Add missing StringScanner methods

[postmodern]

Crystal's StringScanner does not support every method which Ruby's StringScanner does.

• #beginning_of_line?
• #get_byte
• #getch
• #match
• #match?
• #matched?
• #matched_size
• #exist?
• #pos
• #rest?
• #rest_size
• #pre_match
• #post_match
• #<<
• #concat
• #unscan

[postmodern]

I particularly need #getch to step through the string if #scan does not match the current position.

[asterite]

I don't know what you are doing, but << would be impossible to implement because strings are immutable.

[asterite]

Also, could you describe what you are doing?

[postmodern]

I am porting this code to Crystal which walks a string attempt to match a series of regexes at each index, and if one of them matches wraps the matching string in ANSI styling: https://github.com/postmodern/hexdump.rb/blob/ead8dc6b45b281423ad7d4ea86d88f533ea0fbb4/lib/hexdump/theme/rule.rb#L129-L146

I even tried rewriting the code to only use String#match(regex,pos), but match(regex,pos) behaves differently from StringScanner#scan (will only match the regex at the current position, where as String#match will incrementally walk the string until it finds a match).

Yeah mutable methods like #<<, #concat, or even #string= are questionable, but I need #getch.

[HertzDevil]

Have you tried Regex#match with Regex::Options::ANCHORED?

getch should be equivalent to scan(/./), though a Crystal implementation is certainly faster than that.

[postmodern]

@HertzDevil ah clever and now the specs pass. Thank you!

I still think StringScanner should support more of Ruby's StringScanner to make porting to Crystal easier. getch, get_byte, match* methods seem like good candidates.

[asterite]

I agree, though I'll probably name them read_byte, read_char, etc., to match IO

[Kanezoh]

@asterite Hi! I also need those methods when porting Ruby code to Crystal, so tried implementing #read_byte and #read_char methods.
Please take a look at #11785 .