query-engine/src/sql/parser.rs

use std::error::Error;
use std::fmt;

use super::ast::{BinaryOp, Expr, Literal, Select, SelectItem};

/// Errors returned by the minimal SQL parser.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum ParseError {
    UnexpectedEnd,
    ExpectedToken(&'static str),
    ExpectedIdentifier,
    UnexpectedToken(String),
    UnterminatedString,
}

impl fmt::Display for ParseError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::UnexpectedEnd => write!(f, "unexpected end of input"),
            Self::ExpectedToken(token) => write!(f, "expected `{}`", token),
            Self::ExpectedIdentifier => write!(f, "expected identifier"),
            Self::UnexpectedToken(token) => write!(f, "unexpected token `{}`", token),
            Self::UnterminatedString => write!(f, "unterminated string literal"),
        }
    }
}

impl Error for ParseError {}

#[derive(Debug, Clone, PartialEq, Eq)]
enum Token {
    Select,
    From,
    Where,
    As,
    Null,
    Identifier(String),
    String(String),
    Star,
    Comma,
    Eq,
}

/// Parse a `SELECT-FROM-WHERE` query in the current SQL subset.
pub fn parse_select(input: &str) -> Result<Select, ParseError> {
    let tokens = tokenize(input)?;
    let mut parser = Parser::new(tokens);
    parser.parse_select()
}

struct Parser {
    tokens: Vec<Token>,
    index: usize,
}

impl Parser {
    fn new(tokens: Vec<Token>) -> Self {
        Self { tokens, index: 0 }
    }

    fn parse_select(&mut self) -> Result<Select, ParseError> {
        self.expect_keyword(Token::Select, "SELECT")?;
        let projection = self.parse_projection()?;
        self.expect_keyword(Token::From, "FROM")?;
        let from = self.expect_identifier()?;
        let selection = if self.peek() == Some(&Token::Where) {
            self.index += 1;
            Some(self.parse_expr()?)
        } else {
            None
        };

        if let Some(token) = self.peek() {
            return Err(ParseError::UnexpectedToken(render_token(token)));
        }

        Ok(Select {
            projection,
            from,
            selection,
        })
    }

    fn parse_projection(&mut self) -> Result<Vec<SelectItem>, ParseError> {
        let mut items = Vec::new();

        loop {
            let item = match self.peek().ok_or(ParseError::UnexpectedEnd)? {
                Token::Star => {
                    self.index += 1;
                    SelectItem::Wildcard
                }
                _ => {
                    let expr = self.parse_operand()?;
                    let alias = if self.peek() == Some(&Token::As) {
                        self.index += 1;
                        Some(self.expect_identifier()?)
                    } else {
                        None
                    };
                    SelectItem::Expr { expr, alias }
                }
            };
            items.push(item);

            if self.peek() == Some(&Token::Comma) {
                self.index += 1;
                continue;
            }
            break;
        }

        Ok(items)
    }

    fn parse_expr(&mut self) -> Result<Expr, ParseError> {
        let left = self.parse_operand()?;
        match self.next().ok_or(ParseError::UnexpectedEnd)? {
            Token::Eq => {
                let right = self.parse_operand()?;
                Ok(Expr::Binary {
                    left: Box::new(left),
                    op: BinaryOp::Eq,
                    right: Box::new(right),
                })
            }
            other => Err(ParseError::UnexpectedToken(render_token(&other))),
        }
    }

    fn parse_operand(&mut self) -> Result<Expr, ParseError> {
        match self.next().ok_or(ParseError::UnexpectedEnd)? {
            Token::Identifier(name) => Ok(Expr::Identifier(name)),
            Token::String(value) => Ok(Expr::Literal(Literal::String(value))),
            Token::Null => Ok(Expr::Literal(Literal::Null)),
            other => Err(ParseError::UnexpectedToken(render_token(&other))),
        }
    }

    fn expect_keyword(&mut self, token: Token, label: &'static str) -> Result<(), ParseError> {
        let next = self.next().ok_or(ParseError::UnexpectedEnd)?;
        if next == token {
            Ok(())
        } else {
            Err(ParseError::ExpectedToken(label))
        }
    }

    fn expect_identifier(&mut self) -> Result<String, ParseError> {
        match self.next().ok_or(ParseError::UnexpectedEnd)? {
            Token::Identifier(name) => Ok(name),
            _ => Err(ParseError::ExpectedIdentifier),
        }
    }

