inform7/retrospective/6L02/I6T/Flex.i6t

B/flext: Flex Template.
 
@Purpose: To allocate flexible-sized blocks of memory as needed to hold
arbitrary-length strings of text, stored actions or other block values.
 
@-------------------------------------------------------------------------------

@p Blocks.
The purpose of the Flex routines is to manage flexible-sized "blocks" of
memory for any general-purpose use. The main customer for this service is
the system for creating texts, lists, stored actions, and so on, which
are collectively called "block values" because they store their data in
blocks allocated by Flex. But in this section, we're just managing
arrays of memory, and don't need to know what it's for.

A "block" is a continuous range of $2^n$ bytes of memory, where $n\geq 3$
for a 16-bit VM (i.e., for the Z-machine) and $n\geq 4$ for a 32-bit VM
(i.e., on Glulx). Internally, a block is divided into a header followed
by a data section.

The header size depends on the word size and the kind of block (see below). It
always begins with a byte specifying $n$, the binary logarithm of its size,
except that if the top bit is set then we know this isn't a flex-allocated
block, which turns out to be convenient. Thus the largest block representable
is $2^{127}$ bytes long, but somehow I think we can live with that. The second
byte contains a bitmap of (at present) four flags, whose meanings will be
explained below. The second {\it word} of the block, which might be at byte
offset 2 or 4 from the start of the block depending on the word-size of the
VM, is a number specifying the kind of value (if any) which the block contains
data of.

The header also contains a weak kind ID and a reference count, but the Flex
routines do nothing with either of these except to initialise them; they're
provided for the benefit of the |BlockValue| system.

The data section of a block begins at the byte offset |BLK_DATA_OFFSET|
from the address of the block: but see below for how multiple-blocks
behave differently.

These definitions must not be altered without making matching changes
to the compiler.

@c
Constant BLK_HEADER_N = 0;
Constant BLK_HEADER_FLAGS = 1;
Constant BLK_FLAG_MULTIPLE = $$00000001;
Constant BLK_FLAG_16_BIT   = $$00000010;
Constant BLK_FLAG_WORD     = $$00000100;
Constant BLK_FLAG_RESIDENT = $$00001000;
Constant BLK_FLAG_TRUNCMULT = $$00010000;
Constant BLK_HEADER_KOV = 1;
Constant BLK_HEADER_RCOUNT = 2;

Constant BLK_DATA_OFFSET = 3*WORDSIZE;

@p Multiple Blocks.
Some of the data we want to store will be fixed in size, but some of it will
need to expand or contract. The latter can change unpredictably in size and
might at any point overflow their current storage, so they're stored in a
doubly linked list of blocks. The pointer to such a flexible-length array is
by definition the pointer to the block heading this linked list. For instance,
the data in a text

	|"But now I worship a celestiall Sunne"|

might be stored in a list of blocks like so:

	|NULL <-- BN: "But now I wor" <--> BN2: "ship a celestiall Sunne" --> NULL|

Note that the unique pointer to |BN2| is the one in the header of the
|BN| block. When we need to grow such a text, we add additional blocks;
if the text should shrink, blocks at the end can at our discretion be
deallocated. If the entire text should be deallocated, then all of the
blocks used for it are deallocated, starting at the back and working
towards the front.

A multiple-block is one whose flags byte contains the |BLK_FLAG_MULTIPLE|.
This information is redundant since it could in principle be deduced from
the kind of value stored in the block, which is recorded in the
|-->BLK_HEADER_KOV| word, but that would be too slow. |BLK_FLAG_MULTIPLE|
can never change for a currently allocated block, just as it can never
change its KOV.

A multiple-block header is longer than that of an ordinary block, because
it contains two extra words: |-->BLK_NEXT| is the next block in the
doubly-linked list of blocks representing the current value, or |NULL|
if this is the end; |-->BLK_PREV| is the previous block, or |NULL| if this
is the beginning. The need to fit these two extra words in means that the
data section is deferred, and so for a multiple-block data begins at the
byte offset |BLK_DATA_MULTI_OFFSET| rather than |BLK_DATA_OFFSET|.

