Compiler_Parser_Utils.H

/*
                          Aleph_w

  Data structures & Algorithms
  version 2.0.0b
  https://github.com/lrleon/Aleph-w

  This file is part of Aleph-w library

  Copyright (c) 2002-2026 Leandro Rabindranath Leon

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  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
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  copies of the Software, and to permit persons to whom the Software is
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*/

/** @file Compiler_Parser_Utils.H
 *  @brief Reusable token-stream helpers for recursive-descent frontends.
 *
 *  This header provides lightweight utilities that sit between
 *  `Compiler_Lexer.H` and a language-specific parser. It intentionally avoids
 *  AST concerns so that alternate frontends can reuse lookahead, expectations,
 *  separated-list parsing, and statement recovery without adopting the MVP
 *  language surface.
 *
 *  @ingroup Utilities
 */

#ifndef COMPILER_PARSER_UTILS_H
#define COMPILER_PARSER_UTILS_H

#include <string>
#include <string_view>

#include <Compiler_Lexer.H>

namespace Aleph {
/** @brief Returns whether a token kind can start an expression in the MVP grammar.
 *
 *  Frontends may reuse this predicate directly or use it as a reference when
 *  defining a custom expression-entry policy.
 */
inline bool compiler_is_expression_start(const Compiler_Token_Kind kind) noexcept
{
  switch (kind)
    {
    case Compiler_Token_Kind::Identifier:
    case Compiler_Token_Kind::Integer_Literal:
    case Compiler_Token_Kind::String_Literal:
    case Compiler_Token_Kind::Char_Literal:
    case Compiler_Token_Kind::Kw_True:
    case Compiler_Token_Kind::Kw_False:
    case Compiler_Token_Kind::LParen:
    case Compiler_Token_Kind::Bang:
    case Compiler_Token_Kind::Minus:
    case Compiler_Token_Kind::Plus:
    case Compiler_Token_Kind::Tilde:
      return true;
    default:
      return false;
    }
}

/** @brief Returns whether a token kind can start a statement in the MVP grammar. */
inline bool compiler_is_statement_start(const Compiler_Token_Kind kind) noexcept
{
  switch (kind)
    {
    case Compiler_Token_Kind::LBrace:
    case Compiler_Token_Kind::Kw_Let:
    case Compiler_Token_Kind::Kw_Return:
    case Compiler_Token_Kind::Kw_If:
    case Compiler_Token_Kind::Kw_While:
    case Compiler_Token_Kind::Kw_Break:
    case Compiler_Token_Kind::Kw_Continue:
      return true;
    default:
      return compiler_is_expression_start(kind);
    }
}

/** @brief Returns whether a token kind starts one top-level declaration. */
inline bool compiler_is_top_level_declaration_start(const Compiler_Token_Kind kind) noexcept
{
  switch (kind)
    {
    case Compiler_Token_Kind::Kw_Import:
    case Compiler_Token_Kind::Kw_Fn:
    case Compiler_Token_Kind::Kw_Type:
    case Compiler_Token_Kind::Kw_Struct:
    case Compiler_Token_Kind::Kw_Enum:
      return true;
    default:
      return false;
    }
}

/** @brief Removes surrounding double quotes from one string-literal token. */
inline std::string compiler_unquote_string_token(const Compiler_Token &token)
{
  if (token.kind != Compiler_Token_Kind::String_Literal)
    return token.lexeme;
  if (token.lexeme.size() >= 2 and token.lexeme.front() == '"' and token.lexeme.back() == '"')
    return token.lexeme.substr(1, token.lexeme.size() - 2);
  return token.lexeme;
}

/** @brief Lightweight token stream facade for recursive-descent parsers.
 *
 *  The stream wraps one `Compiler_Lexer` and exposes parser-oriented helpers:
 *  lookahead, `match`, `expect`, identifier expectations, separated-list
 *  parsing, and statement-oriented recovery.
 */
class Compiler_Parser_Stream
{
  Compiler_Lexer *lexer = nullptr;
  Diagnostic_Engine *diagnostics = nullptr;

  void emit_error(const Source_Span &span,
                  const std::string &message,
                  const std::string &code,
                  const std::string &note = "",
                  const std::string &help = "") const
  {
    if (diagnostics == nullptr)
      return;

    auto builder = diagnostics->error(span, message).code(code);
    if (not note.empty())
      builder.note(note);
    if (not help.empty())
      builder.help(help);
    builder.emit();
  }

public:
  /** @brief Builds one parser stream over an existing lexer. */
  Compiler_Parser_Stream(Compiler_Lexer &token_source, Diagnostic_Engine *dx = nullptr) noexcept
    : lexer(&token_source), diagnostics(dx)
  {}

