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diff --git a/src/core/idhash.h b/src/core/idhash.h
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+//
+// Copyright 2016 Garrett D'Amore <garrett@damore.org>
+//
+// This software is supplied under the terms of the MIT License, a
+// copy of which should be located in the distribution where this
+// file was obtained (LICENSE.txt).  A copy of the license may also be
+// found online at https://opensource.org/licenses/MIT.
+//
+
+#ifndef CORE_IDHASH_H
+#define CORE_IDHASH_H
+
+#include "core/nng_impl.h"
+
+// We find that we often want to have a list of things listed by a
+// numeric ID, which is generally monotonically increasing.  This is
+// most often a pipe ID.  To help keep collections of these things
+// indexed by their ID (which might start from a very large value),
+// we offer a hash table.  The hash table uses open addressing, but
+// we use a better probe (taken from Python) to avoid hitting the same
+// positions.  Our hash algorithm is just the low order bits, and we
+// use table sizes that are powers of two.  Note that hash items
+// must be non-NULL.  The caller is responsible for providing any
+// locking required.
+
+// In order to make life easy, we just define the ID hash structure
+// directly, and let consumers directly inline it.
+typedef struct {
+	uint32_t	ihe_key;
+	void *		ihe_val;
+} nni_idhash_entry;
+
+typedef struct {
+	uint32_t		ih_cap;
+	uint32_t		ih_count;
+	uint32_t		ih_mincount;
+	uint32_t		ih_maxcount;
+	nni_idhash_entry *	ih_entries;
+} nni_idhash;
+
+// nni_idhash_walkfn is called when walking a hash table.  If the
+// return value is non-zero, then nni_idhash_walk will terminate further
+// process and return that return value.  The function takes the generic
+// opaque value for the walk as its first argument, and the next two
+// arguments are the hash key and the opaque value stored with it.
+typedef int (*nni_idhash_walkfn)(void *, uint32_t, void *);
+extern int nni_idhash_init(nni_idhash *);
+extern void nni_idhash_fini(nni_idhash *);
+extern int nni_idhash_find(nni_idhash *, uint32_t, void **);
+extern int nni_idhash_remove(nni_idhash *, uint32_t);
+extern int nni_idhash_insert(nni_idhash *, uint32_t, void *);
+extern int nni_idhash_count(nni_idhash *, uint32_t *);
+extern int nni_idhash_walk(nni_idhash *, nni_idhash_walkfn, void *);
+
+#endif  // CORE_IDHASH_H