//===-- lib/Parser/preprocessor.cpp ---------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include "flang/Parser/preprocessor.h"

#include "prescan.h"
#include "flang/Common/idioms.h"
#include "flang/Parser/characters.h"
#include "flang/Parser/message.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cinttypes>
#include <cstddef>
#include <ctime>
#include <map>
#include <memory>
#include <optional>
#include <set>
#include <string>
#include <utility>
#include <vector>

namespace Fortran::parser {

Definition::Definition(
    const TokenSequence &repl, std::size_t firstToken, std::size_t tokens)
    : replacement_{Tokenize({}, repl, firstToken, tokens)} {}

Definition::Definition(const std::vector<std::string> &argNames,
    const TokenSequence &repl, std::size_t firstToken, std::size_t tokens,
    bool isVariadic)
    : isFunctionLike_{true}, isVariadic_{isVariadic}, argNames_{argNames},
      replacement_{Tokenize(argNames, repl, firstToken, tokens)} {}

Definition::Definition(const std::string &predefined, AllSources &sources)
    : isPredefined_{true},
      replacement_{
          predefined, sources.AddCompilerInsertion(predefined).start()} {}

Definition::Definition(const TokenSequence &repl)
    : isPredefined_{true}, replacement_{repl} {}

bool Definition::set_isDisabled(bool disable) {
  bool was{isDisabled_};
  isDisabled_ = disable;
  return was;
}

void Definition::Print(llvm::raw_ostream &out, const char *macroName) const {
  if (!isFunctionLike_) {
    // If it's not a function-like macro, then just print the replacement.
    out << ' ' << replacement_.ToString();
    return;
  }

  size_t argCount{argumentCount()};

  out << '(';
  for (size_t i{0}; i != argCount; ++i) {
    if (i != 0) {
      out << ", ";
    }
    out << argNames_[i];
  }
  if (isVariadic_) {
    out << ", ...";
  }
  out << ") ";

  for (size_t i{0}, e{replacement_.SizeInTokens()}; i != e; ++i) {
    std::string tok{replacement_.TokenAt(i).ToString()};
    if (size_t idx{GetArgumentIndex(tok)}; idx < argCount) {
      out << argNames_[idx];
    } else {
      out << tok;
    }
  }
}

static bool IsLegalIdentifierStart(const CharBlock &cpl) {
  return cpl.size() > 0 && IsLegalIdentifierStart(cpl[0]);
}

TokenSequence Definition::Tokenize(const std::vector<std::string> &argNames,
    const TokenSequence &token, std::size_t firstToken, std::size_t tokens) {
  std::map<std::string, std::string> args;
  char argIndex{'A'};
  for (const std::string &arg : argNames) {
    CHECK(args.find(arg) == args.end());
    args[arg] = "~"s + argIndex++;
  }
  TokenSequence result;
  for (std::size_t j{0}; j < tokens; ++j) {
    CharBlock tok{token.TokenAt(firstToken + j)};
    if (IsLegalIdentifierStart(tok)) {
      auto it{args.find(tok.ToString())};
      if (it != args.end()) {
        result.Put(it->second, token.GetTokenProvenance(j));
        continue;
      }
    }
    result.AppendRange(token, firstToken + j, 1);
  }
  return result;
}

std::size_t Definition::GetArgumentIndex(const CharBlock &token) const {
  if (token.size() >= 2 && token[0] == '~') {
    return static_cast<size_t>(token[1] - 'A');
  }
  return argumentCount();
}

static TokenSequence Stringify(
    const TokenSequence &tokens, AllSources &allSources) {
  TokenSequence result;
  Provenance quoteProvenance{allSources.CompilerInsertionProvenance('"')};
  result.PutNextTokenChar('"', quoteProvenance);
  for (std::size_t j{0}; j < tokens.SizeInTokens(); ++j) {
    const CharBlock &token{tokens.TokenAt(j)};
    std::size_t bytes{token.size()};
    for (std::size_t k{0}; k < bytes; ++k) {
      char ch{token[k]};
      Provenance from{tokens.GetTokenProvenance(j, k)};
      if (ch == '"' || ch == '\\') {
        result.PutNextTokenChar(ch, from);
      }
      result.PutNextTokenChar(ch, from);
    }
  }
  result.PutNextTokenChar('"', quoteProvenance);
  result.CloseToken();
  return result;
}

