mdw@git.distorted.org.uk Git - fringe/blob - forth-fringe.fth

   1 \ -*-forth-*-
   2 \
   3 \ Same-fringe solver in Forth.
   4
   5 \ ---------------------------------------------------------------------------
   6 \ Utilities.  Most of these are GForth-specific in some way.
   7
   8 \ String representation conversions.
   9
  10 : string>bounds ( c-addr u -- c-addr-limit c-addr )
  11     \ Convert a string in the usual base/length form to a limit/base form
  12     \ which is better suited to iteration.  The base is left on the top
  13     \ because it's likely to change more frequently.
  14     chars over + swap ;
  15
  16 : bounds>string ( c-addr-limit c-addr -- c-addr u )
  17     \ Convert a string in limit/base form back to base/length form.
  18     tuck - [ 1 chars ] literal / ;
  19
  20 \ Program name.  Want the portion after the rightmost `/'.
  21 \
  22 \ Bodge: gforth doesn't want to hand over the image filename so we'll have to
  23 \ hardwire.
  24
  25 : quis s" forth-fringe" ;
  26
  27 \ Error reporting.
  28
  29 : ouch ( a-addr u -- program exits )
  30     \ Report an error message on stderr and exit with a nonzero status.
  31     quis stderr write-file drop
  32     s" : " stderr write-file drop
  33     2dup stderr write-line drop
  34     1 (bye)                             \ Gforth specific
  35 ;
  36
  37 \ ---------------------------------------------------------------------------
  38 \ Coroutines.  Largely very scary.
  39
  40 \ A coroutine descriptor consists of a single cell containing the coroutine's
  41 \ return-stack pointer.  This cell is only valid when the coroutine is
  42 \ inactive.
  43 \
  44 \ Coroutines have distinct return stacks, but share the main value stack and
  45 \ floating-point stack, which they can use for communication with other
  46 \ coroutines.  A coroutine will therefore typically stash state on the return
  47 \ stack.
  48 \
  49 \ There's no current provision for Gforth's separate locals stack.
  50
  51 \ The amount of return-stack storage we allocate to a coroutine.
  52 256 cells constant cr-space
  53
  54 \ The current coroutine.  This initially points to an uninitialized
  55 \ descriptor which we'll fill in during the first coroutine switch.
  56 variable current-cr
  57 here current-cr ! cell allot
  58
  59 \ The coroutine which invoked this one.  This is used by `yield'.
  60 variable caller-cr
  61
  62 : switch-cr ( cr -- )
  63     \ Make `cr' the current coroutine, and tell it that it was called by this
  64     \ one.
  65     rp@ current-cr @ !
  66     current-cr @ caller-cr !
  67     dup current-cr !
  68     @ rp!
  69 ;
  70
  71 : yield ( -- )
  72     \ Make the calling coroutine current again.
  73     caller-cr @ switch-cr
  74 ;
  75
  76 : start-cr ( cr xt -- )
  77     \ Switch to the new coroutine `cr', and have it execute the token `xt'.
  78     swap
  79     rp@ current-cr @ !
  80     current-cr @ caller-cr !
  81     dup current-cr !
  82     @ rp!
  83     execute
  84 ;
  85
  86 : init-cr ( a-addr -- cr )
  87     \ Initialize a chunk of memory at `a-addr' and turn it into a pointer to
  88     \ a coroutine descriptor `cr' ready for use by `start-cr'.
  89     [ cr-space cell - ] literal +
  90     dup dup !
  91 ;
  92
  93 : [alloc-cr] ( -- cr ; R: -- cr-sys )
  94     \ Compile-time word: adjust the return stack pointer, returning a
  95     \ coroutine descriptor `cr'.  The space can be recovered using
  96     \ `[drop-cr]'.  This must be done at compile time, because returning is
  97     \ hard after you've messed with the return stack pointer.
  98     postpone rp@ postpone cr-space postpone - postpone dup
  99     postpone rp! postpone init-cr
 100 ; immediate
 101
 102 : [drop-cr] ( cr -- ; R: cr-sys -- )
 103     \ Compile-time word: adjust the return-stack pointer to reclaim the space
 104     \ used for the coroutine `cr' and all those above it on the return stack.
 105     postpone cell postpone + postpone rp!
 106 ; immediate
 107
 108 \ ---------------------------------------------------------------------------
 109 \ Iterator protocol.
 110 \
 111 \ An iterator is a coroutine which yields a word and a flag.  While there are
 112 \ items available, it yields items paired with `true' flags; when all items
 113 \ are exhausted, it yields a word and a `false' flag.  After that, invoking
 114 \ the coroutine again is invalid.
 115
 116 : print-iterator ( cr -- )
 117     \ Print the characters returned by the iterator coroutine `cr'.
