chunky_array.nim

type
  TChunkyNode[IX, T] = tuple
    elms:   array[IX, T]
    next:   PChunkyNode[IX, T]

  PChunkyNode[IX, T] = ptr TChunkyNode[IX, T]

  TChunkyArray*[IX, T] = object
    first:  PChunkyNode[IX, T]
    size:   int

template chunk_size*[IX, T](ca: TChunkyArray[IX, T]):   int  = high(IX) + 1
template len*[IX, T](ca: TChunkyArray[IX, T]):          int  = ca.size
template is_empty*[IX, T](ca: TChunkyArray[IX, T]):     bool = len(ca) == 0

template new_node_for[IX, T](ca: TChunkyArray[IX, T]): PChunkyNode[IX,T] =
  cast[PChunkyNode[IX,T]](allocShared(sizeof(TChunkyNode[IX, T])))

template check_init*[IX, T](ca: var TChunkyArray[IX, T]) =
  if ca.first.isNil:
    ca.first = new_node_for(ca)

proc `[]`*[IX, T](ca: TChunkyArray[IX, T], i: int): T =
  if is_empty(ca): raise newException(EInvalidIndex,
    "tried to index empty TChunkyArray")
  var node = ca.first
  for j in 0 .. <(i div chunk_size(ca)):
    if node.next.isNil: raise newException(EInvalidIndex,
      "index out of bounds: " & $i)
    node = node.next
  return node.elms[i mod chunk_size(ca)]

proc `[]=`*[IX, T](ca: var TChunkyArray[IX, T], i: int, e: T) =
  check_init(ca)
  var node = ca.first
  for j in 0 .. <(i div chunk_size(ca)):
    if node.next.isNil:
      node.next = new_node_for(ca)
    node = node.next
  node.elms[i mod chunk_size(ca)] = e # SEGFAULT OCCURS ON THIS LINE (but in gc.nim:206 (canbeCycleRoot(c)))
  if ca.size <= i: ca.size = i + 1

tests.nim

import ChunkyArrays, RedHuntingHat

test("game/ADTs/ChunkyArrays"): # Just ignore these lines
  test("with blocks of 16"):    # that's my test suite lib

    var ca: TChunkyArray[range[0..15], string]

    req(is_empty(ca), "is_empty at init") # req is eqivalent to assert
    req(len(ca) == 0, "len==0 at init")
    ca[0] = "hello"
    req(ca[0] == "hello", "insertion and retrieval at index 0")
    req(len(ca) == 1, "correct length")

    ca[100] = "ugh"
    req(ca[100] == "ugh", "insertion and retrieval at index 100")

    ca[15] = "ugh"
    req(ca[15] == "ugh", "insertion and retrieval at index 15")

    ca[16] = "ugh"
    req(ca[16] == "ugh", "insertion and retrieval at index 16")