Add cow_http_hd:parse_if_range/1

From RFC7233.

src/cow_http_hd.erl

@@ -34,6 +34,7 @@
 -export([parse_if_match/1]).
 -export([parse_if_modified_since/1]).
 -export([parse_if_none_match/1]).
+-export([parse_if_range/1]).
 -export([parse_if_unmodified_since/1]).
 -export([parse_last_modified/1]).
 -export([parse_max_forwards/1]).
@@ -1835,6 +1836,44 @@ horse_parse_if_none_match() ->
 	).
 -endif.
 
+%% @doc Parse the If-Range header.
+
+-spec parse_if_range(binary()) -> etag() | calendar:datetime().
+parse_if_range(<< $W, $/, $", R/bits >>) ->
+	etag(R, weak, <<>>);
+parse_if_range(<< $", R/bits >>) ->
+	etag(R, strong, <<>>);
+parse_if_range(IfRange) ->
+	cow_date:parse_date(IfRange).
+
+-ifdef(TEST).
+parse_if_range_test_() ->
+	Tests = [
+		{<<"W/\"xyzzy\"">>, {weak, <<"xyzzy">>}},
+		{<<"\"xyzzy\"">>, {strong, <<"xyzzy">>}},
+		{<<"Sat, 29 Oct 1994 19:43:31 GMT">>, {{1994, 10, 29}, {19, 43, 31}}}
+	],
+	[{V, fun() -> R = parse_if_range(V) end} || {V, R} <- Tests].
+
+parse_if_range_error_test_() ->
+	Tests = [
+		<<>>
+	],
+	[{V, fun() -> {'EXIT', _} = (catch parse_if_range(V)) end} || V <- Tests].
+-endif.
+
+-ifdef(PERF).
+horse_parse_if_range_etag() ->
+	horse:repeat(200000,
+		parse_if_range(<<"\"xyzzy\"">>)
+	).
+
+horse_parse_if_range_date() ->
+	horse:repeat(200000,
+		parse_if_range(<<"Sat, 29 Oct 1994 19:43:31 GMT">>)
+	).
+-endif.
+
 %% @doc Parse the If-Unmodified-Since header.
 
 -spec parse_if_unmodified_since(binary()) -> calendar:datetime().