NAME
Image::Leptonica::Func::list
VERSION
version 0.04
list.c
list.c
Inserting and removing elements
void listDestroy()
DLLIST *listAddToHead()
l_int32 listAddToTail()
l_int32 listInsertBefore()
l_int32 listInsertAfter()
void *listRemoveElement()
void *listRemoveFromHead()
void *listRemoveFromTail()
Other list operations
DLLIST *listFindElement()
DLLIST *listFindTail()
l_int32 listGetCount()
l_int32 listReverse()
DLLIST *listJoin()
Lists are much harder to handle than arrays. There is
more overhead for the programmer, both cognitive and
codewise, and more likelihood that an error can be made.
For that reason, lists should only be used when it is
inefficient to use arrays, such as when elements are
routinely inserted or deleted from inside arrays whose
average size is greater than about 10.
A list of data structures can be implemented in a number
of ways. The two most popular are:
(1) The list can be composed of a linked list of
pointer cells ("cons cells"), where the data structures
are hung off the cells. This is more difficult
to use because you have to keep track of both
your hanging data and the cell structures.
It requires 3 pointers for every data structure
that is put in a list. There is no problem
cloning (using reference counts) for structures that
are put in such a list. We implement lists by this
method here.
(2) The list pointers can be inserted directly into
the data structures. This is easy to implement
and easier to use, but it adds 2 ptrs of overhead
to every data structure in which the ptrs are embedded.
It also requires special care not to put the ptrs
in any data that is cloned with a reference count;
else your lists will break.
Writing C code that uses list pointers explicitly to make
and alter lists is difficult and prone to error.
Consequently, a generic list utility that handles lists
of arbitrary objects and doesn't force the programmer to
touch the "next" and "prev" pointers, is quite useful.
Such functions are provided here. However, the usual
situation requires traversing a list and applying some
function to one or more of the list elements. Macros
for traversing the list are, in general, necessary, to
achieve the goal of invisibly handling all "next" and "prev"
pointers in generic lists. We provide macros for
traversing a list in both forward and reverse directions.
Because of the typing in C, implementation of a general
list utility requires casting. If macros are used, the
casting can be done implicitly; otherwise, using functions,
some of the casts must be explicit. Fortunately, this
can be implemented with void* so the programmer using
the library will not have to make any casts! (Unless you
compile with g++, in which case the rules on implicit
conversion are more strict.)
For example, to add an arbitrary data structure foo to the
tail of a list, use
listAddToTail(&head, &tail, pfoo);
where head and tail are list cell ptrs and pfoo is
a pointer to the foo object.
And to remove an arbitrary data structure foo from a
list, when you know the list cell element it is hanging from,
use
pfoo = listRemoveElement(&head, elem)
where head and elem are list cell ptrs and pfoo is a pointer
to the foo object. No casts are required for foo in
either direction in ANSI C. (However, casts are
required for ANSI C++).
We use lists that are composed of doubly-linked
cells with data structures hanging off the cells.
We use doubly-linked cells to simplify insertion
and deletion, and to allow operations to proceed in either
direction along the list. With doubly-linked lists,
it is tempting to make them circular, by setting head->prev
to the tail of the list and tail->next to the head.
The circular list costs nothing extra in storage, and
allows operations to proceed from either end of the list
with equal speed. However, the circular link adds
cognitive overhead for the application programmer in
general, and it greatly complicates list traversal when
arbitrary list elements can be added or removed as you
move through. It can be done, but in the spirit of
simplicity, we avoid the temptation. The price to be paid
is the extra cost to find the tail of a list -- a full
traversal -- before the tail can be used. This is a
cheap price to pay to avoid major headaches and buggy code.
When you are only applying some function to each element
in a list, you can go either forwards or backwards.
