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+# ID Map
+
+Internally, _NNG_ uses a map of numeric identifiers to data structures.
+This feature is also exposed for application use, as a "supplemental" feature.
+
+When using these functions, it is necessary to add the `#include <nng/supplemental/util/idhash.h>`
+include file to list of includes.
+
+## ID Map Structure
+
+```c
+#include <nng/nng.h>
+#include <nng/supplemental/util/idhash.h>
+
+typedef struct nng_id_map_s nng_id_map;
+```
+
+The ID map structure, {{i:`nng_id_map`}} provides a table of identifiers mapping
+to user-supplied pointers (which must not be `NULL`). The identifiers can be
+thought of as indices into the table, with the pointers providing the reference
+for the user supplied data.
+
+The values of identifiers can be supplied by the user, or can be allocated automatically
+by `nng_id_map` from a predefined range. The starting point for allocations
+can also be randomly within the range.
+
+The identifiers are 64-bit unsigned integers and can be sparse; the structure
+will use space efficiently even if identifiers are very far apart.
+{{footnote: The ID map is capable of storing at most 2<sup>32</sup> identifiers, even though the identifers may
+themselves be much larger than this.}}
+
+> [!IMPORTANT]
+> The function available for `nng_id_map` are _not_ thread-safe.
+> Callers should use a [mutex][nng_mutex] or similar approach when thread-safety is needed.
+
+## Create ID Map
+
+```c
+#define NNG_MAP_RANDOM 1
+
+int nng_id_map_alloc(nng_id_map **map_p, uint64_t lo, uint64_t hi, int flags);
+```
+
+The {{i:`nng_id_map_alloc`}} function allocates a map without any data in it,
+and returns a pointer to it in _map_p_. When allocating identifiers dynamically,
+the values will be chosen from the range defined by _lo_ and _hi_, inclusive.
+
+The _flags_ argument is a bit mask of flags that can adjust behavior of the map.
+The only flag defined at present
+is `NNG_MAP_RANDOM`, which causes the first identifier allocation to start at a random
+point within the range.
+This is useful to reduce the odds of different instances of an application using
+the same identifiers at the same time.
+
+If both _lo_ and _hi_ are zero, then the values `0` and `0xffffffff` are substituted
+in their place, giving a full range of 32-bit identifiers.
+
+This function can return `NNG_ENOMEM` if it is unable to allocate resources, otherwise
+it returns zero on success.
+
+## Destroy Map
+
+```c
+void nng_id_map_free(nng_id_map *map);
+```
+
+The {{i:`nng_id_map_free`}} function destroys _map_, releasing any resources associated
+with it.
+
+> [!NOTE]
+> The `nng_id_map_free` frees the map itself, but will not free memory associated with
+> any strctures contained within it.
+
+## Store a Value
+
+```c
+int nng_id_set(nng_id_map *map, uint64_t id, void *value);
+```
+
+The {{i:`nng_id_map_set`}} function is used to store the _value_ in the _map_ at
+index _id_.
+
+If another value is already stored at that same location, then it is overwritten with
+_value_.
+
+> [!NOTE]
+> The _value_ must not be `NULL`.
+
+If the table has to grow to accommodate this value, it may fail if insufficient
+memory is available, returning `NNG_ENOMEM`. OtherwiseG it returns zero.
+
+## Lookup a Value
+
+```c
+void *nng_id_get(nng_id_map *map, uint64_t id);
+```
+
+The {{i:`nng_id_get`}} function looks up the entry for _id_ in _map_, returning the
+associated value if present, or `NULL` if no such entry exists.
+
+## Allocate an ID
+
+```c
+int nng_id_alloc(nng_id_map *map, uint64_t *id_p, void *value);
+```
+
+The {{i:`nng_id_alloc`}} stores the _value_ in the _map_, at a newly allocated index,
+and returns the index in _id_p_.
+
+Identifiers are allocated in increasing order, without reusing old identifiers until the
+largest possible identifier is allocated. After wrapping, only identifiers that are no longer
+in use will be considered.
+No effort is made to order the availability of identifiers based on
+when they were freed.{{footnote: The concern about possibly reusing a
+recently released identifier comes into consideration after the range has wrapped.
+Given a sufficiently large range, this is unlikely to be a concern.}}
+
+As with [`nng_id_set`][nng_id_set], this may need to allocate memory and can thus
+fail with `NNG_ENOMEM`.
+
+Additionally, if there are no more free identifiers within the range specified
+when _map_ was created, then it will return `NNG_ENOSPC`.
+
+Otherwise it returns zero, indicating success.
+
+## Remove an ID
+
+```c
+int nng_id_remove(nng_id_map *map, uint64_t id);
+```
+
+The {{i:`nng_id_remove`}} removes the entry at index _id_ from _map_.
+
+If no such entry exist, it will return `NNG_ENOENT`. Otherwise it returns zero.
+
+## Iterating IDs
+
+```c
+bool nng_id_visit(nng_id_map *map, uint64_t *id_p, void **value_p, uint32_t *cursor);
+```
+
+The {{i:`nng_id_visit`}} function is used to iterate over all items in the table.
+The caller starts the iteration by setting the _cursor_ to 0 before calling it.
+For each call, the associated key and value of the next item will be returned in _id_p_,
+and _value_p_ and the _cursor_ will be updated.
+When all items have been iterated, the function returns `false`.
+The order of items returned is not guaranteed to be sequential.
+The caller must not attempt to derive any value of the _cursor_ as it refers to internal table indices.
+
+Entries may be safely removed from _map_ while iterating.
+
+However, if new entries are added to the table while iterating, the result of
+iteration is undefined; entries may be repeated or omitted during such an iteration.
+
+The caller must not attempt to derive any value of the _cursor_ as it refers to internal
+table indices.
+
+[nng_id_set]: #store-a-value
+[nng_mutex]: synch.md#mutual-exclusion-lock