-module(erc_hash).
-export([hash/1]).

-define(REDIS_CLUSTER_HASH_SLOTS, 16384).

-define(CRCDEF, <<16#00,16#00,16#10,16#21,16#20,16#42,16#30,16#63,
16#40,16#84,16#50,16#a5,16#60,16#c6,16#70,16#e7,
16#81,16#08,16#91,16#29,16#a1,16#4a,16#b1,16#6b,
16#c1,16#8c,16#d1,16#ad,16#e1,16#ce,16#f1,16#ef,
16#12,16#31,16#02,16#10,16#32,16#73,16#22,16#52,
16#52,16#b5,16#42,16#94,16#72,16#f7,16#62,16#d6,
16#93,16#39,16#83,16#18,16#b3,16#7b,16#a3,16#5a,
16#d3,16#bd,16#c3,16#9c,16#f3,16#ff,16#e3,16#de,
16#24,16#62,16#34,16#43,16#04,16#20,16#14,16#01,
16#64,16#e6,16#74,16#c7,16#44,16#a4,16#54,16#85,
16#a5,16#6a,16#b5,16#4b,16#85,16#28,16#95,16#09,
16#e5,16#ee,16#f5,16#cf,16#c5,16#ac,16#d5,16#8d,
16#36,16#53,16#26,16#72,16#16,16#11,16#06,16#30,
16#76,16#d7,16#66,16#f6,16#56,16#95,16#46,16#b4,
16#b7,16#5b,16#a7,16#7a,16#97,16#19,16#87,16#38,
16#f7,16#df,16#e7,16#fe,16#d7,16#9d,16#c7,16#bc,
16#48,16#c4,16#58,16#e5,16#68,16#86,16#78,16#a7,
16#08,16#40,16#18,16#61,16#28,16#02,16#38,16#23,
16#c9,16#cc,16#d9,16#ed,16#e9,16#8e,16#f9,16#af,
16#89,16#48,16#99,16#69,16#a9,16#0a,16#b9,16#2b,
16#5a,16#f5,16#4a,16#d4,16#7a,16#b7,16#6a,16#96,
16#1a,16#71,16#0a,16#50,16#3a,16#33,16#2a,16#12,
16#db,16#fd,16#cb,16#dc,16#fb,16#bf,16#eb,16#9e,
16#9b,16#79,16#8b,16#58,16#bb,16#3b,16#ab,16#1a,
16#6c,16#a6,16#7c,16#87,16#4c,16#e4,16#5c,16#c5,
16#2c,16#22,16#3c,16#03,16#0c,16#60,16#1c,16#41,
16#ed,16#ae,16#fd,16#8f,16#cd,16#ec,16#dd,16#cd,
16#ad,16#2a,16#bd,16#0b,16#8d,16#68,16#9d,16#49,
16#7e,16#97,16#6e,16#b6,16#5e,16#d5,16#4e,16#f4,
16#3e,16#13,16#2e,16#32,16#1e,16#51,16#0e,16#70,
16#ff,16#9f,16#ef,16#be,16#df,16#dd,16#cf,16#fc,
16#bf,16#1b,16#af,16#3a,16#9f,16#59,16#8f,16#78,
16#91,16#88,16#81,16#a9,16#b1,16#ca,16#a1,16#eb,
16#d1,16#0c,16#c1,16#2d,16#f1,16#4e,16#e1,16#6f,
16#10,16#80,16#00,16#a1,16#30,16#c2,16#20,16#e3,
16#50,16#04,16#40,16#25,16#70,16#46,16#60,16#67,
16#83,16#b9,16#93,16#98,16#a3,16#fb,16#b3,16#da,
16#c3,16#3d,16#d3,16#1c,16#e3,16#7f,16#f3,16#5e,
16#02,16#b1,16#12,16#90,16#22,16#f3,16#32,16#d2,
16#42,16#35,16#52,16#14,16#62,16#77,16#72,16#56,
16#b5,16#ea,16#a5,16#cb,16#95,16#a8,16#85,16#89,
16#f5,16#6e,16#e5,16#4f,16#d5,16#2c,16#c5,16#0d,
16#34,16#e2,16#24,16#c3,16#14,16#a0,16#04,16#81,
16#74,16#66,16#64,16#47,16#54,16#24,16#44,16#05,
16#a7,16#db,16#b7,16#fa,16#87,16#99,16#97,16#b8,
16#e7,16#5f,16#f7,16#7e,16#c7,16#1d,16#d7,16#3c,
16#26,16#d3,16#36,16#f2,16#06,16#91,16#16,16#b0,
16#66,16#57,16#76,16#76,16#46,16#15,16#56,16#34,
16#d9,16#4c,16#c9,16#6d,16#f9,16#0e,16#e9,16#2f,
16#99,16#c8,16#89,16#e9,16#b9,16#8a,16#a9,16#ab,
16#58,16#44,16#48,16#65,16#78,16#06,16#68,16#27,
16#18,16#c0,16#08,16#e1,16#38,16#82,16#28,16#a3,
16#cb,16#7d,16#db,16#5c,16#eb,16#3f,16#fb,16#1e,
16#8b,16#f9,16#9b,16#d8,16#ab,16#bb,16#bb,16#9a,
16#4a,16#75,16#5a,16#54,16#6a,16#37,16#7a,16#16,
16#0a,16#f1,16#1a,16#d0,16#2a,16#b3,16#3a,16#92,
16#fd,16#2e,16#ed,16#0f,16#dd,16#6c,16#cd,16#4d,
16#bd,16#aa,16#ad,16#8b,16#9d,16#e8,16#8d,16#c9,
16#7c,16#26,16#6c,16#07,16#5c,16#64,16#4c,16#45,
16#3c,16#a2,16#2c,16#83,16#1c,16#e0,16#0c,16#c1,
16#ef,16#1f,16#ff,16#3e,16#cf,16#5d,16#df,16#7c,
16#af,16#9b,16#bf,16#ba,16#8f,16#d9,16#9f,16#f8,
16#6e,16#17,16#7e,16#36,16#4e,16#55,16#5e,16#74,
16#2e,16#93,16#3e,16#b2,16#0e,16#d1,16#1e,16#f0>>).

%% API.
-spec hash(string()) -> integer().
hash(Key) when is_list(Key) ->
    % ensure Key2 is a charlist, not iolist
    Key2 = binary_to_list(iolist_to_binary(Key)),
    crc16(Key2) rem ?REDIS_CLUSTER_HASH_SLOTS;
hash(Key) when is_binary(Key) ->
    hash(binary_to_list(Key)).

-spec crc16(string()) -> integer().
crc16(Key) ->
    crc16(Key,0).

-spec crc16(string(), integer()) -> integer().
crc16([B|T], Crc) ->
    Index = (((Crc bsr 8) bxor B) band 16#ff),
    NewCrc = ((Crc bsl 8) band 16#ffff) bxor crc_index(Index),
    crc16(T,NewCrc);
crc16([],Crc) ->
    Crc.

-spec crc_index(integer()) -> integer().
crc_index(N) ->
    <<Crc:16>> = binary:part(?CRCDEF,N*2,2),
    Crc.