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+/*-------------------------------------------------------------------------
+ *
+ * ilist.h
+ * integrated/inline doubly- and singly-linked lists
+ *
+ * These list types are useful when there are only a predetermined set of
+ * lists that an object could be in. List links are embedded directly into
+ * the objects, and thus no extra memory management overhead is required.
+ * (Of course, if only a small proportion of existing objects are in a list,
+ * the link fields in the remainder would be wasted space. But usually,
+ * it saves space to not have separately-allocated list nodes.)
+ *
+ * None of the functions here allocate any memory; they just manipulate
+ * externally managed memory. The APIs for singly and doubly linked lists
+ * are identical as far as capabilities of both allow.
+ *
+ * Each list has a list header, which exists even when the list is empty.
+ * An empty singly-linked list has a NULL pointer in its header.
+ * There are two kinds of empty doubly linked lists: those that have been
+ * initialized to NULL, and those that have been initialized to circularity.
+ * (If a dlist is modified and then all its elements are deleted, it will be
+ * in the circular state.) We prefer circular dlists because there are some
+ * operations that can be done without branches (and thus faster) on lists
+ * that use circular representation. However, it is often convenient to
+ * initialize list headers to zeroes rather than setting them up with an
+ * explicit initialization function, so we also allow the other case.
+ *
+ * EXAMPLES
+ *
+ * Here's a simple example demonstrating how this can be used. Let's assume
+ * we want to store information about the tables contained in a database.
+ *
+ * #include "lib/ilist.h"
+ *
+ * // Define struct for the databases including a list header that will be
+ * // used to access the nodes in the table list later on.
+ * typedef struct my_database
+ * {
+ * char *datname;
+ * dlist_head tables;
+ * // ...
+ * } my_database;
+ *
+ * // Define struct for the tables. Note the list_node element which stores
+ * // prev/next list links. The list_node element need not be first.
+ * typedef struct my_table
+ * {
+ * char *tablename;
+ * dlist_node list_node;
+ * perm_t permissions;
+ * // ...
+ * } my_table;
+ *
+ * // create a database
+ * my_database *db = create_database();
+ *
+ * // and add a few tables to its table list
+ * dlist_push_head(&db->tables, &create_table(db, "a")->list_node);
+ * ...
+ * dlist_push_head(&db->tables, &create_table(db, "b")->list_node);
+ *
+ *
+ * To iterate over the table list, we allocate an iterator variable and use
+ * a specialized looping construct. Inside a dlist_foreach, the iterator's
+ * 'cur' field can be used to access the current element. iter.cur points to
+ * a 'dlist_node', but most of the time what we want is the actual table
+ * information; dlist_container() gives us that, like so:
+ *
+ * dlist_iter iter;
+ * dlist_foreach(iter, &db->tables)
+ * {
+ * my_table *tbl = dlist_container(my_table, list_node, iter.cur);
+ * printf("we have a table: %s in database %s\n",
+ * tbl->tablename, db->datname);
+ * }
+ *
+ *
+ * While a simple iteration is useful, we sometimes also want to manipulate
+ * the list while iterating. There is a different iterator element and looping
+ * construct for that. Suppose we want to delete tables that meet a certain
+ * criterion:
+ *
+ * dlist_mutable_iter miter;
+ * dlist_foreach_modify(miter, &db->tables)
+ * {
+ * my_table *tbl = dlist_container(my_table, list_node, miter.cur);
+ *
+ * if (!tbl->to_be_deleted)
+ * continue; // don't touch this one
+ *
+ * // unlink the current table from the linked list
+ * dlist_delete(miter.cur);
+ * // as these lists never manage memory, we can still access the table
+ * // after it's been unlinked
+ * drop_table(db, tbl);
+ * }
+ *
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ * src/include/lib/ilist.h
+ *-------------------------------------------------------------------------
+ */
+#ifndef ILIST_H
+#define ILIST_H
+
+/*
+ * Enable for extra debugging. This is rather expensive, so it's not enabled by
+ * default even when USE_ASSERT_CHECKING.
+ */
+/* #define ILIST_DEBUG */
+
+/*
+ * Node of a doubly linked list.
+ *
+ * Embed this in structs that need to be part of a doubly linked list.
+ */
+typedef struct dlist_node dlist_node;
+struct dlist_node
+{
+ dlist_node *prev;
+ dlist_node *next;
+};
+
+/*
+ * Head of a doubly linked list.
+ *
+ * Non-empty lists are internally circularly linked. Circular lists have the
+ * advantage of not needing any branches in the most common list manipulations.
+ * An empty list can also be represented as a pair of NULL pointers, making
+ * initialization easier.