constexpr bool IsTokenPasting(CharBlock opr) {
  return opr.size() == 2 && opr[0] == '#' && opr[1] == '#';
}

static bool AnyTokenPasting(const TokenSequence &text) {
  std::size_t tokens{text.SizeInTokens()};
  for (std::size_t j{0}; j < tokens; ++j) {
    if (IsTokenPasting(text.TokenAt(j))) {
      return true;
    }
  }
  return false;
}

static TokenSequence TokenPasting(TokenSequence &&text) {
  if (!AnyTokenPasting(text)) {
    return std::move(text);
  }
  TokenSequence result;
  std::size_t tokens{text.SizeInTokens()};
  std::optional<CharBlock> before; // last non-blank token before ##
  for (std::size_t j{0}; j < tokens; ++j) {
    CharBlock after{text.TokenAt(j)};
    if (!before) {
      if (IsTokenPasting(after)) {
        while (!result.empty() &&
            result.TokenAt(result.SizeInTokens() - 1).IsBlank()) {
          result.pop_back();
        }
        if (!result.empty()) {
          before = result.TokenAt(result.SizeInTokens() - 1);
        }
      } else {
        result.AppendRange(text, j, 1);
      }
    } else if (after.IsBlank() || IsTokenPasting(after)) {
      // drop it
    } else { // pasting before ## after
      bool doPaste{false};
      char last{before->back()};
      char first{after.front()};
      // Apply basic sanity checking to pasting so avoid constructing a bogus
      // token that might cause macro replacement to fail, like "macro(".
      if (IsLegalInIdentifier(last) && IsLegalInIdentifier(first)) {
        doPaste = true;
      } else if (IsDecimalDigit(first) &&
          (last == '.' || last == '+' || last == '-')) {
        doPaste = true; // 1. ## 0, - ## 1
      } else if (before->size() == 1 && after.size() == 1) {
        if (first == last &&
            (last == '<' || last == '>' || last == '*' || last == '/' ||
                last == '=' || last == '&' || last == '|' || last == ':')) {
          // Fortran **, //, ==, ::
          // C <<, >>, &&, || for use in #if expressions
          doPaste = true;
        } else if (first == '=' && (last == '!' || last == '/')) {
          doPaste = true; // != and /=
        }
      }
      if (doPaste) {
        result.ReopenLastToken();
      }
      result.AppendRange(text, j, 1);
      before.reset();
    }
  }
  return result;
}

constexpr bool IsDefinedKeyword(CharBlock token) {
  return token.size() == 7 && (token[0] == 'd' || token[0] == 'D') &&
      ToLowerCaseLetters(token.ToString()) == "defined";
}

