src/list.c - List aka array routines


List is roughly based on concepts of IntList (thanks to Steve), so I don't repeat them here.

Especially the same invariants hold, except an empty list is really empty, meaning, push does first check for space.

The main differences are:

- List can hold items of different size, it's suitable for ints and PMCs ..., calculations are still done in terms of items. The item_size is specified at list creation time with the "type" argument.

If you later store different item types in the list, as stated initially, you'll get probably not what you want - so don't do this.

- List does auto grow. The caller may implement a different behaviour if she likes.

- Error checking for out of bounds access is minimal, caller knows better, what should be done.

- List structure itself is different from List_chunk, implying:

- list chunks don't have ->start and ->end fields. Instead the list has ->start, which is start of first chunk, and ->cap, the total usable capacity in the list.

- number of items in chunks are not fixed, but there is a mode using same sized chunks

Grow policy ^


All chunks are of MAX_ITEMS size, chosen, when the first access to the array is indexed and beyond MIN_ITEMS and below 10 * MAX_ITEMS

If the first access is beyond 10 * MAX_ITEMS a sparse chunk will be created.

To avoid this - and the performance penalty - set the array size before setting elements.

    new P0, 'Array'
    set P0, 100000  # sets fixed sized, no sparse
This is only meaningful, if a lot of the entries are used too.


Chunk sizes grow from MIN_ITEMS to MAX_ITEMS, this will be selected for pushing data on an empty array.


Mixture of above chunk types and when sparse chunks are present, or after insert and delete.

The chunks hold the information, how many chunks are of the same type, beginning from the current, and how many items are included in this range. See get_chunk below for details.

Sparse lists ^

To save memory, List can handle sparse arrays. This code snippet:

new P0, 'IntList' set P0[1000000], 42

generates 3 List_chunks, one at the beginning of the array, a big sparse chunk and a chunk for the actual data.

Setting values inside sparse chunks changes them to real chunks. For poping/shifting inside sparse chunks, s. return value below.

Chunk types ^


Have allocated space, size is a power of 2, consecutive chunks are same sized.


Same, but consecutive chunks are growing.


Have allocated space but any size.


Only dummy allocation, chunk->items holds the items of this sparse hole.

Data types ^

A List can hold various datatypes. See src/datatypes.h for the enumeration of types.

Not all are yet implemented in list_set/list_item, see the switch().

Arbitrary length data:

Construct initializer with:


item_size (in bytes)

items_per_chunk (rounded up to power of 2, default MAX_ITEMS)

In list_assign the values are copied into the array, list_get returns a pointer as for all other data types.

See src/list_2.t and list_new_init().

Return value ^

List get functions return a (void*) pointer to the location of the stored data. The caller has to extract the value from this pointer.

For non existent data beyond the dimensions of the array a NULL pointer is returned.

For non existing data inside sparse holes, a pointer (void*)-1 is returned.

The caller can decide to assume these data as undef or 0 or whatever is appropriate.

Testing ^

See t/src/{int, }list.c and t/pmc/{int, }list.t.

Also all array usage depends on list.

Functions ^

static List_chunk *allocate_chunk

Makes a new chunk, and allocates size bytes for buffer storage from the generic memory pool. The chunk holds items items. Marks the chunk as being part of list->container, if it exists, for the purposes of GC. Does not install the chunk into list->container yet.

static void rebuild_chunk_ptrs

Rebuilds list and updates/optimizes chunk usage. Deletes empty chunks, counts chunks and fixes prev pointers.

static void rebuild_sparse

Combines together adjacent sparse chunks in list.

static void rebuild_other

Combines together adjacent irregular chunks in list.

static void rebuild_fix_ends

Resets some values in list and the lists's first chunk. Called by rebuild_chunk_list().

static UINTVAL rebuild_chunk_list

Optimises list when it's been modified in some way. Combines adjacent chunks if they are both sparse or irregular. Updates the grow policies and computes list statistics.

static List_chunk *alloc_next_size

Calculates the size and number of items for the next chunk and allocates it. Adds the number of allocated items to the list's total, but does not directly add the chunk to the list.

static List_chunk *add_chunk

Adds a new chunk to the list. If where is 0, the chunk is added to the front of the list. If 0, it is added to the end of the list.


Calculates log2(x), or a useful approximation thereof. Stolen from src/malloc.c.

static List_chunk *get_chunk

Get the chunk for idx, also update the idx to point into the chunk.

This routine will be called for every operation on list, so it's optimized to be fast and needs an up-to-date chunk statistic. rebuild_chunk_list provides the necessary chunk statistics.

The scheme of operations is:

    if all_chunks_are_MAX_ITEMS
         chunk = chunk_list[ idx / MAX_ITEMS ]
         idx =   idx % MAX_ITEMS

    chunk = first
         if (index < chunk->items)

     if (index >= items_in_chunk_block)
         index -= items_in_chunk_block
         chunk += chunks_in_chunk_block

     calc chunk and index in this block
One chunk_block consists of chunks of the same type: fixed, growing or other. So the time to look up a chunk doesn't depend on the array length, but on the complexity of the array. rebuild_chunk_list tries to reduce the complexity, but may fail, if you e.g. do a prime sieve by actually list_deleting the none prime numbers.

The complexity of the array is how many different chunk_blocks are there. They come from:

- initially fixed: 1

- initially growing: 2

- first unshift: 1 except for initially fixed arrays

- insert: 1 - 3

- delete: 1 - 2

- sparse hole: 3 (could be 2, code assumes access at either end now)

There could be some optimizer, that, after detecting almost only indexed access after some time, does reorganize the array to be all MAX_ITEMS sized, when this would improve performance.

static void split_chunk

Splits a sparse chunk, so that we have

- allocated space at idx

if sparse is big:

- MAX_ITEMS near idx and if there is still sparse space after the real chunk, this also n*MAX_ITEMS sized, so that consecutive writing would make MAX_ITEMS sized real chunks.

static void list_set

Sets item of type type in chunk at idx.

static void *list_item

Get the pointer to the item of type type in the chunk at idx.

static void list_append

Adds one or more chunks to end of list.

Public Interface Functions ^

List *list_new

Returns a new list of type type.

void list_pmc_new

Creates a new list containing PMC* values in PMC_data(container).

List *list_new_init

list_new_init() uses these initializers:

    0 ... size (set initial size of list)
    1 ... array dimensions (multiarray)
    2 ... type (overriding type parameter)
    3 ... item_size for enum_type_sized
    4 ... items_per_chunk
After getting these values out of the key/value pairs, a new array with these values is stored in user_data, where the keys are explicit.

void list_pmc_new_init

Creates a new list of PMC* values in PMC_data(container).

List *list_clone

Returns a clone of the other list.

TODO - Barely tested. Optimize new array structure, fixed if big.

void list_mark

Marks the list and its contents as live for the memory management system.

void list_visit

This is used by freeze/thaw to visit the contents of the list.

pinfo is the visit info, (see include/parrot/pmc_freeze.h>).

INTVAL list_length

Returns the length of the list.

void list_set_length

Sets the length of the list to len.

void list_insert

Makes room for n_items at idx.

void list_delete

Deletes n_items at idx.

void list_push

Pushes item of type type on to the end of the list.

void list_unshift

Pushes item of type type on to the start of the list.

void *list_pop

Removes and returns the last item of type type from the end of the list.

void *list_shift

Removes and returns the first item of type type from the start of the list.

void list_assign

Assigns item of type type to index idx.

void *list_get

Returns the item of type type at index idx.

void list_splice

Replaces count items starting at offset with the items in value.

If count is 0 then the items in value will be inserted after offset.