builtin: fix snake_to_camel when passing a SCREAMING_SNAKE_CASE string

[ttytm]

Ref.: #21691 (comment)

[ttytm]

Related to this and potential simplifications, would a .is_upper / .is_lower method for chars make it as a builtin?

[spytheman]

We have u8.is_capital/0 already, which does c >= A && c <= Z, but I could not find a separate implementation for u8.is_lower/0 .

[kbkpbot]

Bug in code:

assert '_ISspace'.camel_to_snake() == '_i_sspace'
but got '__sspace'
lost the first char I

[ttytm]

Good catch. Thanks a lot for the test cases @kbkpbot

[spytheman]

Why assert '_ISspace'.camel_to_snake() == '_i_sspace' ?

I would have expected _ISspace to become _is_space 🤔 .

[ttytm]

Good question. From the short research I did before, the initial implementation (without extracting handling of the first characters to reduce the load in the loop) was very similar to the implementation at https://github.com/iancoleman/strcase. So the initial impl and Go's strcase gives _i_sspace 🤔 .