I originally posted this in a "Re: Last Call: <draft-ietf-appsawg-json-patch-08.txt> (JSON Patch) to Proposed Standard" thread
but now I've read draft-ietf-appsawg-json-pointer-07 and have changed that suggestion to reflect draft-ietf-appsawg-json-pointer-07
Suggestion: allow negative numbers to indicate a position counted from the end of the array instead "array/-".
For example, a/-1 to point to the last item in an array (length > 0), -2 to point to the penultimate item, and so on.
The notation -i is an alias for (array.length - i)
It is an error if |i| > array length.
For the json-patch "add" operation, use
{ "op" : "add", "path" : "...array/-1", "value" : value }
to append to the end, "...array/-2" to insert before the last item, and so on.
The "path" is the path that the item will be addressed by after inserting it.
I.e. after appending to an array of length l at j < 0, the item will be at the zero-based index (l + 1) + j.
E.g. given the document with an array of length 4:
{ "a" : [ 0, 1, 2, 3, ] }
then
{ "op" : "add", "path" : "/a/-1", "value" : 99 }
yields
{ "a" : [ 0, 1, 2, 3, 99] }
i.e. 99 is found at (4 + 1) -1 == 4
{ "op" : "add", "path" : "/a/-2", "value" : 99}
yields
{ "a" : [ 0, 1, 2, 99, 3] }
i.e. 99 is found at (4 + 1) -2 == 3
and finally
{ "op" : "add", "path" : "/a/-5", "value" : 99 }
yields
{ "a" : [ 99, 0, 1, 2, 3] }
i.e. 99 is found at (4 + 1) -5 == 0
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David J. Biesack | Principal API Architect, SAS | @davidbiesack | 919-531-7771 | www.sas.com