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+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * SPDX-License-Identifier: MPL-2.0
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, you can obtain one at https://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+#pragma once
+
+/*****
+***** Module Info
+*****/
+
+/*! \file isc/symtab.h
+ * \brief Provides a simple memory-based symbol table.
+ *
+ * Keys are C strings, and key comparisons are case-insensitive.  A type may
+ * be specified when looking up, defining, or undefining.  A type value of
+ * 0 means "match any type"; any other value will only match the given
+ * type.
+ *
+ * It's possible that a client will attempt to define a <key, type, value>
+ * tuple when a tuple with the given key and type already exists in the table.
+ * What to do in this case is specified by the client.  Possible policies are:
+ *
+ *\li	#isc_symexists_reject	Disallow the define, returning #ISC_R_EXISTS
+ *\li	#isc_symexists_replace	Replace the old value with the new.  The
+ *				undefine action (if provided) will be called
+ *				with the old <key, type, value> tuple.
+ *\li	#isc_symexists_add	Add the new tuple, leaving the old tuple in
+ *				the table.  Subsequent lookups will retrieve
+ *				the most-recently-defined tuple.
+ *
+ * A lookup of a key using type 0 will return the most-recently defined
+ * symbol with that key.  An undefine of a key using type 0 will undefine the
+ * most-recently defined symbol with that key.  Trying to define a key with
+ * type 0 is illegal.
+ *
+ * The symbol table library does not make a copy the key field, so the
+ * caller must ensure that any key it passes to isc_symtab_define() will not
+ * change until it calls isc_symtab_undefine() or isc_symtab_destroy().
+ *
+ * A user-specified action will be called (if provided) when a symbol is
+ * undefined.  It can be used to free memory associated with keys and/or
+ * values.
+ *
+ * A symbol table is implemented as a hash table of lists; the size of the
+ * hash table is set by the 'size' parameter to isc_symtbl_create().  When
+ * the number of entries in the symbol table reaches three quarters of this
+ * value, the hash table is reallocated with size doubled, in order to
+ * optimize lookup performance.  This has a negative effect on insertion
+ * performance, which can be mitigated by sizing the table appropriately
+ * when creating it.
+ *
+ * \li MP:
+ *	The callers of this module must ensure any required synchronization.
+ *
+ * \li Reliability:
+ *	No anticipated impact.
+ *
+ * \li Resources:
+ *	TBS
+ *
+ * \li Security:
+ *	No anticipated impact.
+ *
+ * \li Standards:
+ *	None.
+ */
+
+/***
+ *** Imports.
+ ***/
+
+#include <stdbool.h>
+
+#include <isc/lang.h>
+#include <isc/types.h>
+
+/*
+ *** Symbol Tables.
+ ***/
+/*% Symbol table value. */
+typedef union isc_symvalue {
+	void	    *as_pointer;
+	const void  *as_cpointer;
+	int	     as_integer;
+	unsigned int as_uinteger;
+} isc_symvalue_t;
+
+typedef void (*isc_symtabaction_t)(char *key, unsigned int type,
+				   isc_symvalue_t value, void *userarg);
+/*% Symbol table exists. */
+typedef enum {
+	isc_symexists_reject = 0,  /*%< Disallow the define */
+	isc_symexists_replace = 1, /*%< Replace the old value with the new */
+	isc_symexists_add = 2	   /*%< Add the new tuple */
+} isc_symexists_t;
+
+ISC_LANG_BEGINDECLS
+
+/*% Create a symbol table. */
+isc_result_t
+isc_symtab_create(isc_mem_t *mctx, unsigned int size,
+		  isc_symtabaction_t undefine_action, void *undefine_arg,
+		  bool case_sensitive, isc_symtab_t **symtabp);
+
+/*% Destroy a symbol table. */
+void
+isc_symtab_destroy(isc_symtab_t **symtabp);
+
+/*% Lookup a symbol table. */
+isc_result_t
+isc_symtab_lookup(isc_symtab_t *symtab, const char *key, unsigned int type,
+		  isc_symvalue_t *value);
+
+/*% Define a symbol table. */
+isc_result_t
+isc_symtab_define(isc_symtab_t *symtab, const char *key, unsigned int type,
+		  isc_symvalue_t value, isc_symexists_t exists_policy);
+
+/*% Undefine a symbol table. */
+isc_result_t
+isc_symtab_undefine(isc_symtab_t *symtab, const char *key, unsigned int type);
+
+/*% Return the number of items in a symbol table. */
+unsigned int
+isc_symtab_count(isc_symtab_t *symtab);
+ISC_LANG_ENDDECLS