-module(log).
-export([open/2, append/2, tip/1, replay/3, entries/1]).

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

open(ActorId, BasePath) ->
    {ok, [{actor, ActorId}, {base, BasePath}, {seq, 0}, {entries, []}]}.

append(LogState, Activity) ->
    Seq = field(seq, LogState),
    Entries = field(entries, LogState),
    NewState = replace_field(seq, Seq + 1,
                  replace_field(entries, Entries ++ [Activity], LogState)),
    {ok, NewState, Seq}.

tip(LogState) ->
    field(seq, LogState).

replay(LogState, InitAcc, Fun) ->
    Entries = field(entries, LogState),
    replay_loop(Entries, 0, InitAcc, Fun).

replay_loop([], _, Acc, _) -> Acc;
replay_loop([Act | Rest], Seq, Acc, Fun) ->
    replay_loop(Rest, Seq + 1, Fun(Act, Seq, Acc), Fun).

entries(LogState) ->
    field(entries, LogState).

field(K, [{K, V} | _]) -> V;
field(K, [_ | Rest]) -> field(K, Rest);
field(_, []) -> erlang:error(badkey).

replace_field(K, V, []) -> [{K, V}];
replace_field(K, V, [{K, _} | Rest]) -> [{K, V} | Rest];
replace_field(K, V, [P | Rest]) -> [P | replace_field(K, V, Rest)].