ListWriter

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ListWriter

ListWriter A flexible object-lister taking care of plurals, inventory information, various formats and so on.

§1. Specification. The list-writer is called by one of the following function calls:

WriteListOfMarkedObjects(style), where the set of objects listed is understood to be exactly those with the workflag2 attribute set, and

- the `style` is a sum of `*_BIT` constants as defined in "Definitions.i6t".

WriteListFrom(obj, style, depth, noactivity, iterator), where only the first two parameters are compulsory:
- the set of objects listed begins with obj;
- the style is a sum of *_BIT constants as defined in "Definitions.i6t";
- the depth is the recursion depth within the list — ordinarily 0, but by supplying a higher value, we can simulate a sublist of another list;
- noactivity is a flag which forces the list-writer to ignore the "listing the contents of" activity (in cases where it would otherwise consult this): by default this is false;
- iterator is an iterator function which provides the objects in sequence.

WriteListOfMarkedObjects is simply a front-end which supplies suitable parameters for WriteListFrom.

The iterator function is by default ObjectTreeIterator. This defines the sequence of objects as being the children of the parent of obj, in object tree sequence (that is: child(parent(obj)) is first). Moreover, when using ObjectTreeIterator, the "listing the contents of" activity is carried out, unless noactivity is set.

We also provide the iterator function MarkedListIterator, which defines the sequence of objects as being the list in the word array MarkedObjectArray with length MarkedObjectLength. Here the "listing the contents of" activity is never used, since the objects are not necessarily the contents of any single thing. This of course is the iterator used by WriteListOfMarkedObjects(style): it works by filling this array with all the objects having workflag2 set.

The full specification for iterator functions is given below.

The list-writer automatically groups adjacent and indistinguishable terms in the sequence into plurals, and carries out the "printing the plural name" activity to handle these. Doing this alone would result in text such as "You can see a cake, three coins, an onion, and two coins here", where the five coins are mentioned in two clauses because they happen not to be adjacent in the list. The list-writer therefore carries out the activity "grouping together" to see if the user would like to tie certain objects together into a single entry: what this does is to set suitable list_together properties for the objects. Inform has already given plural objects a similar list_together property. The net effect is that any entries in the list with a non-blank value of list_together must be adjacent to each other.

We could achieve that by sorting the list in order of list_together value, but that would result in drastic movements, whereas we want to upset the original ordering as little as possible. So instead we use a process called coalescing the list. This is such that for every value \INWEBMATH(L\neq 0\INWEBMATH) of list_together, every entry with that value is moved back in the list to follow the first entry with that value. Thus if the original order is \INWEBMATH(x_1, x_2, ..., x_N\INWEBMATH) then \INWEBMATH(x_j\INWEBMATH) still precedes \INWEBMATH(x_k\INWEBMATH) in coalesced order unless there exists \INWEBMATH(i<j<k\INWEBMATH) such that \INWEBMATH(L(i) = L(k) \neq 0\INWEBMATH) and \INWEBMATH(L(j)\neq L(i)\INWEBMATH). This is as stable as it can be while achieving the "interval" property that non-zero \INWEBMATH(L\INWEBMATH) values occur in contiguous runs.

We therefore obtain text such as "You can see a cake, five coins, the tiles W, X, Y and Z from a Scrabble set, and an onion here." where the coins and the Scrabble tiles have been coalesced together in the list.

It's important to note that the default ObjectTreeIterator has the side-effect of actually reordering the object tree: it rearranges the children being listed so that they appear in the tree in coalesced order. The MarkedListIterator used by WriteListOfMarkedObjects has the same side-effect if the marked objects all happen to share a common parent. It might seem odd for a list-writer to have side effects at all, but the idea is that occasional coalescing improves the quality of play in many small ways — for instance, the sequence of matches to TAKE ALL is tidier — and that coalescing has a small speed cost, so we want to do it as little as possible. (The latter was more of a consideration for I6: interpreters are faster nowadays.)

This specification is somewhat stronger than that of the I6 library's traditional list-writer, because (i) it supports arbitrary lists of objects, not just children of specific parents, while still allowing coalesced and grouped lists, (ii) it can be used recursively in all cases, (iii) it uses its own memory, rather than borrowing memory from the parser, so that it can safely be used while the parser is working, and (iv) it manages this memory more flexibly and without silently failing by buffer overruns on unexpectedly large lists.

The I7 version of WriteListFrom, when using ObjectTreeIterator, differs from the I6 version in that the object o is required to be the child of its parent, that is, to be the eldest child. (So in effect it's a function of parents, not children, but we retain the form for familiarity's sake.) In practice this was invariably the way WriteListFrom was used even in I6 days.

Finally, the ISARE_BIT is differently interpreted in I7: instead of printing something like " are ducks and drakes", as it would have done in I6, the initial space is now suppressed and we instead print "are ducks and drakes".

§2. Memory. The list-writer needs to dynamically allocate temporary array space of a known size, in such a way that the array is effectively on the local stack frame: if only either the Z-machine or Glulx supported a stack in memory, this would be no problem, but they do not and we must therefore use the following.

The size of the stack is such that it can support a list which includes every object and recurses in turn to most other objects: in practice, this never happens. It would be nice to allocate more just in case, but the Z-machine is desperately short of array memory.

