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* 3966Kartik K. Agaram2017-07-091-1/+1
* 3536Kartik K. Agaram2016-10-201-0/+4
* 3181Kartik K. Agaram2016-08-131-32/+0
* 3157 - more testable 'random' interfaceKartik K. Agaram2016-07-271-4/+4
* 3154 - reorg before making 'random' more testableKartik K. Agaram2016-07-271-0/+62
n65' href='#n65'>65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781
# A doubly linked list permits bidirectional traversal.

container duplex-list:_elem [
  value:_elem
  next:&:duplex-list:_elem
  prev:&:duplex-list:_elem
]

def push x:_elem, in:&:duplex-list:_elem/contained-in:result -> result:&:duplex-list:_elem [
  local-scope
  load-inputs
  result:&:duplex-list:_elem <- new {(duplex-list _elem): type}
  *result <- merge x, in, null
  return-unless in
  put *in, prev:offset, result
]

def first in:&:duplex-list:_elem -> result:_elem [
  local-scope
  load-inputs
  {
    break-if in
    zero:&:_elem <- new _elem:type
    zero-result:_elem <- copy *zero
    abandon zero
    return zero-result
  }
  result <- get *in, value:offset
]

def next in:&:duplex-list:_elem -> result:&:duplex-list:_elem/contained-in:in [
  local-scope
  load-inputs
  return-unless in, null
  result <- get *in, next:offset
]

def prev in:&:duplex-list:_elem -> result:&:duplex-list:_elem/contained-in:in [
  local-scope
  load-inputs
  return-unless in, null
  result <- get *in, prev:offset
  return result
]

scenario duplex-list-handling [
  run [
    local-scope
    # reserve locations 0-9 to check for missing null check
    10:num/raw <- copy 34
    11:num/raw <- copy 35
    list:&:duplex-list:num <- push 3, null
    list <- push 4, list
    list <- push 5, list
    list2:&:duplex-list:num <- copy list
    20:num/raw <- first list2
    list2 <- next list2
    21:num/raw <- first list2
    list2 <- next list2
    22:num/raw <- first list2
    30:&:duplex-list:num/raw <- next list2
    31:num/raw <- first 30:&:duplex-list:num/raw
    32:&:duplex-list:num/raw <- next 30:&:duplex-list:num/raw
    33:&:duplex-list:num/raw <- prev 30:&:duplex-list:num/raw
    list2 <- prev list2
    40:num/raw <- first list2
    list2 <- prev list2
    41:num/raw <- first list2
    50:bool/raw <- equal list, list2
  ]
  memory-should-contain [
    0 <- 0  # no modifications to null pointers
    10 <- 34
    11 <- 35
    20 <- 5  # scanning next
    21 <- 4
    22 <- 3
    30 <- 0  # null
    31 <- 0  # first of null
    32 <- 0  # next of null
    33 <- 0  # prev of null
    40 <- 4  # then start scanning prev
    41 <- 5
    50 <- 1  # list back at start
  ]
]

def length l:&:duplex-list:_elem -> result:num [
  local-scope
  load-inputs
  result <- copy 0
  {
    break-unless l
    result <- add result, 1
    l <- next l
    loop
  }
]

# insert 'x' after 'in'
def insert x:_elem, in:&:duplex-list:_elem -> in:&:duplex-list:_elem [
  local-scope
  load-inputs
  new-node:&:duplex-list:_elem <- new {(duplex-list _elem): type}
  *new-node <- put *new-node, value:offset, x
  # save old next before changing it
  next-node:&:duplex-list:_elem <- get *in, next:offset
  *in <- put *in, next:offset, new-node
  *new-node <- put *new-node, prev:offset, in
  *new-node <- put *new-node, next:offset, next-node
  return-unless next-node
  *next-node <- put *next-node, prev:offset, new-node
]

scenario inserting-into-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  list <- push 4, list
  list <- push 5, list
  run [
    list2:&:duplex-list:num <- next list  # inside list
    list2 <- insert 6, list2
    # check structure like before
    list2 <- copy list
    10:num/raw <- first list2
    list2 <- next list2
    11:num/raw <- first list2
    list2 <- next list2
    12:num/raw <- first list2
    list2 <- next list2
    13:num/raw <- first list2
    list2 <- prev list2
    20:num/raw <- first list2
    list2 <- prev list2
    21:num/raw <- first list2
    list2 <- prev list2
    22:num/raw <- first list2
    30:bool/raw <- equal list, list2
  ]
  memory-should-contain [
    10 <- 5  # scanning next
    11 <- 4
    12 <- 6  # inserted element
    13 <- 3
    20 <- 6  # then prev
    21 <- 4
    22 <- 5
    30 <- 1  # list back at start
  ]
]