@c
Constant BLK_DATA_MULTI_OFFSET = BLK_DATA_OFFSET + 2*WORDSIZE;
Constant BLK_NEXT 3;
Constant BLK_PREV 4;

! Constant BLKVALUE_TRACE = 1; ! Uncomment this for debugging purposes

@p The Heap.
Properly speaking, a "heap" is a specific kind of structure often used for
managing uneven-sized or unpredictably changing data. We use "heap" here in
the looser sense of being an amorphous-sized collection of blocks of memory,
some free, others allocated; our actual representation of free space on the
heap is not a heap structure in computer science terms. (Though this segment
could easily be rewritten to make it so, or to adopt any other scheme which
might be faster.) The heap begins as a contiguous region of memory, but it
need not remain so: on Glulx we use dynamic memory allocation to extend it.

For I7 purposes we don't need a way to represent allocated memory, only
the free memory. A block is free if and only if it has |-->BLK_HEADER_KOV|
equal to 0, which is never a valid kind of value, and also has the multiple
flag set. We do that because we construct the whole collection of free
blocks, at any given time, as a single, multiple-block "value": a doubly
linked list joined by the |-->BLK_NEXT| and |<--BLK_PREV|.

A single block, at the bottom of memory and never moving, never allocated
to anyone, is preserved in order to be the head of this linked list of
free blocks. This is a 16-byte (i.e., $n=4$) block, which we format when
the heap is initialised in |HeapInitialise()|. Thus the heap is full
if and only if the |-->BLK_NEXT| of the head-free-block is |NULL|.

So far we have described a somewhat lax regime. After many allocations and
deallocations one could imagine the list of free blocks becoming a very
long list of individually small blocks, which would both make it difficult
to allocate large blocks, and also slow to look through the list. To
ameliorate matters, we maintain the following invariants:

(a) In the free blocks list, |B-->BLK_NEXT| is always an address after |B|;
(b) For any contiguous run of free space blocks in memory (excluding the
head-free-block), taking up a total of $T$ bytes, the last block in the run
has size $2^n$ where $n$ is the largest integer such that $2^n\leq T$.

For instance, there can never be two consecutive free blocks of size 128:
they would form a "run" in the sense of rule (b) of size $T = 256$, and
when $T$ is a power of two the run must contain a single block. In general,
it's easy to prove that the number of blocks in the run is exactly the
number of 1s when $T$ is written out as a binary number, and that the
blocks are ordered in memory from small to large (the reverse of the
direction of reading, i.e., rightmost 1 digit first). Maintaining (b)
is a matter of being careful to fragment blocks only from the front when
smaller blocks are needed, and to rejoin from the back when blocks are
freed and added to the free space object.

@c
Array Flex_Heap -> MEMORY_HEAP_SIZE + 16; ! Plus 16 to allow room for head-free-block

@p Initialisation.
To recap: the constant |MEMORY_HEAP_SIZE| has been predefined by the NI compiler,
and is always itself a power of 2, say $2^n$. We therefore have $2^n + 2^4$
bytes available to us, and we format these as a free space list of two
blocks: the $2^4$-sized "head-free-block" described above followed by
a $2^n$-sized block exactly containing the whole of the rest of the heap.

@c
[ HeapInitialise n bsize blk2;
	blk2 = Flex_Heap + 16;
	Flex_Heap->BLK_HEADER_N = 4;
	Flex_Heap-->BLK_HEADER_KOV = 0;
	Flex_Heap-->BLK_HEADER_RCOUNT = MAX_POSITIVE_NUMBER;
	Flex_Heap->BLK_HEADER_FLAGS = BLK_FLAG_MULTIPLE;
	Flex_Heap-->BLK_NEXT = blk2;
	Flex_Heap-->BLK_PREV = NULL;
	for (bsize=1: bsize < MEMORY_HEAP_SIZE: bsize=bsize*2) n++;
	blk2->BLK_HEADER_N = n;
	blk2-->BLK_HEADER_KOV = 0;
	blk2-->BLK_HEADER_RCOUNT = 0;
	blk2->BLK_HEADER_FLAGS = BLK_FLAG_MULTIPLE;
	blk2-->BLK_NEXT = NULL;
	blk2-->BLK_PREV = Flex_Heap;
];

