sql/src/parser.ts

import * as AST from "./ast";
import Error, { isError } from "./error";
import Token, { TokenKind } from "./token";

export default class Parser {
  public tokens: Token[];
  public position: number;

  constructor(tokens: Token[]) {
    this.tokens = tokens;
    this.position = 0;
  }

  public parse(): AST.Statement[] | Error {
    const tree: AST.Statement[] = [];

    while (!this.atEnd()) {
      const stmt = this.statement();
      if (isError(stmt)) {
        return stmt;
      } else {
        tree.push(stmt);
      }
    }

    return tree;
  }

  private statement(): AST.Statement | Error {
    const token = this.currentToken();

    switch (token.kind) {
      case TokenKind.SELECT:
        return this.select();
    }

    return new Error(`Unexpected token ${token.kind}`, token.line);
  }

  private select(): AST.Statement | Error {
    this.eat(TokenKind.SELECT);
    const args = this.args();
    if (isError(args)) {
      return args;
    }

    const from = this.currentToken().kind === TokenKind.FROM ? this.from() : null;
    if (isError(from)) {
      return from;
    }

    const where = this.currentToken().kind === TokenKind.WHERE ? this.where() : null;
    if (isError(where)) {
      return where;
    }

    return new AST.SelectStatement({
      arguments: args,
      from,
      where,
    });
  }

  private args(): AST.Secondary[] | Error {
    const args = [];

    while (true) {
      const arg = this.alias();
      if (isError(arg)) {
        return arg;
      }
      args.push(arg);

      if (this.currentToken().kind === TokenKind.COMMA) {
        this.eat(TokenKind.COMMA);
      } else {
        break;
      }
    }

    return args;
  }

  private from(): AST.Secondary | Error {
    this.eat(TokenKind.FROM);
    return this.alias(this.identifier);
  }

  private where(): AST.Expr | Error {
    this.eat(TokenKind.WHERE);
    return this.expr();
  }

  private expr(): AST.Expr | Error {
    return this.alias();
  }

  private alias(fn: () => AST.Primary | Error = this.primary): AST.Secondary | Error {
    const primary = fn.bind(this)();
    if (isError(primary)) {
      return primary;
    }

    if (this.currentToken().kind === TokenKind.AS) {
      this.eat(TokenKind.AS);
      const name = this.identifier();
      if (isError(name)) {
        return name;
      }
      return new AST.Alias(primary, name);
    }

    return primary;
  }

  private primary(): AST.Primary | Error {
    const token = this.currentToken();
    switch (token.kind) {
      case TokenKind.NUMBER:
        return this.number();
      case TokenKind.IDENTIFIER:
        return this.identifier();
    }

    return new Error(
      `Unexpected token ${token.kind} ${token.value}`,
      token.line,
    );
  }

  private identifier(): AST.Identifier | Error {
    const identifier = this.eat(TokenKind.IDENTIFIER);
    if (isError(identifier)) {
      return identifier;
    }
    return new AST.Identifier(identifier.value || "");
  }

  private number(): AST.Number | Error {
    const n = this.eat(TokenKind.NUMBER);
    if (isError(n)) {
      return n;
    }
    return new AST.Number(parseFloat(n.value || ""));
  }

  private eat(kind: TokenKind) {
    const token = this.currentToken();
    if (token.kind === kind) {
      this.advance();
      return token;
    }

    const repr = `{ kind: ${token.kind}${token.value ? `, value: ${token.value} }` : ""}`;

    return new Error(`Unexpected token: ${repr}`, token.line);
  }

  private currentToken(): Token {
    return this.tokens[this.position];
  }

  private advance() {
    this.position += 1;
  }

  private atEnd(): boolean {
    return this.currentToken().kind === TokenKind.EOF;
  }
}