UTF16iin

UTF16iin is a hypothetical encoding form related to the UTF-16 family. It is not part of the Unicode Standard and has no official specification. In this article, UTF16iin is described as a variant intended to illustrate how a UTF-16-like encoding could incorporate an internal indexing mechanism to speed up character access.

Like UTF-16, UTF16iin encodes text in 16-bit code units. Characters in the Basic Multilingual Plane are represented directly by a single unit, while supplementary code points are encoded using a surrogate pair of high (0xD800–0xDBFF) and low (0xDC00–0xDFFF) surrogates. The "iin" feature proposes an optional internal indexing layer that maps code points to positions within a string, enabling faster lookup and slicing operations in memory-resident processing. This indexing is not embedded in the byte stream by default but can be materialized by conformant implementations.

Endianness is handled as in UTF-16, with a Byte Order Mark indicating the encoding's endianness if present. As with UTF-16, unpaired surrogates are treated as encoding errors; decoders may replace them with the Unicode replacement character, depending on configuration.

Because the external representation mirrors UTF-16, UTF16iin would be largely interoperable with UTF-16 data and tools. The internal indexing feature is intended to improve random-access performance in environments such as in-memory data structures or operations on long strings, while keeping compatibility with existing UTF-16 text when the index is not used or is omitted.

Status: UTF16iin is not an established standard. There is no official specification or central governance, and naming or implementation details may vary. See also: UTF-16, Unicode, surrogate pairs.