[postgresql] / src / backend / nodes / list.c

/*-------------------------------------------------------------------------
 *
 * list.c
 *        implementation for PostgreSQL generic linked list package
 *
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        $PostgreSQL: pgsql/src/backend/nodes/list.c,v 1.69 2008/01/01 19:45:50 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "nodes/pg_list.h"


/*
 * Routines to simplify writing assertions about the type of a list; a
 * NIL list is considered to be an empty list of any type.
 */
#define IsPointerList(l)                ((l) == NIL || IsA((l), List))
#define IsIntegerList(l)                ((l) == NIL || IsA((l), IntList))
#define IsOidList(l)                    ((l) == NIL || IsA((l), OidList))

#ifdef USE_ASSERT_CHECKING
/*
 * Check that the specified List is valid (so far as we can tell).
 */
static void
check_list_invariants(List *list)
{
        if (list == NIL)
                return;

        Assert(list->length > 0);
        Assert(list->head != NULL);
        Assert(list->tail != NULL);

        Assert(list->type == T_List ||
                   list->type == T_IntList ||
                   list->type == T_OidList);

        if (list->length == 1)
                Assert(list->head == list->tail);
        if (list->length == 2)
                Assert(list->head->next == list->tail);
        Assert(list->tail->next == NULL);
}
#else
#define check_list_invariants(l)
#endif   /* USE_ASSERT_CHECKING */

/*
 * Return a freshly allocated List. Since empty non-NIL lists are
 * invalid, new_list() also allocates the head cell of the new list:
 * the caller should be sure to fill in that cell's data.
 */
static List *
new_list(NodeTag type)
{
        List       *new_list;
        ListCell   *new_head;

        new_head = (ListCell *) palloc(sizeof(*new_head));
        new_head->next = NULL;
        /* new_head->data is left undefined! */

        new_list = (List *) palloc(sizeof(*new_list));
        new_list->type = type;
        new_list->length = 1;
        new_list->head = new_head;
        new_list->tail = new_head;

        return new_list;
}

/*
 * Allocate a new cell and make it the head of the specified
 * list. Assumes the list it is passed is non-NIL.
 *
 * The data in the new head cell is undefined; the caller should be
 * sure to fill it in
 */
static void
new_head_cell(List *list)
{
        ListCell   *new_head;

        new_head = (ListCell *) palloc(sizeof(*new_head));
        new_head->next = list->head;

        list->head = new_head;
        list->length++;
}

/*
 * Allocate a new cell and make it the tail of the specified
 * list. Assumes the list it is passed is non-NIL.
 *
 * The data in the new tail cell is undefined; the caller should be
 * sure to fill it in
 */
static void
new_tail_cell(List *list)
{
        ListCell   *new_tail;

        new_tail = (ListCell *) palloc(sizeof(*new_tail));
        new_tail->next = NULL;

        list->tail->next = new_tail;
        list->tail = new_tail;
        list->length++;
}

/*
 * Append a pointer to the list. A pointer to the modified list is
 * returned. Note that this function may or may not destructively
 * modify the list; callers should always use this function's return
 * value, rather than continuing to use the pointer passed as the
 * first argument.
 */
List *
lappend(List *list, void *datum)
{
        Assert(IsPointerList(list));

        if (list == NIL)
                list = new_list(T_List);
        else
                new_tail_cell(list);

        lfirst(list->tail) = datum;
        check_list_invariants(list);
        return list;
}

/*
 * Append an integer to the specified list. See lappend()
 */
List *
lappend_int(List *list, int datum)
{
        Assert(IsIntegerList(list));

        if (list == NIL)
                list = new_list(T_IntList);
        else
                new_tail_cell(list);

        lfirst_int(list->tail) = datum;
        check_list_invariants(list);
        return list;
}

/*
 * Append an OID to the specified list. See lappend()
 */
List *
lappend_oid(List *list, Oid datum)
{
        Assert(IsOidList(list));

        if (list == NIL)
                list = new_list(T_OidList);
        else
                new_tail_cell(list);

        lfirst_oid(list->tail) = datum;
        check_list_invariants(list);
        return list;
}