    fn peek(&self) -> Option<&Token> {
        self.tokens.get(self.index)
    }

    fn next(&mut self) -> Option<Token> {
        let token = self.tokens.get(self.index).cloned();
        if token.is_some() {
            self.index += 1;
        }
        token
    }
}

fn tokenize(input: &str) -> Result<Vec<Token>, ParseError> {
    let mut chars = input.chars().peekable();
    let mut tokens = Vec::new();

    while let Some(ch) = chars.peek().copied() {
        if ch.is_whitespace() {
            chars.next();
            continue;
        }

        match ch {
            '*' => {
                chars.next();
                tokens.push(Token::Star);
            }
            ',' => {
                chars.next();
                tokens.push(Token::Comma);
            }
            '=' => {
                chars.next();
                tokens.push(Token::Eq);
            }
            '\'' => tokens.push(Token::String(parse_string(&mut chars)?)),
            ch if is_identifier_start(ch) => {
                let ident = parse_identifier(&mut chars);
                let token = match ident.to_ascii_uppercase().as_str() {
                    "SELECT" => Token::Select,
                    "FROM" => Token::From,
                    "WHERE" => Token::Where,
                    "AS" => Token::As,
                    "NULL" => Token::Null,
                    _ => Token::Identifier(ident),
                };
                tokens.push(token);
            }
            other => return Err(ParseError::UnexpectedToken(other.to_string())),
        }
    }

    Ok(tokens)
}

fn parse_string<I>(chars: &mut std::iter::Peekable<I>) -> Result<String, ParseError>
where
    I: Iterator<Item = char>,
{
    let mut value = String::new();
    let quote = chars.next();
    if quote != Some('\'') {
        return Err(ParseError::ExpectedToken("'"));
    }

    while let Some(ch) = chars.next() {
        if ch == '\'' {
            if chars.peek() == Some(&'\'') {
                chars.next();
                value.push('\'');
                continue;
            }
            return Ok(value);
        }
        value.push(ch);
    }

    Err(ParseError::UnterminatedString)
}

fn parse_identifier<I>(chars: &mut std::iter::Peekable<I>) -> String
where
    I: Iterator<Item = char>,
{
    let mut ident = String::new();
    while let Some(ch) = chars.peek().copied() {
        if is_identifier_part(ch) {
            ident.push(ch);
            chars.next();
        } else {
            break;
        }
    }
    ident
}

fn is_identifier_start(ch: char) -> bool {
    ch.is_ascii_alphabetic() || ch == '_'
}

fn is_identifier_part(ch: char) -> bool {
    ch.is_ascii_alphanumeric() || ch == '_'
}

fn render_token(token: &Token) -> String {
    match token {
        Token::Select => "SELECT".to_string(),
        Token::From => "FROM".to_string(),
        Token::Where => "WHERE".to_string(),
        Token::As => "AS".to_string(),
        Token::Null => "NULL".to_string(),
        Token::Identifier(name) => name.clone(),
        Token::String(value) => format!("'{}'", value),
        Token::Star => "*".to_string(),
        Token::Comma => ",".to_string(),
        Token::Eq => "=".to_string(),
    }
}

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

    #[test]
    fn parses_select_with_filter() {
        let select = parse_select("SELECT c0 FROM Parent WHERE c1 = 'bob'").unwrap();

        assert_eq!(select.from, "Parent");
        assert_eq!(select.projection.len(), 1);
        assert!(select.selection.is_some());
    }

    #[test]
    fn parses_projection_aliases_and_literals() {
        let select =
            parse_select("SELECT c0 AS parent_name, 'seed' AS label, NULL FROM Parent").unwrap();

        assert_eq!(select.projection.len(), 3);
        assert_eq!(
            select.projection[0],
            SelectItem::Expr {
                expr: Expr::Identifier("c0".to_string()),
                alias: Some("parent_name".to_string()),
            }
        );
        assert_eq!(
            select.projection[1],
            SelectItem::Expr {
                expr: Expr::Literal(Literal::String("seed".to_string())),
                alias: Some("label".to_string()),
            }
        );
        assert_eq!(
            select.projection[2],
            SelectItem::Expr {
                expr: Expr::Literal(Literal::Null),
                alias: None,
            }
        );
    }
}