Module DynArray

module DynArray: sig .. end
Dynamic arrays.

A dynamic array is equivalent to a OCaml array that will resize itself when elements are added or removed, except that floats are boxed and that no initialization element is required.

type 'a t 
exception Invalid_arg of int * string * string
When an operation on an array fails, Invalid_arg is raised. The integer is the value that made the operation fail, the first string contains the function name that has been called and the second string contains the parameter name that made the operation fail.

Array creation

val create : unit -> 'a t
create() returns a new empty dynamic array.
val make : int -> 'a t
make count returns an array with some memory already allocated so up to count elements can be stored into it without resizing.
val init : int -> (int -> 'a) -> 'a t
init n f returns an array of n elements filled with values returned by f 0 , f 1, ... f (n-1).

Array manipulation functions

val empty : 'a t -> bool
Return true if the number of elements in the array is 0.
val length : 'a t -> int
Return the number of elements in the array.
val get : 'a t -> int -> 'a
get darr idx gets the element in darr at index idx. If darr has len elements in it, then the valid indexes range from 0 to len-1.
val last : 'a t -> 'a
last darr returns the last element of darr.
val set : 'a t -> int -> 'a -> unit
set darr idx v sets the element of darr at index idx to value v. The previous value is overwritten.
val insert : 'a t -> int -> 'a -> unit
insert darr idx v inserts v into darr at index idx. All elements of darr with an index greater than or equal to idx have their index incremented (are moved up one place) to make room for the new element.
val add : 'a t -> 'a -> unit
add darr v appends v onto darr. v becomes the new last element of darr.
val append : 'a t -> 'a t -> unit
append src dst adds all elements of src to the end of dst.
val delete : 'a t -> int -> unit
delete darr idx deletes the element of darr at idx. All elements with an index greater than idx have their index decremented (are moved down one place) to fill in the hole.
val delete_last : 'a t -> unit
delete_last darr deletes the last element of darr. This is equivalent of doing delete darr ((length darr) - 1).
val delete_range : 'a t -> int -> int -> unit
delete_range darr p len deletes len elements starting at index p. All elements with an index greater than p+len are moved to fill in the hole.
val clear : 'a t -> unit
remove all elements from the array and resize it to 0.
val blit : 'a t -> int -> 'a t -> int -> int -> unit
blit src srcidx dst dstidx len copies len elements from src starting with index srcidx to dst starting at dstidx.
val compact : 'a t -> unit
compact darr ensures that the space allocated by the array is minimal.

Array copy and conversion

val to_list : 'a t -> 'a list
to_list darr returns the elements of darr in order as a list.
val to_array : 'a t -> 'a array
to_array darr returns the elements of darr in order as an array.
val enum : 'a t -> 'a Enum.t
enum darr returns the enumeration of darr elements.
val of_list : 'a list -> 'a t
of_list lst returns a dynamic array with the elements of lst in it in order.
val of_array : 'a array -> 'a t
of_array arr returns an array with the elements of arr in it in order.
val of_enum : 'a Enum.t -> 'a t
of_enum e returns an array that holds, in order, the elements of e.
val copy : 'a t -> 'a t
copy src returns a fresh copy of src, such that no modification of src affects the copy, or vice versa (all new memory is allocated for the copy).
val sub : 'a t -> int -> int -> 'a t
sub darr start len returns an array holding the subset of len elements from darr starting with the element at index idx.

Array functional support

val iter : ('a -> unit) -> 'a t -> unit
iter f darr calls the function f on every element of darr. It is equivalent to for i = 0 to length darr - 1 do f (get darr i) done;
val iteri : (int -> 'a -> unit) -> 'a t -> unit
iter f darr calls the function f on every element of darr. It is equivalent to for i = 0 to length darr - 1 do f i (get darr i) done;
val map : ('a -> 'b) -> 'a t -> 'b t
map f darr applies the function f to every element of darr and creates a dynamic array from the results - similar to or
val mapi : (int -> 'a -> 'b) -> 'a t -> 'b t
mapi f darr applies the function f to every element of darr and creates a dynamic array from the results - similar to List.mapi or Array.mapi.
val fold_left : ('a -> 'b -> 'a) -> 'a -> 'b t -> 'a
fold_left f x darr computes f ( ... ( f ( f (get darr 0) x) (get darr 1) ) ... ) (get darr n-1), similar to Array.fold_left or List.fold_left.
val fold_right : ('a -> 'b -> 'b) -> 'a t -> 'b -> 'b
fold_right f darr x computes f (get darr 0) (f (get darr 1) ( ... ( f (get darr n-1) x ) ... ) ) similar to Array.fold_right or List.fold_right.
val index_of : ('a -> bool) -> 'a t -> int
index_of f darr returns the index of the first element x in darr such as f x returns true or raise Not_found if not found.
val filter : ('a -> bool) -> 'a t -> unit

