From 5a9cdd3cc89507d4d74f8bded56ce5e037b3b56e Mon Sep 17 00:00:00 2001
From: Trygve Laugstøl <trygvis@inamo.no>
Date: Fri, 23 Feb 2024 07:08:18 +0100
Subject: wip

---
 learn-you-some-erlang/reminder/src/event.erl | 61 ++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)
 create mode 100644 learn-you-some-erlang/reminder/src/event.erl

(limited to 'learn-you-some-erlang/reminder/src/event.erl')

diff --git a/learn-you-some-erlang/reminder/src/event.erl b/learn-you-some-erlang/reminder/src/event.erl
new file mode 100644
index 0000000..58b5b57
--- /dev/null
+++ b/learn-you-some-erlang/reminder/src/event.erl
@@ -0,0 +1,61 @@
+-module(event).
+-export([start/2, start_link/2, cancel/1]).
+-export([init/3, loop/1]).
+-record(state, {server,
+                name="",
+                to_go=0}).
+
+%%% Public interface
+start(EventName, DateTime) ->
+    spawn(?MODULE, init, [self(), EventName, DateTime]).
+
+start_link(EventName, DateTime) ->
+    spawn_link(?MODULE, init, [self(), EventName, DateTime]).
+
+cancel(Pid) ->
+    %% Monitor in case the process is already dead
+    Ref = erlang:monitor(process, Pid),
+    Pid ! {self(), Ref, cancel},
+    receive
+        {Ref, ok} ->
+            erlang:demonitor(Ref, [flush]),
+            ok;
+        {'DOWN', Ref, process, Pid, _Reason} ->
+            ok
+    end.
+
+%%% Event's innards
+init(Server, EventName, DateTime) ->
+    loop(#state{server=Server,
+                name=EventName,
+                to_go=time_to_go(DateTime)}).
+
+%% Loop uses a list for times in order to go around the ~49 days limit
+%% on timeouts.
+loop(S = #state{server=Server, to_go=[T|Next]}) ->
+    receive
+        {Server, Ref, cancel} ->
+            Server ! {Ref, ok}
+    after T*1000 ->
+        if Next =:= [] ->
+            Server ! {done, S#state.name};
+           Next =/= [] ->
+            loop(S#state{to_go=Next})
+        end
+    end.
+
+%%% private functions
+time_to_go(TimeOut={{_,_,_}, {_,_,_}}) ->
+    Now = calendar:local_time(),
+    ToGo = calendar:datetime_to_gregorian_seconds(TimeOut) -
+           calendar:datetime_to_gregorian_seconds(Now),
+    Secs = if ToGo > 0  -> ToGo;
+              ToGo =< 0 -> 0
+           end,
+    normalize(Secs).
+
+%% Because Erlang is limited to about 49 days (49*24*60*60*1000) in
+%% milliseconds, the following function is used
+normalize(N) ->
+    Limit = 49*24*60*60,
+    [N rem Limit | lists:duplicate(N div Limit, Limit)].
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