@p Net Free Space.
"Net" in the sense of "after deductions for the headers": this is the
actual number of free bytes left on the heap which could be used for data.
Note that it is used to predict whether it is possible to fit something
further in: so there are two answers, depending on whether the something
is multiple-block data (with a larger header and therefore less room for
data) or single-block data (smaller header, more room).

@c
[ HeapNetFreeSpace multiple txb asize;
	for (txb=Flex_Heap-->BLK_NEXT: txb~=NULL: txb=txb-->BLK_NEXT) {
		asize = asize + FlexSize(txb);
		if (multiple) asize = asize - BLK_DATA_MULTI_OFFSET;
		else asize = asize - BLK_DATA_OFFSET;
	}
	return asize;
];

@p Make Space.
The following routine determines if there is enough free space to accommodate
another |size| bytes of data, given that it has to be multiple-block data if
the |multiple| flag is set. If the answer turns out to be "no", we see if
the host virtual machine is able to allocate more for us: if it is, then
we ask for $2^m$ further bytes, where $2^m$ is at least |size| plus the
worst-case header storage requirement (16 bytes), and in addition is large
enough to make it worth while allocating. We don't want to bother the VM
by asking for trivial amounts of memory.

This looks to be more memory than is needed, since after all we've asked
for enough that the new data can fit entirely into the new block allocated,
and we might have been able to squeeze some of it into the existing free
space. But it ensures that heap invariant (b) above is preserved, and
besides, running out of memory tends to be something you don't do only once.

(The code below is a refinement on the original, suggested by Jesse McGrew,
which handles non-multiple blocks better.)

@c
Constant SMALLEST_BLK_WORTH_ALLOCATING = 12; ! i.e. 2^12 = 4096 bytes

[ HeapMakeSpace size multiple  newblocksize newblock B n;
	for (::) {
		if (multiple) {
			if (HeapNetFreeSpace(multiple) >= size) rtrue;
		} else {
			if (HeapLargestFreeBlock(0) >= size) rtrue;
		}
		newblocksize = 1;
		for (n=0: (n<SMALLEST_BLK_WORTH_ALLOCATING) || (newblocksize<size): n++)
			newblocksize = newblocksize*2;
		while (newblocksize < size+16) newblocksize = newblocksize*2;
		newblock = VM_AllocateMemory(newblocksize);
		if (newblock == 0) rfalse;
		newblock->BLK_HEADER_N = n;
		newblock-->BLK_HEADER_KOV = 0;
		newblock-->BLK_HEADER_RCOUNT = 0;
		newblock->BLK_HEADER_FLAGS = BLK_FLAG_MULTIPLE;
		newblock-->BLK_NEXT = NULL;
		newblock-->BLK_PREV = NULL;
		for (B = Flex_Heap-->BLK_NEXT:B ~= NULL:B = B-->BLK_NEXT)
			if (B-->BLK_NEXT == NULL) {
				B-->BLK_NEXT = newblock;
				newblock-->BLK_PREV = B;
				jump Linked;
			}
		Flex_Heap-->BLK_NEXT = newblock;
		newblock-->BLK_PREV = Flex_Heap;
		.Linked; ;
		#ifdef BLKVALUE_TRACE;
		print "Increasing heap to free space map: "; FlexDebugDecomposition(Flex_Heap, 0);
		#endif;
	}
	rtrue;
];

[ HeapLargestFreeBlock multiple txb asize best;
	best = 0;
	for (txb=Flex_Heap-->BLK_NEXT: txb~=NULL: txb=txb-->BLK_NEXT) {
		asize = FlexSize(txb);
		if (multiple) asize = asize - BLK_DATA_MULTI_OFFSET;
		else asize = asize - BLK_DATA_OFFSET;
		if (asize > best) best = asize;
	}
	return best;
];