scenario inserting-at-end-of-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  list <- push 4, list
  list <- push 5, list
  run [
    list2:&:duplex-list:num <- next list  # inside list
    list2 <- next list2  # now at end of list
    list2 <- insert 6, list2
    # check structure like before
    list2 <- copy list
    10:num/raw <- first list2
    list2 <- next list2
    11:num/raw <- first list2
    list2 <- next list2
    12:num/raw <- first list2
    list2 <- next list2
    13:num/raw <- first list2
    list2 <- prev list2
    20:num/raw <- first list2
    list2 <- prev list2
    21:num/raw <- first list2
    list2 <- prev list2
    22:num/raw <- first list2
    30:bool/raw <- equal list, list2
  ]
  memory-should-contain [
    10 <- 5  # scanning next
    11 <- 4
    12 <- 3
    13 <- 6  # inserted element
    20 <- 3  # then prev
    21 <- 4
    22 <- 5
    30 <- 1  # list back at start
  ]
]

scenario inserting-after-start-of-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  list <- push 4, list
  list <- push 5, list
  run [
    list <- insert 6, list
    # check structure like before
    list2:&:duplex-list:num <- copy list
    10:num/raw <- first list2
    list2 <- next list2
    11:num/raw <- first list2
    list2 <- next list2
    12:num/raw <- first list2
    list2 <- next list2
    13:num/raw <- first list2
    list2 <- prev list2
    20:num/raw <- first list2
    list2 <- prev list2
    21:num/raw <- first list2
    list2 <- prev list2
    22:num/raw <- first list2
    30:bool/raw <- equal list, list2
  ]
  memory-should-contain [
    10 <- 5  # scanning next
    11 <- 6  # inserted element
    12 <- 4
    13 <- 3
    20 <- 4  # then prev
    21 <- 6
    22 <- 5
    30 <- 1  # list back at start
  ]
]

# remove 'x' from its surrounding list 'in'
#
# Returns null if and only if list is empty. Beware: in that case any other
# pointers to the head are now invalid.
def remove x:&:duplex-list:_elem/contained-in:in, in:&:duplex-list:_elem -> in:&:duplex-list:_elem [
  local-scope
  load-inputs
  # if 'x' is null, return
  return-unless x
  next-node:&:duplex-list:_elem <- get *x, next:offset
  prev-node:&:duplex-list:_elem <- get *x, prev:offset
  # null x's pointers
  *x <- put *x, next:offset, null
  *x <- put *x, prev:offset, null
  # if next-node is not null, set its prev pointer
  {
    break-unless next-node
    *next-node <- put *next-node, prev:offset, prev-node
  }
  # if prev-node is not null, set its next pointer and return
  {
    break-unless prev-node
    *prev-node <- put *prev-node, next:offset, next-node
    return
  }
  # if prev-node is null, then we removed the head node at 'in'
  # return the new head rather than the old 'in'
  return next-node
]

scenario removing-from-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  list <- push 4, list
  list <- push 5, list
  run [
    list2:&:duplex-list:num <- next list  # second element
    list <- remove list2, list
    10:bool/raw <- equal list2, null
    # check structure like before
    list2 <- copy list
    11:num/raw <- first list2
    list2 <- next list2
    12:num/raw <- first list2
    20:&:duplex-list:num/raw <- next list2
    list2 <- prev list2
    30:num/raw <- first list2
    40:bool/raw <- equal list, list2
  ]
  memory-should-contain [
    10 <- 0  # remove returned non-null
    11 <- 5  # scanning next, skipping deleted element
    12 <- 3
    20 <- 0  # no more elements
    30 <- 5  # prev of final element
    40 <- 1  # list back at start
  ]
]

scenario removing-from-start-of-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  list <- push 4, list
  list <- push 5, list
  run [
    list <- remove list, list
    # check structure like before
    list2:&:duplex-list:num <- copy list
    10:num/raw <- first list2
    list2 <- next list2
    11:num/raw <- first list2
    20:&:duplex-list:num/raw <- next list2
    list2 <- prev list2
    30:num/raw <- first list2
    40:bool/raw <- equal list, list2
  ]
  memory-should-contain [
    10 <- 4  # scanning next, skipping deleted element
    11 <- 3
    20 <- 0  # no more elements
    30 <- 4  # prev of final element
    40 <- 1  # list back at start
  ]
]