+ */
+typedef struct dlist_head
+{
+ /*
+ * head.next either points to the first element of the list; to &head if
+ * it's a circular empty list; or to NULL if empty and not circular.
+ *
+ * head.prev either points to the last element of the list; to &head if
+ * it's a circular empty list; or to NULL if empty and not circular.
+ */
+ dlist_node head;
+} dlist_head;
+
+
+/*
+ * Doubly linked list iterator.
+ *
+ * Used as state in dlist_foreach() and dlist_reverse_foreach(). To get the
+ * current element of the iteration use the 'cur' member.
+ *
+ * Iterations using this are *not* allowed to change the list while iterating!
+ *
+ * NB: We use an extra "end" field here to avoid multiple evaluations of
+ * arguments in the dlist_foreach() macro.
+ */
+typedef struct dlist_iter
+{
+ dlist_node *cur; /* current element */
+ dlist_node *end; /* last node we'll iterate to */
+} dlist_iter;
+
+/*
+ * Doubly linked list iterator allowing some modifications while iterating.
+ *
+ * Used as state in dlist_foreach_modify(). To get the current element of the
+ * iteration use the 'cur' member.
+ *
+ * Iterations using this are only allowed to change the list at the current
+ * point of iteration. It is fine to delete the current node, but it is *not*
+ * fine to insert or delete adjacent nodes.
+ *
+ * NB: We need a separate type for mutable iterations so that we can store
+ * the 'next' node of the current node in case it gets deleted or modified.
+ */
+typedef struct dlist_mutable_iter
+{
+ dlist_node *cur; /* current element */
+ dlist_node *next; /* next node we'll iterate to */
+ dlist_node *end; /* last node we'll iterate to */
+} dlist_mutable_iter;
+
+/*
+ * Node of a singly linked list.
+ *
+ * Embed this in structs that need to be part of a singly linked list.
+ */
+typedef struct slist_node slist_node;
+struct slist_node
+{
+ slist_node *next;
+};
+
+/*
+ * Head of a singly linked list.
+ *
+ * Singly linked lists are not circularly linked, in contrast to doubly linked
+ * lists; we just set head.next to NULL if empty. This doesn't incur any
+ * additional branches in the usual manipulations.
+ */
+typedef struct slist_head
+{
+ slist_node head;
+} slist_head;
+
+/*
+ * Singly linked list iterator.
+ *
+ * Used as state in slist_foreach(). To get the current element of the
+ * iteration use the 'cur' member.
+ *
+ * It's allowed to modify the list while iterating, with the exception of
+ * deleting the iterator's current node; deletion of that node requires
+ * care if the iteration is to be continued afterward. (Doing so and also
+ * deleting or inserting adjacent list elements might misbehave; also, if
+ * the user frees the current node's storage, continuing the iteration is
+ * not safe.)
+ *
+ * NB: this wouldn't really need to be an extra struct, we could use an
+ * slist_node * directly. We prefer a separate type for consistency.
+ */
+typedef struct slist_iter
+{
+ slist_node *cur;
+} slist_iter;
+
+/*
+ * Singly linked list iterator allowing some modifications while iterating.
+ *
+ * Used as state in slist_foreach_modify(). To get the current element of the
+ * iteration use the 'cur' member.
+ *
+ * The only list modification allowed while iterating is to remove the current
+ * node via slist_delete_current() (*not* slist_delete()). Insertion or
+ * deletion of nodes adjacent to the current node would misbehave.
+ */
+typedef struct slist_mutable_iter
+{
+ slist_node *cur; /* current element */
+ slist_node *next; /* next node we'll iterate to */
+ slist_node *prev; /* prev node, for deletions */
+} slist_mutable_iter;
+
+
+/* Static initializers */
+#define DLIST_STATIC_INIT(name) {{&(name).head, &(name).head}}
+#define SLIST_STATIC_INIT(name) {{NULL}}
+
+
+/* Prototypes for functions too big to be inline */
+
+/* Caution: this is O(n); consider using slist_delete_current() instead */
+extern void slist_delete(slist_head *head, slist_node *node);
+
+#ifdef ILIST_DEBUG
+extern void dlist_check(dlist_head *head);
+extern void slist_check(slist_head *head);
+#else
+/*
+ * These seemingly useless casts to void are here to keep the compiler quiet
+ * about the argument being unused in many functions in a non-debug compile,
+ * in which functions the only point of passing the list head pointer is to be
+ * able to run these checks.
+ */
+#define dlist_check(head) ((void) (head))
+#define slist_check(head) ((void) (head))
+#endif /* ILIST_DEBUG */
+
+/* doubly linked list implementation */
+
+/*
+ * Initialize a doubly linked list.