/*
 * Add a new cell to the list, in the position after 'prev_cell'. The
 * data in the cell is left undefined, and must be filled in by the
 * caller. 'list' is assumed to be non-NIL, and 'prev_cell' is assumed
 * to be non-NULL and a member of 'list'.
 */
static ListCell *
add_new_cell(List *list, ListCell *prev_cell)
{
        ListCell   *new_cell;

        new_cell = (ListCell *) palloc(sizeof(*new_cell));
        /* new_cell->data is left undefined! */
        new_cell->next = prev_cell->next;
        prev_cell->next = new_cell;

        if (list->tail == prev_cell)
                list->tail = new_cell;

        list->length++;

        return new_cell;
}

/*
 * Add a new cell to the specified list (which must be non-NIL);
 * it will be placed after the list cell 'prev' (which must be
 * non-NULL and a member of 'list'). The data placed in the new cell
 * is 'datum'. The newly-constructed cell is returned.
 */
ListCell *
lappend_cell(List *list, ListCell *prev, void *datum)
{
        ListCell   *new_cell;

        Assert(IsPointerList(list));

        new_cell = add_new_cell(list, prev);
        lfirst(new_cell) = datum;
        check_list_invariants(list);
        return new_cell;
}

ListCell *
lappend_cell_int(List *list, ListCell *prev, int datum)
{
        ListCell   *new_cell;

        Assert(IsIntegerList(list));

        new_cell = add_new_cell(list, prev);
        lfirst_int(new_cell) = datum;
        check_list_invariants(list);
        return new_cell;
}

ListCell *
lappend_cell_oid(List *list, ListCell *prev, Oid datum)
{
        ListCell   *new_cell;

        Assert(IsOidList(list));

        new_cell = add_new_cell(list, prev);
        lfirst_oid(new_cell) = datum;
        check_list_invariants(list);
        return new_cell;
}

/*
 * Prepend a new element to the list. A pointer to the modified list
 * is returned. Note that this function may or may not destructively
 * modify the list; callers should always use this function's return
 * value, rather than continuing to use the pointer passed as the
 * second argument.
 *
 * Caution: before Postgres 8.0, the original List was unmodified and
 * could be considered to retain its separate identity.  This is no longer
 * the case.
 */
List *
lcons(void *datum, List *list)
{
        Assert(IsPointerList(list));

        if (list == NIL)
                list = new_list(T_List);
        else
                new_head_cell(list);

        lfirst(list->head) = datum;
        check_list_invariants(list);
        return list;
}

/*
 * Prepend an integer to the list. See lcons()
 */
List *
lcons_int(int datum, List *list)
{
        Assert(IsIntegerList(list));

        if (list == NIL)
                list = new_list(T_IntList);
        else
                new_head_cell(list);

        lfirst_int(list->head) = datum;
        check_list_invariants(list);
        return list;
}

/*
 * Prepend an OID to the list. See lcons()
 */
List *
lcons_oid(Oid datum, List *list)
{
        Assert(IsOidList(list));

        if (list == NIL)
                list = new_list(T_OidList);
        else
                new_head_cell(list);

        lfirst_oid(list->head) = datum;
        check_list_invariants(list);
        return list;
}

/*
 * Concatenate list2 to the end of list1, and return list1. list1 is
 * destructively changed. Callers should be sure to use the return
 * value as the new pointer to the concatenated list: the 'list1'
 * input pointer may or may not be the same as the returned pointer.
 *
 * The nodes in list2 are merely appended to the end of list1 in-place
 * (i.e. they aren't copied; the two lists will share some of the same
 * storage). Therefore, invoking list_free() on list2 will also
 * invalidate a portion of list1.
 */
List *
list_concat(List *list1, List *list2)
{
        if (list1 == NIL)
                return list2;
        if (list2 == NIL)
                return list1;
        if (list1 == list2)
                elog(ERROR, "cannot list_concat() a list to itself");

