Robin Hood Hashing (Papers We Love SF August 2017)

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Robin Hood Hashing (Pedro Celis, 1986) Papers We Love

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André Arko this is me, @indirect

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Robin Hood Hashing

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Robin Hood Hashing (1986) example hash table data LEIA HAN REY LUKE : ALDERAAN : CORELLIA : JAKKU : TATTOOINE

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Robin Hood Hashing (1986) example hash table data LEIA HAN REY LUKE

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Robin Hood Hashing (1986) example hash table data LEIA HAN REY LUKE

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Robin Hood Hashing (1986) keys hashed to index values LEIA HAN REY LUKE

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Robin Hood Hashing (1986) keys hashed to index values LEIA HAN REY LUKE

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Robin Hood Hashing (1986) keys hashed to index values LEIA HAN REY LUKE HASHING ALGORITHM

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Robin Hood Hashing (1986) keys hashed to index values LEIA HAN REY LUKE HASHING ALGORITHM

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Robin Hood Hashing (1986) keys hashed to index values LEIA HAN REY LUKE HASHING ALGORITHM 2 4 0 4

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Robin Hood Hashing (1986) keys hashed to index values LEIA HAN REY LUKE 2 4 0 4

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Robin Hood Hashing (1986) keys hashed to index values LEIA HAN REY LUKE 2 4 0 4

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data stored in an array LEIA HAN REY LUKE 2 4 0 4 Robin Hood Hashing (1986)

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data stored in an array LEIA HAN REY LUKE 2 4 0 4 0 1 2 3 4 Robin Hood Hashing (1986)

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data stored in an array LEIA HAN REY LUKE 2 4 0 4 0 1 2 3 4 Robin Hood Hashing (1986)

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data stored in an array LEIA HAN REY LUKE 2 4 0 4 0 1 2 3 4 LEIA:ALDERAAN Robin Hood Hashing (1986)

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data stored in an array LEIA HAN REY LUKE 2 4 0 4 0 1 2 3 4 LEIA:ALDERAAN Robin Hood Hashing (1986)

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data stored in an array LEIA HAN REY LUKE 2 4 0 4 0 1 2 3 4 LEIA:ALDERAAN HAN:CORELLIA Robin Hood Hashing (1986)

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data stored in an array LEIA HAN REY LUKE 2 4 0 4 0 1 2 3 4 LEIA:ALDERAAN HAN:CORELLIA Robin Hood Hashing (1986)

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data stored in an array LEIA HAN REY LUKE 2 4 0 4 0 1 2 3 4 LEIA:ALDERAAN HAN:CORELLIA REY:JAKKU Robin Hood Hashing (1986)

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hash collisions LEIA HAN REY LUKE 2 4 0 4 0 1 2 3 4 LEIA:ALDERAAN HAN:CORELLIA REY:JAKKU Robin Hood Hashing (1986)

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resolving collisions: chaining LEIA HAN REY LUKE 2 4 0 4 0 1 2 3 4 LEIA:ALDERAAN HAN:CORELLIA REY:JAKKU Robin Hood Hashing (1986)

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0 1 2 3 4 5 resolving collisions: open addressing LEIA HAN REY LUKE 2 4 0 4 LEIA:ALDERAAN HAN:CORELLIA REY:JAKKU Robin Hood Hashing (1986)

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0 1 2 3 4 5 resolving collisions: open addressing LEIA HAN REY LUKE 2 4 0 4 LEIA:ALDERAAN HAN:CORELLIA + REY:JAKKU LUKE:TATTOOINE Robin Hood Hashing (1986)

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0 1 2 3 4 5 resolving collisions: open addressing LEIA HAN REY LUKE FINN 2 4 0 4 5 LEIA:ALDERAAN HAN:CORELLIA + REY:JAKKU LUKE:TATTOOINE Robin Hood Hashing (1986)

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0 1 2 3 4 5 6 Robin Hood Hashing (1986) resolving collisions: open addressing LEIA HAN REY LUKE FINN 2 4 0 4 5 LEIA:ALDERAAN HAN:CORELLIA + REY:JAKKU LUKE:TATTOOINE + FINN:CORUSCANT

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0 1 2 3 4 5 6 7 8 9 Robin Hood Hashing (1986) resolving collisions: open addressing LEIA HAN REY LUKE FINN CHEWIE LANDO MAUL 2 4 0 4 5 9 6 8 LEIA:ALDERAAN HAN:CORELLIA + REY:JAKKU LUKE:TATTOOINE + FINN:CORUSCANT+ CHEWIE:KASHYYK LANDO:BESPIN MAUL:DATHOMIR

