nac3/nac3parser/src/fstring.rs

use std::iter;
use std::mem;
use std::str;

use crate::ast::{Constant, ConversionFlag, Expr, ExprKind, Location};
use crate::error::{FStringError, FStringErrorType, ParseError};
use crate::parser::parse_expression;

use self::FStringErrorType::*;

struct FStringParser<'a> {
    chars: iter::Peekable<str::Chars<'a>>,
    str_location: Location,
}

impl<'a> FStringParser<'a> {
    fn new(source: &'a str, str_location: Location) -> Self {
        Self { chars: source.chars().peekable(), str_location }
    }

    #[inline]
    fn expr(&self, node: ExprKind) -> Expr {
        Expr::new(self.str_location, node)
    }

    fn parse_formatted_value(&mut self) -> Result<Vec<Expr>, FStringErrorType> {
        let mut expression = String::new();
        let mut spec = None;
        let mut delims = Vec::new();
        let mut conversion = None;
        let mut pred_expression_text = String::new();
        let mut trailing_seq = String::new();

        while let Some(ch) = self.chars.next() {
            match ch {
                // can be integrated better with the remainign code, but as a starting point ok
                // in general I would do here a tokenizing of the fstrings to omit this peeking.
                '!' if self.chars.peek() == Some(&'=') => {
                    expression.push_str("!=");
                    self.chars.next();
                }

                '=' if self.chars.peek() == Some(&'=') => {
                    expression.push_str("==");
                    self.chars.next();
                }

                '>' if self.chars.peek() == Some(&'=') => {
                    expression.push_str(">=");
                    self.chars.next();
                }

                '<' if self.chars.peek() == Some(&'=') => {
                    expression.push_str("<=");
                    self.chars.next();
                }

                '!' if delims.is_empty() && self.chars.peek() != Some(&'=') => {
                    if expression.trim().is_empty() {
                        return Err(EmptyExpression);
                    }

                    conversion = Some(match self.chars.next() {
                        Some('s') => ConversionFlag::Str,
                        Some('a') => ConversionFlag::Ascii,
                        Some('r') => ConversionFlag::Repr,
                        Some(_) => {
                            return Err(InvalidConversionFlag);
                        }
                        None => {
                            return Err(ExpectedRbrace);
                        }
                    });

                    if let Some(&peek) = self.chars.peek() {
                        if peek != '}' && peek != ':' {
                            return Err(ExpectedRbrace);
                        }
                    } else {
                        return Err(ExpectedRbrace);
                    }
                }

                // match a python 3.8 self documenting expression
                // format '{' PYTHON_EXPRESSION '=' FORMAT_SPECIFIER? '}'
                '=' if self.chars.peek() != Some(&'=') && delims.is_empty() => {
                    pred_expression_text = expression.to_string(); // safe expression before = to print it
                }

