list.c File Reference

#include <string.h>
#include "allheaders.h"

Go to the source code of this file.

Functions
void	listDestroy (DLLIST **phead)
l_ok	listAddToHead (DLLIST **phead, void *data)
l_ok	listAddToTail (DLLIST **phead, DLLIST **ptail, void *data)
l_ok	listInsertBefore (DLLIST **phead, DLLIST *elem, void *data)
l_ok	listInsertAfter (DLLIST **phead, DLLIST *elem, void *data)
void *	listRemoveElement (DLLIST **phead, DLLIST *elem)
void *	listRemoveFromHead (DLLIST **phead)
void *	listRemoveFromTail (DLLIST **phead, DLLIST **ptail)
DLLIST *	listFindElement (DLLIST *head, void *data)
DLLIST *	listFindTail (DLLIST *head)
l_int32	listGetCount (DLLIST *head)
l_ok	listReverse (DLLIST **phead)
l_ok	listJoin (DLLIST **phead1, DLLIST **phead2)

Detailed Description

 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).
  

Definition in file list.c.

Function Documentation

listAddToHead()

l_ok listAddToHead	(	DLLIST **	phead,
		void *	data
	)

listAddToHead()

Parameters

[in,out]	phead	[optional] input head
[in]	data	void* ptr, to be added

Returns

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.

Definition at line 277 of file list.c.

Referenced by listReverse().

listAddToTail()

l_ok listAddToTail	(	DLLIST **	phead,
		DLLIST **	ptail,
		void *	data
	)

listAddToTail()

Parameters

[in,out]	phead	[may be updated], can be NULL
[in,out]	ptail	[updated], can be NULL
[in]	data	void* ptr, to be hung on tail cons cell

Returns

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.

Definition at line 331 of file list.c.

References listFindTail().

Referenced by listJoin().

listDestroy()

void listDestroy

(

DLLIST **

phead

)

listDestroy()

Parameters

[in,out]

phead

to be nulled; head of list

Returns

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.

Definition at line 236 of file list.c.

listFindElement()

DLLIST* listFindElement	(	DLLIST *	head,
		void *	data
	)

listFindElement()

Parameters

[in]	head	list head
[in]	data	void* address, to be searched for

Returns

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?

Definition at line 668 of file list.c.

listFindTail()

DLLIST* listFindTail

(

DLLIST *

head

)

listFindTail()

Parameters

[in]

head

Returns

tail, or NULL on error

Definition at line 696 of file list.c.

Referenced by listAddToTail(), listJoin(), and listRemoveFromTail().

listGetCount()

l_int32 listGetCount

(

DLLIST *

head

)

listGetCount()

Parameters

[in]

head

of list

Returns

number of elements; 0 if no list or on error

Definition at line 721 of file list.c.

listInsertAfter()

l_ok listInsertAfter	(	DLLIST **	phead,
		DLLIST *	elem,
		void *	data
	)

listInsertAfter()

Parameters

[in,out]	phead	[optional] input head
[in]	elem	list element to be inserted after; must be NULL if head is NULL
[in]	data	void* ptr, to be added

Returns

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

Definition at line 459 of file list.c.

listInsertBefore()

l_ok listInsertBefore	(	DLLIST **	phead,
		DLLIST *	elem,
		void *	data
	)

listInsertBefore()

Parameters

[in,out]	phead	[optional] input head
[in]	elem	list element to be inserted in front of; must be NULL if head is NULL
[in]	data	void* address, to be added

Returns

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

Definition at line 394 of file list.c.

listJoin()

l_ok listJoin	(	DLLIST **	phead1,
		DLLIST **	phead2
	)

listJoin()

Parameters

[in,out]	phead1	[may be changed] head of first list
[in,out]	phead2	to be nulled; head of second list

Returns

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.

Definition at line 788 of file list.c.

References listAddToTail(), listFindTail(), and listRemoveFromHead().

listRemoveElement()

void* listRemoveElement	(	DLLIST **	phead,
		DLLIST *	elem
	)

listRemoveElement()

Parameters

[in,out]	phead	[can be changed] input head
[in]	elem	list element to be removed

Returns

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);

Definition at line 514 of file list.c.

listRemoveFromHead()

void* listRemoveFromHead

(

DLLIST **

phead

)

listRemoveFromHead()

Parameters

[in,out]

phead

head of list [to be updated]

Returns

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);

Definition at line 566 of file list.c.

Referenced by listJoin(), and listReverse().

listRemoveFromTail()

void* listRemoveFromTail	(	DLLIST **	phead,
		DLLIST **	ptail
	)

listRemoveFromTail()

Parameters

[in,out]	phead	[may be changed], head must NOT be NULL
[in,out]	ptail	[always updated], tail may be NULL

Returns

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);

Definition at line 614 of file list.c.

References listFindTail().

listReverse()

l_ok listReverse

(

DLLIST **

phead

)

listReverse()

Parameters

[in,out]

phead

[may be changed] list head

Returns

0 if OK, 1 on error

Notes:
     (1) This reverses the list in-place.

Definition at line 751 of file list.c.

References listAddToHead(), and listRemoveFromHead().