100 lines
2.6 KiB
Forth
100 lines
2.6 KiB
Forth
# Linked Lists
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A linked list is a linear collection of data elements, where the
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order is not given by their physical locations in memory. Each
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element (called a node) has a secondary field that points to the
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next.
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+----+---+ +----+---+
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| 11 | ----->| 22 | x |
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+----+---+ +----+---+
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This implements a basic linked list in RETRO. The words here are
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in a `ll` namespace. I'm *not* focusing on short names here; I
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expect that anything making extensive use of this will provide a
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set of aliases or extensions to make things more readable. E.g.,
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Rather than:
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#1 ll:node-n #2 ll:node-n ll:append #3 ll:node-n
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ll:append #4 ll:node-n ll:append
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It's easy to write wrappers that allow for things like:
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#1 < #2 > #3 > #4 >
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# Code
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A node is a simple structure with two fields. The first (which I
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am calling `head`) holds a value and the second (`next`) holds a
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pointer to the next node in the list. If the `next` field points
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to an address of zero it is considered the end of the list.
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So:
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ll:head (a-a) return the address of the head field for a node
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ll:next (a-a) return the address of the next field for a node
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ll:node-n (n-a) create a new node with a head of n and a next of 0
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ll:node (-a) create a new node with both fields set to 0
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~~~
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:ll:head ;
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:ll:next n:inc ;
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:ll:node-n here swap , #0 , ;
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:ll:node here #0 , #0 , ;
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~~~
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Next is a helper word to find the end of a list and one to add a node
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to the end of the list.
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ll:to-end (a-a) return the address of the last node in the list
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ll:append (aa-a) set the next field of a1 to the address of a2,
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return a1
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~~~
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:ll:to-end [ ll:next dup fetch n:zero? ] until n:dec ;
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:ll:append swap [ ll:to-end ll:next store ] sip ;
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~~~
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I have a `for-each` combinator to handle performing an action on
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each node in a list. As an example:
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:< ll:node-n ;
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:> ll:node-n ll:append ;
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#1 < #2 > #3 > #5 > #1 > #2 > #3 >
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[ ll:head fetch n:put sp ] ll:for-each
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The docstring for this is:
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ll:for-each (aq-a) run q once for each node in the list, passing
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the node address to q each time.
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~~~
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{{
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'Action var
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:perform
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[ @Action call ] sip ;
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:at-end?
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ll:next fetch dup n:zero? ;
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---reveal---
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:ll:for-each
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&Action [ !Action [ perform at-end? ] until drop ] v:preserve ;
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~~~
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# Undocumented
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~~~
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:ll:length
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#0 swap [ [ n:inc ] dip ll:next dup fetch n:zero? ] until drop
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n:dec #1 n:max ;
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~~~
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Tests
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```
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:< ll:node-n ;
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:> ll:node-n ll:append ;
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#1 < #2 > #3 > #5 > #1 > #2 > #3 > [ ll:head fetch n:put sp ] ll:for-each
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```
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