                ':' if delims.is_empty() => {
                    let mut nested = false;
                    let mut in_nested = false;
                    let mut spec_expression = String::new();
                    while let Some(&next) = self.chars.peek() {
                        match next {
                            '{' => {
                                if in_nested {
                                    return Err(ExpressionNestedTooDeeply);
                                }
                                in_nested = true;
                                nested = true;
                                self.chars.next();
                                continue;
                            }
                            '}' => {
                                if in_nested {
                                    in_nested = false;
                                    self.chars.next();
                                }
                                break;
                            }
                            _ => (),
                        }
                        spec_expression.push(next);
                        self.chars.next();
                    }
                    if in_nested {
                        return Err(UnclosedLbrace);
                    }
                    spec = Some(if nested {
                        Box::new(
                            self.expr(ExprKind::FormattedValue {
                                value: Box::new(
                                    parse_fstring_expr(&spec_expression)
                                        .map_err(|e| InvalidExpression(Box::new(e.error)))?,
                                ),
                                conversion: None,
                                format_spec: None,
                            }),
                        )
                    } else {
                        Box::new(self.expr(ExprKind::Constant {
                            value: spec_expression.clone().into(),
                            kind: None,
                        }))
                    });
                }
                '(' | '{' | '[' => {
                    expression.push(ch);
                    delims.push(ch);
                }
                ')' => {
                    if delims.pop() != Some('(') {
                        return Err(MismatchedDelimiter);
                    }
                    expression.push(ch);
                }
                ']' => {
                    if delims.pop() != Some('[') {
                        return Err(MismatchedDelimiter);
                    }
                    expression.push(ch);
                }
                '}' if !delims.is_empty() => {
                    if delims.pop() != Some('{') {
                        return Err(MismatchedDelimiter);
                    }
                    expression.push(ch);
                }
                '}' => {
                    if expression.is_empty() {
                        return Err(EmptyExpression);
                    }
                    let ret = if pred_expression_text.is_empty() {
                        vec![self.expr(ExprKind::FormattedValue {
                            value: Box::new(
                                parse_fstring_expr(&expression)
                                    .map_err(|e| InvalidExpression(Box::new(e.error)))?,
                            ),
                            conversion,
                            format_spec: spec,
                        })]
                    } else {
                        vec![
                            self.expr(ExprKind::Constant {
                                value: Constant::Str(pred_expression_text + "="),
                                kind: None,
                            }),
                            self.expr(ExprKind::Constant {
                                value: trailing_seq.into(),
                                kind: None,
                            }),
                            self.expr(ExprKind::FormattedValue {
                                value: Box::new(
                                    parse_fstring_expr(&expression)
                                        .map_err(|e| InvalidExpression(Box::new(e.error)))?,
                                ),
                                conversion,
                                format_spec: spec,
                            }),
                        ]
                    };
                    return Ok(ret);
                }
                '"' | '\'' => {
                    expression.push(ch);
                    for next in &mut self.chars {
                        expression.push(next);
                        if next == ch {
                            break;
                        }
                    }
                }
                ' ' if !pred_expression_text.is_empty() => {
                    trailing_seq.push(ch);
                }
                _ => {
                    expression.push(ch);
                }
            }
        }
        Err(UnclosedLbrace)
    }

    fn parse(mut self) -> Result<Expr, FStringErrorType> {
        let mut content = String::new();
        let mut values = vec![];

        while let Some(ch) = self.chars.next() {
            match ch {
                '{' => {
                    if let Some('{') = self.chars.peek() {
                        self.chars.next();
                        content.push('{');
                    } else {
                        if !content.is_empty() {
                            values.push(self.expr(ExprKind::Constant {
                                value: mem::take(&mut content).into(),
                                kind: None,
                            }));
                        }

                        values.extend(self.parse_formatted_value()?);
                    }
                }
                '}' => {
                    if let Some('}') = self.chars.peek() {
                        self.chars.next();
                        content.push('}');
                    } else {
                        return Err(UnopenedRbrace);
                    }
                }
                _ => {
                    content.push(ch);
                }
            }
        }

        if !content.is_empty() {
            values.push(self.expr(ExprKind::Constant { value: content.into(), kind: None }));
        }

        let s = match values.len() {
            0 => self.expr(ExprKind::Constant { value: String::new().into(), kind: None }),
            1 => values.into_iter().next().unwrap(),
            _ => self.expr(ExprKind::JoinedStr { values }),
        };
        Ok(s)
    }
}

fn parse_fstring_expr(source: &str) -> Result<Expr, ParseError> {
    let fstring_body = format!("({source})");
    parse_expression(&fstring_body)
}

/// Parse an fstring from a string, located at a certain position in the sourcecode.
/// In case of errors, we will get the location and the error returned.
pub fn parse_located_fstring(source: &str, location: Location) -> Result<Expr, FStringError> {
    FStringParser::new(source, location).parse().map_err(|error| FStringError { error, location })
}

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

    fn parse_fstring(source: &str) -> Result<Expr, FStringErrorType> {
        FStringParser::new(source, Location::default()).parse()
    }

