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Michael Raitza 2020-02-24 15:44:07 +01:00
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USING: critbit tools.test ;
IN: critbit.tests
{ t } [ <critbit> empty? ] unit-test
{ 0 } [ 0 0b11111011 direction ] unit-test
{ 1 } [ 4 0b11111011 direction ] unit-test

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! Copyright (C) 2019 Michael Raitza
!
! * Crit-bit trees
! ** Rationale
! Critbit trees are described in [[https://cr.yp.to/critbit.html][djb's crit-bit tree]]. They are an evolution of
! PATRICIA trees showing that fast insertion, deletion, exact searching and suffix
! searching is possible with this data structure.
! The strength of this datastructure, according to its author, lies in its simple
! design and its optimisation to be machine parsable using machine word-sized
! operations where possible. Like PATRICIA trees, crit-bit trees are
! prefix-compressed, with internal nodes storing next decision point (the critical
! bit) in a length field (encoded as an integer and a mask) and two successor
! pointers. Arbitrary data objects make up its leaves.
USING: accessors fry kernel math sequences serialize ;
IN: critbit
TUPLE: critbit root ;
<PRIVATE
: new-critbit ( class -- critbit )
new
f >>root ; inline
PRIVATE>
: <critbit> ( -- critbit )
critbit new-critbit ;
<PRIVATE
TUPLE: node left right { byte# integer } { bits integer } ;
: empty? ( tree -- ? )
root>> f = ; inline
: new-node ( bits byte# class -- node )
new
swap >>byte#
swap >>bits ; inline
: <node> ( bits byte# -- node )
node new-node ;
: get-child ( node direction -- child )
0 = [ right>> ] [ left>> ] if ;
: set-child ( node child direction -- node )
0 = [ >>right ] [ >>left ] if ;
: set-child-opposite ( node child direction -- node )
1 swap - set-child ;
: direction ( byte bits -- direction )
bitor 1 + -8 shift ;
! ** Walking the tree
! Extract the split byte number and bit from node and return the proper child the
! proper child corresponding to the bytestring's prefix.
: successor ( bytes node -- bytes successor via-direction )
2dup [ byte#>> ] [ bits>> ] bi
[ swap ?nth [ 0 ] unless* ] dip
direction [ get-child ] keep ;
! Walk the tree until we find a leaf.
: walk-critbit ( obj tree -- objbytes leaf )
[ object>bytes ] [ root>> ] bi*
[ dup node? ] [ successor drop ] while ;
PRIVATE>
: member? ( obj tree -- ? )
dup critbit?
[ walk-critbit = ]
[ 2drop f ] if ;
<PRIVATE
! Produce a byte with all bits set except the msb from bits*.
! See MAGIC Algorithms for rationale.
: msb0 ( bits* -- bits )
dup -1 shift bitor
dup -2 shift bitor
dup -4 shift bitor
dup -1 shift bitnot bitand 255 bitxor ;
: byte-diff ( newbyte oldbyte -- direction bits )
swap over
bitxor msb0
[ direction ] keep ;
: node-params ( newbytes oldbytes -- direction bits byte# )
2dup mismatch
[ [ '[ _ swap nth ] bi@ byte-diff ] keep ]
! Equal prefix over full (shorter) byte sequence.
[ [ 1 255 ] 2dip shorter length 1 - ] if* ;
: derive-node ( obj newbytes leaf -- node direction )
object>bytes
node-params <node> -rot [ set-child-opposite ] keep ;
: insert-node ( node split-node direction -- successor ? )
[
[ [ byte#>> ] bi@ - [ 0 < ] [ 0 = ] bi ] 2keep
[ bits>> ] > and
or
] dip drop
;
: insertion-walk ( tree split-node new-direction -- tree )
! FIXME Unfinished. We need obj to calculate succession :(
rot [
root>> dup node?
[ ] [ 2over insert-node ] while 3drop
] keep ;
:: walk-on? ( split-node node -- ? )
node node? not
node byte#>> split-node byte#>> >
or
node byte#>> split-node byte#>> =
node bits>> split-node bits>> >
and
or
not ;
:: insertion-walk ( obj split-node new-direction -- ? )
obj obj>bytes :> objbytes
tree root>> :> node
[ split-node node walk-on? ] [
node :> predecessor
objbytes node successor :> node-direction :> node drop
] while
node split-node new-direction set-child
;
! :: insertion-successor ( objbytes bits byte# node -- split-node )
! node node? not
! [ node ]
! [
! node byte#>>
! [ byte# > ] keep
! byte# = node bits>> bits > and
! or
! ! objbytes no longer matches prefix?
! [ node ]
! [
! ! FIXME: complete node insertion
! ] if
! ] if ;
! :: (insert-node) ( obj tree objbytes treebytes direction bits byte# -- tree )
! bits byte# <node>
! direction 0 = [ obj >>right ] [ obj >>left ] if :> node
! tree root>> [ dup node? ] [ insertion-successor]
! ;
! : insert-node ( obj tree objbytes treebytes -- tree )
! 2dup node-params (insert-node)
! ;
PRIVATE>
: (put) ( tree obj -- tree )
swap dup empty?
[ swap >>root ]
[ [ 2dup walk-critbit
derive-node
! FIXME Unfinished, this node must now be inserted into the tree
] 2keep
2drop ! <- remove;
! insert-node
] if ;
: put ( obj tree -- tree )
dup critbit? [ (put) ] [ 2drop "Not a critbit tree" throw ] if ;