To run through a list forwards, use:
for (elem = head; elem; elem = nextelem) {
nextelem = elem->next; (in case we destroy elem)
<do something with elem->data>
}
To run through a list backwards, find the tail and use:
for (elem = tail; elem; elem = prevelem) {
# prevelem = elem->prev; (in case we destroy elem)
<do something with elem->data>
}
Even though these patterns are very simple, they are so common
that we've provided macros for them in list.h. Using the
macros, this becomes:
L_BEGIN_LIST_FORWARD(head, elem)
<do something with elem->data>
L_END_LIST
L_BEGIN_LIST_REVERSE(tail, elem)
<do something with elem->data>
L_END_LIST
Note again that with macros, the application programmer does
not need to refer explicitly to next and prev fields. Also,
in the reverse case, note that we do not explicitly
show the head of the list. However, the head of the list
is always in scope, and functions can be called within the
iterator that change the head.
Some special cases are simpler. For example, when
removing all items from the head of the list, you can use
while (head) {
obj = listRemoveFromHead(&head);
<do something with obj>
}
Removing successive elements from the tail is equally simple:
while (tail) {
obj = listRemoveFromTail(&head, &tail);
<do something with obj>
}
When removing an arbitrary element from a list, use
obj = listRemoveElement(&head, elem);
All the listRemove*() functions hand you the object,
destroy the list cell to which it was attached, and
reset the list pointers if necessary.
Several other list operations, that do not involve
inserting or removing objects, are also provided.
The function listFindElement() locates a list pointer
by matching the object hanging on it to a given
object. The function listFindTail() gets a handle
to the tail list ptr, allowing backwards traversals of
the list. listGetCount() gives the number of elements
in a list. Functions that reverse a list and concatenate
two lists are also provided.
These functions can be modified for efficiency in the
situation where there is a large amount of creation and
destruction of list cells. If millions of cells are
made and destroyed, but a relatively small number are
around at any time, the list cells can be stored for
later re-use in a stack (see the generic stack functions
in stack.c).FUNCTIONS
listAddToHead
l_int32 listAddToHead ( DLLIST **phead, void *data )
listAddToHead()
Input: &head (<optional> input head)
data (void* ptr, to be added)
Return: 0 if OK; 1 on error
Notes:
(1) This makes a new cell, attaches the data, and adds the
cell to the head of the list.
(2) When consing from NULL, be sure to initialize head to NULL
before calling this function.listAddToTail
l_int32 listAddToTail ( DLLIST **phead, DLLIST **ptail, void *data )
listAddToTail()
Input: &head (<may be updated>, head can be null)
&tail (<updated>, tail can be null)
data (void* ptr, to be hung on tail cons cell)
Return: 0 if OK; 1 on error
Notes:
(1) This makes a new cell, attaches the data, and adds the
cell to the tail of the list.
(2) &head is input to allow the list to be "cons'd" up from NULL.
(3) &tail is input to allow the tail to be updated
for efficient sequential operation with this function.
(4) We assume that if *phead and/or *ptail are not NULL,
then they are valid addresses. Therefore:
(a) when consing from NULL, be sure to initialize both
head and tail to NULL.
(b) when tail == NULL for an existing list, the tail
will be found and updated.listDestroy
void listDestroy ( DLLIST **phead )
listDestroy()
Input: &head (<to be nulled> head of list)
Return: void
Notes:
(1) This only destroys the cons cells. Before destroying
the list, it is necessary to remove all data and set the
data pointers in each cons cell to NULL.
(2) listDestroy() will give a warning message for each data
ptr that is not NULL.listFindElement
DLLIST * listFindElement ( DLLIST *head, void *data )
listFindElement()
Input: head (list head)
data (void* address, to be searched for)
Return: cell (the containing cell, or null if not found or on error)
Notes:
(1) This returns a ptr to the cell, which is still embedded in
the list.
