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+%% Copyright (c) 2012, Magnus Klaar <klaar@ninenines.eu>
+%%
+%% Permission to use, copy, modify, and/or distribute this software for any
+%% purpose with or without fee is hereby granted, provided that the above
+%% copyright notice and this permission notice appear in all copies.
+%%
+%% THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+%% WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+%% MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+%% ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+%% WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+%% ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+%% OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+%% @doc Accessor function for goldrush event terms.
+-module(gre).
+
+-export([
+    make/2,
+    has/2,
+    fetch/2,
+    append/2,
+    merge/2,
+    find/2,
+    keys/1,
+    pairs/1
+]).
+
+-type event() :: {list, [{atom(), term()}]}.
+-export_type([event/0]).
+
+%% @doc Construct an event term.
+-spec make(term(), [list]) -> event().
+make(Term, [Type]) ->
+    {Type, Term}.
+
+
+%% @doc Check if a field exists in an event.
+-spec has(atom(), event()) -> boolean().
+has(Key, {list, List}) ->
+    lists:keymember(Key, 1, List).
+
+-spec append(term(), event()) -> event().
+append(KeyVal, {list, List}) ->
+    {list, [KeyVal|List]}.
+
+-spec merge(event(), event()) -> event().
+merge({list, AList}, {list, BList}) ->
+    {list, lists:merge(AList, BList)}.
+
+%% @doc Get the value of a field in an event.
+%% The field is expected to exist in the event.
+-spec fetch(atom(), event()) -> term().
+fetch(Key, {list, List}=Event) ->
+    case lists:keyfind(Key, 1, List) of
+        {_, Value} -> Value;
+        false -> erlang:error(badarg, [Key, Event])
+    end.
+
+
+%% @doc Find the value of a field in an event.
+%% This is equivalent to testing if a field exists using {@link has/2}
+%% before accessing the value of the field using {@link fetch/2}.
+-spec find(atom(), event()) -> {true, term()} | false.
+find(Key, {list, List}) ->
+    case lists:keyfind(Key, 1, List) of
+        {_, Value} -> {true, Value};
+        false -> false
+    end.
+
+
+%% @doc Get the names of all fields in an event.
+-spec keys(event()) -> [atom()].
+keys({list, List}) ->
+    kv_keys_(List).
+
+%% @private Get the names of all fields in a key-value list.
+-spec kv_keys_([{atom(), term()}]) -> [atom()].
+kv_keys_([{Key, _}|T]) ->
+    [Key|kv_keys_(T)];
+kv_keys_([]) ->
+    [].
+
+%% @doc Get the name and value of all fields in an event.
+-spec pairs(event()) -> [{atom(), term()}].
+pairs({list, List}) ->
+    lists:sort(List).
+
+
+-ifdef(TEST).
+-include_lib("eunit/include/eunit.hrl").
+
+gre_test_() ->
+    [?_assert(gre:has(a, gre:make([{a,1}], [list]))),
+     ?_assertNot(gre:has(b, gre:make([{a,1}], [list]))),
+     ?_assertEqual(1, gre:fetch(a, gre:make([{a,1}], [list]))),
+     ?_assertError(badarg, gre:fetch(a, gre:make([], [list]))),
+     ?_assertEqual([], gre:keys(gre:make([], [list]))),
+     ?_assertEqual([a], gre:keys(gre:make([{a,1}], [list]))),
+     ?_assertEqual([a,b], gre:keys(gre:make([{a,1},{b,2}], [list]))),
+     ?_assertEqual([{a,1},{b,2}], gre:pairs(gre:make([{b,2},{a,1}], [list])))
+    ].
+
+-endif.