Editing Hash function (section)

=== Identity hash function ===
If the data to be hashed is small enough, then one can use the data itself (reinterpreted as an integer) as the hashed value. The cost of computing this ''[[identity function|identity]]'' hash function is effectively zero. This hash function is [[Perfect hash function|perfect]], as it maps each input to a distinct hash value.

The meaning of "small enough" depends on the size of the type that is used as the hashed value. For example, in [[Java (programming language)|Java]], the hash code is a 32-bit integer. Thus the 32-bit integer <code>Integer</code> and 32-bit floating-point <code>Float</code> objects can simply use the value directly, whereas the 64-bit integer <code>Long</code> and 64-bit floating-point <code>Double</code> cannot.

Other types of data can also use this hashing scheme. For example, when mapping [[character string]]s between [[letter case|upper and lower case]], one can use the binary encoding of each character, interpreted as an integer, to index a table that gives the alternative form of that character ("A" for "a", "8" for "8", etc.).  If each character is stored in 8 bits (as in [[extended ASCII]]<ref group=Notes>Plain [[ASCII]] is a 7-bit character encoding, although it is often stored in 8-bit bytes with the highest-order bit always clear (zero).  Therefore, for plain ASCII, the bytes have only 2<sup>7</sup> = 128 valid values, and the character translation table has only this many entries.</ref> or [[ISO Latin 1]]), the table has only 2<sup>8</sup> = 256 entries; in the case of [[Unicode]] characters, the table would have 17 × 2<sup>16</sup> = {{Val|1114112}} entries.

The same technique can be used to map [[ISO 3166-1 alpha-2|two-letter country codes]] like "us" or "za" to country names (26<sup>2</sup> = 676 table entries), 5-digit [[ZIP Code|ZIP codes]] like 13083 to city names ({{val|100000}} entries), etc. Invalid data values (such as the country code "xx" or the ZIP code 00000) may be left undefined in the table or mapped to some appropriate "null" value.