Constant REQUISITION_STACK_SIZE = 3*ICOUNT_OBJECT;
Array requisition_stack --> REQUISITION_STACK_SIZE;
Global requisition_stack_pointer = 0;

[ RequisitionStack len top addr;
    top = requisition_stack_pointer + len;
    if (top > REQUISITION_STACK_SIZE) return false;
    addr = requisition_stack + requisition_stack_pointer*WORDSIZE;
    ! print "Allocating ", addr, " at pointer ", requisition_stack_pointer, "^";
    requisition_stack_pointer = top;
    return addr;
];

[ FreeStack addr;
    if (addr == 0) return;
    requisition_stack_pointer = (addr - requisition_stack)/WORDSIZE;
];

§3. WriteListOfMarkedObjects. This routine will use the MarkedListIterator. That means it has to create an array containing the object numbers of every object with the workflag2 attribute set, placing the address of this array in MarkedObjectArray and the length in MarkedObjectLength. Note that we preserve any existing marked list on the stack (using the assembly-language instructions @push and @pull) for the duration of our use.

While the final order of this list will depend on what it looks like after coalescing, the initial order is also important. If all of the objects have a common parent in the object tree, then we coalesce those objects and place the list in object tree order. But if not, we place the list in object number order (which is essentially the order of traversal of the initial state of the object tree: thus objects in Room A will all appear before objects in Room B if A was created before B in the source text).

Global MarkedObjectArray = 0;
Global MarkedObjectLength = 0;

+replacing(from BasicInformKit) [ WriteListOfMarkedObjects in_style
    obj common_parent first mixed_parentage length g gc;

    gc = -2;
    objectloop (obj ofclass Object && obj has workflag2) {
        length++;
        if (first == nothing) { first = obj; common_parent = parent(obj); }
        else { if (parent(obj) ~= common_parent) mixed_parentage = true; }
        g = GetGNAOfObject(obj); g = g%3;
        if (gc == -2) gc = g;
        else if (gc ~= g) gc = -1;
    }
    if (mixed_parentage) common_parent = nothing;

    if (length == 0) {
        if (in_style & ISARE_BIT ~= 0) LW_Response('W');
        else if (in_style & CFIRSTART_BIT ~= 0) LW_Response('X');
        else LW_Response('Y');
    } else {
        @push MarkedObjectArray; @push MarkedObjectLength;
        MarkedObjectArray = RequisitionStack(length);
        MarkedObjectLength = length;
        if (MarkedObjectArray == 0)
            return IssueRTP("ListWriterRanOutOfMemory",
                "The list-writer has run out of memory.^", BasicInformKitRTPs);

        if (common_parent) {
            ObjectTreeCoalesce(child(common_parent));
            length = 0;
            objectloop (obj in common_parent) ! object tree order
                if (obj has workflag2) MarkedObjectArray-->length++ = obj;
        } else {
            length = 0;
            objectloop (obj ofclass Object) ! object number order
                if (obj has workflag2) MarkedObjectArray-->length++ = obj;
        }

        WriteListFrom(first, in_style, 0, false, MarkedListIterator);

        FreeStack(MarkedObjectArray);
        @pull MarkedObjectLength; @pull MarkedObjectArray;
    }
    prior_named_list = length;
    prior_named_list_gender = gc;
    return;
];

§4. List Number and Gender. As a brief parenthesis, the same algorithm can be used to work out the prior named list number and gender for everything matching a description.

[ RegardingMarkedObjects
    obj length g gc;
    gc = -2;
    objectloop (obj ofclass Object && obj has workflag2) {
        length++;
        g = GetGNAOfObject(obj); g = g%3;
        if (gc == -2) {
            gc = g;
            prior_named_noun = obj;
        } else if (gc ~= g) gc = -1;
    }
    prior_named_list = length;
    prior_named_list_gender = gc;
    if (length == 0) { prior_named_noun = nothing; prior_named_list_gender = -1; }
    return;
];

+replacing(from BasicInformKit) [ PNToVP gna;
    if (prior_named_noun == player) return story_viewpoint;
    if (prior_named_noun) gna = GetGNAOfObject(prior_named_noun);
    if (((gna%6)/3 == 1) || (prior_named_list >= 2)) return 6;
    return 3;
];

§5. List Writer Regard Storage. This is somewhat hacky, but enables "[regarding list writer internals]".

Array LWI_Storage --> 1 (-1) nothing;
[ SetLWI a b c;
    LWI_Storage-->0 = a;
    LWI_Storage-->1 = b;
    LWI_Storage-->2 = c;
];
[ RegardingLWI;
    prior_named_list = LWI_Storage-->0;
    prior_named_list_gender = LWI_Storage-->1;
    prior_named_noun = LWI_Storage-->2;
];

§6. Response Printing. From which:

[ ResponseViaActivity R;
    @push prior_named_noun; @push prior_named_list; @push prior_named_list_gender;
    RegardingSingleObject(nothing);
    CarryOutActivity(PRINTING_RESPONSE_ACT, R);
    @pull prior_named_list_gender; @pull prior_named_list; @pull prior_named_noun;
];

§7. About Iterator Functions. Suppose Iter is an iterator function and that we have a "raw list" \INWEBMATH(x_1, x_2, ..., x_n\INWEBMATH) of objects. Of these, the iterator function will choose a sublist of "qualifying" objects. It is called with arguments

	Iter(obj, depth, L, function)

where the obj is required to be \INWEBMATH(x_j\INWEBMATH) for some \INWEBMATH(j\INWEBMATH) and the function is one of the *_ITF constants defined below:

On START_ITF, we return \INWEBMATH(x_1\INWEBMATH), or nothing if the list is empty.
On SEEK_ITF, we return the smallest \INWEBMATH(k\geq j\INWEBMATH) such that \INWEBMATH(x_k\INWEBMATH) qualifies, or nothing if none of \INWEBMATH(x_j, x_{j+1}, ..., x_n\INWEBMATH) qualify.
On ADVANCE_ITF, we return the smallest \INWEBMATH(k > j\INWEBMATH) such that \INWEBMATH(x_k\INWEBMATH) qualifies, or nothing if none of \INWEBMATH(x_{j+1}, x_{j+2} ..., x_n\INWEBMATH) qualify.
On COALESCE_ITF, we coalesce the entire list (not merely the qualifying entries) and return the new \INWEBMATH(x_1\INWEBMATH), or nothing if the list is empty.

