`JsonPointer.head()` throws `StringIndexOutOfBoundsException` for valid JSON Pointers

[sergeykad]

Problem Description:
After upgrading to Jackson 2.19.0, valid JsonPointer strings are causing StringIndexOutOfBoundsException when calling head().
The crash happens due to a negative suffixLength calculated in _constructHead().
This was not an issue in previous versions (2.18.x and earlier).

Caused by: java.lang.StringIndexOutOfBoundsException: begin 0, end 14, length 6
    at java.base/java.lang.String.checkBoundsBeginEnd(String.java:4606)
    at java.base/java.lang.String.substring(String.java:2709)
    at com.fasterxml.jackson.core.JsonPointer._constructHead(JsonPointer.java:853)
    at com.fasterxml.jackson.core.JsonPointer.head(JsonPointer.java:606)

Reproducer:

import com.fasterxml.jackson.core.JsonPointer;

public class JsonPointerBugTest {
    public static void main(String[] args) {
        JsonPointer head = JsonPointer.compile("/a/b/0/qwerty");
        head = head.head();
        head = head.head(); // Exception happens here
        System.out.println(head);
    }
}

Version:
Jackson Core 2.19.0

[pjfanning]

I've reproduced this issue and it appears to be related to #1354.
The new code looks pretty complicated and I'm not sure that I even understand what it is doing.

[cowtowncoder]

Ok, first of all, thank you for reporting @sergeykad.

Too bad this was not found during release-candidate phase. But it is what it is.

FWTW, verified this does not fail on 2.18.3.

[cowtowncoder]

Ok, figured it out. Indeed offset miscalculated, needed to change.