PATH Mac OS 8 and 9 Developer Documentation > Text Encoding Conversion Manager
Programming With the Text Encoding Conversion Manager

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Strict and Loose Mapping

A strict mapping preserves the information content of text and permits round-trip fidelity. A loose mapping preserves the information content of text but does not permit round-trip fidelity. A mapping table has both strict equivalence and loose mapping sections that identify how a mapping is to occur. Loose and strict mappings occur within the context of multiple semantics and multiple representations.

First, an example that illustrates the difference in the case of multiple semantics. The ASCII character at 0x2D is called HYPHEN-MINUS. Unicode includes a HYPHEN-MINUS character at U+002D for ASCII compatibility. However, Unicode also has separate characters HYPHEN (U+2010) and MINUS SIGN (U+2212); each of these characters represents one aspect of the meaning of HYPHEN-MINUS.

The ASCII character HYPHEN-MINUS is typically mapped to Unicode HYPHEN-MINUS. All three of the Unicode characters--HYPHEN-MINUS, HYPHEN, and MINUS SIGN--should typically be mapped to ASCII HYPHEN-MINUS, since it includes all of their meanings. The mapping from Unicode HYPHEN-MINUS to ASCII is strict, since mapping from ASCII back to Unicode produces the original Unicode character. However, the mappings from Unicode HYPHEN and MINUS SIGN to ASCII are loose, since they do not provide round-trip fidelity. The mapping from ASCII HYPHEN-MINUS to Unicode is, of course, strict.

Second, an example that illustrates the difference in the case of multiple representation. The Latin-1 character LATIN SMALL LETTER A WITH ACUTE (0xE1) is typically mapped to Unicode LATIN SMALL LETTER A WITH ACUTE (U+00E1), so the reverse is a strict mapping. However, the Unicode sequence LATIN SMALL LETTER A plus COMBINING ACUTE ACCENT can also be mapped to the Latin-1 character as a loose mapping.

There are two important things to note here. First, calling a mapping from one character set to another strict or loose depends on how the second character set is mapped back to the first; strictness or looseness depends on the mappings in both directions. Second, neither strict nor loose mappings necessarily preserve the number of characters; either can map a sequence of one or more characters in the source encoding to one or more characters in the destination encoding.