%% (where it becomes the segment-file directory).
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%%
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%% open/2 takes ActorId + BasePath and returns {ok, LogState} starting
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%% with seq=0 and no entries.
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%%
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%% append/2 returns {ok, NewLogState, AssignedSeq}.
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%%
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%% tip/1 returns the next seq the log would assign (== count of entries).
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%%
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%% replay/3 folds Fun(Activity, AssignedSeq, Acc) over every entry in
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%% append order. Three-arity rather than two-arity because the plan's
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%% example test is "sequence numbers gap-free across replay" — having
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%% the seq number visible in the fold makes that test direct.
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%%
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%% entries/1 is a debug accessor returning [Activity, ...] in append
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%% order. Not part of the public API contract.
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open(ActorId, BasePath) ->
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{ok, [{actor, ActorId}, {base, BasePath}, {seq, 0}, {entries, []}]}.
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append(LogState, Activity) ->
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Seq = field(seq, LogState),
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Entries = field(entries, LogState),
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NewState = replace_field(seq, Seq + 1,
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replace_field(entries, Entries ++ [Activity], LogState)),
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{ok, NewState, Seq}.
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tip(LogState) ->
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field(seq, LogState).
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replay(LogState, InitAcc, Fun) ->
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Entries = field(entries, LogState),
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replay_loop(Entries, 0, InitAcc, Fun).
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replay_loop([], _, Acc, _) -> Acc;
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replay_loop([Act | Rest], Seq, Acc, Fun) ->
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replay_loop(Rest, Seq + 1, Fun(Act, Seq, Acc), Fun).
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entries(LogState) ->
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field(entries, LogState).
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field(K, [{K, V} | _]) -> V;
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field(K, [_ | Rest]) -> field(K, Rest);
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field(_, []) -> erlang:error(badkey).
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replace_field(K, V, []) -> [{K, V}];
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replace_field(K, V, [{K, _} | Rest]) -> [{K, V} | Rest];
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replace_field(K, V, [P | Rest]) -> [P | replace_field(K, V, Rest)].
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