[ HeapDebug full;
	if (full) {
		print "Managing a heap of initially ", MEMORY_HEAP_SIZE+16, " bytes.^";
		print HeapNetFreeSpace(false), " bytes currently free.^";
		print "Free space decomposition: "; FlexDebugDecomposition(Flex_Heap);
		print "Free space map: "; FlexDebug(Flex_Heap);
	} else {
		print HeapNetFreeSpace(false), " of ", MEMORY_HEAP_SIZE+16, " bytes free.^";
	}
];

@p Block Allocation.
Now for the Flex routines. Those with names ending in |Internal| are private
and should only be called by other Flex routines. Even the public ones must
be used with care, or memory leaks or crashes will occur.

The routine |FlexAllocate(N, K, F)| allocates a block with room for |size|
net bytes of data, which will have kind of value |K| and with flags |F|. If
the flags include |BLK_FLAG_MULTIPLE|, this may be either a list of blocks
or a single block. It returns either the address of the block or else throws
run-time problem message and returns 0.

If it does succeed and return a nonzero address, then the caller must be
able to guarantee that |FlexFree| will later be called, exactly once, on
this address. In other words, |FlexAllocate| and |FlexFree| behave somewhat
like C's |malloc| and |free| routines, with all the advantages and hazards
that implies.

In allocation, we try to find a block which is as close as possible to the
right size, and we may have to subdivide blocks: see case II below. For
instance, if a block of size $2^n$ is available and we only need a block of
size $2^k$ where $k<n$ then we break it up in memory as a sequence of
blocks of size $2^k, 2^k, 2^{k+1}, 2^{k+2}, ..., 2^{n-1}$: note that the
sum of these sizes is the $2^n$ we started with. We then use the first
block of size $2^k$. To continue the comparison with binary arithmetic,
this is like a subtraction with repeated carries:
$$ 10000000_2 - 00001000_2 = 01111000_2 $$

@c
[ FlexAllocate size kov flags
	dsize n m free_block min_m max_m smallest_oversized_block secondhalf i hsize head tail;
	
	if (HeapMakeSpace(size, flags & BLK_FLAG_MULTIPLE) == false) FlexError("ran out");

	! Calculate the header size for a block of this KOV
	if (flags & BLK_FLAG_MULTIPLE) hsize = BLK_DATA_MULTI_OFFSET;
	else hsize = BLK_DATA_OFFSET;
	! Calculate the data size
	n=0; for (dsize=1: ((dsize < hsize+size) || (n<3+(WORDSIZE/2))): dsize=dsize*2) n++;

	! Seek a free block closest to the correct size, but starting from the
	! block after the fixed head-free-block, which we can't touch
	min_m = 10000; max_m = 0;
	for (free_block = Flex_Heap-->BLK_NEXT:
		free_block ~= NULL:
		free_block = free_block-->BLK_NEXT) {
		m = free_block->BLK_HEADER_N;
		! Current block the ideal size
		if (m == n) jump CorrectSizeFound;
		! Current block too large: find the smallest which is larger than needed
		if (m > n) {
			if (min_m > m) {
				min_m = m;
				smallest_oversized_block = free_block;
			}
		}
		! Current block too small: find the largest which is smaller than needed
		if (m < n) {
			if (max_m < m) {
				max_m = m;
			}
		}
	}

	if (min_m == 10000) {
		! Case I: No block is large enough to hold the entire size
		if (flags & BLK_FLAG_MULTIPLE == 0) FlexError("too fragmented");
		! Set dsize to the size in bytes if the largest block available
		for (dsize=1: max_m > 0: dsize=dsize*2) max_m--;
		! Split as a head (dsize-hsize), which we can be sure fits into one block,
		! plus a tail (size-(dsize-hsize), which might be a list of blocks
		head = FlexAllocate(dsize-hsize, kov, flags);
		if (head == 0) FlexError("for head block not available");
		tail = FlexAllocate(size-(dsize-hsize), kov, flags);
		if (tail == 0) FlexError("for tail block not available");
		head-->BLK_NEXT = tail;
		tail-->BLK_PREV = head;
		return head;
	}

