use std::borrow::Cow;

use regex_syntax::ast::parse::Parser;
use regex_syntax::ast::{self, *};

// covert ecma regex to rust regex if possible
// see https://262.ecma-international.org/11.0/#sec-regexp-regular-expression-objects
pub(crate) fn convert(pattern: &str) -> Result<Cow<'_, str>, Box<dyn std::error::Error>> {
  let mut pattern = Cow::Borrowed(pattern);

  let mut ast = loop {
    match Parser::new().parse(pattern.as_ref()) {
      Ok(ast) => break ast,
      Err(e) => {
        if let Some(s) = fix_error(&e) {
          pattern = Cow::Owned(s);
        } else {
          Err(e)?;
        }
      }
    }
  };

  loop {
    let translator = Translator {
      pat: pattern.as_ref(),
      out: None,
    };
    if let Some(updated_pattern) = ast::visit(&ast, translator)? {
      match Parser::new().parse(&updated_pattern) {
        Ok(updated_ast) => {
          pattern = Cow::Owned(updated_pattern);
          ast = updated_ast;
        }
        Err(e) => {
          debug_assert!(
            false,
            "ecma::translate changed {:?} to {:?}: {e}",
            pattern, updated_pattern
          );
          break;
        }
      }
    } else {
      break;
    }
  }
  Ok(pattern)
}

fn fix_error(e: &Error) -> Option<String> {
  if let ErrorKind::EscapeUnrecognized = e.kind() {
    let (start, end) = (e.span().start.offset, e.span().end.offset);
    let s = &e.pattern()[start..end];
    if let r"\c" = s {
      // handle \c{control_letter}
      if let Some(control_letter) = e.pattern()[end..].chars().next() {
        if control_letter.is_ascii_alphabetic() {
          return Some(format!(
            "{}{}{}",
            &e.pattern()[..start],
            ((control_letter as u8) % 32) as char,
            &e.pattern()[end + 1..],
          ));
        }
      }
    }
  }
  None
}

/**
handles following translations:
- \d should ascii digits only. so replace with [0-9]
- \D should match everything but ascii digits. so replace with [^0-9]
- \w should match ascii letters only. so replace with [a-zA-Z0-9_]
- \W should match everything but ascii letters. so replace with [^a-zA-Z0-9_]
- \s and \S differences
- \a is not an ECMA 262 control escape
*/
struct Translator<'a> {
  pat: &'a str,
  out: Option<String>,
}

impl Translator<'_> {
  fn replace(&mut self, span: &Span, with: &str) {
    let (start, end) = (span.start.offset, span.end.offset);
    self.out = Some(format!("{}{with}{}", &self.pat[..start], &self.pat[end..]));
  }

  fn replace_class_class(&mut self, perl: &ClassPerl) {
    match perl.kind {
      ClassPerlKind::Digit => {
        self.replace(&perl.span, if perl.negated { "[^0-9]" } else { "[0-9]" });
      }
      ClassPerlKind::Word => {
        let with = &if perl.negated {
          "[^A-Za-z0-9_]"
        } else {
          "[A-Za-z0-9_]"
        };
        self.replace(&perl.span, with);
      }
      ClassPerlKind::Space => {
        let with = &if perl.negated {
          "[^ \t\n\r\u{000b}\u{000c}\u{00a0}\u{feff}\u{2003}\u{2029}]"
        } else {
          "[ \t\n\r\u{000b}\u{000c}\u{00a0}\u{feff}\u{2003}\u{2029}]"
        };
        self.replace(&perl.span, with);
      }
    }
  }
}

impl Visitor for Translator<'_> {
  type Output = Option<String>;
  type Err = &'static str;

  fn finish(self) -> Result<Self::Output, Self::Err> {
    Ok(self.out)
  }

  fn visit_class_set_item_pre(&mut self, ast: &ast::ClassSetItem) -> Result<(), Self::Err> {
    if let ClassSetItem::Perl(perl) = ast {
      self.replace_class_class(perl);
    }
    Ok(())
  }

  fn visit_post(&mut self, ast: &Ast) -> Result<(), Self::Err> {
    if self.out.is_some() {
      return Ok(());
    }
    match ast {
      Ast::ClassPerl(perl) => {
        self.replace_class_class(perl);
      }
      Ast::Literal(ref literal) => {
        if let Literal {
          kind: LiteralKind::Special(SpecialLiteralKind::Bell),
          ..
        } = literal.as_ref()
        {
          return Err("\\a is not an ECMA 262 control escape");
        }
      }
      _ => (),
    }
    Ok(())
  }
}

#[cfg(test)]
mod tests {
  use super::*;

  #[test]
  fn test_ecma_compat_valid() {
    // println!("{:#?}", Parser::new().parse(r#"a\a"#));
    let tests = [
      (r"ab\cAcde\cBfg", "ab\u{1}cde\u{2}fg"), // \c{control_letter}
      (r"\\comment", r"\\comment"),      // there is no \c
      (r"ab\def", r#"ab[0-9]ef"#),       // \d
      (r"ab[a-z\d]ef", r#"ab[a-z[0-9]]ef"#),  // \d inside classSet
      (r"ab\Def", r#"ab[^0-9]ef"#),      // \d
      (r"ab[a-z\D]ef", r#"ab[a-z[^0-9]]ef"#), // \D inside classSet
    ];
    for (input, want) in tests {
      match convert(input) {
        Ok(got) => {
          if got.as_ref() != want {
            panic!("convert({input:?}): got: {got:?}, want: {want:?}");
          }
        }
        Err(e) => {
          panic!("convert({input:?}) failed: {e}");
        }
      }
    }
  }

  #[test]
  fn test_ecma_compat_invalid() {
    // println!("{:#?}", Parser::new().parse(r#"a\a"#));
    let tests = [
      r"\c\n",   // \c{invalid_char}
      r"abc\adef", // \a is not valid
    ];
    for input in tests {
      if convert(input).is_ok() {
        panic!("convert({input:?}) mut fail");
      }
    }
  }
}