        Assert(list1->type == list2->type);

        list1->length += list2->length;
        list1->tail->next = list2->head;
        list1->tail = list2->tail;

        check_list_invariants(list1);
        return list1;
}

/*
 * Truncate 'list' to contain no more than 'new_size' elements. This
 * modifies the list in-place! Despite this, callers should use the
 * pointer returned by this function to refer to the newly truncated
 * list -- it may or may not be the same as the pointer that was
 * passed.
 *
 * Note that any cells removed by list_truncate() are NOT pfree'd.
 */
List *
list_truncate(List *list, int new_size)
{
        ListCell   *cell;
        int                     n;

        if (new_size <= 0)
                return NIL;                             /* truncate to zero length */

        /* If asked to effectively extend the list, do nothing */
        if (new_size >= list_length(list))
                return list;

        n = 1;
        foreach(cell, list)
        {
                if (n == new_size)
                {
                        cell->next = NULL;
                        list->tail = cell;
                        list->length = new_size;
                        check_list_invariants(list);
                        return list;
                }
                n++;
        }

        /* keep the compiler quiet; never reached */
        Assert(false);
        return list;
}

/*
 * Locate the n'th cell (counting from 0) of the list.  It is an assertion
 * failure if there is no such cell.
 */
static ListCell *
list_nth_cell(List *list, int n)
{
        ListCell   *match;

        Assert(list != NIL);
        Assert(n >= 0);
        Assert(n < list->length);
        check_list_invariants(list);

        /* Does the caller actually mean to fetch the tail? */
        if (n == list->length - 1)
                return list->tail;

        for (match = list->head; n-- > 0; match = match->next)
                ;

        return match;
}

/*
 * Return the data value contained in the n'th element of the
 * specified list. (List elements begin at 0.)
 */
void *
list_nth(List *list, int n)
{
        Assert(IsPointerList(list));
        return lfirst(list_nth_cell(list, n));
}

/*
 * Return the integer value contained in the n'th element of the
 * specified list.
 */
int
list_nth_int(List *list, int n)
{
        Assert(IsIntegerList(list));
        return lfirst_int(list_nth_cell(list, n));
}

/*
 * Return the OID value contained in the n'th element of the specified
 * list.
 */
Oid
list_nth_oid(List *list, int n)
{
        Assert(IsOidList(list));
        return lfirst_oid(list_nth_cell(list, n));
}

/*
 * Return true iff 'datum' is a member of the list. Equality is
 * determined via equal(), so callers should ensure that they pass a
 * Node as 'datum'.
 */
bool
list_member(List *list, void *datum)
{
        ListCell   *cell;

        Assert(IsPointerList(list));
        check_list_invariants(list);

        foreach(cell, list)
        {
                if (equal(lfirst(cell), datum))
                        return true;
        }

        return false;
}

/*
 * Return true iff 'datum' is a member of the list. Equality is
 * determined by using simple pointer comparison.
 */
bool
list_member_ptr(List *list, void *datum)
{
        ListCell   *cell;

        Assert(IsPointerList(list));
        check_list_invariants(list);

        foreach(cell, list)
        {
                if (lfirst(cell) == datum)
                        return true;
        }

        return false;
}

/*
 * Return true iff the integer 'datum' is a member of the list.
 */
bool
list_member_int(List *list, int datum)
{
        ListCell   *cell;

        Assert(IsIntegerList(list));
        check_list_invariants(list);

        foreach(cell, list)
        {
                if (lfirst_int(cell) == datum)
                        return true;
        }

        return false;
}

/*
 * Return true iff the OID 'datum' is a member of the list.
 */
bool
list_member_oid(List *list, Oid datum)
{
        ListCell   *cell;

        Assert(IsOidList(list));
        check_list_invariants(list);

        foreach(cell, list)
        {
                if (lfirst_oid(cell) == datum)
                        return true;
        }

        return false;
}

/*
 * Delete 'cell' from 'list'; 'prev' is the previous element to 'cell'
 * in 'list', if any (i.e. prev == NULL iff list->head == cell)
 *
 * The cell is pfree'd, as is the List header if this was the last member.
 */
List *
list_delete_cell(List *list, ListCell *cell, ListCell *prev)
{
        check_list_invariants(list);
        Assert(prev != NULL ? lnext(prev) == cell : list_head(list) == cell);