TokenSequence Definition::Apply(const std::vector<TokenSequence> &args,
    const Prescanner &prescanner, bool inIfExpression) const {
  TokenSequence result;
  bool skipping{false};
  int parenthesesNesting{0};
  std::size_t tokens{replacement_.SizeInTokens()};
  for (std::size_t j{0}; j < tokens; ++j) {
    CharBlock token{replacement_.TokenAt(j)};
    std::size_t bytes{token.size()};
    if (skipping) {
      char ch{token.OnlyNonBlank()};
      if (ch == '(') {
        ++parenthesesNesting;
      } else if (ch == ')') {
        if (parenthesesNesting > 0) {
          --parenthesesNesting;
        }
        skipping = parenthesesNesting > 0;
      }
      continue;
    }
    if (bytes == 2 && token[0] == '~') { // argument substitution
      std::size_t index{GetArgumentIndex(token)};
      if (index >= args.size()) {
        continue;
      }
      std::size_t prev{j};
      while (prev > 0 && replacement_.TokenAt(prev - 1).IsBlank()) {
        --prev;
      }
      if (prev > 0 && replacement_.TokenAt(prev - 1).size() == 1 &&
          replacement_.TokenAt(prev - 1)[0] ==
              '#') { // stringify argument without macro replacement
        std::size_t resultSize{result.SizeInTokens()};
        while (resultSize > 0 && result.TokenAt(resultSize - 1).IsBlank()) {
          result.pop_back();
          --resultSize;
        }
        CHECK(resultSize > 0 &&
            result.TokenAt(resultSize - 1) == replacement_.TokenAt(prev - 1));
        result.pop_back();
        AllSources &allSources{
            *const_cast<AllSources *>(&prescanner.allSources())};
        result.CopyAll(Stringify(args[index], allSources));
      } else {
        const TokenSequence *arg{&args[index]};
        std::optional<TokenSequence> replaced;
        // Don't replace macros in the actual argument if it is preceded or
        // followed by the token-pasting operator ## in the replacement text,
        // or if we have to worry about "defined(X)"/"defined X" in an
        // #if/#elif expression.
        if (!inIfExpression &&
            (prev == 0 || !IsTokenPasting(replacement_.TokenAt(prev - 1)))) {
          auto next{replacement_.SkipBlanks(j + 1)};
          if (next >= tokens || !IsTokenPasting(replacement_.TokenAt(next))) {
            // Apply macro replacement to the actual argument
            Preprocessor &preprocessor{
                *const_cast<Preprocessor *>(&prescanner.preprocessor())};
            replaced = preprocessor.MacroReplacement(
                *arg, prescanner, nullptr, inIfExpression);
            if (replaced) {
              arg = &*replaced;
            }
          }
        }
        result.CopyAll(DEREF(arg));
      }
    } else if (bytes == 11 && isVariadic_ &&
        token.ToString() == "__VA_ARGS__") {
      AllSources &allSources{
          *const_cast<AllSources *>(&prescanner.allSources())};
      Provenance commaProvenance{allSources.CompilerInsertionProvenance(',')};
      for (std::size_t k{argumentCount()}; k < args.size(); ++k) {
        if (k > argumentCount()) {
          result.Put(","s, commaProvenance);
        }
        result.CopyAll(args[k]);
      }
    } else if (bytes == 10 && isVariadic_ && token.ToString() == "__VA_OPT__" &&
        j + 2 < tokens && replacement_.TokenAt(j + 1).OnlyNonBlank() == '(' &&
        parenthesesNesting == 0) {
      parenthesesNesting = 1;
      skipping = args.size() == argumentCount();
      ++j;
    } else {
      if (parenthesesNesting > 0) {
        char ch{token.OnlyNonBlank()};
        if (ch == '(') {
          ++parenthesesNesting;
        } else if (ch == ')') {
          if (--parenthesesNesting == 0) {
            skipping = false;
            continue;
          }
        }
      }
      result.AppendRange(replacement_, j);
    }
  }
  return TokenPasting(std::move(result));
}

static std::string FormatTime(const std::time_t &now, const char *format) {
  char buffer[16];
  return {buffer,
      std::strftime(buffer, sizeof buffer, format, std::localtime(&now))};
}

Preprocessor::Preprocessor(AllSources &allSources) : allSources_{allSources} {}

void Preprocessor::DefineStandardMacros() {
  // Capture current local date & time once now to avoid having the values
  // of __DATE__ or __TIME__ change during compilation.
  std::time_t now;
  std::time(&now);
  Define("__DATE__"s, FormatTime(now, "\"%h %e %Y\"")); // e.g., "Jun 16 1904"
  Define("__TIME__"s, FormatTime(now, "\"%T\"")); // e.g., "23:59:60"
  // The values of these predefined macros depend on their invocation sites.
  Define("__FILE__"s, "__FILE__"s);
  Define("__LINE__"s, "__LINE__"s);
  Define("__TIMESTAMP__"s, "__TIMESTAMP__"s);
  Define("__COUNTER__"s, "__COUNTER__"s);
}

static const std::string idChars{
    "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_0123456789"s};