	! Case II: No block is the right size, but some exist which are too big
	! Set dsize to the size in bytes of the smallest oversized block
	for (dsize=1,m=1: m<=min_m: dsize=dsize*2) m++;
	free_block = smallest_oversized_block;
	while (min_m > n) {
		! Repeatedly halve free_block at the front until the two smallest
		! fragments left are the correct size: then take the frontmost
		dsize = dsize/2;
		! print "Halving size to ", dsize, "^";
		secondhalf = free_block + dsize;
		secondhalf-->BLK_NEXT = free_block-->BLK_NEXT;
		if (secondhalf-->BLK_NEXT ~= NULL)
			(secondhalf-->BLK_NEXT)-->BLK_PREV = secondhalf;
		secondhalf-->BLK_PREV = free_block;
		free_block-->BLK_NEXT = secondhalf;
		free_block->BLK_HEADER_N = (free_block->BLK_HEADER_N) - 1;
		secondhalf->BLK_HEADER_N = free_block->BLK_HEADER_N;
		secondhalf-->BLK_HEADER_KOV = free_block-->BLK_HEADER_KOV;
		secondhalf-->BLK_HEADER_RCOUNT = 0;
		secondhalf->BLK_HEADER_FLAGS = free_block->BLK_HEADER_FLAGS;
		min_m--;
	}
	
	! Once that is done, free_block points to a block which is exactly the
	! right size, so we can fall into...
	
	! Case III: There is a free block which has the correct size.
	.CorrectSizeFound;
	! Delete the free block from the double linked list of free blocks: note
	! that it cannot be the head of this list, which is fixed
	if (free_block-->BLK_NEXT == NULL) {
		! We remove final block, so previous is now final
		(free_block-->BLK_PREV)-->BLK_NEXT = NULL;
	} else {
		! We remove a middle block, so join previous to next
		(free_block-->BLK_PREV)-->BLK_NEXT = free_block-->BLK_NEXT;
		(free_block-->BLK_NEXT)-->BLK_PREV = free_block-->BLK_PREV;
	}
	free_block-->BLK_HEADER_KOV = KindAtomic(kov);
	free_block-->BLK_HEADER_RCOUNT = 1;
	free_block->BLK_HEADER_FLAGS = flags;
	if (flags & BLK_FLAG_MULTIPLE) {
		free_block-->BLK_NEXT = NULL;
		free_block-->BLK_PREV = NULL;
	}
	
	! Zero out the data bytes in the memory allocated
	for (i=hsize:i<dsize:i++) free_block->i=0;
	return free_block;
];

@p Errors.
In the event that |FlexAllocate| returns 0, the caller may not be able
to survive, so the following is provided as a standardised way to halt
the virtual machine.

@c
[ FlexError reason;
	print "*** Memory ", (string) reason, " ***^";
	RunTimeProblem(RTP_HEAPERROR);
	@quit;
];

@p Merging.
Given a free block |block|, find the maximal contiguous run of free blocks
which contains it, and then call |FlexRecutInternal| to recut it to conform to
invariant (b) above.

@c
[ FlexMergeInternal block first last pv nx;
	first = block; last = block;
	while (last-->BLK_NEXT == last+FlexSize(last))
		last = last-->BLK_NEXT;
	while ((first-->BLK_PREV + FlexSize(first-->BLK_PREV) == first) &&
		(first-->BLK_PREV ~= Flex_Heap))
		first = first-->BLK_PREV;
	pv = first-->BLK_PREV;
	nx = last-->BLK_NEXT;
	#ifdef BLKVALUE_TRACE;
	print "Merging: "; FlexDebugDecomposition(pv-->BLK_NEXT, nx); print "^";
	#endif;
	if (FlexRecutInternal(first, last)) {
		#ifdef BLKVALUE_TRACE;
		print " --> "; FlexDebugDecomposition(pv-->BLK_NEXT, nx); print "^";
		#endif;
	}
];

@p Recutting.
Given a segment of the free block list, containing blocks known to be contiguous
in memory, we recut into a sequence of blocks satisfying invariant (b): we
repeatedly cut the largest $2^m$-sized chunk off the back end until it is all
used up.