  /** @brief Returns the next token without consuming it. */
  [[nodiscard]] Compiler_Token peek()
  {
    return lexer->peek();
  }

  /** @brief Consumes and returns the next token. */
  [[nodiscard]] Compiler_Token next()
  {
    return lexer->next();
  }

  /** @brief Returns whether the next token has the requested kind. */
  [[nodiscard]] bool check(const Compiler_Token_Kind kind)
  {
    return peek().kind == kind;
  }

  /** @brief Consumes the next token if it matches `kind`. */
  bool match(const Compiler_Token_Kind kind)
  {
    if (not check(kind))
      return false;
    (void) next();
    return true;
  }

  /** @brief Consumes a token of kind `kind` or emits one diagnostic error. */
  [[nodiscard]] Compiler_Token expect(const Compiler_Token_Kind kind,
                                      const std::string &message,
                                      const std::string &code,
                                      const std::string &note = "",
                                      const std::string &help = "")
  {
    const auto tok = peek();
    if (tok.kind == kind)
      return next();

    emit_error(tok.span, message, code, note, help);
    return {kind, "", tok.span};
  }

  /** @brief Consumes one identifier token or emits one diagnostic error. */
  [[nodiscard]] Compiler_Token expect_identifier(const std::string_view context,
                                                 const std::string &code = "PAR002")
  {
    const auto tok = peek();
    if (tok.kind == Compiler_Token_Kind::Identifier)
      return next();

    emit_error(tok.span, "expected identifier " + std::string(context), code);
    return {Compiler_Token_Kind::Identifier, "", tok.span};
  }

  /** @brief Parses one separated list until a terminator token is reached.
   *
   *  @param separator Separator token between items, such as `Comma`.
   *  @param terminator Closing token that ends the list.
   *  @param parse_item Callback that parses and records one item.
   *  @param allow_trailing_separator If `true`, `a, b, )` is accepted.
   *  @return `false` if `parse_item()` reports failure; `true` otherwise.
   */
  template <typename Parse_Item>
  bool parse_separated_list(const Compiler_Token_Kind separator,
                            const Compiler_Token_Kind terminator,
                            Parse_Item &&parse_item,
                            const bool allow_trailing_separator = false)
  {
    if (check(terminator))
      return true;

    while (true)
      {
        if (not parse_item())
          return false;

        if (not match(separator))
          return true;

        if (allow_trailing_separator and check(terminator))
          return true;
      }
  }

  /** @brief Skips tokens until a statement boundary suitable for recovery.
   *
   *  The method stops at EOF, `}`, the next top-level declaration keyword, or
   *  the next structural statement starter (`{`, `let`, `return`, `if`,
   *  `while`, `break`, `continue`). If a semicolon is encountered, it is
   *  consumed before returning so that callers can resume after the failed
   *  statement.
   */
  void synchronize_statement()
  {
    while (true)
      {
        const auto tok = peek();
        if (tok.is_eof() or tok.kind == Compiler_Token_Kind::RBrace
            or compiler_is_top_level_declaration_start(tok.kind))
          return;

        switch (tok.kind)
          {
          case Compiler_Token_Kind::LBrace:
          case Compiler_Token_Kind::Kw_Let:
          case Compiler_Token_Kind::Kw_Return:
          case Compiler_Token_Kind::Kw_If:
          case Compiler_Token_Kind::Kw_While:
          case Compiler_Token_Kind::Kw_Break:
          case Compiler_Token_Kind::Kw_Continue:
            return;
          default:
            break;
          }

        if (tok.kind == Compiler_Token_Kind::Semicolon)
          {
            (void) next();
            return;
          }

        (void) next();
      }
  }
};
}  // namespace Aleph

#endif