Batteries user guide : BatDynArray

# Module BatDynArray

    module BatDynArray: sig .. end

Dynamic arrays.

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

For all the traversal functions (iter, fold, map, etc.), what happens when the array that is being traversed is mutated is not defined.
Author(s): Brian Hurt, Nicolas Cannasse, David Teller (boilerplate code)

    type 'a t

    include BatEnum.Enumerable

    include BatInterfaces.Mappable

    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 BatEnum.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 BatEnum.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 `List.map` or `Array.map`.

    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 a0 x) a1) ) ... ) aN`, where `a0,a1..aN` are the indexed elements of `darr`.

    val fold_right : ('a -> 'b -> 'b) -> 'a t -> 'b -> 'b

`fold_right f darr x` computes `f a0 (f a1 ( ... ( f aN x ) ... ) )` , where `a0,a1..aN` are the indexed elements of `darr`.

    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 keep : ('a -> bool) -> 'a t -> unit

`keep p darr` removes in place all the element `x` of `darr` such that `p x = false`

Note In previous versions, this function used to be called `BatDynArray.filter`. As this caused incompatibilities with comprehension of dynamic arrays, the function name has been changed.

    val filter : ('a -> bool) -> 'a t -> 'a t

`filter p a` returns all the elements of the array `a` that satisfy the predicate `p`. The order of the elements in the input array is preserved.

Note This function replaces another function called `filter`, available in previous versions of the library. As the old function was incompatible with comprehension of dynamic arrays, its name was changed to `BatDynArray.keep`.

    val filter_map : ('a -> 'b option) -> 'a t -> 'b t

`filter_map f e` returns an array consisting of all elements `x` such that `f y` returns `Some x` , where `y` is an element of `e`.

###### 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`.

    val create_with : resizer_t -> 'a t

create a new dynamic array that uses the given resizer.
Since 2.3.0

###### Unsafe operations

*

    val unsafe_get : 'a t -> int -> 'a

    val unsafe_set : 'a t -> int -> 'a -> unit

###### Boilerplate code

Printing

    val print : ?first:string ->
           ?last:string ->
           ?sep:string ->
           ('a BatInnerIO.output -> 'b -> unit) ->
           'a BatInnerIO.output -> 'b t -> unit