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   1 /*
   2  * Copyright © 2016 Keith Packard <keithp@keithp.com>
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License as published by
   6  * the Free Software Foundation, either version 2 of the License, or
   7  * (at your option) any later version.
   8  *
   9  * This program is distributed in the hope that it will be useful, but
  10  * WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12  * General Public License for more details.
  13  */
  14
  15 #include "ao_scheme.h"
  16 #include <assert.h>
  17
  18 struct ao_scheme_stack          *ao_scheme_stack;
  19 ao_poly                         ao_scheme_v;
  20
  21 ao_poly
  22 ao_scheme_set_cond(struct ao_scheme_cons *c)
  23 {
  24         ao_scheme_stack->state = eval_cond;
  25         ao_scheme_stack->sexprs = ao_scheme_cons_poly(c);
  26         return AO_SCHEME_NIL;
  27 }
  28
  29 static int
  30 func_type(ao_poly func)
  31 {
  32         if (func == AO_SCHEME_NIL)
  33                 return ao_scheme_error(AO_SCHEME_INVALID, "func is nil");
  34         switch (ao_scheme_poly_type(func)) {
  35         case AO_SCHEME_BUILTIN:
  36                 return ao_scheme_poly_builtin(func)->args & AO_SCHEME_FUNC_MASK;
  37         case AO_SCHEME_LAMBDA:
  38                 return ao_scheme_poly_lambda(func)->args;
  39         case AO_SCHEME_STACK:
  40                 return AO_SCHEME_FUNC_LAMBDA;
  41         default:
  42                 ao_scheme_error(AO_SCHEME_INVALID, "not a func");
  43                 return -1;
  44         }
  45 }
  46
  47 /*
  48  * Flattened eval to avoid stack issues
  49  */
  50
  51 /*
  52  * Evaluate an s-expression
  53  *
  54  * For a list, evaluate all of the elements and
  55  * then execute the resulting function call.
  56  *
  57  * Each element of the list is evaluated in
  58  * a clean stack context.
  59  *
  60  * The current stack state is set to 'formal' so that
  61  * when the evaluation is complete, the value
  62  * will get appended to the values list.
  63  *
  64  * For other types, compute the value directly.
  65  */
  66
  67 static int
  68 ao_scheme_eval_sexpr(void)
  69 {
  70         DBGI("sexpr: %v\n", ao_scheme_v);
  71         switch (ao_scheme_poly_type(ao_scheme_v)) {
  72         case AO_SCHEME_CONS:
  73                 if (ao_scheme_v == AO_SCHEME_NIL) {
  74                         if (!ao_scheme_stack->values) {
  75                                 /*
  76                                  * empty list evaluates to empty list
  77                                  */
  78                                 ao_scheme_v = AO_SCHEME_NIL;
  79                                 ao_scheme_stack->state = eval_val;
  80                         } else {
  81                                 /*
  82                                  * done with arguments, go execute it
  83                                  */
  84                                 ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->values)->car;
  85                                 ao_scheme_stack->state = eval_exec;
  86                         }
  87                 } else {
  88                         if (!ao_scheme_stack->values)
  89                                 ao_scheme_stack->list = ao_scheme_v;
  90                         /*
  91                          * Evaluate another argument and then switch
  92                          * to 'formal' to add the value to the values
  93                          * list
  94                          */
  95                         ao_scheme_stack->sexprs = ao_scheme_v;
  96                         ao_scheme_stack->state = eval_formal;
  97                         if (!ao_scheme_stack_push())
  98                                 return 0;
  99                         /*
 100                          * push will reset the state to 'sexpr', which
 101                          * will evaluate the expression
 102                          */
 103                         ao_scheme_v = ao_scheme_poly_cons(ao_scheme_v)->car;
 104                 }
 105                 break;
 106         case AO_SCHEME_ATOM:
 107                 DBGI("..frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 108                 ao_scheme_v = ao_scheme_atom_get(ao_scheme_v);
 109                 /* fall through */
 110         default:
 111                 ao_scheme_stack->state = eval_val;
 112                 break;
 113         }
 114         DBGI(".. result "); DBG_POLY(ao_scheme_v); DBG("\n");
 115         return 1;
 116 }
 117
 118 /*
 119  * A value has been computed.
 120  *
 121  * If the value was computed from a macro,
 122  * then we want to reset the current context
 123  * to evaluate the macro result again.
 124  *
 125  * If not a macro, then pop the stack.
 126  * If the stack is empty, we're done.
 127  * Otherwise, the stack will contain
 128  * the next state.