@c
[ FlexRecutInternal first last tsize backsize mfrom mto bnext backend n dsize fine_so_far;
	if (first == last) rfalse;
	mfrom = first; mto = last + FlexSize(last);
	bnext = last-->BLK_NEXT;
	fine_so_far = true;
	for (:mto>mfrom: mto = mto - backsize) {
		for (n=0, backsize=1: backsize*2 <= mto-mfrom: n++) backsize=backsize*2;
		if ((fine_so_far) && (backsize == FlexSize(last))) {
			bnext = last; last = last-->BLK_PREV;
			bnext-->BLK_PREV = last;
			last-->BLK_NEXT = bnext;
			continue;
		}
		fine_so_far = false; ! From this point, "last" is meaningless
		backend = mto - backsize;
		backend->BLK_HEADER_N = n;
		backend-->BLK_HEADER_KOV = 0;
		backend-->BLK_HEADER_RCOUNT = 0;
		backend->BLK_HEADER_FLAGS = BLK_FLAG_MULTIPLE;
		backend-->BLK_NEXT = bnext;
		if (bnext ~= NULL) {
			bnext-->BLK_PREV = backend;
			bnext = backend;
		}
	}
	if (fine_so_far) rfalse;
	rtrue;
];

@p Deallocation.
As noted above, |FlexFree| must be called exactly once on each nonzero
pointer returned by |FlexAllocate|.

There are two complications: first, when we free a multiple block we need
to free all of the blocks in the list, starting from the back end and
working forwards to the front -- this is the job of |FlexFree|. Second,
when any given block is freed it has to be put into the free block list
at the correct position to preserve invariant (a): it might either come
after all of the currently free blocks in memory, and have to be added to
the end of the list, or in between two, and have to be inserted mid-list,
but it can't be before all of them because the head-free-block is kept
lowest in memory of all possible blocks. (Note that Glulx can't allocate
memory dynamically which undercuts the ordinary array space created by I6:
I6 arrays fill up memory from the bottom.)

Certain blocks {\it outside} the heap are marked as "resident" in memory,
that is, are indestructible. This enables Inform to compile constant values.

@c
[ FlexFree block fromtxb ptxb;
	if (block == 0) return;
	if ((block->BLK_HEADER_FLAGS) & BLK_FLAG_RESIDENT) return;
	if ((block->BLK_HEADER_N) & $80) return; ! not a flexible block at all
	if ((block->BLK_HEADER_FLAGS) & BLK_FLAG_MULTIPLE) {
		if (block-->BLK_PREV ~= NULL) (block-->BLK_PREV)-->BLK_NEXT = NULL;
		fromtxb = block;
		for (:(block-->BLK_NEXT)~=NULL:block = block-->BLK_NEXT) ;
		while (block ~= fromtxb) {
			ptxb = block-->BLK_PREV; FlexFreeSingleBlockInternal(block); block = ptxb;
		}
	}
	FlexFreeSingleBlockInternal(block);
];

[ FlexFreeSingleBlockInternal block free nx;
	block-->BLK_HEADER_KOV = 0;
	block-->BLK_HEADER_RCOUNT = 0;
	block->BLK_HEADER_FLAGS = BLK_FLAG_MULTIPLE;
	for (free = Flex_Heap:free ~= NULL:free = free-->BLK_NEXT) {
		nx = free-->BLK_NEXT;
		if (nx == NULL) {
			free-->BLK_NEXT = block;
			block-->BLK_PREV = free;
			block-->BLK_NEXT = NULL;
			FlexMergeInternal(block);
			return;
		}
		if (UnsignedCompare(nx, block) == 1) {
			free-->BLK_NEXT = block;
			block-->BLK_PREV = free;
			block-->BLK_NEXT = nx;
			nx-->BLK_PREV = block;
			FlexMergeInternal(block);
			return;
		}
	}
];

@p Resizing.
A block which has been allocated, but not yet freed, can sometimes have
its data capacity changed by |FlexResize|.

