RCR allow indexing last n items

Hello

As  discussed in thread starting with ruby-talk # 284001 there is an
inconsistency in handling positive and negative range indexes to
arrays and strings.

Currently the requirement is that the first number in the range does
fall into the string.

This allows "asd'[0...10] to return the first (at most) 10 characters
of the string.

However, the same cannot be done on the end:

"asd"[-10..-1] returns nil even though this could mean the last (at
most) 10 characters as direct opposite of the above. However, the
first number (-10) yields an index length-10 = -7 which is outside of
the string.

To make it possible to index the end of the string as easily as the
beginning the rules for indexing should be changed slightly.

currently they can be described as follows:

1) convert any negative indexes by substracting the value of the index
from length
2) if the first index does not fall into 0...length return nil
3) if the second index is >= length change it to (length - 1)
4) if first index >= second return "" (or [])
5) return the slice first..second

To provide for indexing of the end

0) note if both indexes are positive pos = (first >= 0) && (second >= 0)
or both negative neg = (first < 0) && (last < 0)
1) (as above) convert any negative indexes by substracting them from length
first = length - first if first < 0
2) return nil if both indexes were positive and the first index does
not fall into 0...length or both indexes were negative and the second
index does not fall into 0...length

return nil if pos && ! (0..length).include? first
return nil if neg && ! (0..length).include? second

Now the indexes either specify an empty string ( first >= second ), or
they were both the same sign, and we just ensured that they specify a
part of the string, or they were different sign so they must specify a
part of the string because they count from both ends towards each
other.

3) adjust any indexes that reach too far
first = 0 if first < 0
second = length - 1 if second >= length

4-5 as above.


I would submit this to rcrchive but for some reason I cannot sign up for it.

Thanks

Michal