static std::optional<std::vector<std::string>> TokenizeMacroNameAndArgs(
    const std::string &str) {
  // TODO: variadic macros on the command line (?)
  std::vector<std::string> names;
  for (std::string::size_type at{0};;) {
    auto nameStart{str.find_first_not_of(" "s, at)};
    if (nameStart == str.npos) {
      return std::nullopt;
    }
    auto nameEnd{str.find_first_not_of(idChars, nameStart)};
    if (nameEnd == str.npos) {
      return std::nullopt;
    }
    auto punc{str.find_first_not_of(" "s, nameEnd)};
    if (punc == str.npos) {
      return std::nullopt;
    }
    if ((at == 0 && str[punc] != '(') ||
        (at > 0 && str[punc] != ',' && str[punc] != ')')) {
      return std::nullopt;
    }
    names.push_back(str.substr(nameStart, nameEnd - nameStart));
    at = punc + 1;
    if (str[punc] == ')') {
      if (str.find_first_not_of(" "s, at) != str.npos) {
        return std::nullopt;
      } else {
        return names;
      }
    }
  }
}

TokenSequence Preprocessor::TokenizeMacroBody(const std::string &str) {
  TokenSequence tokens;
  Provenance provenance{allSources_.AddCompilerInsertion(str).start()};
  auto end{str.size()};
  for (std::string::size_type at{0}; at < end;) {
    char ch{str.at(at)};
    if (IsWhiteSpace(ch)) {
      ++at;
      continue;
    }
    std::string::size_type start{at};
    if (IsLegalIdentifierStart(ch)) {
      for (++at; at < end && IsLegalInIdentifier(str.at(at)); ++at) {
      }
    } else if (IsDecimalDigit(ch) || ch == '.') {
      for (++at; at < end; ++at) {
        ch = str.at(at);
        if (!IsDecimalDigit(ch) && ch != '.') {
          break;
        }
      }
      if (at < end) {
        ch = ToUpperCaseLetter(str.at(at));
        if (ch == 'E' || ch == 'D' || ch == 'Q') {
          if (++at < end) {
            ch = str.at(at);
            if (ch == '+' || ch == '-') {
              ++at;
            }
            for (; at < end && IsDecimalDigit(str.at(at)); ++at) {
            }
          }
        }
      }
    } else if (ch == '\'' || ch == '"') {
      for (++at; at < end && str.at(at) != ch; ++at) {
      }
      if (at < end) {
        ++at;
      }
    } else {
      ++at; // single-character token
    }
    if (at >= end || at == str.npos) {
      tokens.Put(str.substr(start), provenance + start);
      break;
    }
    tokens.Put(str.substr(start, at - start), provenance + start);
  }
  return tokens;
}

void Preprocessor::Define(const std::string &macro, const std::string &value) {
  TokenSequence rhs{TokenizeMacroBody(value)};
  if (auto lhs{TokenizeMacroNameAndArgs(macro)}) {
    // function-like macro
    CharBlock macroName{SaveTokenAsName(lhs->front())};
    auto iter{lhs->begin()};
    ++iter;
    std::vector<std::string> argNames{iter, lhs->end()};
    definitions_.emplace(std::make_pair(macroName,
        Definition{
            argNames, rhs, 0, rhs.SizeInTokens(), /*isVariadic=*/false}));
  } else { // keyword macro
    definitions_.emplace(SaveTokenAsName(macro), Definition{rhs});
  }
}

void Preprocessor::Undefine(std::string macro) { definitions_.erase(macro); }

std::optional<TokenSequence> Preprocessor::MacroReplacement(
    const TokenSequence &input, const Prescanner &prescanner,
    std::optional<std::size_t> *partialFunctionLikeMacro, bool inIfExpression) {
  // Do quick scan for any use of a defined name.
  if (!inIfExpression && definitions_.empty()) {
    return std::nullopt;
  }
  std::size_t tokens{input.SizeInTokens()};
  std::size_t j{0};
  for (; j < tokens; ++j) {
    CharBlock token{input.TokenAt(j)};
    if (!token.empty() && IsLegalIdentifierStart(token[0]) &&
        (IsNameDefined(token) || (inIfExpression && IsDefinedKeyword(token)))) {
      break;
    }
  }
  if (j == tokens) {
    return std::nullopt; // input contains nothing that would be replaced
  }
  TokenSequence result{input, 0, j};