scenario removing-from-end-of-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  list <- push 4, list
  list <- push 5, list
  run [
    # delete last element
    list2:&:duplex-list:num <- next list
    list2 <- next list2
    list <- remove list2, list
    10:bool/raw <- equal list2, null
    # check structure like before
    list2 <- copy list
    11:num/raw <- first list2
    list2 <- next list2
    12:num/raw <- first list2
    20:&:duplex-list:num/raw <- next list2
    list2 <- prev list2
    30:num/raw <- first list2
    40:bool/raw <- equal list, list2
  ]
  memory-should-contain [
    10 <- 0  # remove returned non-null
    11 <- 5  # scanning next, skipping deleted element
    12 <- 4
    20 <- 0  # no more elements
    30 <- 5  # prev of final element
    40 <- 1  # list back at start
  ]
]

scenario removing-from-singleton-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  run [
    list <- remove list, list
    1:num/raw <- deaddress list
  ]
  memory-should-contain [
    1 <- 0  # back to an empty list
  ]
]

def remove x:&:duplex-list:_elem/contained-in:in, n:num, in:&:duplex-list:_elem -> in:&:duplex-list:_elem [
  local-scope
  load-inputs
  i:num <- copy 0
  curr:&:duplex-list:_elem <- copy x
  {
    done?:bool <- greater-or-equal i, n
    break-if done?
    break-unless curr
    next:&:duplex-list:_elem <- next curr
    in <- remove curr, in
    curr <- copy next
    i <- add i, 1
    loop
  }
]

scenario removing-multiple-from-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 3, null
  list <- push 4, list
  list <- push 5, list
  run [
    list2:&:duplex-list:num <- next list  # second element
    list <- remove list2, 2, list
    stash list
  ]
  trace-should-contain [
    app: 5
  ]
]

# remove values between 'start' and 'end' (both exclusive).
# also clear pointers back out from start/end for hygiene.
# set end to 0 to delete everything past start.
# can't set start to 0 to delete everything before end, because there's no
# clean way to return the new head pointer.
def remove-between start:&:duplex-list:_elem, end:&:duplex-list:_elem/contained-in:start -> start:&:duplex-list:_elem [
  local-scope
  load-inputs
  next:&:duplex-list:_elem <- get *start, next:offset
  nothing-to-delete?:bool <- equal next, end
  return-if nothing-to-delete?
  assert next, [malformed duplex list]
  # start->next->prev = 0
  # start->next = end
  *next <- put *next, prev:offset, null
  *start <- put *start, next:offset, end
  {
    break-if end
    stash [spliced:] next
    return
  }
  # end->prev->next = 0
  # end->prev = start
  prev:&:duplex-list:_elem <- get *end, prev:offset
  assert prev, [malformed duplex list - 2]
  *prev <- put *prev, next:offset, null
  stash [spliced:] next
  *end <- put *end, prev:offset, start
]

scenario remove-range [
  # construct a duplex list with six elements [13, 14, 15, 16, 17, 18]
  local-scope
  list:&:duplex-list:num <- push 18, null
  list <- push 17, list
  list <- push 16, list
  list <- push 15, list
  list <- push 14, list
  list <- push 13, list
  run [
    # delete 16 onwards
    # first pointer: to the third element
    list2:&:duplex-list:num <- next list
    list2 <- next list2
    list2 <- remove-between list2, null
    # now check the list
    10:num/raw <- get *list, value:offset
    list <- next list
    11:num/raw <- get *list, value:offset
    list <- next list
    12:num/raw <- get *list, value:offset
    20:&:duplex-list:num/raw <- next list
  ]
  memory-should-contain [
    10 <- 13
    11 <- 14
    12 <- 15
    20 <- 0
  ]
  trace-should-contain [
    app: spliced: 16 <-> 17 <-> 18
  ]
]