        /*
         * If we're about to delete the last node from the list, free the whole
         * list instead and return NIL, which is the only valid representation of
         * a zero-length list.
         */
        if (list->length == 1)
        {
                list_free(list);
                return NIL;
        }

        /*
         * Otherwise, adjust the necessary list links, deallocate the particular
         * node we have just removed, and return the list we were given.
         */
        list->length--;

        if (prev)
                prev->next = cell->next;
        else
                list->head = cell->next;

        if (list->tail == cell)
                list->tail = prev;

        pfree(cell);
        return list;
}

/*
 * Delete the first cell in list that matches datum, if any.
 * Equality is determined via equal().
 */
List *
list_delete(List *list, void *datum)
{
        ListCell   *cell;
        ListCell   *prev;

        Assert(IsPointerList(list));
        check_list_invariants(list);

        prev = NULL;
        foreach(cell, list)
        {
                if (equal(lfirst(cell), datum))
                        return list_delete_cell(list, cell, prev);

                prev = cell;
        }

        /* Didn't find a match: return the list unmodified */
        return list;
}

/* As above, but use simple pointer equality */
List *
list_delete_ptr(List *list, void *datum)
{
        ListCell   *cell;
        ListCell   *prev;

        Assert(IsPointerList(list));
        check_list_invariants(list);

        prev = NULL;
        foreach(cell, list)
        {
                if (lfirst(cell) == datum)
                        return list_delete_cell(list, cell, prev);

                prev = cell;
        }

        /* Didn't find a match: return the list unmodified */
        return list;
}

/* As above, but for integers */
List *
list_delete_int(List *list, int datum)
{
        ListCell   *cell;
        ListCell   *prev;

        Assert(IsIntegerList(list));
        check_list_invariants(list);

        prev = NULL;
        foreach(cell, list)
        {
                if (lfirst_int(cell) == datum)
                        return list_delete_cell(list, cell, prev);

                prev = cell;
        }

        /* Didn't find a match: return the list unmodified */
        return list;
}

/* As above, but for OIDs */
List *
list_delete_oid(List *list, Oid datum)
{
        ListCell   *cell;
        ListCell   *prev;

        Assert(IsOidList(list));
        check_list_invariants(list);

        prev = NULL;
        foreach(cell, list)
        {
                if (lfirst_oid(cell) == datum)
                        return list_delete_cell(list, cell, prev);

                prev = cell;
        }

        /* Didn't find a match: return the list unmodified */
        return list;
}

/*
 * Delete the first element of the list.
 *
 * This is useful to replace the Lisp-y code "list = lnext(list);" in cases
 * where the intent is to alter the list rather than just traverse it.
 * Beware that the removed cell is freed, whereas the lnext() coding leaves
 * the original list head intact if there's another pointer to it.
 */
List *
list_delete_first(List *list)
{
        check_list_invariants(list);

        if (list == NIL)
                return NIL;                             /* would an error be better? */

        return list_delete_cell(list, list_head(list), NULL);
}

/*
 * Generate the union of two lists. This is calculated by copying
 * list1 via list_copy(), then adding to it all the members of list2
 * that aren't already in list1.
 *
 * Whether an element is already a member of the list is determined
 * via equal().
 *
 * The returned list is newly-allocated, although the content of the
 * cells is the same (i.e. any pointed-to objects are not copied).
 *
 * NB: this function will NOT remove any duplicates that are present
 * in list1 (so it only performs a "union" if list1 is known unique to
 * start with).  Also, if you are about to write "x = list_union(x, y)"
 * you probably want to use list_concat_unique() instead to avoid wasting
 * the list cells of the old x list.
 *
 * This function could probably be implemented a lot faster if it is a
 * performance bottleneck.
 */
List *
list_union(List *list1, List *list2)
{
        List       *result;
        ListCell   *cell;