  // After rescanning after macro replacement has failed due to an unclosed
  // function-like macro call (no left parenthesis yet, or no closing
  // parenthesis), if tokens remain in the input, append them to the
  // replacement text and attempt to proceed.  Otherwise, return, so that
  // the caller may try again with remaining tokens in its input.
  auto CompleteFunctionLikeMacro{
      [this, &input, &prescanner, &result, &partialFunctionLikeMacro,
          inIfExpression](std::size_t after, const TokenSequence &replacement,
          std::size_t pFLMOffset) {
        if (after < input.SizeInTokens()) {
          result.AppendRange(replacement, 0, pFLMOffset);
          TokenSequence suffix;
          suffix.AppendRange(
              replacement, pFLMOffset, replacement.SizeInTokens() - pFLMOffset);
          suffix.AppendRange(input, after, input.SizeInTokens() - after);
          auto further{ReplaceMacros(
              suffix, prescanner, partialFunctionLikeMacro, inIfExpression)};
          if (partialFunctionLikeMacro && *partialFunctionLikeMacro) {
            // still not closed
            **partialFunctionLikeMacro += result.SizeInTokens();
          }
          result.CopyAll(further);
          return true;
        } else {
          if (partialFunctionLikeMacro) {
            *partialFunctionLikeMacro = pFLMOffset + result.SizeInTokens();
          }
          return false;
        }
      }};

