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98 lines
3.0 KiB
Erlang
98 lines
3.0 KiB
Erlang
-module(projection).
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-behaviour(gen_server).
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-export([new/2, new/3, fold_activity/2, replay/2,
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name/1, state/1, fold_fn/1]).
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-export([start_link/3, async_fold/2, query/1, stop/1]).
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-export([init/1, handle_call/3, handle_cast/2, handle_info/2]).
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%% Pure-functional projection driver per design §10.
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%%
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%% A projection is a property list:
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%% [{name, atom}, {state, term}, {fold, fun}]
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%%
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%% The fold function is `fun (Activity, State) -> NewState`. v1
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%% uses Erlang funs as the fold body — the genesis bundle's SX
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%% `:fold` bodies are stored as binaries; an SX-source eval
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%% bridge will plug them into the same projection record once
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%% it lands (Step 7d). For now, callers supply Erlang funs
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%% directly when constructing a projection.
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%%
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%% `replay/2` is the cold-start primitive: fold an activity
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%% list (e.g. `log:entries/1`) through the projection from its
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%% initial state.
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new(Name, InitialState) ->
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new(Name, InitialState, fun (_Activity, S) -> S end).
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new(Name, InitialState, FoldFn) ->
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[{name, Name}, {state, InitialState}, {fold, FoldFn}].
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fold_activity(Proj, Activity) ->
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Fn = fold_fn(Proj),
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S0 = state(Proj),
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S1 = Fn(Activity, S0),
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set_field(state, S1, Proj).
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replay(Proj, Activities) ->
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fold_each(Proj, Activities).
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fold_each(Proj, []) -> Proj;
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fold_each(Proj, [A | Rest]) ->
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fold_each(fold_activity(Proj, A), Rest).
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%% Accessors
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name(Proj) -> field(name, Proj).
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state(Proj) -> field(state, Proj).
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fold_fn(Proj) -> field(fold, Proj).
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%% Internal
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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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set_field(K, V, [{K, _} | Rest]) -> [{K, V} | Rest];
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set_field(K, V, [P | Rest]) -> [P | set_field(K, V, Rest)];
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set_field(K, V, []) -> [{K, V}].
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%% ── Step 7b: gen_server wrapper ─────────────────────────────────
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%%
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%% Each projection runs in its own gen_server, registered under the
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%% projection's Name atom. `async_fold/2` casts an activity into the
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%% process; `query/1` synchronously fetches the current state.
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%%
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%% Port notes (mirroring Step 5b on the registry): `gen_server:start_link`
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%% returns the raw Pid; `?MODULE` macro is unsupported; spawned
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%% processes don't survive across separate `erlang-eval-ast` calls
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%% so tests must inline start_link with their operations.
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start_link(Name, InitialState, FoldFn) ->
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Pid = gen_server:start_link(projection, [Name, InitialState, FoldFn]),
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erlang:register(Name, Pid),
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Pid.
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async_fold(Name, Activity) ->
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gen_server:cast(Name, {fold, Activity}).
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query(Name) ->
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gen_server:call(Name, get_state).
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stop(Name) ->
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R = gen_server:call(Name, '$gen_stop'),
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erlang:unregister(Name),
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R.
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%% gen_server callbacks
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init([Name, InitialState, FoldFn]) ->
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{ok, new(Name, InitialState, FoldFn)}.
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handle_call(get_state, _From, Proj) ->
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{reply, state(Proj), Proj}.
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handle_cast({fold, Activity}, Proj) ->
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{noreply, fold_activity(Proj, Activity)}.
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handle_info(_, Proj) -> {noreply, Proj}.
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