From 67ad47b7f86d97f75808e1a9336d72d96d18dd2c Mon Sep 17 00:00:00 2001
From: Mark Wooding <mdw@distorted.org.uk>
Date: Mon, 5 Jul 2010 22:56:11 +0100
Subject: [PATCH] forth: New language.

Very unpleasant coroutine hacking.

I had a neat word for parsing the basename out of argv[0], but Gforth
doesn't actually let you get at the image name, so it just said `gforth'
which isn't very helpful, really.
---
 Makefile         |   9 ++
 forth-fringe.fth | 251 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 260 insertions(+)
 create mode 100644 forth-fringe.fth

diff --git a/Makefile b/Makefile
index e83ae5e..4f41be2 100644
--- a/Makefile
+++ b/Makefile
@@ -179,6 +179,15 @@ smalltalk-fringe:
 	$(V_HIDE)chmod +x $@.new && mv $@.new $@
 
 ###--------------------------------------------------------------------------
+### Forth.
+
+LANGS			+= forth
+TARGETS			+= forth-fringe
+SOURCES			+= forth-fringe.fth
+forth-fringe: forth-fringe.fth
+	$(call v_echo,FORTHI)gforthmi $@ $<
+
+###--------------------------------------------------------------------------
 ### Erlang.
 
 ERLC			 = erlc
diff --git a/forth-fringe.fth b/forth-fringe.fth
new file mode 100644
index 0000000..bab2eda
--- /dev/null
+++ b/forth-fringe.fth
@@ -0,0 +1,251 @@
+\ -*-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" ;
+
+\ 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
+
+\ 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 ! cell 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 @ !
+    current-cr @ caller-cr !
+    dup current-cr !
+    @ 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 @ !
+    current-cr @ caller-cr !
+    dup current-cr !
+    @ 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 cell - ] literal +
+    dup dup !
+;
+
+: [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] ( cr -- ; R: cr-sys -- )
+    \ Compile-time word: adjust the return-stack pointer to reclaim the space
+    \ used for the coroutine `cr' and all those above it on the return stack.
+    postpone cell 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.
+
+: 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
+;
+
+\ A leaf is an empty tree.  The address of this variable is important; its
+\ contents are not.
+variable leaf
+
+\ Binary tree structure.
+: tree-left ( a-addr -- a-addr' ) ;
+: tree-datum ( a-addr -- a-addr' ) cell+ ;
+: tree-right ( a-addr -- a-addr' ) [ 2 cells ] literal + ;
+3 constant tree-ncells
+
+: 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
+	    dup 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
+	    dup
+	    [alloc-cr] 2 arg parse-tree over ['] tree-fringe start-cr
+	    same-iterators-p
+	    swap [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
+
+\ ---------------------------------------------------------------------------
-- 
2.11.0