Thus, given any \INWEBMATH(x_i\INWEBMATH), we can produce the sublist of qualifying entries by performing START_ITF on \INWEBMATH(x_i\INWEBMATH), then SEEK_ITF, to produce \INWEBMATH(q_1\INWEBMATH); then ADVANCE_ITF to produce \INWEBMATH(q_2\INWEBMATH), and so on until nothing is returned, when there are no more qualifying entries. SEEK_ITF and ADVANCE_ITF always return qualifying objects, or nothing; but START_ITF and COALESCE_ITF may return a non-qualifying object, since there's no reason why \INWEBMATH(x_1\INWEBMATH) should qualify in any ordering.

The iterator can make its own choice of which entries qualify, and of what the raw list is; depth is supplied to help in that decision. But if L is non-zero then the iterator is required to disqualify any entry whose list_together value is not L.

Constant SEEK_ITF = 0;
Constant ADVANCE_ITF = 1;
Constant COALESCE_ITF = 2;
Constant START_ITF = 3;

§8. Tracing. Uncomment this line and rebuild the kit to enable tracing of what the algorithm below is doing. (This constant should not be used anywhere except in this file, where #Ifdef on it will have the expected effect: elsewhere, it might not.)

! Constant LKTRACE_LIST_WRITER;

§9. Marked List Iterator. Here the raw list is provided by the MarkedObjectArray, which is convenient for coalescing, but not so helpful for translating the obj parameter into the \INWEBMATH(i\INWEBMATH) such that it is \INWEBMATH(x_i\INWEBMATH). We simply search from the beginning to do this, which combined with other code using the iterator makes for some unnecessary \INWEBMATH(O(n^2)\INWEBMATH) calculations. But it saves memory and nuisance, and the iterator is used only with printing to the screen, which never needs to be very rapid anyway (because the player can only read very slowly).

[ MarkedListIterator obj depth required_lt function i;
    if (obj == nothing) return nothing;
    if (required_lt == 0) required_lt = EMPTY_TEXT_VALUE;
    switch(function) {
        START_ITF: return MarkedObjectArray-->0;
        COALESCE_ITF: return MarkedListCoalesce();
        SEEK_ITF, ADVANCE_ITF:
            for (i=0: i<MarkedObjectLength: i++)
                if (MarkedObjectArray-->i == obj) {
                    if (function == ADVANCE_ITF) i++;
                    for (:i<MarkedObjectLength: i++) {
                        obj = MarkedObjectArray-->i;
                        if ((LT_Compare(required_lt, EMPTY_TEXT_VALUE) ~= 0) &&
                            (LT_Compare(obj.list_together, required_lt) ~= 0)) continue;
                        if ((c_style & WORKFLAG_BIT) && (depth==0) && (obj hasnt workflag))
                            continue;
                        if ((c_style & CONCEAL_BIT) && (ConcealedFromLists(obj))) continue;
                        return obj;
                    }
                    return nothing;
                }
    }
    return nothing;
];

§10. Concealment. This is the definition of concealment used by the list-writer, and means that, for example, the "deciding the concealed possessions" activity is taken into account.

[ ConcealedFromLists obj c;
    if ((obj has concealed) || (obj has scenery)) rtrue;
    c = parent(obj);
    if ((c) && (c ofclass K5_container or K6_supporter) && (TestConcealment(c, obj))) rtrue;
    rfalse;
];

§11. Having Evident Content. This is true when something has at least one unconcealed thing in it. It's false when something is truly empty or when all the things in it are concealed or undescribed, and is used to ensure that output is consistent in both those cases. The player shouldn't count for these purposes, but since the player is undescribed, they're excluded anyway.

[ ObviouslyOccupied o x;
  if (~~((o ofclass K5_container) || (o ofclass K6_supporter))) rfalse;
  if (o has container && o hasnt open && o hasnt transparent) rfalse; ! opaque closed container's status can never be obvious
  x = child(o);
  while (x) {
    if ((x hasnt concealed) && (~~TestConcealment(o, x))) rtrue;
    x = sibling(x);
  }
  rfalse;
];

§12. Coalesce Marked List. The return value is the new first entry in the raw list.

[ MarkedListCoalesce o i lt l swap m;
    for (i=0: i<MarkedObjectLength: i++) {
        lt = (MarkedObjectArray-->i).list_together;
        if (LT_Compare(lt, EMPTY_TEXT_VALUE) ~= 0) {
            ! Find first object in list after contiguous run with this list_together value:
            for (i++: (i<MarkedObjectLength) &&
                (LT_Compare((MarkedObjectArray-->i).list_together, lt) == 0): i++) ;
            ! If the contiguous run extends to end of list, the list is now perfect:
            if (i == MarkedObjectLength) return MarkedObjectArray-->0;
            ! And otherwise we look to see if any future entries belong in the earlier run:
            for (l=i+1: l<MarkedObjectLength: l++)
                if (LT_Compare((MarkedObjectArray-->l).list_together, lt) == 0) {
                    ! Yes, they do: so we perform a rotation to insert it before element i:
                    swap = MarkedObjectArray-->l;
                    for (m=l: m>i: m--) MarkedObjectArray-->m = MarkedObjectArray-->(m-1);
                    MarkedObjectArray-->i = swap;
                    ! And now the run is longer:
                    i++;
                    if (i == MarkedObjectLength) return MarkedObjectArray-->0;
                }
            i--;
        }
    }
    return MarkedObjectArray-->0;
];

§13. Object Tree Iterator. Here the raw list is the list of all children of a given parent: since the argument obj is required to be a member of the raw list, we can use parent(obj) to find it. Now seeking and advancing are fast, but coalescing is slower.