+ * Previous state will be thrown away without any cleanup.
+ */
+static inline void
+dlist_init(dlist_head *head)
+{
+ head->head.next = head->head.prev = &head->head;
+}
+
+/*
+ * Is the list empty?
+ *
+ * An empty list has either its first 'next' pointer set to NULL, or to itself.
+ */
+static inline bool
+dlist_is_empty(dlist_head *head)
+{
+ dlist_check(head);
+
+ return head->head.next == NULL || head->head.next == &(head->head);
+}
+
+/*
+ * Insert a node at the beginning of the list.
+ */
+static inline void
+dlist_push_head(dlist_head *head, dlist_node *node)
+{
+ if (head->head.next == NULL) /* convert NULL header to circular */
+ dlist_init(head);
+
+ node->next = head->head.next;
+ node->prev = &head->head;
+ node->next->prev = node;
+ head->head.next = node;
+
+ dlist_check(head);
+}
+
+/*
+ * Insert a node at the end of the list.
+ */
+static inline void
+dlist_push_tail(dlist_head *head, dlist_node *node)
+{
+ if (head->head.next == NULL) /* convert NULL header to circular */
+ dlist_init(head);
+
+ node->next = &head->head;
+ node->prev = head->head.prev;
+ node->prev->next = node;
+ head->head.prev = node;
+
+ dlist_check(head);
+}
+
+/*
+ * Insert a node after another *in the same list*
+ */
+static inline void
+dlist_insert_after(dlist_node *after, dlist_node *node)
+{
+ node->prev = after;
+ node->next = after->next;
+ after->next = node;
+ node->next->prev = node;
+}
+
+/*
+ * Insert a node before another *in the same list*
+ */
+static inline void
+dlist_insert_before(dlist_node *before, dlist_node *node)
+{
+ node->prev = before->prev;
+ node->next = before;
+ before->prev = node;
+ node->prev->next = node;
+}
+
+/*
+ * Delete 'node' from its list (it must be in one).
+ */
+static inline void
+dlist_delete(dlist_node *node)
+{
+ node->prev->next = node->next;
+ node->next->prev = node->prev;
+}
+
+/*
+ * Remove and return the first node from a list (there must be one).
+ */
+static inline dlist_node *
+dlist_pop_head_node(dlist_head *head)
+{
+ dlist_node *node;
+
+ Assert(!dlist_is_empty(head));
+ node = head->head.next;
+ dlist_delete(node);
+ return node;
+}
+
+/*
+ * Move element from its current position in the list to the head position in
+ * the same list.
+ *
+ * Undefined behaviour if 'node' is not already part of the list.
+ */
+static inline void
+dlist_move_head(dlist_head *head, dlist_node *node)
+{
+ /* fast path if it's already at the head */
+ if (head->head.next == node)
+ return;
+
+ dlist_delete(node);
+ dlist_push_head(head, node);
+
+ dlist_check(head);
+}
+
+/*
+ * Check whether 'node' has a following node.
+ * Caution: unreliable if 'node' is not in the list.
+ */
+static inline bool
+dlist_has_next(dlist_head *head, dlist_node *node)
+{
+ return node->next != &head->head;
+}
+
+/*
+ * Check whether 'node' has a preceding node.
+ * Caution: unreliable if 'node' is not in the list.
+ */
+static inline bool
+dlist_has_prev(dlist_head *head, dlist_node *node)
+{
+ return node->prev != &head->head;
+}
+
+/*
+ * Return the next node in the list (there must be one).
+ */
+static inline dlist_node *
+dlist_next_node(dlist_head *head, dlist_node *node)
+{
+ Assert(dlist_has_next(head, node));
+ return node->next;
+}
+
+/*
+ * Return previous node in the list (there must be one).
+ */
+static inline dlist_node *
+dlist_prev_node(dlist_head *head, dlist_node *node)
+{
+ Assert(dlist_has_prev(head, node));
+ return node->prev;
+}
+
+/* internal support function to get address of head element's struct */
+static inline void *
+dlist_head_element_off(dlist_head *head, size_t off)
+{
+ Assert(!dlist_is_empty(head));
+ return (char *) head->head.next - off;
+}
+
+/*
+ * Return the first node in the list (there must be one).
+ */
+static inline dlist_node *
+dlist_head_node(dlist_head *head)
+{
+ return (dlist_node *) dlist_head_element_off(head, 0);
+}
+
+/* internal support function to get address of tail element's struct */
+static inline void *
+dlist_tail_element_off(dlist_head *head, size_t off)
+{
+ Assert(!dlist_is_empty(head));
+ return (char *) head->head.prev - off;
+}
+
+/*
+ * Return the last node in the list (there must be one).