When the data being stored stretches or shrinks, we will sometimes need
to change the size of the block(s) containing the data -- though not always:
we might sometimes need to resize a 1052-byte text to a 1204-byte text and
find that we are sitting in a 2048-byte block in any case. We either shed
blocks from the end of the chain, or add new blocks at the end, that being
the simplest thing to do. Sometimes it might mean preserving a not very
efficient block division, but it minimises the churn of blocks being
allocated and freed, which is probably good.

@c
[ FlexResize block req newsize dsize newblk kov n i otxb flags;
	if (block == 0) FlexError("failed resizing null block");
	kov = block-->BLK_HEADER_KOV;
	flags = block->BLK_HEADER_FLAGS;
	if (flags & BLK_FLAG_MULTIPLE == 0) FlexError("failed resizing inextensible block");
	otxb = block;
	newsize = req;
	for (:: block = block-->BLK_NEXT) {
		n = block->BLK_HEADER_N;
		for (dsize=1: n>0: n--) dsize = dsize*2;
		i = dsize - BLK_DATA_MULTI_OFFSET;
		newsize = newsize - i;
		if (newsize > 0) {
			if (block-->BLK_NEXT ~= NULL) continue;
			newblk = FlexAllocate(newsize, kov, flags);
			if (newblk == 0) rfalse;
			block-->BLK_NEXT = newblk;
			newblk-->BLK_PREV = block;
			return;
		}
		if (block-->BLK_NEXT ~= NULL) {
			FlexFree(block-->BLK_NEXT);
			block-->BLK_NEXT = NULL;
		}
		return;
	}
];

@p Block Size.
These two routines are provided for the use of the |BlockValue| routines
only.

@c
[ FlexSize txb bsize n; ! Size of an individual block, including header
	if (txb == 0) return 0;
	for (bsize=1: n<txb->BLK_HEADER_N: bsize=bsize*2) n++;
	return bsize;
];

[ FlexTotalSize txb size_in_bytes; ! Combined size of multiple-blocks for a value
	if (txb == 0) return 0;
	if ((txb->BLK_HEADER_FLAGS) & BLK_FLAG_MULTIPLE == 0)
		return FlexSize(txb) - BLK_DATA_OFFSET;
	for (:txb~=NULL:txb=txb-->BLK_NEXT) {
		size_in_bytes = size_in_bytes + FlexSize(txb) - BLK_DATA_MULTI_OFFSET;
	}
	return size_in_bytes;
];

@p Debugging Routines.
These two routines are purely for testing the above code.

@c
[ FlexDebug txb n k i bsize tot dtot kov;
	if (txb == 0) "Block never created.";
	kov = txb-->BLK_HEADER_KOV;
	print "Block ", txb, " (kov ", kov, "): ";
	for (:txb~=NULL:txb = txb-->BLK_NEXT) {
		if (k++ == 100) " ... and so on.";
		if (txb-->BLK_HEADER_KOV ~= kov)
			print "*Wrong kov=", txb-->BLK_HEADER_KOV, "* ";
		n = txb->BLK_HEADER_N;
		for (bsize=1:n>0:n--) bsize=bsize*2;
		i = bsize - BLK_DATA_OFFSET;
		dtot = dtot+i;
		tot = tot+bsize;
		print txb, "(", bsize, ") > ";
	}
	print dtot, " data in ", tot, " bytes^";
];

[ FlexDebugDecomposition from to txb pf;
	if (to==0) to = NULL;
	for (txb=from:(txb~=to) && (txb~=NULL):txb=txb-->BLK_NEXT) {
		if (pf) print "+";
		print FlexSize(txb);
		pf = true;
	}
	print "^";
];