(2) This handle and the attached data have not been copied or
reference counted, so they must not be destroyed. This
violates our basic rule that every handle returned from a
function is owned by that function and must be destroyed,
but if rules aren't there to be broken, why have them?listFindTail
DLLIST * listFindTail ( DLLIST *head )
listFindTail()
Input: head
Return: tail, or null on errorlistGetCount
l_int32 listGetCount ( DLLIST *head )
listGetCount()
Input: head (of list)
Return: number of elements; 0 if no list or on errorlistInsertAfter
l_int32 listInsertAfter ( DLLIST **phead, DLLIST *elem, void *data )
listInsertAfter()
Input: &head (<optional> input head)
elem (list element to be inserted after;
must be null if head is null)
data (void* ptr, to be added)
Return: 0 if OK; 1 on error
Notes:
(1) This can be called on a null list, in which case both
head and elem must be null. The head is included
in the call to allow "consing" up from NULL.
(2) If you are searching through a list, looking for a condition
to add an element, you can do something like this:
L_BEGIN_LIST_FORWARD(head, elem)
<identify an elem to insert after>
listInsertAfter(&head, elem, data);
L_END_LISTlistInsertBefore
l_int32 listInsertBefore ( DLLIST **phead, DLLIST *elem, void *data )
listInsertBefore()
Input: &head (<optional> input head)
elem (list element to be inserted in front of;
must be null if head is null)
data (void* address, to be added)
Return: 0 if OK; 1 on error
Notes:
(1) This can be called on a null list, in which case both
head and elem must be null.
(2) If you are searching through a list, looking for a condition
to add an element, you can do something like this:
L_BEGIN_LIST_FORWARD(head, elem)
<identify an elem to insert before>
listInsertBefore(&head, elem, data);
L_END_LISTlistJoin
l_int32 listJoin ( DLLIST **phead1, DLLIST **phead2 )
listJoin()
Input: &head1 (<may be changed> head of first list)
&head2 (<to be nulled> head of second list)
Return: 0 if OK, 1 on error
Notes:
(1) The concatenated list is returned with head1 as the new head.
(2) Both input ptrs must exist, though either can have the value NULL.listRemoveElement
void * listRemoveElement ( DLLIST **phead, DLLIST *elem )
listRemoveElement()
Input: &head (<can be changed> input head)
elem (list element to be removed)
Return: data (void* struct on cell)
Notes:
(1) in ANSI C, it is not necessary to cast return to actual type; e.g.,
pix = listRemoveElement(&head, elem);
but in ANSI C++, it is necessary to do the cast:
pix = (Pix *)listRemoveElement(&head, elem);listRemoveFromHead
void * listRemoveFromHead ( DLLIST **phead )
listRemoveFromHead()
Input: &head (<to be updated> head of list)
Return: data (void* struct on cell), or null on error
Notes:
(1) in ANSI C, it is not necessary to cast return to actual type; e.g.,
pix = listRemoveFromHead(&head);
but in ANSI C++, it is necessary to do the cast; e.g.,
pix = (Pix *)listRemoveFromHead(&head);listRemoveFromTail
void * listRemoveFromTail ( DLLIST **phead, DLLIST **ptail )
listRemoveFromTail()
Input: &head (<may be changed>, head must NOT be null)
&tail (<always updated>, tail may be null)
Return: data (void* struct on cell) or null on error
Notes:
(1) We include &head so that it can be set to NULL if
if the only element in the list is removed.
(2) The function is relying on the fact that if tail is
not NULL, then is is a valid address. You can use
this function with tail == NULL for an existing list, in
which case the tail is found and updated, and the
removed element is returned.
(3) In ANSI C, it is not necessary to cast return to actual type; e.g.,
pix = listRemoveFromTail(&head, &tail);
but in ANSI C++, it is necessary to do the cast; e.g.,
pix = (Pix *)listRemoveFromTail(&head, &tail);listReverse
l_int32 listReverse ( DLLIST **phead )
listReverse()
Input: &head (<may be changed> list head)
Return: 0 if OK, 1 on error
Notes:
(1) This reverses the list in-place.AUTHOR
Zakariyya Mughal <zmughal@cpan.org>
COPYRIGHT AND LICENSE
This software is copyright (c) 2014 by Zakariyya Mughal.
This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.