Array resizers

type resizer_t = currslots:int -> oldlength:int -> newlength:int -> int 
The type of a resizer function.

Resizer functions are called whenever elements are added to or removed from the dynamic array to determine what the current number of storage spaces in the array should be. The three named arguments passed to a resizer are the current number of storage spaces in the array, the length of the array before the elements are added or removed, and the length the array will be after the elements are added or removed. If elements are being added, newlength will be larger than oldlength, if elements are being removed, newlength will be smaller than oldlength. If the resizer function returns exactly oldlength, the size of the array is only changed when adding an element while there is not enough space for it.

By default, all dynamic arrays are created with the default_resizer. When a dynamic array is created from another dynamic array (using copy, map , etc. ) the resizer of the copy will be the same as the original dynamic array resizer. To change the resizer, use the set_resizer function.

val set_resizer : 'a t -> resizer_t -> unit
Change the resizer for this array.
val get_resizer : 'a t -> resizer_t
Get the current resizer function for a given array
val default_resizer : resizer_t
The default resizer function the library is using - in this version of DynArray, this is the exponential_resizer but should change in next versions.
val exponential_resizer : resizer_t
The exponential resizer- The default resizer except when the resizer is being copied from some other darray.

exponential_resizer works by doubling or halving the number of slots until they "fit". If the number of slots is less than the new length, the number of slots is doubled until it is greater than the new length (or Sys.max_array_size is reached).

If the number of slots is more than four times the new length, the number of slots is halved until it is less than four times the new length.

Allowing darrays to fall below 25% utilization before shrinking them prevents "thrashing". Consider the case where the caller is constantly adding a few elements, and then removing a few elements, causing the length to constantly cross above and below a power of two. Shrinking the array when it falls below 50% would causing the underlying array to be constantly allocated and deallocated. A few elements would be added, causing the array to be reallocated and have a usage of just above 50%. Then a few elements would be remove, and the array would fall below 50% utilization and be reallocated yet again. The bulk of the array, untouched, would be copied and copied again. By setting the threshold at 25% instead, such "thrashing" only occurs with wild swings- adding and removing huge numbers of elements (more than half of the elements in the array).

exponential_resizer is a good performing resizer for most applications. A list allocates 2 words for every element, while an array (with large numbers of elements) allocates only 1 word per element (ignoring unboxed floats). On insert, exponential_resizer keeps the amount of wasted "extra" array elements below 50%, meaning that less than 2 words per element are used. Even on removals where the amount of wasted space is allowed to rise to 75%, that only means that darray is using 4 words per element. This is generally not a significant overhead.

Furthermore, exponential_resizer minimizes the number of copies needed- appending n elements into an empty darray with initial size 0 requires between n and 2n elements of the array be copied- O(n) work, or O(1) work per element (on average). A similar argument can be made that deletes from the end of the array are O(1) as well (obviously deletes from anywhere else are O(n) work- you have to move the n or so elements above the deleted element down).

val step_resizer : int -> resizer_t
The stepwise resizer- another example of a resizer function, this time of a parameterized resizer.

The resizer returned by step_resizer step returns the smallest multiple of step larger than newlength if currslots is less then newlength-step or greater than newlength.

For example, to make an darray with a step of 10, a length of len, and a null of null, you would do: make ~resizer:(step_resizer 10) len null

val conservative_exponential_resizer : resizer_t
conservative_exponential_resizer is an example resizer function which uses the oldlength parameter. It only shrinks the array on inserts- no deletes shrink the array, only inserts. It does this by comparing the oldlength and newlength parameters. Other than that, it acts like exponential_resizer.

Unsafe operations
val unsafe_get : 'a t -> int -> 'a
val unsafe_set : 'a t -> int -> 'a -> unit