Image::Leptonica::Func::list
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).
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.
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.
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.
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?
DLLIST * listFindTail ( DLLIST *head )
listFindTail() Input: head Return: tail, or null on error
l_int32 listGetCount ( DLLIST *head )
listGetCount() Input: head (of list) Return: number of elements; 0 if no list or on error
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_LIST
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_LIST
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.
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);
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);
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);
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.
Zakariyya Mughal <zmughal@cpan.org>
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.
To install Image::Leptonica, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Image::Leptonica
CPAN shell
perl -MCPAN -e shell install Image::Leptonica
For more information on module installation, please visit the detailed CPAN module installation guide.