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143 lines
3.4 KiB
Forth
143 lines
3.4 KiB
Forth
# Forward Linked Lists
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(c) Arland Childers
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~~~
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:comma , ;
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~~~
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The basic building block of our lists is a *cons cell*. This is
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a tuple with two values. The first of these is the *car*, and
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the second is the *cdr*. (Naming of these comes from LISP, and
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some very old computer processors)
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In memory a cons cell with two values (1, 2) would look like:
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+---+-----+
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| 1 | car |
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+---+-----+
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| 2 | cdr |
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+---+-----+
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We provide `cons` to make a cons cell, `car` to get a pointer to
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the first element, and `cdr` to get a pointer to the second.
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We also provide `car@` and `car!` to read or modify the car
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value, and `cdr@` and `cdr!` for doing the same to the cdr.
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~~~
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:cons (car,cdr-ptr) here [ swap comma comma ] dip ;
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:car (cons-ptr) ;
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:cdr (cons-ptr) n:inc ;
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:car@ (cons-value) car fetch ;
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:car! (value,cons-) car store ;
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:cdr@ (cons-value) cdr fetch ;
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:cdr! (value,cons-) cdr store ;
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~~~
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With this out of the way, we turn to lists. A list, in this
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model, is a series of cons cells. We are working with forward
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linked lists, so each cons cell points to the next one in the
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chain. (Back linked lists, or doubly linked lists, are not
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implemented at this time). A list of values 1, 2, and 3 would
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look like:
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+---+---+ +---+---+ +---+---+
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| 1 | =====> | 2 | =====> | 3 | =====> END
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+---+---+ +---+---+ +---+---+
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`END` is an empty word that we use to denote the end of a list.
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We'll compare the contents of the CDR field to this to determine
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if we've reached the end. This could equally well be zero, but
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we decided it was better for readability to have a named element.
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~~~
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:END (-) ;
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~~~
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Creation of a list begins with `fll:create`. This takes in a
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value, and creates a cons cell with the value, and a pointer
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to END in the cdr.
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~~~
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:fll:create (v-p) &END cons ;
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~~~
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Many operations will need to access the end of the list. We have
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`fll:to-end` to seek this. It'll walk through the list, returning
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a pointer to the final cons cell.
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~~~
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{{
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'r var
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---reveal---
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:fll:to-end (p-p)
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dup !r [ cdr@ dup &END -eq? dup
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[ over !r ] [ nip ] choose ] while @r ;
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}}
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~~~
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Appening a value is simple. Create a new cons with the value in
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the car and a pointer to END in the cdr. Find the end of the
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list, and update the cdr to point to the new cons.
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~~~
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:fll:append/value (pv-) &END cons swap fll:to-end cdr! ;
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~~~
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Given a specific node in the list, follow the cdr chain until
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the node number is reached. Starts at zero.
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An issue with this: it does not currently check the length to
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make sure that the index is valid.
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~~~
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:fll:to-index (pn-p) [ cdr@ ] times ;
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~~~
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~~~
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:fll:del
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dup-pair n:dec fll:to-index [ n:inc fll:to-index ] dip cdr! ;
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~~~
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The `fll:for-each` runs a quote once for each value in a list.
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~~~
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{{
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'Action var
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---reveal---
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:fll:for-each (aq-)
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!Action [ [ car@ @Action call ] sip cdr@ dup &END -eq? ]
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while drop ;
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}}
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~~~
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To obtain the length of a list (in number of cons) use
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`fll:length`.
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~~~
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:fll:length (p-n) #0 swap [ drop n:inc ] fll:for-each n:dec ;
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~~~
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`fll:drop` removes the a specified cons from a list.
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~~~
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:fll:drop dup fll:length n:dec fll:to-index &END swap cdr! ;
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~~~
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`fll:inject` adds a new cons to a list, inserting at a specified
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point.
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~~~
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{{
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'i var
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---reveal---
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:fll:inject (piv-)
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fll:create !i dup-pair n:dec fll:to-index
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&fll:to-index dip @i swap cdr! @i cdr! ;
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:fll:put (a-) [ n:put sp ] fll:for-each ;
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}}
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~~~
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