  for (; j < tokens; ++j) {
    CharBlock token{input.TokenAt(j)};
    if (token.IsBlank() || !IsLegalIdentifierStart(token[0])) {
      result.AppendRange(input, j);
      continue;
    }
    // Process identifier in replacement text.
    auto it{definitions_.find(token)};
    // Is in the X in "defined(X)" or "defined X" in an #if/#elif expression?
    if (inIfExpression) {
      if (auto prev{result.SkipBlanksBackwards(result.SizeInTokens())}) {
        bool ok{true};
        std::optional<std::size_t> rightParenthesis;
        if (result.TokenAt(*prev).OnlyNonBlank() == '(') {
          prev = result.SkipBlanksBackwards(*prev);
          rightParenthesis = input.SkipBlanks(j + 1);
          ok = *rightParenthesis < tokens &&
              input.TokenAt(*rightParenthesis).OnlyNonBlank() == ')';
        }
        if (ok && prev && IsDefinedKeyword(result.TokenAt(*prev))) {
          result = TokenSequence{result, 0, *prev}; // trims off "defined ("
          char truth{it != definitions_.end() ? '1' : '0'};
          result.Put(&truth, 1, allSources_.CompilerInsertionProvenance(truth));
          j = rightParenthesis.value_or(j);
          continue;
        }
      }
    }
    if (it == definitions_.end()) {
      result.AppendRange(input, j);
      continue;
    }
    Definition *def{&it->second};
    if (def->isDisabled()) {
      result.AppendRange(input, j);
      continue;
    }
    if (!def->isFunctionLike()) {
      if (def->isPredefined() && !def->replacement().empty()) {
        std::string repl;
        std::string name{def->replacement().TokenAt(0).ToString()};
        if (name == "__FILE__") {
          repl = "\""s +
              allSources_.GetPath(prescanner.GetCurrentProvenance()) + '"';
        } else if (name == "__LINE__") {
          std::string buf;
          llvm::raw_string_ostream ss{buf};
          ss << allSources_.GetLineNumber(prescanner.GetCurrentProvenance());
          repl = ss.str();
        } else if (name == "__TIMESTAMP__") {
          auto path{allSources_.GetPath(
              prescanner.GetCurrentProvenance(), /*topLevel=*/true)};
          llvm::sys::fs::file_status status;
          repl = "??? ??? ?? ??:??:?? ????";
          if (!llvm::sys::fs::status(path, status)) {
            auto modTime{llvm::sys::toTimeT(status.getLastModificationTime())};
            if (std::string time{std::asctime(std::localtime(&modTime))};
                time.size() > 1 && time[time.size() - 1] == '\n') {
              time.erase(time.size() - 1); // clip terminal '\n'
              repl = "\""s + time + '"';
            }
          }
        } else if (name == "__COUNTER__") {
          repl = std::to_string(counterVal_++);
        }
        if (!repl.empty()) {
          ProvenanceRange insert{allSources_.AddCompilerInsertion(repl)};
          ProvenanceRange call{allSources_.AddMacroCall(
              insert, input.GetTokenProvenanceRange(j), repl)};
          result.Put(repl, call.start());
          continue;
        }
      }
      std::optional<std::size_t> partialFLM;
      def->set_isDisabled(true);
      TokenSequence replaced{TokenPasting(ReplaceMacros(
          def->replacement(), prescanner, &partialFLM, inIfExpression))};
      def->set_isDisabled(false);
      if (partialFLM &&
          CompleteFunctionLikeMacro(j + 1, replaced, *partialFLM)) {
        return result;
      }
      if (!replaced.empty()) {
        ProvenanceRange from{def->replacement().GetProvenanceRange()};
        ProvenanceRange use{input.GetTokenProvenanceRange(j)};
        ProvenanceRange newRange{
            allSources_.AddMacroCall(from, use, replaced.ToString())};
        result.CopyWithProvenance(replaced, newRange);
      }
    } else {
      // Possible function-like macro call.  Skip spaces and newlines to see
      // whether '(' is next.
      std::size_t k{j};
      bool leftParen{false};
      while (++k < tokens) {
        const CharBlock &lookAhead{input.TokenAt(k)};
        if (!lookAhead.IsBlank() && lookAhead[0] != '\n') {
          leftParen = lookAhead[0] == '(' && lookAhead.size() == 1;
          break;
        }
      }
      if (!leftParen) {
        if (partialFunctionLikeMacro) {
          *partialFunctionLikeMacro = result.SizeInTokens();
          result.AppendRange(input, j, tokens - j);
          return result;
        } else {
          result.AppendRange(input, j);
          continue;
        }
      }
      std::vector<std::size_t> argStart{++k};
      for (int nesting{0}; k < tokens; ++k) {
        CharBlock token{input.TokenAt(k)};
        char ch{token.OnlyNonBlank()};
        if (ch == '(') {
          ++nesting;
        } else if (ch == ')') {
          if (nesting == 0) {
            break;
          }
          --nesting;
        } else if (ch == ',' && nesting == 0) {
          argStart.push_back(k + 1);
        }
      }
      if (argStart.size() == 1 && k == argStart[0] &&
          def->argumentCount() == 0) {
        // Subtle: () is zero arguments, not one empty argument,
        // unless one argument was expected.
        argStart.clear();
      }
      if (k >= tokens && partialFunctionLikeMacro) {
        *partialFunctionLikeMacro = result.SizeInTokens();
        result.AppendRange(input, j, tokens - j);
        return result;
      } else if (k >= tokens || argStart.size() < def->argumentCount() ||
          (argStart.size() > def->argumentCount() && !def->isVariadic())) {
        result.AppendRange(input, j);
        continue;
      }
      std::vector<TokenSequence> args;
      for (std::size_t n{0}; n < argStart.size(); ++n) {
        std::size_t at{argStart[n]};
        std::size_t count{
            (n + 1 == argStart.size() ? k : argStart[n + 1] - 1) - at};
        args.emplace_back(TokenSequence(input, at, count));
      }
      TokenSequence applied{def->Apply(args, prescanner, inIfExpression)};
      std::optional<std::size_t> partialFLM;
      def->set_isDisabled(true);
      TokenSequence replaced{ReplaceMacros(
          std::move(applied), prescanner, &partialFLM, inIfExpression)};
      def->set_isDisabled(false);
      if (partialFLM &&
          CompleteFunctionLikeMacro(k + 1, replaced, *partialFLM)) {
        return result;
      }
      if (!replaced.empty()) {
        ProvenanceRange from{def->replacement().GetProvenanceRange()};
        ProvenanceRange use{input.GetIntervalProvenanceRange(j, k - j)};
        ProvenanceRange newRange{
            allSources_.AddMacroCall(from, use, replaced.ToString())};
        result.CopyWithProvenance(replaced, newRange);
      }
      j = k; // advance to the terminal ')'
    }
  }
  return result;
}

