fed-sx-m1: Step 3a — in-memory log:open/append/tip/replay + 12 tests

next/kernel/log.erl

@@ -0,0 +1,63 @@
+-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)].