 129  */
 130
 131 static int
 132 ao_scheme_eval_val(void)
 133 {
 134         DBGI("val: "); DBG_POLY(ao_scheme_v); DBG("\n");
 135         /*
 136          * Value computed, pop the stack
 137          * to figure out what to do with the value
 138          */
 139         ao_scheme_stack_pop();
 140         DBGI("..state %d\n", ao_scheme_stack ? ao_scheme_stack->state : -1);
 141         return 1;
 142 }
 143
 144 /*
 145  * A formal has been computed.
 146  *
 147  * If this is the first formal, then check to see if we've got a
 148  * lamda, macro or nlambda.
 149  *
 150  * For lambda, go compute another formal.  This will terminate
 151  * when the sexpr state sees nil.
 152  *
 153  * For macro/nlambda, we're done, so move the sexprs into the values
 154  * and go execute it.
 155  *
 156  * Macros have an additional step of saving a stack frame holding the
 157  * macro value execution context, which then gets the result of the
 158  * macro to run
 159  */
 160
 161 static int
 162 ao_scheme_eval_formal(void)
 163 {
 164         ao_poly                 formal;
 165         struct ao_scheme_stack  *prev;
 166
 167         DBGI("formal: "); DBG_POLY(ao_scheme_v); DBG("\n");
 168
 169         /* Check what kind of function we've got */
 170         if (!ao_scheme_stack->values) {
 171                 switch (func_type(ao_scheme_v)) {
 172                 case AO_SCHEME_FUNC_LAMBDA:
 173                         DBGI(".. lambda\n");
 174                         break;
 175                 case AO_SCHEME_FUNC_MACRO:
 176                         /* Evaluate the result once more */
 177                         ao_scheme_stack->state = eval_macro;
 178                         if (!ao_scheme_stack_push())
 179                                 return 0;
 180
 181                         /* After the function returns, take that
 182                          * value and re-evaluate it
 183                          */
 184                         prev = ao_scheme_poly_stack(ao_scheme_stack->prev);
 185                         ao_scheme_stack->sexprs = prev->sexprs;
 186
 187                         DBGI(".. start macro\n");
 188                         DBGI("\t.. sexprs       "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 189                         DBGI("\t.. values       "); DBG_POLY(ao_scheme_stack->values); DBG("\n");
 190                         DBG_FRAMES();
 191
 192                         /* fall through ... */
 193                 case AO_SCHEME_FUNC_NLAMBDA:
 194                         DBGI(".. nlambda or macro\n");
 195
 196                         /* use the raw sexprs as values */
 197                         ao_scheme_stack->values = ao_scheme_stack->sexprs;
 198                         ao_scheme_stack->values_tail = AO_SCHEME_NIL;
 199                         ao_scheme_stack->state = eval_exec;
 200
 201                         /* ready to execute now */
 202                         return 1;
 203                 case -1:
 204                         return 0;
 205                 }
 206         }
 207
 208         /* Append formal to list of values */
 209         formal = ao_scheme__cons(ao_scheme_v, AO_SCHEME_NIL);
 210         if (!formal)
 211                 return 0;
 212
 213         if (ao_scheme_stack->values_tail)
 214                 ao_scheme_poly_cons(ao_scheme_stack->values_tail)->cdr = formal;
 215         else
 216                 ao_scheme_stack->values = formal;
 217         ao_scheme_stack->values_tail = formal;
 218
 219         DBGI(".. values "); DBG_POLY(ao_scheme_stack->values); DBG("\n");
 220
 221         /*
 222          * Step to the next argument, if this is last, then
 223          * 'sexpr' will end up switching to 'exec'
 224          */
 225         ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->cdr;
 226
 227         ao_scheme_stack->state = eval_sexpr;
 228
 229         DBGI(".. "); DBG_POLY(ao_scheme_v); DBG("\n");
 230         return 1;
 231 }
 232
 233 /*
 234  * Start executing a function call
 235  *
 236  * Most builtins are easy, just call the function.
 237  * 'cond' is magic; it sticks the list of clauses
 238  * in 'sexprs' and switches to 'cond' state. That
 239  * bit of magic is done in ao_scheme_set_cond.
 240  *
 241  * Lambdas build a new frame to hold the locals and
 242  * then re-use the current stack context to evaluate
 243  * the s-expression from the lambda.
