String class lacks convenient test for substring at position

[rakudrama]

I'm looking for something that does the equivalent of
   s1.substring(pos, pos + s2.length) == s2
but without doing any allocation.

Python's str class has s1.startswith(s2, pos)

[DartBot]

This comment was originally written by ngeoffray@google.com

---

How about s1.indexOf(s2, pos) ?

---

Set owner to ngeoffray@google.com.
Added Area-Library label.

[DartBot]

This comment was originally written by drfibonacci@google.com

---

Removed Type-Defect label.
Added Type-Enhancement, Triaged labels.

[rakudrama]

s1.indexOf(s2, pos)==pos will do a search which is O(|s1|) (MB) instead of O(|s2|) (2-20 characters).

I have a similar problem with RegExp matching against strings - there is no way to restrict the match to a substring.

I see two ways forward for string APIs.
One way is to give constant time performance guarantees on substring (achieved by structure sharing, like in Java).
Another way is to uniformly pass in optional start and end positions to allow any string operation to be restricted to the substring [start,end).
There may be other options that I don't see at present.

[DartBot]

This comment was originally written by ngeoffray@google.com

---

Handing that bug to Josh.

---

Set owner to jjb@google.com.

[sethladd]

Removed the owner.

[rakudrama]

String has startsWith method.

---

Added Library-Core, Fixed labels.

[lrhn]

The startsWith methods doesn't have an optional position argument, so it can't be used to check for the occurrence of a string in another at a position greater than zero, so it doesn't solve this request.

---

Added Triaged label.

[lrhn]

I've added an optional start position to String.startsWith, so now it can be used:
  s1.startsWith(s2, index) == true
iff
  index + s2.length <= s1.length && s1.substring(index, index + s2.length) == s2

---

Added Fixed label.