Global list_filter_routine;

[ ObjectTreeIterator obj depth required_lt function;
    if ((obj == nothing) || (parent(obj) == nothing)) return nothing;
    if (function == START_ITF) obj = child(parent(obj));
    if (function == COALESCE_ITF) return ObjectTreeCoalesce(obj);
    if (function == ADVANCE_ITF) obj = sibling(obj);
    if (required_lt == 0) required_lt = EMPTY_TEXT_VALUE;
    for (:: obj = sibling(obj)) {
        if (obj == nothing) return nothing;
!if (function == ADVANCE_ITF) print "Considering ", (the) obj, ": ", (TEXT_TY_Say) obj.list_together, ": ", (TEXT_TY_Say) required_lt, ": ", ": ", (TEXT_TY_Say) lt_value, ": ", LT_Compare(obj.list_together, required_lt), "^";
        if ((LT_Compare(required_lt, EMPTY_TEXT_VALUE) ~= 0) &&
            (LT_Compare(obj.list_together, required_lt) ~= 0)) continue;
        if ((c_style & WORKFLAG_BIT) && (depth==0) && (obj hasnt workflag)) continue;
        if (obj hasnt list_filter_permits) continue;
        if ((c_style & CONCEAL_BIT) && (ConcealedFromLists(obj))) continue;
        return obj;
    }
];

§14. Coalesce Object Tree. Again, the return value is the new first entry in the raw list.

[ ObjectTreeCoalesce obj memb lt later;
    #Ifdef LKTRACE_LIST_WRITER; print "^^Sorting out: "; DiagnoseSortList(obj); #Endif;
    .StartAgain;
    memb = obj;
    while (memb ~= nothing) {
        ! Find the next object in a list_together run.
        lt = memb.list_together;
        if (LT_Compare(lt, EMPTY_TEXT_VALUE) == 0) {
            memb = sibling(memb);
            continue;
        }
        ! Find first object after the contiguous run with this list_together value:
        for (memb=sibling(memb):
        (memb) && (LT_Compare(memb.list_together, lt) == 0): memb = sibling(memb)) ;
        ! If the contiguous run extends to end of list, the list is now perfect:
        if (memb == 0) {
            return obj;
        }
        ! And otherwise we look to see if any future entries belong in the run:
        for (later=sibling(memb): later: later=sibling(later)) {
            if (LT_Compare(later.list_together, lt) == 0) {
                ! Yes, they do: so we perform a regrouping of the list and start again:
                obj = GroupChildren(parent(obj), lt);
                #Ifdef LKTRACE_LIST_WRITER; print "^^Sorted to: "; DiagnoseSortList(obj); #Endif;
                jump StartAgain;
            }
        }
        ! No, that run is complete. Continue with the element that followed the run.
    }
    return obj;
];

§15. GroupChildren. The following runs through the child-objects of par in the I6 object tree, and moves those having a given list_property property value together, to become the eldest children. It preserves the ordering in between those objects, and also in between those not having that property value.

We do this by temporarily moving objects into and out of in_obj and out_obj, objects which in all other circumstances never have children in the tree.

[ GroupChildren par value;
    while (child(par) ~= 0) {
        if (LT_Compare(child(par).list_together, value) ~= 0)
            move child(par) to out_obj;
        else
            move child(par) to in_obj;
    }
    while (child(in_obj) ~= 0)  move child(in_obj) to par;
    while (child(out_obj) ~= 0) move child(out_obj) to par;
    return child(par);
];

#Ifdef LKTRACE_LIST_WRITER;
[ DiagnoseSortList obj memb;
    for (memb=obj: memb~=nothing: memb=sibling(memb)) print memb, " --> "; new_line;
];
#Endif;

§16. WriteListFrom. And here we go at last. Or at any rate we initialise the quartet of global variables detailing the current list-writing process, and begin.

[ WriteListFrom first in_style depth noactivity iter a ol;
    @push c_iterator; @push c_style; @push c_depth; @push c_margin;
    if (iter) c_iterator = iter; else c_iterator = ObjectTreeIterator;
    c_style = in_style; c_depth = depth;
    c_margin = 0; if (in_style & EXTRAINDENT_BIT) c_margin = 1;

    objectloop (a ofclass Object) {
        give a list_filter_permits;
        if ((list_filter_routine) && (list_filter_routine(a) == false))
            give a ~list_filter_permits;
    }

    first = c_iterator(first, depth, 0, START_ITF);
    if (first == nothing) {
        if (in_style & ISARE_BIT ~= 0) LW_Response('W');
        else LW_Response('Y');
        if (in_style & NEWLINE_BIT ~= 0) new_line;
    } else {
        if ((noactivity) || (iter)) {
            WriteListR(first, c_depth, true);
            say__p = 1;
        } else {
            objectloop (ol provides list_together)
                CopyPV(ol.list_together, EMPTY_TEXT_VALUE);
            CarryOutActivity(LISTING_CONTENTS_ACT, parent(first));
        }
    }

    @pull c_margin; @pull c_depth; @pull c_style; @pull c_iterator;
];

§17. Standard Contents Listing Rule. The default for the listing contents activity is to call this rule in its "for" stage: note that this suppresses the use of the activity, to avoid an infinite regress. The activity is used only for the default ObjectTreeIterator, so there is no need to specify which is used.