+ */
+static inline dlist_node *
+dlist_tail_node(dlist_head *head)
+{
+ return (dlist_node *) dlist_tail_element_off(head, 0);
+}
+
+/*
+ * Return the containing struct of 'type' where 'membername' is the dlist_node
+ * pointed at by 'ptr'.
+ *
+ * This is used to convert a dlist_node * back to its containing struct.
+ */
+#define dlist_container(type, membername, ptr) \
+ (AssertVariableIsOfTypeMacro(ptr, dlist_node *), \
+ AssertVariableIsOfTypeMacro(((type *) NULL)->membername, dlist_node), \
+ ((type *) ((char *) (ptr) - offsetof(type, membername))))
+
+/*
+ * Return the address of the first element in the list.
+ *
+ * The list must not be empty.
+ */
+#define dlist_head_element(type, membername, lhead) \
+ (AssertVariableIsOfTypeMacro(((type *) NULL)->membername, dlist_node), \
+ (type *) dlist_head_element_off(lhead, offsetof(type, membername)))
+
+/*
+ * Return the address of the last element in the list.
+ *
+ * The list must not be empty.
+ */
+#define dlist_tail_element(type, membername, lhead) \
+ (AssertVariableIsOfTypeMacro(((type *) NULL)->membername, dlist_node), \
+ ((type *) dlist_tail_element_off(lhead, offsetof(type, membername))))
+
+/*
+ * Iterate through the list pointed at by 'lhead' storing the state in 'iter'.
+ *
+ * Access the current element with iter.cur.
+ *
+ * It is *not* allowed to manipulate the list during iteration.
+ */
+#define dlist_foreach(iter, lhead) \
+ for (AssertVariableIsOfTypeMacro(iter, dlist_iter), \
+ AssertVariableIsOfTypeMacro(lhead, dlist_head *), \
+ (iter).end = &(lhead)->head, \
+ (iter).cur = (iter).end->next ? (iter).end->next : (iter).end; \
+ (iter).cur != (iter).end; \
+ (iter).cur = (iter).cur->next)
+
+/*
+ * Iterate through the list pointed at by 'lhead' storing the state in 'iter'.
+ *
+ * Access the current element with iter.cur.
+ *
+ * Iterations using this are only allowed to change the list at the current
+ * point of iteration. It is fine to delete the current node, but it is *not*
+ * fine to insert or delete adjacent nodes.
+ */
+#define dlist_foreach_modify(iter, lhead) \
+ for (AssertVariableIsOfTypeMacro(iter, dlist_mutable_iter), \
+ AssertVariableIsOfTypeMacro(lhead, dlist_head *), \
+ (iter).end = &(lhead)->head, \
+ (iter).cur = (iter).end->next ? (iter).end->next : (iter).end, \
+ (iter).next = (iter).cur->next; \
+ (iter).cur != (iter).end; \
+ (iter).cur = (iter).next, (iter).next = (iter).cur->next)
+
+/*
+ * Iterate through the list in reverse order.
+ *
+ * It is *not* allowed to manipulate the list during iteration.
+ */
+#define dlist_reverse_foreach(iter, lhead) \
+ for (AssertVariableIsOfTypeMacro(iter, dlist_iter), \
+ AssertVariableIsOfTypeMacro(lhead, dlist_head *), \
+ (iter).end = &(lhead)->head, \
+ (iter).cur = (iter).end->prev ? (iter).end->prev : (iter).end; \
+ (iter).cur != (iter).end; \
+ (iter).cur = (iter).cur->prev)
+
+
+/* singly linked list implementation */
+
+/*
+ * Initialize a singly linked list.
+ * Previous state will be thrown away without any cleanup.
+ */
+static inline void
+slist_init(slist_head *head)
+{
+ head->head.next = NULL;
+}
+
+/*
+ * Is the list empty?
+ */
+static inline bool
+slist_is_empty(slist_head *head)
+{
+ slist_check(head);
+
+ return head->head.next == NULL;
+}
+
+/*
+ * Insert a node at the beginning of the list.
+ */
+static inline void
+slist_push_head(slist_head *head, slist_node *node)
+{
+ node->next = head->head.next;
+ head->head.next = node;
+
+ slist_check(head);
+}
+
+/*
+ * Insert a node after another *in the same list*
+ */
+static inline void
+slist_insert_after(slist_node *after, slist_node *node)
+{
+ node->next = after->next;
+ after->next = node;
+}
+
+/*
+ * Remove and return the first node from a list (there must be one).
