String optimization and addition of Suffix()

[algoidurovic]

Optimization of Substring() and Extract() functions when int args are both immediate and fit in one byte. This is achieved by utilizing the substring and extract opcodes rather than substring3 and extract3.

[CLAassistant]

All committers have signed the CLA.

[algochoi]

Looking good! Minor comment about the tests.

[jasonpaulos]

This is looking good, but I have some suggestions for further improvements. Specifically, there are 2 cases where the current implementation behaves sub-optimally:

• Substring(Bytes("..."), Int(255), Int(257)) currently translates to byte "..."; int 255; int 257; substring3. If the extract opcode is available, it would be more efficient to translate this to byte "..."; extract 255 2

• Suffix(Bytes("..."), Int(256)) currently translates to byte "..."; int 256; byte "..."; len; substring 3. If the dig opcode is available, it would be more efficient & correct to translate this to byte "..."; int 256; dig 1; len; substring 3 (the dig opcode copies a value that's already on the stack).

  • It would be more efficient because the input string might be the result of a computation (e.g. Concat(Bytes("a"), Bytes("b"), Bytes("c"))), so we should avoid doing the computation twice.
  • Additionally, it would be more correct because the input string can be the result of a computation that has other sides effects, like Seq(Log(Bytes("hello")), Bytes("myString")). So duplicating the computation would add unintended behavior.
    • You could also add a workaround using other opcodes in case someone is compiling a program for TEAL v2 where dig is not available, but I think it would be fine for this expression to just raise an error in that case.

I have some suggestions for how to address the above issues. The first, mainly for organization, is to create a new file, something like pyteal/ast/substring.py, and move all substring, extract, and suffix expressions into there, to keep ternaryexpr.py simple.

The second would be to turn the existing Substring, Extract, and Suffix functions into full classes which extend the Expr base class. These classes would essentially do the same inspection of arguments that you've already implemented, but they should happen inside the __teal__ method when the AST is being compiled, rather than when the AST is being created. This is beneficial because in the __teal__ method, you'll have access to the current TEAL version that's being targeted during compilation, so you can choose the most efficient opcodes for the given arguments and TEAL version.

[jasonpaulos]

This is very good! I have a few minor comments, the most important of which is the case where l = 0 in SubstringExpr

[jasonpaulos]

Looks great!