        Assert(IsPointerList(list1));
        Assert(IsPointerList(list2));

        result = list_copy(list1);
        foreach(cell, list2)
        {
                if (!list_member(result, lfirst(cell)))
                        result = lappend(result, lfirst(cell));
        }

        check_list_invariants(result);
        return result;
}

/*
 * This variant of list_union() determines duplicates via simple
 * pointer comparison.
 */
List *
list_union_ptr(List *list1, List *list2)
{
        List       *result;
        ListCell   *cell;

        Assert(IsPointerList(list1));
        Assert(IsPointerList(list2));

        result = list_copy(list1);
        foreach(cell, list2)
        {
                if (!list_member_ptr(result, lfirst(cell)))
                        result = lappend(result, lfirst(cell));
        }

        check_list_invariants(result);
        return result;
}

/*
 * This variant of list_union() operates upon lists of integers.
 */
List *
list_union_int(List *list1, List *list2)
{
        List       *result;
        ListCell   *cell;

        Assert(IsIntegerList(list1));
        Assert(IsIntegerList(list2));

        result = list_copy(list1);
        foreach(cell, list2)
        {
                if (!list_member_int(result, lfirst_int(cell)))
                        result = lappend_int(result, lfirst_int(cell));
        }

        check_list_invariants(result);
        return result;
}

/*
 * This variant of list_union() operates upon lists of OIDs.
 */
List *
list_union_oid(List *list1, List *list2)
{
        List       *result;
        ListCell   *cell;

        Assert(IsOidList(list1));
        Assert(IsOidList(list2));

        result = list_copy(list1);
        foreach(cell, list2)
        {
                if (!list_member_oid(result, lfirst_oid(cell)))
                        result = lappend_oid(result, lfirst_oid(cell));
        }

        check_list_invariants(result);
        return result;
}

/*
 * Return a list that contains all the cells in list1 that are not in
 * list2. The returned list is freshly allocated via palloc(), but the
 * cells themselves point to the same objects as the cells of the
 * input lists.
 *
 * This variant works on lists of pointers, and determines list
 * membership via equal()
 */
List *
list_difference(List *list1, List *list2)
{
        ListCell   *cell;
        List       *result = NIL;

        Assert(IsPointerList(list1));
        Assert(IsPointerList(list2));

        if (list2 == NIL)
                return list_copy(list1);

        foreach(cell, list1)
        {
                if (!list_member(list2, lfirst(cell)))
                        result = lappend(result, lfirst(cell));
        }

        check_list_invariants(result);
        return result;
}

/*
 * This variant of list_difference() determines list membership via
 * simple pointer equality.
 */
List *
list_difference_ptr(List *list1, List *list2)
{
        ListCell   *cell;
        List       *result = NIL;

        Assert(IsPointerList(list1));
        Assert(IsPointerList(list2));

        if (list2 == NIL)
                return list_copy(list1);

        foreach(cell, list1)
        {
                if (!list_member_ptr(list2, lfirst(cell)))
                        result = lappend(result, lfirst(cell));
        }

        check_list_invariants(result);
        return result;
}

/*
 * This variant of list_difference() operates upon lists of integers.
 */
List *
list_difference_int(List *list1, List *list2)
{
        ListCell   *cell;
        List       *result = NIL;

        Assert(IsIntegerList(list1));
        Assert(IsIntegerList(list2));

        if (list2 == NIL)
                return list_copy(list1);

        foreach(cell, list1)
        {
                if (!list_member_int(list2, lfirst_int(cell)))
                        result = lappend_int(result, lfirst_int(cell));
        }

        check_list_invariants(result);
        return result;
}

/*
 * This variant of list_difference() operates upon lists of OIDs.
 */
List *
list_difference_oid(List *list1, List *list2)
{
        ListCell   *cell;
        List       *result = NIL;

