Control table: Difference between revisions

Revision as of 10:02, 9 August 2025

Revision as of 10:09, 9 August 2025

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A table can be structured in a variety of ways. It may have one or multiple dimensions and be of fixed or [[variable length code |variable length]]. The structure of the table may be similar to a [[multimap (data structure)|multimap]] [[associative array]], where a data value (or combination of data values) may be mapped to one or more functions to be performed. Often, the structure allows tabular data is [[software portability |portable]] between [[computer platform]]s as long as a compatible interpreter exists on each platform.

===SparseTrival one-dimensionalhash arrayfunction===

A relatively simple implementation consists of a [[sparse file |sparse]], one-dimensional array of values. The index space is fully covered by the array such that lookup involves indexing into the array by the input value, and a value is found even for an index that is not intended to be used; preventing an error that might otherwise occur for an unused index value. The lookup is achieved in [[constant time]]; without searching. In most [[computer architecture |architecture]]s, this can be accomplished in two or three [[machine instruction]]s. The technique is known as a "[[trivial hash function]]" or, when used specifically for branch tables, "[[double dispatch]]".

To be feasible, the range of index values should be relatively small. In the example below, the control table is indexed by ASCII value so it has 256 entries; an entry for each ASCII value; omitted entries are shown as '...'. Only the values for the letter A, D, M, and S are important. All other values are uninteresting and set to 0. A two-byte index would require a minimum of 65,536 entries to handle all input possibilities.

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In automata-based programming and [[pseudoconversational transaction]] processing, if the number of distinct program states is small, a "dense sequence" control variable can be used to efficiently dictate the entire flow of the main program loop.

A two byte raw data value would require a ''minimum'' table size of 65,536 bytes – to handle all input possibilities – whilst allowing just 256 different output values. However, this direct translation technique provides an extremely fast [[data validation|validation]] & conversion to a (relative) subroutine pointer if the [[heuristic]]s, together with sufficient fast access memory, permits its use.

===Branch tables===