-module(flow_store).
-export([start_link/0, start_link/1, stop/0,
         register_flow/2, resolve_flow/1, registered_flows/0,
         start/2, resume/2, status/1, instances/0]).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2]).
-behaviour(gen_server).

%% flow-on-erlang durable store — the named-flow registry plus the
%% instance table that makes suspend/resume durable. flow.erl is the
%% pure replay driver; this gen_server is the stateful shell around it,
%% holding the registry (so triggers can reference flows by name, and
%% so an instance can be re-resolved + replayed after a restart) and
%% each instance's accumulated replay log.
%%
%% Crucially the driver stays OUT of any blocking context: start/resume
%% call flow:drive/3 (pure — no receive, no gen_server:call) from inside
%% handle_call, and the only message-passing is the caller's
%% gen_server:call into this store. (A blocking receive inside a `try`
%% deadlocks this cooperative scheduler, so the engine never does one.)
%%
%% State: {Registry, Instances, NextId}
%%   Registry  = [{Name, FlowFun}, ...]
%%   Instances = [{Id, {Name, Input, Log, Status}}, ...]
%%   Status    = {suspended, Tag} | {done, Result}
%%   Log       = [{Tag, ResolvedValue}, ...]  (the replay log — plain
%%               data, so an instance is fully described by its log and
%%               survives process restart by re-driving the named flow)
%%
%% v1 backs the store in gen_server memory; persisting the instance
%% logs to the kernel's durable log (so flows survive an OS restart) is
%% a later layer — the data shape is already restart-ready.

start_link() ->
    start_link([]).

start_link(InitialFlows) ->
    Pid = gen_server:start_link(flow_store, [InitialFlows]),
    erlang:register(flow_store, Pid),
    Pid.

stop() ->
    R = gen_server:call(flow_store, '$gen_stop'),
    erlang:unregister(flow_store),
    R.

%% register_flow(Name, Flow) — register a named flow (a node fun). Named
%% rather than `register` to avoid the erlang:register/2 auto-import.
register_flow(Name, Flow) ->
    gen_server:call(flow_store, {register_flow, Name, Flow}).

resolve_flow(Name) ->
    gen_server:call(flow_store, {resolve_flow, Name}).

registered_flows() ->
    gen_server:call(flow_store, registered_flows).

%% start(Name, Input) -> {ok, Id, Result} | {error, no_such_flow}.
%% Result is {flow_done, V} | {flow_suspended, Tag}; the instance is
%% recorded either way so a suspended flow can be resumed by Id.
start(Name, Input) ->
    gen_server:call(flow_store, {start, Name, Input}).

%% resume(Id, Value) -> {ok, Result} | {error, Reason}. Resolves the
%% instance's current suspend tag with Value (appends {Tag, Value} to
%% its replay log) and re-drives from the top.
resume(Id, Value) ->
    gen_server:call(flow_store, {resume, Id, Value}).

%% status(Id) -> {ok, {suspended, Tag}} | {ok, {done, Result}} | not_found
status(Id) ->
    gen_server:call(flow_store, {status, Id}).

instances() ->
    gen_server:call(flow_store, instances).

%% ── gen_server ──────────────────────────────────────────────────

init([InitialFlows]) ->
    {ok, {InitialFlows, [], 1}}.

handle_call({register_flow, Name, Flow}, _From, {Reg, Ins, N}) ->
    {reply, ok, {set_keyed(Name, Flow, Reg), Ins, N}};
handle_call({resolve_flow, Name}, _From, {Reg, Ins, N}) ->
    {reply, find_keyed(Name, Reg), {Reg, Ins, N}};
handle_call(registered_flows, _From, {Reg, Ins, N}) ->
    {reply, [Name || {Name, _} <- Reg], {Reg, Ins, N}};
handle_call({start, Name, Input}, _From, {Reg, Ins, N}) ->
    case find_keyed(Name, Reg) of
        not_found ->
            {reply, {error, no_such_flow}, {Reg, Ins, N}};
        {ok, Flow} ->
            R = flow:drive(Flow, Input, []),
            Status = result_status(R),
            Ins2 = set_keyed(N, {Name, Input, [], Status}, Ins),
            {reply, {ok, N, R}, {Reg, Ins2, N + 1}}
    end;
handle_call({resume, Id, Value}, _From, {Reg, Ins, N}) ->
    case find_keyed(Id, Ins) of
        not_found ->
            {reply, {error, no_such_instance}, {Reg, Ins, N}};
        {ok, {_Name, _Input, _Log, {done, _}}} ->
            {reply, {error, already_done}, {Reg, Ins, N}};
        {ok, {Name, Input, Log, {suspended, Tag}}} ->
            case find_keyed(Name, Reg) of
                not_found ->
                    {reply, {error, no_such_flow}, {Reg, Ins, N}};
                {ok, Flow} ->
                    NewLog = log_append(Log, Tag, Value),
                    R = flow:drive(Flow, Input, NewLog),
                    Status = result_status(R),
                    Ins2 = set_keyed(Id, {Name, Input, NewLog, Status}, Ins),
                    {reply, {ok, R}, {Reg, Ins2, N}}
            end
    end;
handle_call({status, Id}, _From, {Reg, Ins, N}) ->
    case find_keyed(Id, Ins) of
        {ok, {_Name, _Input, _Log, Status}} -> {reply, {ok, Status}, {Reg, Ins, N}};
        not_found                           -> {reply, not_found, {Reg, Ins, N}}
    end;
handle_call(instances, _From, {Reg, Ins, N}) ->
    {reply, [Id || {Id, _} <- Ins], {Reg, Ins, N}}.

handle_cast(_, S) -> {noreply, S}.

handle_info(_, S) -> {noreply, S}.

%% ── helpers ─────────────────────────────────────────────────────

result_status({flow_done, R})      -> {done, R};
result_status({flow_suspended, T})  -> {suspended, T}.

log_append([], Tag, Value) -> [{Tag, Value}];
log_append([H | T], Tag, Value) -> [H | log_append(T, Tag, Value)].

find_keyed(_, []) -> not_found;
find_keyed(K, [{K, V} | _]) -> {ok, V};
find_keyed(K, [_ | Rest]) -> find_keyed(K, Rest).

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