        Assert(IsOidList(list1));
        Assert(IsOidList(list2));

        if (list2 == NIL)
                return list_copy(list1);

        foreach(cell, list1)
        {
                if (!list_member_oid(list2, lfirst_oid(cell)))
                        result = lappend_oid(result, lfirst_oid(cell));
        }

        check_list_invariants(result);
        return result;
}

/*
 * Append datum to list, but only if it isn't already in the list.
 *
 * Whether an element is already a member of the list is determined
 * via equal().
 */
List *
list_append_unique(List *list, void *datum)
{
        if (list_member(list, datum))
                return list;
        else
                return lappend(list, datum);
}

/*
 * This variant of list_append_unique() determines list membership via
 * simple pointer equality.
 */
List *
list_append_unique_ptr(List *list, void *datum)
{
        if (list_member_ptr(list, datum))
                return list;
        else
                return lappend(list, datum);
}

/*
 * This variant of list_append_unique() operates upon lists of integers.
 */
List *
list_append_unique_int(List *list, int datum)
{
        if (list_member_int(list, datum))
                return list;
        else
                return lappend_int(list, datum);
}

/*
 * This variant of list_append_unique() operates upon lists of OIDs.
 */
List *
list_append_unique_oid(List *list, Oid datum)
{
        if (list_member_oid(list, datum))
                return list;
        else
                return lappend_oid(list, datum);
}

/*
 * Append to list1 each member of list2 that isn't already in list1.
 *
 * Whether an element is already a member of the list is determined
 * via equal().
 *
 * This is almost the same functionality as list_union(), but list1 is
 * modified in-place rather than being copied.  Note also that list2's cells
 * are not inserted in list1, so the analogy to list_concat() isn't perfect.
 */
List *
list_concat_unique(List *list1, List *list2)
{
        ListCell   *cell;

        Assert(IsPointerList(list1));
        Assert(IsPointerList(list2));

        foreach(cell, list2)
        {
                if (!list_member(list1, lfirst(cell)))
                        list1 = lappend(list1, lfirst(cell));
        }

        check_list_invariants(list1);
        return list1;
}

/*
 * This variant of list_concat_unique() determines list membership via
 * simple pointer equality.
 */
List *
list_concat_unique_ptr(List *list1, List *list2)
{
        ListCell   *cell;

        Assert(IsPointerList(list1));
        Assert(IsPointerList(list2));

        foreach(cell, list2)
        {
                if (!list_member_ptr(list1, lfirst(cell)))
                        list1 = lappend(list1, lfirst(cell));
        }

        check_list_invariants(list1);
        return list1;
}

/*
 * This variant of list_concat_unique() operates upon lists of integers.
 */
List *
list_concat_unique_int(List *list1, List *list2)
{
        ListCell   *cell;

        Assert(IsIntegerList(list1));
        Assert(IsIntegerList(list2));

        foreach(cell, list2)
        {
                if (!list_member_int(list1, lfirst_int(cell)))
                        list1 = lappend_int(list1, lfirst_int(cell));
        }

        check_list_invariants(list1);
        return list1;
}

/*
 * This variant of list_concat_unique() operates upon lists of OIDs.
 */
List *
list_concat_unique_oid(List *list1, List *list2)
{
        ListCell   *cell;

        Assert(IsOidList(list1));
        Assert(IsOidList(list2));

        foreach(cell, list2)
        {
                if (!list_member_oid(list1, lfirst_oid(cell)))
                        list1 = lappend_oid(list1, lfirst_oid(cell));
        }

        check_list_invariants(list1);
        return list1;
}

/*
 * Free all storage in a list, and optionally the pointed-to elements
 */
static void
list_free_private(List *list, bool deep)
{
        ListCell   *cell;

        check_list_invariants(list);

        cell = list_head(list);
        while (cell != NULL)
        {
                ListCell   *tmp = cell;

                cell = lnext(cell);
                if (deep)
                        pfree(lfirst(tmp));
                pfree(tmp);
        }

        if (list)
                pfree(list);
}

/*
 * Free all the cells of the list, as well as the list itself. Any
 * objects that are pointed-to by the cells of the list are NOT
 * free'd.
 *
 * On return, the argument to this function has been freed, so the
 * caller would be wise to set it to NIL for safety's sake.
 */
void
list_free(List *list)
{
        list_free_private(list, false);
}