[ STANDARD_CONTENTS_LISTING_R;
    WriteListFrom(child(parameter_value), c_style, c_depth, true);
];

§18. Partitioning. Given qualifying objects \INWEBMATH(x_1, ..., x_j\INWEBMATH), we partition them into classes of the equivalence relation \INWEBMATH(x_i\sim x_j\INWEBMATH) if and only

they both have a plural property (not necessarily the same), and
neither will cause the list-maker to recurse downwards to show the objects inside or on top of them, and
if they cause the list-maker to add information about whether they are worn, lighted or open, then it will add the same information about both, and
they are considered identical by the parser, in that they respond to the same syntaxes of name: so that it is impossible to find a TAKE X command such that X matches one and not the other.

The equivalence classes are numbered consecutively upwards from 1 to \INWEBMATH(n\INWEBMATH), in order of first appearance in the list. For each object \INWEBMATH(x_i\INWEBMATH), partition_classes->(i-1) is the number of its equivalence class. For each equivalence class number \INWEBMATH(c\INWEBMATH), partition_class_sizes->c is the number of objects in this class.

#Ifdef LKTRACE_LIST_WRITER;
Global DBLW_no_classes; Global DBLW_no_objs;
[ DebugPartition partition_class_sizes partition_classes first depth i k o;
    print "[Length of list is ", DBLW_no_objs, " with ", k, " plural.]^";
    print "[Partitioned into ", DBLW_no_classes, " equivalence classes.]^";
    for (i=1: i<=DBLW_no_classes : i++) {
        print "Class ", i, " has size ", partition_class_sizes->i, "^";
    }
    for (k=0, o=first: k<DBLW_no_objs : k++, o = c_iterator(o, depth, lt_value, ADVANCE_ITF)) {
        print "Entry ", k, " has class ", partition_classes->k,
            " represented by ", o, " with L=", o.list_together, "^";
    }
];
#Endif;

§19. Partition List. The following creates the partition_classes and partition_class_sizes accordingly. We return \INWEBMATH(n\INWEBMATH), the number of classes.

[ PartitionList first no_objs depth partition_classes partition_class_sizes
    i k l n m;
    for (i=0: i<no_objs: i++) partition_classes->i = 0;
    n = 1;
    for (i=first, k=0: k<no_objs: i=c_iterator(i, depth, lt_value, ADVANCE_ITF), k++)
        if (partition_classes->k == 0) {
            partition_classes->k = n; partition_class_sizes->n = 1;
            for (l=c_iterator(i, depth, lt_value, ADVANCE_ITF), m=k+1:
                (l~=0) && (m<no_objs):
                l=c_iterator(l, depth, lt_value, ADVANCE_ITF), m++) {
                if ((partition_classes->m == 0) && (ListEqual(i, l))) {
                    if (partition_class_sizes->n < 255) (partition_class_sizes->n)++;
                    partition_classes->m = n;
                }
            }
            if (n < 255) n++;
        }
    n--;
    #Ifdef LKTRACE_LIST_WRITER;
    DBLW_no_classes = n; DBLW_no_objs = no_objs;
    DebugPartition(partition_class_sizes, partition_classes, first, depth);
    #Endif;
    return n;
];

§20. Equivalence Relation. The above algorithm will fail unless ListEqual is indeed reflexive, symmetric and transitive, which ultimately depends on the care with which Identical is implemented, which in turn hangs on the parse_noun properties compiled by Inform. But this seems to be sound.

[ ListEqual o1 o2;
  if (o1.KD_Count ~= o2.KD_Count) rfalse;
  if ((o1.plural == 0) || (o2.plural == 0)) rfalse;
  if (ObviouslyOccupied(o1) && WillRecurs(o1)) rfalse;
  if (ObviouslyOccupied(o2) && WillRecurs(o2)) rfalse;
  if (c_style & (FULLINV_BIT + PARTINV_BIT) ~= 0) {
    if ((o1 hasnt worn && o2 has worn) || (o2 hasnt worn && o1 has worn)) rfalse;
    if ((o1 hasnt light && o2 has light) || (o2 hasnt light && o1 has light)) rfalse;
    if ((o1 has open && o2 hasnt open) || (o2 has open && o1 hasnt open)) rfalse;
  }
  return Identical(o1, o2);
];

[ WillRecurs o;
    if (c_style & ALWAYS_BIT ~= 0) rtrue;
    if (c_style & RECURSE_BIT == 0) rfalse;
    if ((o has supporter) || ((o has container) && (o has open or transparent))) rtrue;
    rfalse;
];

§21. Grouping. A "group" is a maximally-sized run of one or more adjacent partition classes in the list whose first members have a common value of list_together which is a routine or string, and which is not equal to lt_value, the current grouping value. (As we see below, it's by setting lt_value that we restrict attention to a particular group: if we reacted to that as a list_together value here, then we would simply go around in circles, and never be able to see the individual objects in the group.)

For instance, suppose we have objects with list_together values as follows, where \INWEBMATH(R_1\INWEBMATH) and \INWEBMATH(R_2\INWEBMATH) are addresses of routines:

coin (\INWEBMATH(R_1\INWEBMATH)), coin (\INWEBMATH(R_1\INWEBMATH)), box (\INWEBMATH(R_2\INWEBMATH)), statuette (0), coin (\INWEBMATH(R_1\INWEBMATH)), box (\INWEBMATH(R_2\INWEBMATH))

Then the partition is 1, 1, 2, 3, 1, 2, so that the final output will be something like "three coins, two boxes and a statuette" — with three grouped terms. Here each partition class is the only member in its group, so the number of groups is the same as the number of classes. (The above list is not coalesced, so the \INWEBMATH(R_1\INWEBMATH) values, for instance, are not contiguous: we need to work in general with non-coalesced lists because not every iterator function will be able to coalesce fully.)

