! Copyright (C) 2020 Michael Raitza
! See http://factorcode.org/license.txt for BSD license.
!
! * 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 kernel math sequences serialize trees trees.private ;
IN: trees.cb;

TUPLE: cb < tree ;

: <cb> ( -- tree ) cb new-tree ; inline

<PRIVATE

TUPLE: cb-node < node byte# bits ;

: <cb-node> ( byte# bits key value -- node )
    cb-node new-node
    >>bits
    >>byte# ; inline

! 0 = left
! 1 = right
: direction ( byte bits -- direction )
    bitor 1 + -8 shift ; inline

: key-side ( byte bits -- side )
    direction 0 = -1 1 ? ;

: get-byte-at ( byte# key -- byte/0 )
    object>bytes ?nth [ 0 ] unless* ;

! ** Insertion
! Explain...

! Keep the byte sequence of the current key in =key-bytes= and provide a working
! environment for it with =with-key=.
SYMBOL: key-bytes

: with-key ( key quot -- )
    [ key-bytes ] dip with-variable ; inline

DEFER: cb-set

! TODO Rewrite with =with-key=.
: cb-insert ( value key node -- node taller? created? )
    2dup [ byte#>> ] [ bits>> ] bi
    [ swap get-byte-at ] dip key-side [
        node-link cb-set
    ] with-side ;

! Implement this and cb-insert using SYMBOL for current key bytes while
! traversing the tree.
: (cb-set) ( value key node -- node taller? created? )
    ;

: cb-set ( value key node -- node taller? created? )
    [ (cb-set) ] [ swap [ [ 0 get-byte-at 0xfe ] keep ] dip <cb-node> t t ] if* ;

PRIVATE>