1 files changed, 121 insertions, 0 deletions
diff --git a/src/util/ring.c b/src/util/ring.c
new file mode 100644
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+++ b/src/util/ring.c
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+/*++
+/* NAME
+/*	ring 3
+/* SUMMARY
+/*	circular list management
+/* SYNOPSIS
+/*	#include <ring.h>
+/*
+/*	void	ring_init(list)
+/*	RING	*list;
+/*
+/*	void	ring_prepend(list, element)
+/*	RING	*list;
+/*	RING	*element;
+/*
+/*	void	ring_append(list, element)
+/*	RING	*list;
+/*	RING	*element;
+/*
+/*	RING	*ring_pred(element)
+/*	RING	*element;
+/*
+/*	RING	*ring_succ(element)
+/*	RING	*element;
+/*
+/*	void	ring_detach(element)
+/*	RING	*element;
+/*
+/*	RING_FOREACH(RING *element, RING *head)
+/* DESCRIPTION
+/*	This module manages circular, doubly-linked, lists. It provides
+/*	operations to initialize a list, to add or remove an element,
+/*	and to iterate over a list. Although the documentation appears
+/*	to emphasize the special role of the list head, each operation
+/*	can be applied to each list member.
+/*
+/*	Examples of applications: any sequence of objects such as queue,
+/*	unordered list, or stack. Typically, an application embeds a RING
+/*	structure into its own data structure, and uses the RING primitives
+/*	to maintain the linkage between application-specific data objects.
+/*
+/*	ring_init() initializes its argument to a list of just one element.
+/*
+/*	ring_append() appends the named element to the named list head.
+/*
+/*	ring_prepend() prepends the named element to the named list head.
+/*
+/*	ring_succ() returns the list element that follows its argument.
+/*
+/*	ring_pred() returns the list element that precedes its argument.
+/*
+/*	ring_detach() disconnects a list element from its neighbors
+/*	and closes the hole. This routine performs no implicit ring_init()
+/*	on the removed element.
+/*
+/*	RING_FOREACH() is a macro that expands to a for (... ; ... ; ...)
+/*	statement that iterates over each list element in forward order.
+/*	Upon completion, the \fIelement\fR variable is set equal to
+/*	\fIhead\fR. The list head itself is not treated as a list member.
+/* LICENSE
+/* .ad
+/* .fi
+/*	The Secure Mailer license must be distributed with this software.
+/* AUTHOR(S)
+/*	Wietse Venema
+/*	IBM T.J. Watson Research
+/*	P.O. Box 704
+/*	Yorktown Heights, NY 10598, USA
+/*--*/
+
+/* System libraries. */
+
+/* Application-specific. */
+
+#include "ring.h"
+
+/* ring_init - initialize ring head */
+
+void    ring_init(ring)
+RING   *ring;
+{
+    ring->pred = ring->succ = ring;
+}
+
+/* ring_append - insert entry after ring head */
+
+void    ring_append(ring, entry)
+RING   *ring;
+RING   *entry;
+{
+    entry->succ = ring->succ;
+    entry->pred = ring;
+    ring->succ->pred = entry;
+    ring->succ = entry;
+}
+
+/* ring_prepend - insert new entry before ring head */
+
+void    ring_prepend(ring, entry)
+RING   *ring;
+RING   *entry;
+{
+    entry->pred = ring->pred;
+    entry->succ = ring;
+    ring->pred->succ = entry;
+    ring->pred = entry;
+}
+
+/* ring_detach - remove entry from ring */
+
+void    ring_detach(entry)
+RING   *entry;
+{
+    RING   *succ = entry->succ;
+    RING   *pred = entry->pred;
+
+    pred->succ = succ;
+    succ->pred = pred;
+
+    entry->succ = entry->pred = 0;
+}