 118     begin dup switch-cr while emit repeat
 119     drop
 120 ;
 121
 122 : same-iterators-p ( cr0 cr1 -- f )
 123     \ Report true if the iterator coroutines `cr0' and `cr1' return the same
 124     \ items in the same order, as determined by `='.
 125     begin
 126         over switch-cr ( cr0 cr1 x0 f0 )
 127         2 pick switch-cr ( cr0 cr1 x0 f0 x1 f1 )
 128         rot ( cr0 cr1 x0 x1 f1 f0 )
 129         over <> if 2drop 2drop drop false exit then
 130         0= if 2drop 2drop true exit then
 131         <> if 2drop false exit then
 132     again
 133 ;
 134
 135 \ ---------------------------------------------------------------------------
 136 \ Binary trees.
 137
 138 : make-tree ( a-addr-left w-datum a-addr-right -- a-addr-tree )
 139     \ Construct a binary tree from components on the stack, returning the
 140     \ address of the tree node.
 141     here >r                             \ stash pointer
 142     swap rot , , ,                      \ reorder and store
 143     r>                                  \ recover pointer
 144 ;
 145
 146 \ A leaf is an empty tree.  The address of this variable is important; its
 147 \ contents are not.
 148 variable leaf
 149
 150 \ Binary tree structure.
 151 : tree-left ( a-addr -- a-addr' ) ;
 152 : tree-datum ( a-addr -- a-addr' ) cell+ ;
 153 : tree-right ( a-addr -- a-addr' ) [ 2 cells ] literal + ;
 154 3 constant tree-ncells
 155
 156 : parse-subtree ( c-addr-limit c-addr -- c-addr-limit c-addr' tree )
 157     \ Parse a subtree from the string on the stack (in limit/base form).
 158     \ Update the string to reflect how much we consumed, and leave the tree
 159     \ address for the caller.  See `parse-tree' for the syntax.
 160     2dup > if dup c@ [char] ( <> else true then if
 161         leaf
 162     else
 163         char+
 164         leaf 0 leaf make-tree >r
 165         recurse r@ tree-left !
 166         2dup <= if s" no data" ouch then
 167         dup c@ r@ tree-datum ! char+
 168         recurse r@ tree-right !
 169         2dup <= if true else dup c@ [char] ) <> then if
 170             s" missing )" ouch
 171         then
 172         char+
 173         r>
 174     then
 175 ;
 176
 177 : parse-tree ( c-addr u -- tree )
 178     \ Parse a tree from the string on the stack.
 179     \
 180     \ The syntax is simple:
 181     \
 182     \   tree :: empty | `(' tree char tree `)'
 183     \
 184     \ The ambiguity is resolved by always treating `(' as a tree when a tree
 185     \ is expected.
 186     string>bounds
 187     parse-subtree >r
 188     <> if s" trailing junk" ouch then
 189     r>
 190 ;
 191
 192 : do-tree-fringe ( tree -- yields: x f )
 193     \ Helper word for `tree-fringe' below.  Recursively yields up the items
 194     \ of the subtree rooted at `tree'.
 195     dup leaf = if
 196         drop
 197     else
 198         >r
 199         r@ tree-left @ recurse
 200         r@ tree-datum @ true yield
 201         r> tree-right @ recurse
 202     then
 203 ;
 204
 205 : tree-fringe ( tree -- yields: x f )
 206     \ Yield up the items of `tree' in order, according to the iteration
 207     \ protocol.
 208     >r yield
 209     r> do-tree-fringe
 210     0 false yield
 211 ;
 212
 213 \ ---------------------------------------------------------------------------
 214 \ Main program.
 215
 216 : main
 217     \ Main program: parse arguments and do what's asked for.
 218     argc @ case
 219
 220         2 of
 221             \ One proper argument: parse a tree and print its fringe.
 222             [alloc-cr]
 223             1 arg parse-tree over ['] tree-fringe start-cr
 224             dup print-iterator cr
 225             [drop-cr]
 226         endof
 227
 228         3 of
 229             \ Two arguments: parse two trees and compare them.
 230             [alloc-cr] 1 arg parse-tree over ['] tree-fringe start-cr
 231             dup
 232             [alloc-cr] 2 arg parse-tree over ['] tree-fringe start-cr
 233             same-iterators-p
 234             swap [drop-cr]
 235             if ." match" else ." no match" then cr
 236         endof
 237
 238         \ Default.
 239         s" bad args" ouch
 240
 241     endcase
 242 ;
 243
 244 \ Gforth image magic.
 245 :noname
 246     defers 'cold
 247     main
 248     bye
 249 ; is 'cold
 250
 251 \ ---------------------------------------------------------------------------