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25.16 Replicate and rotate

Function: %xsubstring from to str start past
Macro: xsubstring S from to

Extended substring procedure replicating the selected substring “up and down” index space, in both the positive and negative directions.

The call:

(%xsubstring from to "abcdefg" 3 6)

selects the substring "def" and defines the conceptual bidirectionally–infinite string:

...  e  f  d  e  f  d  e  f  d  e  f ...
... -5 -4 -3 -2 -1  0 +1 +2 +3 +4 +5 ...

that is def repeated in both directions. %xsubstring returns the substring of this string beginning at index from, and ending at to.

We can use this function to perform a variety of tasks:

Note that:

It is an error if start equals past.

Function: xstring-xcopy! from to target target-start source source-start source-past
Macro: string-xcopy! target-S source-S from to

Exactly the same as %xsubstring, but the extracted text is written into the selected substring of target.

This operation is not defined if (eq? target start); we cannot copy a string on top of itself.

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