scenario remove-range-to-final [
  local-scope
  # construct a duplex list with six elements [13, 14, 15, 16, 17, 18]
  list:&:duplex-list:num <- push 18, null
  list <- push 17, list
  list <- push 16, list
  list <- push 15, list
  list <- push 14, list
  list <- push 13, list
  run [
    # delete 15, 16 and 17
    # start pointer: to the second element
    list2:&:duplex-list:num <- next list
    # end pointer: to the last (sixth) element
    end:&:duplex-list:num <- next list2
    end <- next end
    end <- next end
    end <- next end
    remove-between list2, end
    # now check the list
    10:num/raw <- get *list, value:offset
    list <- next list
    11:num/raw <- get *list, value:offset
    list <- next list
    12:num/raw <- get *list, value:offset
    20:&:duplex-list:num/raw <- next list
  ]
  memory-should-contain [
    10 <- 13
    11 <- 14
    12 <- 18
    20 <- 0  # no more elements
  ]
  trace-should-contain [
    app: spliced: 15 <-> 16 <-> 17
  ]
]

scenario remove-range-to-penultimate [
  local-scope
  # construct a duplex list with six elements [13, 14, 15, 16, 17, 18]
  list:&:duplex-list:num <- push 18, null
  list <- push 17, list
  list <- push 16, list
  list <- push 15, list
  list <- push 14, list
  list <- push 13, list
  run [
    # delete 15 and 16
    # start pointer: to the second element
    list2:&:duplex-list:num <- next list
    # end pointer: to the last (sixth) element
    end:&:duplex-list:num <- next list2
    end <- next end
    end <- next end
    remove-between list2, end
    # now check the list
    10:num/raw <- get *list, value:offset
    list <- next list
    11:num/raw <- get *list, value:offset
    list <- next list
    12:num/raw <- get *list, value:offset
    list <- next list
    13:num/raw <- get *list, value:offset
    20:&:duplex-list:num/raw <- next list
  ]
  memory-should-contain [
    10 <- 13
    11 <- 14
    12 <- 17
    13 <- 18
    20 <- 0  # no more elements
  ]
  trace-should-contain [
    app: spliced: 15 <-> 16
  ]
]

scenario remove-range-empty [
  local-scope
  # construct a duplex list with three elements [13, 14, 15]
  list:&:duplex-list:num <- push 15, null
  list <- push 14, list
  list <- push 13, list
  run [
    # delete between first and second element (i.e. nothing)
    list2:&:duplex-list:num <- next list
    remove-between list, list2
    # now check the list
    10:num/raw <- get *list, value:offset
    list <- next list
    11:num/raw <- get *list, value:offset
    list <- next list
    12:num/raw <- get *list, value:offset
    20:&:duplex-list:num/raw <- next list
  ]
  # no change
  memory-should-contain [
    10 <- 13
    11 <- 14
    12 <- 15
    20 <- 0
  ]
]

scenario remove-range-to-end [
  local-scope
  # construct a duplex list with six elements [13, 14, 15, 16, 17, 18]
  list:&:duplex-list:num <- push 18, null
  list <- push 17, list
  list <- push 16, list
  list <- push 15, list
  list <- push 14, list
  list <- push 13, list
  run [
    # remove the third element and beyond
    list2:&:duplex-list:num <- next list
    remove-between list2, null
    # now check the list
    10:num/raw <- get *list, value:offset
    list <- next list
    11:num/raw <- get *list, value:offset
    20:&:duplex-list:num/raw <- next list
  ]
  memory-should-contain [
    10 <- 13
    11 <- 14
    20 <- 0
  ]
]

# insert list beginning at 'start' after 'in'
def splice in:&:duplex-list:_elem, start:&:duplex-list:_elem/contained-in:in -> in:&:duplex-list:_elem [
  local-scope
  load-inputs
  return-unless in
  return-unless start
  end:&:duplex-list:_elem <- last start
  next:&:duplex-list:_elem <- next in
  {
    break-unless next
    *end <- put *end, next:offset, next
    *next <- put *next, prev:offset, end
  }
  *in <- put *in, next:offset, start
  *start <- put *start, prev:offset, in
]