+ */
+static inline slist_node *
+slist_pop_head_node(slist_head *head)
+{
+ slist_node *node;
+
+ Assert(!slist_is_empty(head));
+ node = head->head.next;
+ head->head.next = node->next;
+ slist_check(head);
+ return node;
+}
+
+/*
+ * Check whether 'node' has a following node.
+ */
+static inline bool
+slist_has_next(slist_head *head, slist_node *node)
+{
+ slist_check(head);
+
+ return node->next != NULL;
+}
+
+/*
+ * Return the next node in the list (there must be one).
+ */
+static inline slist_node *
+slist_next_node(slist_head *head, slist_node *node)
+{
+ Assert(slist_has_next(head, node));
+ return node->next;
+}
+
+/* internal support function to get address of head element's struct */
+static inline void *
+slist_head_element_off(slist_head *head, size_t off)
+{
+ Assert(!slist_is_empty(head));
+ return (char *) head->head.next - off;
+}
+
+/*
+ * Return the first node in the list (there must be one).
+ */
+static inline slist_node *
+slist_head_node(slist_head *head)
+{
+ return (slist_node *) slist_head_element_off(head, 0);
+}
+
+/*
+ * Delete the list element the iterator currently points to.
+ *
+ * Caution: this modifies iter->cur, so don't use that again in the current
+ * loop iteration.
+ */
+static inline void
+slist_delete_current(slist_mutable_iter *iter)
+{
+ /*
+ * Update previous element's forward link. If the iteration is at the
+ * first list element, iter->prev will point to the list header's "head"
+ * field, so we don't need a special case for that.
+ */
+ iter->prev->next = iter->next;
+
+ /*
+ * Reset cur to prev, so that prev will continue to point to the prior
+ * valid list element after slist_foreach_modify() advances to the next.
+ */
+ iter->cur = iter->prev;
+}
+
+/*
+ * Return the containing struct of 'type' where 'membername' is the slist_node
+ * pointed at by 'ptr'.
+ *
+ * This is used to convert a slist_node * back to its containing struct.
+ */
+#define slist_container(type, membername, ptr) \
+ (AssertVariableIsOfTypeMacro(ptr, slist_node *), \
+ AssertVariableIsOfTypeMacro(((type *) NULL)->membername, slist_node), \
+ ((type *) ((char *) (ptr) - offsetof(type, membername))))
+
+/*
+ * Return the address of the first element in the list.
+ *
+ * The list must not be empty.
+ */
+#define slist_head_element(type, membername, lhead) \
+ (AssertVariableIsOfTypeMacro(((type *) NULL)->membername, slist_node), \
+ (type *) slist_head_element_off(lhead, offsetof(type, membername)))
+
+/*
+ * Iterate through the list pointed at by 'lhead' storing the state in 'iter'.
+ *
+ * Access the current element with iter.cur.
+ *
+ * It's allowed to modify the list while iterating, with the exception of
+ * deleting the iterator's current node; deletion of that node requires
+ * care if the iteration is to be continued afterward. (Doing so and also
+ * deleting or inserting adjacent list elements might misbehave; also, if
+ * the user frees the current node's storage, continuing the iteration is
+ * not safe.)
+ */
+#define slist_foreach(iter, lhead) \
+ for (AssertVariableIsOfTypeMacro(iter, slist_iter), \
+ AssertVariableIsOfTypeMacro(lhead, slist_head *), \
+ (iter).cur = (lhead)->head.next; \
+ (iter).cur != NULL; \
+ (iter).cur = (iter).cur->next)
+
+/*
+ * Iterate through the list pointed at by 'lhead' storing the state in 'iter'.
+ *
+ * Access the current element with iter.cur.
+ *
+ * The only list modification allowed while iterating is to remove the current
+ * node via slist_delete_current() (*not* slist_delete()). Insertion or
+ * deletion of nodes adjacent to the current node would misbehave.
+ */
+#define slist_foreach_modify(iter, lhead) \
+ for (AssertVariableIsOfTypeMacro(iter, slist_mutable_iter), \
+ AssertVariableIsOfTypeMacro(lhead, slist_head *), \
+ (iter).prev = &(lhead)->head, \
+ (iter).cur = (iter).prev->next, \
+ (iter).next = (iter).cur ? (iter).cur->next : NULL; \
+ (iter).cur != NULL; \
+ (iter).prev = (iter).cur, \
+ (iter).cur = (iter).next, \
+ (iter).next = (iter).next ? (iter).next->next : NULL)
+
+#endif /* ILIST_H */
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