But if we have something like this:

coin (\INWEBMATH(R_1\INWEBMATH)), Q (\INWEBMATH(R_2\INWEBMATH)), W (\INWEBMATH(R_2\INWEBMATH)), coin (\INWEBMATH(R_1\INWEBMATH)), statuette (0), E (\INWEBMATH(R_2\INWEBMATH)), R (\INWEBMATH(R_2\INWEBMATH))

then the partition is 1, 2, 3, 1, 4, 5, 6 and we have six classes in all. But classes 2 and 3 are grouped together, as are classes 5 and 6, so we end up with a list having just four groups: "two coins, the letters Q and W from a Scrabble set, a statuette and the letters E and R from a Scrabble set". (Again, this list has not been fully coalesced: if it had been, it would be reordered coin, coin, Q, W, E, R, statuette, with partition 1, 1, 2, 3, 4, 5, 6, and three groups: "two coins, the letters Q, W, E and R from a Scrabble set and a statuette".)

The reason we do not group together classes with a common non-zero list_together which is not a routine or string is that low values of list_together are used in coalescing the list into a pleasing order (say, moving all of the key-like items together) but not in grouping: see list_together in the Inform Designer's Manual, 4th edition.

We calculate the number of groups by starting with the number of classes and then subtracting one each time two adjacent classes share list_together in the above sense.

[ NumberOfGroupsInList o no_classes depth partition_classes partition_class_sizes
    no_groups cl memb k current_lt lt;
    current_lt = EMPTY_TEXT_VALUE;
    lt = EMPTY_TEXT_VALUE;
    no_groups = no_classes;
    for (cl=1, memb=o, k=0: cl<=no_classes: cl++) {
        ! Advance to first member of class number cl
        while (partition_classes->k ~= cl) {
            k++; memb = c_iterator(memb, depth, lt_value, ADVANCE_ITF);
        }
        if (memb) { ! In case of accidents, but should always happen
            lt = memb.list_together;
            if ((LT_Compare(lt, lt_value) ~= 0) &&
                (LT_Compare(lt, EMPTY_TEXT_VALUE) ~= 0) &&
                (LT_Compare(lt, current_lt) == 0)) {
                no_groups--;
            }
            current_lt = lt;
        }
    }
    #Ifdef LKTRACE_LIST_WRITER; print "[There are ", no_groups, " groups.]^"; #Endif;
    return no_groups;
];

[ LT_Compare lt1 lt2;
    if (lt1 == lt2) return 0;
    if (lt1 == 0) lt1 = EMPTY_TEXT_VALUE;
    if (lt2 == 0) lt2 = EMPTY_TEXT_VALUE;
    if (TEXT_TY_IsSubstituted(lt1) == false) {
        if (TEXT_TY_IsSubstituted(lt2) == false) return (lt1-->1)-(lt2-->1);
        return -1;
    }
    if (TEXT_TY_IsSubstituted(lt2) == false) {
        return -1;
    }
    return ComparePV(lt1, lt2);
];

§22. Write List Recursively. The big one: WriteListR is the heart of the list-writer.

[ WriteListR o depth from_start
    partition_classes partition_class_sizes
    cl memb index k2 l m no_classes q groups_to_do current_lt;
    if (o == nothing) return; ! An empty list: no output

    if (from_start) {
        o = c_iterator(o, depth, 0, COALESCE_ITF); ! Coalesce list and choose new start
    }
    o = c_iterator(o, depth, 0, SEEK_ITF); ! Find first entry in list from o
    if (o == nothing) return;

    ! Count index = length of list
    for (memb=o, index=0: memb: memb=c_iterator(memb, depth, lt_value, ADVANCE_ITF)) index++;

    if (c_style & ISARE_BIT ~= 0) {
        SetLWI(index, -1, o);
        LW_Response('V', o);
        if (c_style & NEWLINE_BIT ~= 0)   print ":^";
        else                              print (char) ' ';
        c_style = c_style - ISARE_BIT;
    }

    partition_classes = RequisitionStack(index/WORDSIZE + 2);
    partition_class_sizes = RequisitionStack(index/WORDSIZE + 2);
    if ((partition_classes == 0) || (partition_class_sizes == 0))
        return IssueRTP("ListWriterRanOutOfMemory",
            "The list-writer has run out of memory.^", BasicInformKitRTPs);

    no_classes =
        PartitionList(o, index, depth, partition_classes, partition_class_sizes);

    groups_to_do =
        NumberOfGroupsInList(o, no_classes, depth, partition_classes, partition_class_sizes);

    for (cl=1, memb=o, index=0, current_lt=EMPTY_TEXT_VALUE: groups_to_do>0: cl++) {
        ! Set memb to first object of partition class cl
        while (partition_classes->index ~= cl) {
            index++; memb=c_iterator(memb, depth, lt_value, ADVANCE_ITF);
            if (memb==0) { print "*** Error in list-writer ***^"; return; }
        }

        #Ifdef LKTRACE_LIST_WRITER;
        ! DebugPartition(partition_class_sizes, partition_classes, o, depth);
        print "^[Class ", cl, " of ", no_classes, ": first object ", memb,
            " (", memb.list_together, "); groups_to_do ", groups_to_do, ",
            current_lt=", current_lt, " listing_size=", listing_size,
            " lt_value=", lt_value, " memb.list_together=", memb.list_together, "]^";
        #Endif;

        if ((LT_Compare(memb.list_together, lt_value) == 0) ||
            (LT_Compare(memb.list_together, EMPTY_TEXT_VALUE) == 0)) current_lt = EMPTY_TEXT_VALUE;
        else {
            if (LT_Compare(memb.list_together, current_lt) == 0) continue;