/*
 * Free all the cells of the list, the list itself, and all the
 * objects pointed-to by the cells of the list (each element in the
 * list must contain a pointer to a palloc()'d region of memory!)
 *
 * On return, the argument to this function has been freed, so the
 * caller would be wise to set it to NIL for safety's sake.
 */
void
list_free_deep(List *list)
{
        /*
         * A "deep" free operation only makes sense on a list of pointers.
         */
        Assert(IsPointerList(list));
        list_free_private(list, true);
}

/*
 * Return a shallow copy of the specified list.
 */
List *
list_copy(List *oldlist)
{
        List       *newlist;
        ListCell   *newlist_prev;
        ListCell   *oldlist_cur;

        if (oldlist == NIL)
                return NIL;

        newlist = new_list(oldlist->type);
        newlist->length = oldlist->length;

        /*
         * Copy over the data in the first cell; new_list() has already allocated
         * the head cell itself
         */
        newlist->head->data = oldlist->head->data;

        newlist_prev = newlist->head;
        oldlist_cur = oldlist->head->next;
        while (oldlist_cur)
        {
                ListCell   *newlist_cur;

                newlist_cur = (ListCell *) palloc(sizeof(*newlist_cur));
                newlist_cur->data = oldlist_cur->data;
                newlist_prev->next = newlist_cur;

                newlist_prev = newlist_cur;
                oldlist_cur = oldlist_cur->next;
        }

        newlist_prev->next = NULL;
        newlist->tail = newlist_prev;

        check_list_invariants(newlist);
        return newlist;
}

/*
 * Return a shallow copy of the specified list, without the first N elements.
 */
List *
list_copy_tail(List *oldlist, int nskip)
{
        List       *newlist;
        ListCell   *newlist_prev;
        ListCell   *oldlist_cur;

        if (nskip < 0)
                nskip = 0;                              /* would it be better to elog? */

        if (oldlist == NIL || nskip >= oldlist->length)
                return NIL;

        newlist = new_list(oldlist->type);
        newlist->length = oldlist->length - nskip;

        /*
         * Skip over the unwanted elements.
         */
        oldlist_cur = oldlist->head;
        while (nskip-- > 0)
                oldlist_cur = oldlist_cur->next;

        /*
         * Copy over the data in the first remaining cell; new_list() has already
         * allocated the head cell itself
         */
        newlist->head->data = oldlist_cur->data;

        newlist_prev = newlist->head;
        oldlist_cur = oldlist_cur->next;
        while (oldlist_cur)
        {
                ListCell   *newlist_cur;

                newlist_cur = (ListCell *) palloc(sizeof(*newlist_cur));
                newlist_cur->data = oldlist_cur->data;
                newlist_prev->next = newlist_cur;

                newlist_prev = newlist_cur;
                oldlist_cur = oldlist_cur->next;
        }

        newlist_prev->next = NULL;
        newlist->tail = newlist_prev;

        check_list_invariants(newlist);
        return newlist;
}

/*
 * When using non-GCC compilers, we can't define these as inline
 * functions in pg_list.h, so they are defined here.
 *
 * TODO: investigate supporting inlining for some non-GCC compilers.
 */
#ifndef __GNUC__

ListCell *
list_head(List *l)
{
        return l ? l->head : NULL;
}

ListCell *
list_tail(List *l)
{
        return l ? l->tail : NULL;
}

int
list_length(List *l)
{
        return l ? l->length : 0;
}
#endif   /* ! __GNUC__ */

/*
 * Temporary compatibility functions
 *
 * In order to avoid warnings for these function definitions, we need
 * to include a prototype here as well as in pg_list.h. That's because
 * we don't enable list API compatibility in list.c, so we
 * don't see the prototypes for these functions.
 */

/*
 * Given a list, return its length. This is merely defined for the
 * sake of backward compatibility: we can't afford to define a macro
 * called "length", so it must be a function. New code should use the
 * list_length() macro in order to avoid the overhead of a function
 * call.
 */
int                     length(List *list);

int
length(List *list)
{
        return list_length(list);
}