 244  */
 245
 246 static int
 247 ao_scheme_eval_exec(void)
 248 {
 249         ao_poly v;
 250         struct ao_scheme_builtin        *builtin;
 251
 252         DBGI("exec: "); DBG_POLY(ao_scheme_v); DBG(" values "); DBG_POLY(ao_scheme_stack->values); DBG ("\n");
 253         ao_scheme_stack->sexprs = AO_SCHEME_NIL;
 254         switch (ao_scheme_poly_type(ao_scheme_v)) {
 255         case AO_SCHEME_BUILTIN:
 256                 ao_scheme_stack->state = eval_val;
 257                 builtin = ao_scheme_poly_builtin(ao_scheme_v);
 258                 v = ao_scheme_func(builtin) (
 259                         ao_scheme_poly_cons(ao_scheme_poly_cons(ao_scheme_stack->values)->cdr));
 260                 DBG_DO(if (!ao_scheme_exception && ao_scheme_poly_builtin(ao_scheme_v)->func == builtin_set) {
 261                                 struct ao_scheme_cons *cons = ao_scheme_poly_cons(ao_scheme_stack->values);
 262                                 ao_poly atom = ao_scheme_arg(cons, 1);
 263                                 ao_poly val = ao_scheme_arg(cons, 2);
 264                                 DBGI("set "); DBG_POLY(atom); DBG(" = "); DBG_POLY(val); DBG("\n");
 265                         });
 266                 builtin = ao_scheme_poly_builtin(ao_scheme_v);
 267                 if (builtin && (builtin->args & AO_SCHEME_FUNC_FREE_ARGS) && !ao_scheme_stack_marked(ao_scheme_stack)) {
 268                         struct ao_scheme_cons *cons = ao_scheme_poly_cons(ao_scheme_stack->values);
 269                         ao_scheme_stack->values = AO_SCHEME_NIL;
 270                         ao_scheme_cons_free(cons);
 271                 }
 272
 273                 ao_scheme_v = v;
 274                 ao_scheme_stack->values = AO_SCHEME_NIL;
 275                 ao_scheme_stack->values_tail = AO_SCHEME_NIL;
 276                 DBGI(".. result "); DBG_POLY(ao_scheme_v); DBG ("\n");
 277                 DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 278                 break;
 279         case AO_SCHEME_LAMBDA:
 280                 DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 281                 ao_scheme_stack->state = eval_begin;
 282                 v = ao_scheme_lambda_eval();
 283                 ao_scheme_stack->sexprs = v;
 284                 ao_scheme_stack->values = AO_SCHEME_NIL;
 285                 ao_scheme_stack->values_tail = AO_SCHEME_NIL;
 286                 DBGI(".. sexprs "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 287                 DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 288                 break;
 289         case AO_SCHEME_STACK:
 290                 DBGI(".. stack "); DBG_POLY(ao_scheme_v); DBG("\n");
 291                 ao_scheme_v = ao_scheme_stack_eval();
 292                 DBGI(".. value "); DBG_POLY(ao_scheme_v); DBG("\n");
 293                 DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 294                 break;
 295         }
 296         return 1;
 297 }
 298
 299 /*
 300  * Finish setting up the apply evaluation
 301  *
 302  * The value is the list to execute
 303  */
 304 static int
 305 ao_scheme_eval_apply(void)
 306 {
 307         struct ao_scheme_cons   *cons = ao_scheme_poly_cons(ao_scheme_v);
 308         struct ao_scheme_cons   *cdr, *prev;
 309
 310         /* Glue the arguments into the right shape. That's all but the last
 311          * concatenated onto the last
 312          */
 313         cdr = cons;
 314         for (;;) {
 315                 prev = cdr;
 316                 cdr = ao_scheme_poly_cons(prev->cdr);
 317                 if (cdr->cdr == AO_SCHEME_NIL)
 318                         break;
 319         }
 320         DBGI("before mangling: "); DBG_POLY(ao_scheme_v); DBG("\n");
 321         prev->cdr = cdr->car;
 322         ao_scheme_stack->values = ao_scheme_v;
 323         ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->values)->car;
 324         DBGI("apply: "); DBG_POLY(ao_scheme_stack->values); DBG ("\n");
 325         ao_scheme_stack->state = eval_exec;
 326         ao_scheme_stack_mark(ao_scheme_stack);
 327         return 1;
 328 }
 329
 330 /*
 331  * Start evaluating the next cond clause
 332  *
 333  * If the list of clauses is empty, then
 334  * the result of the cond is nil.
 335  *
 336  * Otherwise, set the current stack state to 'cond_test' and create a
 337  * new stack context to evaluate the test s-expression. Once that's
 338  * complete, we'll land in 'cond_test' to finish the clause.