# insert contents of 'new' after 'in'
def insert in:&:duplex-list:_elem, new:&:@:_elem -> in:&:duplex-list:_elem [
  local-scope
  load-inputs
  return-unless in
  return-unless new
  len:num <- length *new
  return-unless len
  curr:&:duplex-list:_elem <- copy in
  idx:num <- copy 0
  {
    done?:bool <- greater-or-equal idx, len
    break-if done?
    c:_elem <- index *new, idx
    insert c, curr
    # next iter
    curr <- next curr
    idx <- add idx, 1
    loop
  }
]

def append in:&:duplex-list:_elem, new:&:duplex-list:_elem/contained-in:in -> in:&:duplex-list:_elem [
  local-scope
  load-inputs
  last:&:duplex-list:_elem <- last in
  *last <- put *last, next:offset, new
  return-unless new
  *new <- put *new, prev:offset, last
]

def last in:&:duplex-list:_elem -> result:&:duplex-list:_elem [
  local-scope
  load-inputs
  result <- copy in
  {
    next:&:duplex-list:_elem <- next result
    break-unless next
    result <- copy next
    loop
  }
]

# does a duplex list start with a certain sequence of elements?
def match x:&:duplex-list:_elem, y:&:@:_elem -> result:bool [
  local-scope
  load-inputs
  i:num <- copy 0
  max:num <- length *y
  {
    done?:bool <- greater-or-equal i, max
    break-if done?
    expected:_elem <- index *y, i
    return-unless x, false/no-match
    curr:_elem <- first x
    curr-matches?:bool <- equal curr, expected
    return-unless curr-matches?, false/no-match
    x <- next x
    i <- add i, 1
    loop
  }
  return true/successful-match
]

scenario duplex-list-match [
  local-scope
  list:&:duplex-list:char <- push 97/a, null
  list <- push 98/b, list
  list <- push 99/c, list
  list <- push 100/d, list
  run [
    10:bool/raw <- match list, []
    11:bool/raw <- match list, [d]
    12:bool/raw <- match list, [dc]
    13:bool/raw <- match list, [dcba]
    14:bool/raw <- match list, [dd]
    15:bool/raw <- match list, [dcbax]
  ]
  memory-should-contain [
    10 <- 1  # matches []
    11 <- 1  # matches [d]
    12 <- 1  # matches [dc]
    13 <- 1  # matches [dcba]
    14 <- 0  # does not match [dd]
    15 <- 0  # does not match [dcbax]
  ]
]

# helper for debugging
def dump-from x:&:duplex-list:_elem [
  local-scope
  load-inputs
  $print x, [: ]
  {
    break-unless x
    c:_elem <- get *x, value:offset
    $print c, [ ]
    x <- next x
    {
      is-newline?:bool <- equal c, 10/newline
      break-unless is-newline?
      $print 10/newline
      $print x, [: ]
    }
    loop
  }
  $print 10/newline, [---], 10/newline
]

scenario stash-duplex-list [
  local-scope
  list:&:duplex-list:num <- push 1, null
  list <- push 2, list
  list <- push 3, list
  run [
    stash [list:], list
  ]
  trace-should-contain [
    app: list: 3 <-> 2 <-> 1
  ]
]

def to-text in:&:duplex-list:_elem -> result:text [
  local-scope
  load-inputs
  buf:&:buffer:char <- new-buffer 80
  buf <- to-buffer in, buf
  result <- buffer-to-array buf
]

# variant of 'to-text' which stops printing after a few elements (and so is robust to cycles)
def to-text-line in:&:duplex-list:_elem -> result:text [
  local-scope
  load-inputs
  buf:&:buffer:char <- new-buffer 80
  buf <- to-buffer in, buf, 6  # max elements to display
  result <- buffer-to-array buf
]

def to-buffer in:&:duplex-list:_elem, buf:&:buffer:char -> buf:&:buffer:char [
  local-scope
  load-inputs
  {
    break-if in
    buf <- append buf, [[]]
    return
  }
  # append in.value to buf
  val:_elem <- get *in, value:offset
  buf <- append buf, val
  # now prepare next
  next:&:duplex-list:_elem <- next in
  nextn:num <- deaddress next
  return-unless next
  buf <- append buf, [ <-> ]
  # and recurse
  remaining:num, optional-input-found?:bool <- next-input
  {
    break-if optional-input-found?
    # unlimited recursion
    buf <- to-buffer next, buf
    return
  }
  {
    break-unless remaining
    # limited recursion
    remaining <- subtract remaining, 1
    buf <- to-buffer next, buf, remaining
    return
  }
  # past recursion depth; insert ellipses and stop
  append buf, [...]
]

scenario stash-empty-duplex-list [
  local-scope
  x:&:duplex-list:num <- copy null
  run [
    stash x
  ]
  trace-should-contain [
    app: []
  ]
]