\ -*-forth-*-
\
\ Same-fringe solver in Forth.

\ ---------------------------------------------------------------------------
\ Utilities.  Most of these are GForth-specific in some way.

\ String representation conversions.

: string>bounds ( c-addr u -- c-addr-limit c-addr )
    \ Convert a string in the usual base/length form to a limit/base form
    \ which is better suited to iteration.  The base is left on the top
    \ because it's likely to change more frequently.
    chars over + swap ;

: bounds>string ( c-addr-limit c-addr -- c-addr u )
    \ Convert a string in limit/base form back to base/length form.
    tuck - [ 1 chars ] literal / ;

\ Program name.  Want the portion after the rightmost `/'.
\
\ Bodge: gforth doesn't want to hand over the image filename so we'll have to
\ hardwire.

: quis s" forth-fringe" ;

\ Structures.

: defstruct ( -- struct-sys )
    \ Commence a new structure.
    0 ;

: slot ( "name" struct-sys u -- struct-sys' )
    \ Add a new slot called `name', `u' units in size.  The word `name'
    \ applies the necessary offset to find the slot given the structure's
    \ base address.
    create over , + does> @ + ;

: endstruct ( "name" struct-sys' -- )
    \ End a structure definition.  The word `name' becomes a constant
    \ containing the requires size of the structure.
    create , does> @ ;

\ Error reporting.

: ouch ( a-addr u -- program exits )
    \ Report an error message on stderr and exit with a nonzero status.
    quis stderr write-file drop
    s" : " stderr write-file drop
    2dup stderr write-line drop
    1 (bye)				\ Gforth specific
;

\ ---------------------------------------------------------------------------
\ Coroutines.  Largely very scary.

\ A coroutine descriptor consists of a single cell containing the coroutine's
\ return-stack pointer.  This cell is only valid when the coroutine is
\ inactive.
\
\ Coroutines have distinct return stacks, but share the main value stack and
\ floating-point stack, which they can use for communication with other
\ coroutines.  A coroutine will therefore typically stash state on the return
\ stack.
\
\ There's no current provision for Gforth's separate locals stack.

\ The amount of return-stack storage we allocate to a coroutine.
256 cells constant cr-space

\ Coroutine descriptors.
defstruct
    cell slot cr-sp
endstruct cr-size

\ The current coroutine.  This initially points to an uninitialized
\ descriptor which we'll fill in during the first coroutine switch.
variable current-cr
here current-cr ! cr-size allot

\ The coroutine which invoked this one.  This is used by `yield'.
variable caller-cr

: switch-cr ( cr -- )
    \ Make `cr' the current coroutine, and tell it that it was called by this
    \ one.
    rp@ current-cr @ cr-sp !
    current-cr @ caller-cr !
    dup current-cr !
    cr-sp @ rp!
;

: yield ( -- )
    \ Make the calling coroutine current again.
    caller-cr @ switch-cr
;

: start-cr ( cr xt -- )
    \ Switch to the new coroutine `cr', and have it execute the token `xt'.
    swap
    rp@ current-cr @ cr-sp !
    current-cr @ caller-cr !
    dup current-cr !
    cr-sp @ rp!
    execute
;

: init-cr ( a-addr -- cr )
    \ Initialize a chunk of memory at `a-addr' and turn it into a pointer to
    \ a coroutine descriptor `cr' ready for use by `start-cr'.
    [ cr-space cr-size - ] literal +
    dup dup cr-sp !
;

: [alloc-cr] ( -- cr ; R: -- cr-sys )
    \ Compile-time word: adjust the return stack pointer, returning a
    \ coroutine descriptor `cr'.  The space can be recovered using
    \ `[drop-cr]'.  This must be done at compile time, because returning is
    \ hard after you've messed with the return stack pointer.
    postpone rp@ postpone cr-space postpone - postpone dup
    postpone rp! postpone init-cr
; immediate

: [drop-cr] ( R: cr-sys -- )
    \ Compile-time word: adjust the return-stack pointer to reclaim the space
    \ used by a coroutine.
    postpone rp@ postpone cr-space postpone + postpone rp!
; immediate

\ ---------------------------------------------------------------------------
\ Iterator protocol.
\
\ An iterator is a coroutine which yields a word and a flag.  While there are
\ items available, it yields items paired with `true' flags; when all items
\ are exhausted, it yields a word and a `false' flag.  After that, invoking
\ the coroutine again is invalid.