 339  */
 340 static int
 341 ao_scheme_eval_cond(void)
 342 {
 343         DBGI("cond: "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 344         DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 345         DBGI(".. saved frame "); DBG_POLY(ao_scheme_stack->frame); DBG("\n");
 346         if (!ao_scheme_stack->sexprs) {
 347                 ao_scheme_v = _ao_scheme_bool_false;
 348                 ao_scheme_stack->state = eval_val;
 349         } else {
 350                 ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->car;
 351                 if (!ao_scheme_v || !AO_SCHEME_IS_CONS(ao_scheme_v)) {
 352                         ao_scheme_error(AO_SCHEME_INVALID, "invalid cond clause");
 353                         return 0;
 354                 }
 355                 ao_scheme_v = ao_scheme_poly_cons(ao_scheme_v)->car;
 356                 if (ao_scheme_v == _ao_scheme_atom_else)
 357                         ao_scheme_v = _ao_scheme_bool_true;
 358                 ao_scheme_stack->state = eval_cond_test;
 359                 if (!ao_scheme_stack_push())
 360                         return 0;
 361         }
 362         return 1;
 363 }
 364
 365 /*
 366  * Finish a cond clause.
 367  *
 368  * Check the value from the test expression, if
 369  * non-nil, then set up to evaluate the value expression.
 370  *
 371  * Otherwise, step to the next clause and go back to the 'cond'
 372  * state
 373  */
 374 static int
 375 ao_scheme_eval_cond_test(void)
 376 {
 377         DBGI("cond_test: "); DBG_POLY(ao_scheme_v); DBG(" sexprs "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 378         DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 379         DBGI(".. saved frame "); DBG_POLY(ao_scheme_stack->frame); DBG("\n");
 380         if (ao_scheme_v != _ao_scheme_bool_false) {
 381                 struct ao_scheme_cons *car = ao_scheme_poly_cons(ao_scheme_poly_cons(ao_scheme_stack->sexprs)->car);
 382                 ao_poly c = car->cdr;
 383
 384                 if (c) {
 385                         ao_scheme_stack->state = eval_begin;
 386                         ao_scheme_stack->sexprs = c;
 387                 } else
 388                         ao_scheme_stack->state = eval_val;
 389         } else {
 390                 ao_scheme_stack->sexprs = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->cdr;
 391                 DBGI("next cond: "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 392                 ao_scheme_stack->state = eval_cond;
 393         }
 394         return 1;
 395 }
 396
 397 /*
 398  * Evaluate a list of sexprs, returning the value from the last one.
 399  *
 400  * ao_scheme_begin records the list in stack->sexprs, so we just need to
 401  * walk that list. Set ao_scheme_v to the car of the list and jump to
 402  * eval_sexpr. When that's done, it will land in eval_val. For all but
 403  * the last, leave a stack frame with eval_begin set so that we come
 404  * back here. For the last, don't add a stack frame so that we can
 405  * just continue on.
 406  */
 407 static int
 408 ao_scheme_eval_begin(void)
 409 {
 410         DBGI("begin: "); DBG_POLY(ao_scheme_v); DBG(" sexprs "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 411         DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 412         DBGI(".. saved frame "); DBG_POLY(ao_scheme_stack->frame); DBG("\n");
 413
 414         if (!ao_scheme_stack->sexprs) {
 415                 ao_scheme_v = AO_SCHEME_NIL;
 416                 ao_scheme_stack->state = eval_val;
 417         } else {
 418                 ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->car;
 419                 ao_scheme_stack->sexprs = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->cdr;
 420
 421                 /* If there are more sexprs to do, then come back here, otherwise
 422                  * return the value of the last one by just landing in eval_sexpr
 423                  */
 424                 if (ao_scheme_stack->sexprs) {
 425                         ao_scheme_stack->state = eval_begin;
 426                         if (!ao_scheme_stack_push())
 427                                 return 0;
 428                 }
 429                 ao_scheme_stack->state = eval_sexpr;
 430         }
 431         return 1;
 432 }
 433
 434 /*
 435  * Conditionally execute a list of sexprs while the first is true
 436  */
 437 static int
 438 ao_scheme_eval_while(void)
 439 {
 440         DBGI("while: "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 441         DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 442         DBGI(".. saved frame "); DBG_POLY(ao_scheme_stack->frame); DBG("\n");