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0 1 2 3 4 5 6 7 8 9 Robin Hood Hashing (1986) worst-case performance: it’s really bad LEIA HAN REY LUKE FINN CHEWIE LANDO MAUL WICKET 2 4 0 4 5 9 6 8 4 HAN:CORELLIA + REY:JAKKU LUKE:TATTOOINE + FINN:CORUSCANT+ CHEWIE:KASHYYK LANDO:BESPIN MAUL:DATHOMIR LEIA:ALDERAAN

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0 1 2 3 4 5 6 7 8 9 10 Robin Hood Hashing (1986) worst-case performance: it’s really bad LEIA HAN REY LUKE FINN CHEWIE LANDO MAUL WICKET 2 4 0 4 5 9 6 8 4 HAN:CORELLIA + REY:JAKKU LUKE:TATTOOINE + FINN:CORUSCANT+ CHEWIE:KASHYYK+ LANDO:BESPIN+ MAUL:DATHOMIR+ WICKET:ENDOR LEIA:ALDERAAN

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0 1 2 3 4 5 6 7 8 9 10 Robin Hood Hashing (1986) data 6 slots away from ideal LEIA HAN REY LUKE FINN CHEWIE LANDO MAUL WICKET 2 4 0 4 5 9 6 8 4 HAN:CORELLIA + REY:JAKKU LUKE:TATTOOINE + FINN:CORUSCANT+ CHEWIE:KASHYYK+ LANDO:BESPIN+ MAUL:DATHOMIR+ WICKET:ENDOR LEIA:ALDERAAN

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Robin Hood Hashing (1986) as you can probably imagine, this gets very bad, very quickly

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Robin Hood Hashing (1986) but a hero is on the horizon! Robin Hood Hashing offers vastly more efﬁcient storage

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Robin Hood Hashing (1986) how much better is it, really? O(ln n) probes, where n is items in the hash

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0 1 2 3 4 5 6 7 8 9 10 Robin Hood Hashing (1986) data stored in an array with robin hood hashing LEIA HAN REY LUKE FINN CHEWIE LANDO MAUL WICKET 2 4 0 4 5 9 6 8 4

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0 1 2 3 4 5 6 7 8 9 10 Robin Hood Hashing (1986) data stored in an array with robin hood hashing LEIA HAN REY LUKE FINN CHEWIE LANDO MAUL WICKET 2 4 0 4 5 9 6 8 4 REY:JAKKU 0

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0 1 2 3 4 5 6 7 8 9 10 Robin Hood Hashing (1986) data stored in an array with robin hood hashing LEIA HAN REY LUKE FINN CHEWIE LANDO MAUL WICKET 2 4 0 4 5 9 6 8 4 REY:JAKKU 0 LEIA:ALDERAAN 0

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0 1 2 3 4 5 6 7 8 9 10 Robin Hood Hashing (1986) data stored in an array with robin hood hashing LEIA HAN REY LUKE FINN CHEWIE LANDO MAUL WICKET 2 4 0 4 5 9 6 8 4 HAN:CORELLIA 0 REY:JAKKU 0 LEIA:ALDERAAN 0

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0 1 2 3 4 5 6 7 8 9 10 Robin Hood Hashing (1986) data stored in an array with robin hood hashing LEIA HAN REY LUKE FINN CHEWIE LANDO MAUL WICKET 2 4 0 4 5 9 6 8 4 HAN:CORELLIA 0 REY:JAKKU 0 LUKE:TATTOOINE 1 LEIA:ALDERAAN 0

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0 1 2 3 4 5 6 7 8 9 10 Robin Hood Hashing (1986) only 2 slots away from ideal LEIA HAN REY LUKE FINN CHEWIE LANDO MAUL WICKET 2 4 0 4 5 9 6 8 4 HAN:CORELLIA 0 REY:JAKKU 0 LUKE:TATTOOINE 1 WICKET:ENDOR 2 MAUL:DATHOMIR 1 FINN:CORUSCANT 2 LANDO:BESPIN 2 LEIA:ALDERAAN 0 CHEWIE:KASHYYK 1

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Robin Hood Hashing (1986) stealing from the rich and giving to the poor? that’s Robin Hood all over

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Robin Hood Hashing (1986) further reading ‣ Robin Hood Hashing (1986 original paper) ‣ Robin Hood Hashing with Linear Probing paper (2005) ‣ Paul Khuong experimenting with hashing options (2009) ‣ Paul’s follow-up and conclusions (2011) ‣ Sebastian Sylvan saying robin hood should be the default (2013) ‣ Sebastian following up on slowness after deletions (2013) ‣ Emmanuel Goossaert benchmarking in C++ (2013) ‣ Paul Kuhong again, on linear probing for performance (2013) ‣ Emmanuel benchmarking again after tweaking deletions (2013)

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Robin Hood Hashing (1986) thanks! slides  bit.ly/indirect-robin-hood-hashing twitter  @indirect email  andre@arko.net