            ! Otherwise this class begins a new group
            @push listing_size;
            q = memb; listing_size = 1; l = index; m = cl;
            while (m < no_classes &&
                (LT_Compare(q.list_together, memb.list_together) == 0)) {
                m++;
                while (partition_classes->l ~= m) {
                    l++; q = c_iterator(q, depth, lt_value, ADVANCE_ITF);
                }
                if (LT_Compare(q.list_together, memb.list_together) == 0)
                    listing_size++;
            }

            if (listing_size > 1) {
                ! The new group contains more than one partition class
                WriteMultiClassGroup(cl, memb, depth, partition_class_sizes);
                current_lt = memb.list_together;
                jump GroupComplete;
            }
            current_lt = EMPTY_TEXT_VALUE;
            @pull listing_size;
        }

        WriteSingleClassGroup(cl, memb, depth, partition_class_sizes->cl);

        .GroupComplete;
        groups_to_do--;
        if (c_style & ENGLISH_BIT ~= 0) {
            if (groups_to_do == 1) {
                if (BasicInformKit`SERIAL_COMMA_CFGF) {
                    if (cl > 1) print ",";
                }
                LW_Response('C');
            }
            if (groups_to_do > 1) print ", ";
        }
    }

    FreeStack(partition_class_sizes);
    FreeStack(partition_classes);
]; ! end of WriteListR

§23. Write Multiple Class Group. The text of a single group which contains more than one partition class. We carry out the "grouping together" activity, so that the user can add text fore and aft — this is how groups of objects such as "X, Y and Z" can be fluffed up to "the letters X, Y and Z from a Scrabble set" — but the default is simple: we print the grouped items by calling WriteListR once again, but this time starting from X, and where lt_value is set to the common list_together value of X, Y and Z. (That restricts the list to just the objects in this group.) Because lt_value is set to this value, the grouping code won't then group X, Y and Z again, and they will instead be individual classes in the new list — so each will end up being sent, in turn, to WriteSingleClassGroup below, and that is where they are printed.

[ WriteMultiClassGroup cl memb depth partition_class_sizes pv q k2 l;
    ! Save the style, because the activity below is allowed to change it
    q = c_style;
    if (c_style & INDENT_BIT ~= 0) PrintSpaces(2*(depth+c_margin));

    BeginActivity(GROUPING_TOGETHER_ACT, memb);

    if (ForActivity(GROUPING_TOGETHER_ACT, memb)) {
        c_style = c_style &~ NEWLINE_BIT;
    } else {
        pv = memb.list_together;
        if (TEXT_TY_IsSubstituted(pv) == false) {
            inventory_stage = 1;
            parser_one = memb; parser_two = depth + c_margin;
            if ((pv-->1)() == 1) jump Omit__Sublist2;
        } else if (pv) {
            ! Set k2 to the number of objects covered by the group
            k2 = 0;
            for (l=0 : l<listing_size : l++) k2 = k2 + partition_class_sizes->(l+cl);
            EnglishNumber(k2); print " ";
            print (TEXT_TY_Say) pv;
            if (c_style & ENGLISH_BIT ~= 0) print " (";
            if (c_style & INDENT_BIT ~= 0)  print ":^";
        }

        c_margin++;
        @push lt_value; @push listing_together; @push listing_size;

        lt_value = memb.list_together; listing_together = memb;
        #Ifdef LKTRACE_LIST_WRITER; print "^^DOWN lt_value = ", lt_value, " listing_together = ", memb, "^^";
        @push DBLW_no_classes; @push DBLW_no_objs; #Endif;
        WriteListR(memb, depth, false);
        #Ifdef LKTRACE_LIST_WRITER; print "^^UP^^"; @pull DBLW_no_objs; @pull DBLW_no_classes; #Endif;

        @pull listing_size; @pull listing_together; @pull lt_value;
        c_margin--;

        pv = memb.list_together;
        if (TEXT_TY_IsSubstituted(pv) == false) {
            inventory_stage = 2;
            parser_one = memb; parser_two = depth+c_margin;
            (pv-->1)();
        } else if (LT_Compare(pv, EMPTY_TEXT_VALUE) ~= 0) {
            if (q & ENGLISH_BIT ~= 0) print ")";
        }
        .Omit__Sublist2;
    }

    EndActivity(GROUPING_TOGETHER_ACT, memb);

    ! If the NEWLINE_BIT has been forced by the activity, act now
    ! before it vanishes...
    if (q & NEWLINE_BIT ~= 0 && c_style & NEWLINE_BIT == 0) new_line;

    ! ...when the original style is restored again:
    c_style = q;
];

§24. Write Single Class Group. The text of a single group which contains exactly one partition class. Because of the way the multiple-class case recurses, every class ends up in this routine sooner or later; this is the place where the actual name of an object is printed, at long last.