 443
 444         ao_scheme_stack->values = ao_scheme_v;
 445         if (!ao_scheme_stack->sexprs) {
 446                 ao_scheme_v = AO_SCHEME_NIL;
 447                 ao_scheme_stack->state = eval_val;
 448         } else {
 449                 ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->car;
 450                 ao_scheme_stack->state = eval_while_test;
 451                 if (!ao_scheme_stack_push())
 452                         return 0;
 453         }
 454         return 1;
 455 }
 456
 457 /*
 458  * Check the while condition, terminate the loop if nil. Otherwise keep going
 459  */
 460 static int
 461 ao_scheme_eval_while_test(void)
 462 {
 463         DBGI("while_test: "); DBG_POLY(ao_scheme_v); DBG(" sexprs "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 464         DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 465         DBGI(".. saved frame "); DBG_POLY(ao_scheme_stack->frame); DBG("\n");
 466
 467         if (ao_scheme_v != _ao_scheme_bool_false) {
 468                 ao_scheme_stack->values = ao_scheme_v;
 469                 ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->cdr;
 470                 ao_scheme_stack->state = eval_while;
 471                 if (!ao_scheme_stack_push())
 472                         return 0;
 473                 ao_scheme_stack->state = eval_begin;
 474                 ao_scheme_stack->sexprs = ao_scheme_v;
 475         }
 476         else
 477         {
 478                 ao_scheme_stack->state = eval_val;
 479                 ao_scheme_v = ao_scheme_stack->values;
 480         }
 481         return 1;
 482 }
 483
 484 /*
 485  * Replace the original sexpr with the macro expansion, then
 486  * execute that
 487  */
 488 static int
 489 ao_scheme_eval_macro(void)
 490 {
 491         DBGI("macro: "); DBG_POLY(ao_scheme_v); DBG(" sexprs "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 492
 493         if (ao_scheme_v == AO_SCHEME_NIL)
 494                 ao_scheme_abort();
 495         if (AO_SCHEME_IS_CONS(ao_scheme_v)) {
 496                 *ao_scheme_poly_cons(ao_scheme_stack->sexprs) = *ao_scheme_poly_cons(ao_scheme_v);
 497                 ao_scheme_v = ao_scheme_stack->sexprs;
 498                 DBGI("sexprs rewritten to: "); DBG_POLY(ao_scheme_v); DBG("\n");
 499         }
 500         ao_scheme_stack->sexprs = AO_SCHEME_NIL;
 501         ao_scheme_stack->state = eval_sexpr;
 502         return 1;
 503 }
 504
 505 static int (*const evals[])(void) = {
 506         [eval_sexpr] = ao_scheme_eval_sexpr,
 507         [eval_val] = ao_scheme_eval_val,
 508         [eval_formal] = ao_scheme_eval_formal,
 509         [eval_exec] = ao_scheme_eval_exec,
 510         [eval_apply] = ao_scheme_eval_apply,
 511         [eval_cond] = ao_scheme_eval_cond,
 512         [eval_cond_test] = ao_scheme_eval_cond_test,
 513         [eval_begin] = ao_scheme_eval_begin,
 514         [eval_while] = ao_scheme_eval_while,
 515         [eval_while_test] = ao_scheme_eval_while_test,
 516         [eval_macro] = ao_scheme_eval_macro,
 517 };
 518
 519 const char * const ao_scheme_state_names[] = {
 520         [eval_sexpr] = "sexpr",
 521         [eval_val] = "val",
 522         [eval_formal] = "formal",
 523         [eval_exec] = "exec",
 524         [eval_apply] = "apply",
 525         [eval_cond] = "cond",
 526         [eval_cond_test] = "cond_test",
 527         [eval_begin] = "begin",
 528         [eval_while] = "while",
 529         [eval_while_test] = "while_test",
 530         [eval_macro] = "macro",
 531 };
 532
 533 /*
 534  * Called at restore time to reset all execution state
 535  */
 536
 537 void
 538 ao_scheme_eval_clear_globals(void)
 539 {
 540         ao_scheme_stack = NULL;
 541         ao_scheme_frame_current = NULL;
 542         ao_scheme_v = AO_SCHEME_NIL;
 543 }
 544
 545 int
 546 ao_scheme_eval_restart(void)
 547 {
 548         return ao_scheme_stack_push();
 549 }
 550
 551 ao_poly
 552 ao_scheme_eval(ao_poly _v)
 553 {
 554         ao_scheme_v = _v;
 555
 556         ao_scheme_frame_init();
 557
 558         if (!ao_scheme_stack_push())
 559                 return AO_SCHEME_NIL;
 560
 561         while (ao_scheme_stack) {
 562                 if (!(*evals[ao_scheme_stack->state])() || ao_scheme_exception) {
 563                         ao_scheme_stack_clear();
 564                         return AO_SCHEME_NIL;
 565                 }
 566         }
 567         DBG_DO(if (ao_scheme_frame_current) {DBGI("frame left as "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");});
 568         ao_scheme_frame_current = NULL;
 569         return ao_scheme_v;
 570 }