: print-iterator ( cr -- )
    \ Print the characters returned by the iterator coroutine `cr'.
    begin dup switch-cr while emit repeat
    drop
;

: same-iterators-p ( cr0 cr1 -- f )
    \ Report true if the iterator coroutines `cr0' and `cr1' return the same
    \ items in the same order, as determined by `='.
    begin
	over switch-cr ( cr0 cr1 x0 f0 )
	2 pick switch-cr ( cr0 cr1 x0 f0 x1 f1 )
	rot ( cr0 cr1 x0 x1 f1 f0 )
	over <> if 2drop 2drop drop false exit then
	0= if 2drop 2drop true exit then
	<> if 2drop false exit then
    again
;

\ ---------------------------------------------------------------------------
\ Binary trees.

\ A leaf is an empty tree.  The address of this variable is important; its
\ contents are not.
variable leaf

\ Binary tree structure.
defstruct
    cell slot tree-left
    cell slot tree-datum
    cell slot tree-right
endstruct tree-size

: make-tree ( a-addr-left w-datum a-addr-right -- a-addr-tree )
    \ Construct a binary tree from components on the stack, returning the
    \ address of the tree node.
    here >r				\ stash pointer
    swap rot , , ,			\ reorder and store
    r>					\ recover pointer
;

: parse-subtree ( c-addr-limit c-addr -- c-addr-limit c-addr' tree )
    \ Parse a subtree from the string on the stack (in limit/base form).
    \ Update the string to reflect how much we consumed, and leave the tree
    \ address for the caller.  See `parse-tree' for the syntax.
    2dup > if dup c@ [char] ( <> else true then if
	leaf
    else
	char+
	leaf 0 leaf make-tree >r
	recurse r@ tree-left !
	2dup <= if s" no data" ouch then
	dup c@ r@ tree-datum ! char+
	recurse r@ tree-right !
	2dup <= if true else dup c@ [char] ) <> then if
	    s" missing )" ouch
	then
	char+
	r>
    then
;

: parse-tree ( c-addr u -- tree )
    \ Parse a tree from the string on the stack.
    \
    \ The syntax is simple:
    \
    \	tree :: empty | `(' tree char tree `)'
    \
    \ The ambiguity is resolved by always treating `(' as a tree when a tree
    \ is expected.
    string>bounds
    parse-subtree >r
    <> if s" trailing junk" ouch then
    r>
;

: do-tree-fringe ( tree -- yields: x f )
    \ Helper word for `tree-fringe' below.  Recursively yields up the items
    \ of the subtree rooted at `tree'.
    dup leaf = if
	drop
    else
	>r
	r@ tree-left @ recurse
	r@ tree-datum @ true yield
	r> tree-right @ recurse
    then
;

: tree-fringe ( tree -- yields: x f )
    \ Yield up the items of `tree' in order, according to the iteration
    \ protocol.
    >r yield
    r> do-tree-fringe
    0 false yield
;

\ ---------------------------------------------------------------------------
\ Main program.

: main
    \ Main program: parse arguments and do what's asked for.
    argc @ case

	2 of
	    \ One proper argument: parse a tree and print its fringe.
	    [alloc-cr]
	    1 arg parse-tree over ['] tree-fringe start-cr
	    print-iterator cr
	    [drop-cr]
	endof

	3 of
	    \ Two arguments: parse two trees and compare them.
	    [alloc-cr] 1 arg parse-tree over ['] tree-fringe start-cr
	    [alloc-cr] 2 arg parse-tree over ['] tree-fringe start-cr
	    same-iterators-p
	    [drop-cr] [drop-cr]
	    if ." match" else ." no match" then cr
	endof

	\ Default.
	s" bad args" ouch

    endcase
;

\ Gforth image magic.
:noname
    defers 'cold
    main
    bye
; is 'cold

\ ---------------------------------------------------------------------------