[ WriteSingleClassGroup cl memb depth size q;
    q = c_style;
    if (c_style & INDENT_BIT) PrintSpaces(2*(depth+c_margin));
    if (size == 1) {
        if (c_style & NOARTICLE_BIT ~= 0) print (name) memb;
        else {
            if (c_style & DEFART_BIT) {
                if ((cl == 1) && (c_style & CFIRSTART_BIT)) print (The) memb;
                else print (the) memb;
            } else {
                if ((cl == 1) && (c_style & CFIRSTART_BIT)) print (CIndefArt) memb;
                else print (a) memb;
            }
        }
    } else {
        if (c_style & DEFART_BIT) {
            if ((cl == 1) && (c_style & CFIRSTART_BIT)) PrefaceByArticle(memb, 0, size);
            else PrefaceByArticle(memb, 1, size);
        }
        @push listing_size; listing_size = size;
        CarryOutActivity(PRINTING_A_NUMBER_OF_ACT, memb);
        @pull listing_size;
    }
    if ((size > 1) && (memb hasnt pluralname)) {
        give memb pluralname;
        WriteAfterEntry(memb, depth);
        give memb ~pluralname;
    } else WriteAfterEntry(memb, depth);
    c_style = q;
];

§25. Write After Entry. Each entry can be followed by supplementary, usually parenthetical, information: exactly what, depends on the style. The extreme case is when the style, and the object, call for recursion to list the object-tree contents: this is achieved by calling WriteListR, using the ObjectTreeIterator (whatever the iterator used at the top level) and increasing the depth by 1.

[ WriteAfterEntry o depth
    p recurse_flag parenth_flag eldest_child child_count combo;

    inventory_stage = 2;
    if (c_style & PARTINV_BIT) {
        BeginActivity(PRINTING_ROOM_DESC_DETAILS_ACT, o);
        if (ForActivity(PRINTING_ROOM_DESC_DETAILS_ACT, o) == false) {
            combo = 0;
            if (o has light && location hasnt light) combo=combo+1;
            if (o has container && o hasnt open)     combo=combo+2;
            if ((o has container && (o has open || o has transparent))
                && (child(o)==0))                    combo=combo+4;
            if (combo) LW_Response('A'); ! space and open bracket
            switch (combo) {
                1: LW_Response('D', o);
                2: LW_Response('E', o);
                3: LW_Response('H', o);
                4: LW_Response('F', o);
                5: LW_Response('I', o);
                6: LW_Response('G', o);
                7: LW_Response('J', o);
            }
            if (combo) LW_Response('B'); ! close bracket
        }
        EndActivity(PRINTING_ROOM_DESC_DETAILS_ACT, o);
    }   ! end of PARTINV_BIT processing

    if (c_style & FULLINV_BIT) {
        BeginActivity(PRINTING_INVENTORY_DETAILS_ACT, o);
        if (ForActivity(PRINTING_INVENTORY_DETAILS_ACT, o) == false) {
            if (o has light && o has worn) { LW_Response('A'); LW_Response('K', o);  parenth_flag = true; }
            else {
                if (o has light)           { LW_Response('A'); LW_Response('D', o);  parenth_flag = true; }
                if (o has worn)            { LW_Response('A'); LW_Response('L', o); parenth_flag = true; }
            }

            if (o has container) {
                if (o has openable) {
                    if (parenth_flag) {
                        if (BasicInformKit`SERIAL_COMMA_CFGF)
                            print ",";
                        LW_Response('C');
                    } else            LW_Response('A', o);
                    if (o has open) {
                        if (child(o)) LW_Response('M', o);
                        else          LW_Response('N', o);
                    } else {
                        if (o has lockable && o has locked) LW_Response('P', o);
                        else                                LW_Response('O', o);
                    }
                    parenth_flag = true;
                }
                else {
                    if (child(o)==0 && o has transparent) {
                        if (parenth_flag) { LW_Response('C'); LW_Response('F'); }
                        else              { LW_Response('A'); LW_Response('F'); LW_Response('B'); }
                    }
                }
            }
            if (parenth_flag) LW_Response('B');
        }
        EndActivity(PRINTING_INVENTORY_DETAILS_ACT, o);
    }   ! end of FULLINV_BIT processing

    child_count = 0;
    eldest_child = nothing;
    objectloop (p in o)
        if ((c_style & CONCEAL_BIT == 0) || (ConcealedFromLists(p) == false))
            if (p has list_filter_permits) {
                child_count++;
                if (eldest_child == nothing) eldest_child = p;
            }

    if (child_count && (c_style & ALWAYS_BIT)) {
        if (c_style & ENGLISH_BIT) { print " "; LW_Response('Q', o); print " "; }
        recurse_flag = true;
    }

    if (child_count && (c_style & RECURSE_BIT)) {
        if (o has supporter) {
            if (c_style & ENGLISH_BIT) {
                if (c_style & TERSE_BIT) {
                    LW_Response('A', o);
                    LW_Response('R', o);
                } else LW_Response('S', o);
            }
            recurse_flag = true;
        }
        if (o has container && (o has open || o has transparent)) {
            if (c_style & ENGLISH_BIT) {
                if (c_style & TERSE_BIT) {
                    LW_Response('A', o);
                    LW_Response('T', o);
                } else LW_Response('U', o);
            }
            recurse_flag = true;
        }
    }

    if (recurse_flag && (c_style & ENGLISH_BIT)) {
        SetLWI(child_count, -1, eldest_child);
        LW_Response('V', o); print " ";
    }

    if (c_style & NEWLINE_BIT) new_line;

    if (recurse_flag) {
        o = child(o);
        @push lt_value; @push listing_together; @push listing_size;
        @push c_iterator;
        c_iterator = ObjectTreeIterator;
        lt_value = EMPTY_TEXT_VALUE; listing_together = 0; listing_size = 0;
        WriteListR(o, depth+1, true);
        @pull c_iterator;
        @pull listing_size; @pull listing_together; @pull lt_value;
        if (c_style & TERSE_BIT) LW_Response('B');
    }
];

§26. Internal Rule. This rule does nothing in itself; it exists as a placeholder for the response texts used by the list-writer.

[ LIST_WRITER_INTERNAL_R;
];

+replacing(from BasicInformKit) [ LW_Response X o;
    return LIST_WRITER_INTERNAL_RM(X, o);
];