UTF-32 (32-bit Unicode Transformation Format) is a fixed-length encoding used to encode Unicode code points that uses exactly 32 bits (four bytes) per code point (but a number of leading bits must be zero as there are far fewer than 232 Unicode code points, needing actually only 21 bits).[1] UTF-32 is a fixed-length encoding, in contrast to all other Unicode transformation formats, which are variable-length encodings. Each 32-bit value in UTF-32 represents one Unicode code point and is exactly equal to that code point's numerical value.
The main advantage of UTF-32 is that the Unicode code points are directly indexed. Finding the Nth code point in a sequence of code points is a constant-time operation. In contrast, a variable-length code requires linear-time to count N code points from the start of the string. This makes UTF-32 a simple replacement in code that uses integers that are incremented by one to examine each location in a string, as was commonly done for ASCII. However, Unicode code points are rarely processed in complete isolation, such as combining character sequences and for emoji.[2]
The main disadvantage of UTF-32 is that it is space-inefficient, using four bytes per code point, including 11 bits that are always zero. Characters beyond the BMP are relatively rare in most texts (except for e.g. texts with some popular emojis), and can typically be ignored for sizing estimates. This makes UTF-32 close to twice the size of UTF-16. It can be up to four times the size of UTF-8 depending on how many of the characters are in the ASCII subset.[2]
The original ISO/IEC 10646 standard defines a 32-bit encoding form called UCS-4, in which each code point in the Universal Character Set (UCS) is represented by a 31-bit value from 0 to 0x7FFFFFFF (the sign bit was unused and zero). In November 2003, Unicode was restricted by RFC 3629 to match the constraints of the UTF-16 encoding: explicitly prohibiting code points greater than U+10FFFF (and also the high and low surrogates U+D800 through U+DFFF). This limited subset defines UTF-32.[3][1] Although the ISO standard had (as of 1998 in Unicode 2.1) "reserved for private use" 0xE00000 to 0xFFFFFF, and 0x60000000 to 0x7FFFFFFF[4] these areas were removed in later versions. Because the Principles and Procedures document of ISO/IEC JTC 1/SC 2 Working Group 2 states that all future assignments of code points will be constrained to the Unicode range, UTF-32 will be able to represent all UCS code points and UTF-32 and UCS-4 are identical.
Though a fixed number of bytes per code point seems convenient, it is not as useful as it appears. It makes truncation easier but not significantly so compared to UTF-8 and UTF-16 (both of which can search backwards for the point to truncate by looking at 2–4 code units at most).[a][citation needed]
It is extremely rare that code wishes to find the Nth character in a string without first having to calculate N by examining all the characters between this position and one end of the string.[5] For instance any LALR parser (such as for XML) needs to look at all the previous code points before being able to do anything with the Nth code point. An integer index that is incremented by 1 for each character can be replaced with an integer offset, measured in code units and incremented by the number of code units as each character is examined. This removes any speed advantage of UTF-32.[citation needed]