    #[test]
    fn test_parse_fstring() {
        let source = "{a}{ b }{{foo}}";
        let parse_ast = parse_fstring(source).unwrap();

        insta::assert_debug_snapshot!(parse_ast);
    }

    #[test]
    fn test_parse_fstring_nested_spec() {
        let source = "{foo:{spec}}";
        let parse_ast = parse_fstring(source).unwrap();

        insta::assert_debug_snapshot!(parse_ast);
    }

    #[test]
    fn test_parse_fstring_not_nested_spec() {
        let source = "{foo:spec}";
        let parse_ast = parse_fstring(source).unwrap();

        insta::assert_debug_snapshot!(parse_ast);
    }

    #[test]
    fn test_parse_empty_fstring() {
        insta::assert_debug_snapshot!(parse_fstring("").unwrap());
    }

    #[test]
    fn test_fstring_parse_selfdocumenting_base() {
        let src = "{user=}";
        let parse_ast = parse_fstring(src).unwrap();

        insta::assert_debug_snapshot!(parse_ast);
    }

    #[test]
    fn test_fstring_parse_selfdocumenting_base_more() {
        let src = "mix {user=} with text and {second=}";
        let parse_ast = parse_fstring(src).unwrap();

        insta::assert_debug_snapshot!(parse_ast);
    }

    #[test]
    fn test_fstring_parse_selfdocumenting_format() {
        let src = "{user=:>10}";
        let parse_ast = parse_fstring(src).unwrap();

        insta::assert_debug_snapshot!(parse_ast);
    }

    #[test]
    fn test_parse_invalid_fstring() {
        assert_eq!(parse_fstring("{5!a"), Err(ExpectedRbrace));
        assert_eq!(parse_fstring("{5!a1}"), Err(ExpectedRbrace));
        assert_eq!(parse_fstring("{5!"), Err(ExpectedRbrace));
        assert_eq!(parse_fstring("abc{!a 'cat'}"), Err(EmptyExpression));
        assert_eq!(parse_fstring("{!a"), Err(EmptyExpression));
        assert_eq!(parse_fstring("{ !a}"), Err(EmptyExpression));

        assert_eq!(parse_fstring("{5!}"), Err(InvalidConversionFlag));
        assert_eq!(parse_fstring("{5!x}"), Err(InvalidConversionFlag));

        assert_eq!(parse_fstring("{a:{a:{b}}"), Err(ExpressionNestedTooDeeply));

        assert_eq!(parse_fstring("{a:b}}"), Err(UnopenedRbrace));
        assert_eq!(parse_fstring("}"), Err(UnopenedRbrace));
        assert_eq!(parse_fstring("{a:{b}"), Err(UnclosedLbrace));
        assert_eq!(parse_fstring("{"), Err(UnclosedLbrace));

        assert_eq!(parse_fstring("{}"), Err(EmptyExpression));

        // TODO: check for InvalidExpression enum?
        assert!(parse_fstring("{class}").is_err());
    }

    #[test]
    fn test_parse_fstring_not_equals() {
        let source = "{1 != 2}";
        let parse_ast = parse_fstring(source).unwrap();
        insta::assert_debug_snapshot!(parse_ast);
    }

    #[test]
    fn test_parse_fstring_equals() {
        let source = "{42 == 42}";
        let parse_ast = parse_fstring(source).unwrap();
        insta::assert_debug_snapshot!(parse_ast);
    }

    #[test]
    fn test_parse_fstring_selfdoc_prec_space() {
        let source = "{x   =}";
        let parse_ast = parse_fstring(source).unwrap();
        insta::assert_debug_snapshot!(parse_ast);
    }

    #[test]
    fn test_parse_fstring_selfdoc_trailing_space() {
        let source = "{x=   }";
        let parse_ast = parse_fstring(source).unwrap();
        insta::assert_debug_snapshot!(parse_ast);
    }

    #[test]
    fn test_parse_fstring_yield_expr() {
        let source = "{yield}";
        let parse_ast = parse_fstring(source).unwrap();
        insta::assert_debug_snapshot!(parse_ast);
    }
}