carbon-lang/common/string_helpers.cpp

// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include "common/string_helpers.h"

#include <algorithm>
#include <optional>

#include "common/check.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/ADT/StringRef.h"

namespace Carbon {

namespace {

constexpr llvm::StringRef TripleQuotes = "\"\"\"";
constexpr llvm::StringRef HorizontalWhitespaceChars = " \t";

// Carbon only takes uppercase hex input.
auto FromHex(char c) -> std::optional<char> {
  if (c >= '0' && c <= '9') {
    return c - '0';
  }
  if (c >= 'A' && c <= 'F') {
    return 10 + c - 'A';
  }
  return std::nullopt;
}

// Creates an error instance with the specified `message`.
llvm::Expected<std::string> MakeError(llvm::Twine message) {
  return llvm::createStringError(llvm::inconvertibleErrorCode(), message);
}

}  // namespace

auto UnescapeStringLiteral(llvm::StringRef source, bool is_block_string)
    -> std::optional<std::string> {
  std::string ret;
  ret.reserve(source.size());
  size_t i = 0;
  while (i < source.size()) {
    char c = source[i];
    switch (c) {
      case '\\':
        ++i;
        if (i == source.size()) {
          return std::nullopt;
        }
        switch (source[i]) {
          case 'n':
            ret.push_back('\n');
            break;
          case 'r':
            ret.push_back('\r');
            break;
          case 't':
            ret.push_back('\t');
            break;
          case '0':
            if (i + 1 < source.size() && llvm::isDigit(source[i + 1])) {
              // \0[0-9] is reserved.
              return std::nullopt;
            }
            ret.push_back('\0');
            break;
          case '"':
            ret.push_back('"');
            break;
          case '\'':
            ret.push_back('\'');
            break;
          case '\\':
            ret.push_back('\\');
            break;
          case 'x': {
            i += 2;
            if (i >= source.size()) {
              return std::nullopt;
            }
            std::optional<char> c1 = FromHex(source[i - 1]);
            std::optional<char> c2 = FromHex(source[i]);
            if (c1 == std::nullopt || c2 == std::nullopt) {
              return std::nullopt;
            }
            ret.push_back(16 * *c1 + *c2);
            break;
          }
          case 'u':
            FATAL() << "\\u is not yet supported in string literals";
          case '\n':
            if (!is_block_string) {
              return std::nullopt;
            }
            break;
          default:
            // Unsupported.
            return std::nullopt;
        }
        break;

      case '\t':
        // Disallow non-` ` horizontal whitespace:
        // https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/lexical_conventions/whitespace.md
        // TODO: This doesn't handle unicode whitespace.
        return std::nullopt;

      default:
        ret.push_back(c);
        break;
    }
    ++i;
  }
  return ret;
}

auto ParseBlockStringLiteral(llvm::StringRef source)
    -> llvm::Expected<std::string> {
  llvm::SmallVector<llvm::StringRef> lines;
  source.split(lines, '\n', /*MaxSplit=*/-1, /*KeepEmpty=*/true);
  if (lines.size() < 2) {
    return MakeError("Too few lines");
  }

  llvm::StringRef first = lines[0];
  if (!first.consume_front(TripleQuotes)) {
    return MakeError("Should start with triple quotes: " + first);
  }
  first = first.rtrim(HorizontalWhitespaceChars);
  // Remaining chars, if any, are a file type indicator.
  if (first.find_first_of("\"#") != llvm::StringRef::npos ||
      first.find_first_of(HorizontalWhitespaceChars) != llvm::StringRef::npos) {
    return MakeError("Invalid characters in file type indicator: " + first);
  }

  llvm::StringRef last = lines[lines.size() - 1];
  const size_t last_length = last.size();
  last = last.ltrim(HorizontalWhitespaceChars);
  const size_t indent = last_length - last.size();
  if (last != TripleQuotes) {
    return MakeError("Should end with triple quotes: " + last);
  }

  std::string parsed;
  for (size_t i = 1; i < lines.size() - 1; ++i) {
    llvm::StringRef line = lines[i];
    const size_t first_non_ws =
        line.find_first_not_of(HorizontalWhitespaceChars);
    if (first_non_ws == llvm::StringRef::npos) {
      // Empty or whitespace-only line.
      line = "";
    } else {
      if (first_non_ws < indent) {
        return MakeError("Wrong indent for line: " + line + ", expected " +
                         llvm::Twine(indent));
      }
      line = line.drop_front(indent).rtrim(HorizontalWhitespaceChars);
    }
    // Unescaping with \n appended to handle things like \\<newline>.
    llvm::SmallVector<char> buffer;
    std::optional<std::string> unescaped = UnescapeStringLiteral(
        (line + "\n").toStringRef(buffer), /*is_block_string=*/true);
    if (!unescaped.has_value()) {
      return MakeError("Invalid escaping in " + line);
    }
    // A \<newline> string collapses into nothing.
    if (!unescaped->empty()) {
      parsed.append(*unescaped);
    }
  }
  return parsed;
}

}  // namespace Carbon