TokenSequence Preprocessor::ReplaceMacros(const TokenSequence &tokens,
    const Prescanner &prescanner,
    std::optional<std::size_t> *partialFunctionLikeMacro, bool inIfExpression) {
  if (std::optional<TokenSequence> repl{MacroReplacement(
          tokens, prescanner, partialFunctionLikeMacro, inIfExpression)}) {
    return std::move(*repl);
  }
  return tokens;
}

void Preprocessor::Directive(const TokenSequence &dir, Prescanner &prescanner) {
  std::size_t tokens{dir.SizeInTokens()};
  std::size_t j{dir.SkipBlanks(0)};
  if (j == tokens) {
    return;
  }
  if (dir.TokenAt(j).ToString() != "#") {
    prescanner.Say(dir.GetTokenProvenanceRange(j), "missing '#'"_err_en_US);
    return;
  }
  j = dir.SkipBlanks(j + 1);
  while (tokens > 0 && dir.TokenAt(tokens - 1).IsBlank()) {
    --tokens;
  }
  if (j == tokens) {
    return;
  }
  if (IsDecimalDigit(dir.TokenAt(j)[0]) || dir.TokenAt(j)[0] == '"') {
    LineDirective(dir, j, prescanner);
    return;
  }
  std::size_t dirOffset{j};
  std::string dirName{ToLowerCaseLetters(dir.TokenAt(dirOffset).ToString())};
  j = dir.SkipBlanks(j + 1);
  CharBlock nameToken;
  if (j < tokens && IsLegalIdentifierStart(dir.TokenAt(j)[0])) {
    nameToken = dir.TokenAt(j);
  }
  if (dirName == "line") {
    LineDirective(dir, j, prescanner);
  } else if (dirName == "define") {
    if (nameToken.empty()) {
      prescanner.Say(dir.GetTokenProvenanceRange(j < tokens ? j : tokens - 1),
          "#define: missing or invalid name"_err_en_US);
      return;
    }
    nameToken = SaveTokenAsName(nameToken);
    definitions_.erase(nameToken);
    if (++j < tokens && dir.TokenAt(j).OnlyNonBlank() == '(') {
      j = dir.SkipBlanks(j + 1);
      std::vector<std::string> argName;
      bool isVariadic{false};
      if (dir.TokenAt(j).OnlyNonBlank() != ')') {
        while (true) {
          std::string an{dir.TokenAt(j).ToString()};
          if (an == "...") {
            isVariadic = true;
          } else {
            if (an.empty() || !IsLegalIdentifierStart(an[0])) {
              prescanner.Say(dir.GetTokenProvenanceRange(j),
                  "#define: missing or invalid argument name"_err_en_US);
              return;
            }
            argName.push_back(an);
          }
          j = dir.SkipBlanks(j + 1);
          if (j == tokens) {
            prescanner.Say(dir.GetTokenProvenanceRange(tokens - 1),
                "#define: malformed argument list"_err_en_US);
            return;
          }
          char punc{dir.TokenAt(j).OnlyNonBlank()};
          if (punc == ')') {
            break;
          }
          if (isVariadic || punc != ',') {
            prescanner.Say(dir.GetTokenProvenanceRange(j),
                "#define: malformed argument list"_err_en_US);
            return;
          }
          j = dir.SkipBlanks(j + 1);
          if (j == tokens) {
            prescanner.Say(dir.GetTokenProvenanceRange(tokens - 1),
                "#define: malformed argument list"_err_en_US);
            return;
          }
        }
        if (std::set<std::string>(argName.begin(), argName.end()).size() !=
            argName.size()) {
          prescanner.Say(dir.GetTokenProvenance(dirOffset),
              "#define: argument names are not distinct"_err_en_US);
          return;
        }
      }
      j = dir.SkipBlanks(j + 1);
      definitions_.emplace(std::make_pair(
          nameToken, Definition{argName, dir, j, tokens - j, isVariadic}));
    } else {
      j = dir.SkipBlanks(j + 1);
      definitions_.emplace(
          std::make_pair(nameToken, Definition{dir, j, tokens - j}));
    }
  } else if (dirName == "undef") {
    if (nameToken.empty()) {
      prescanner.Say(
          dir.GetIntervalProvenanceRange(dirOffset, tokens - dirOffset),
          "# missing or invalid name"_err_en_US);
    } else {
      if (dir.IsAnythingLeft(++j)) {
        prescanner.Warn(common::UsageWarning::Portability,
            dir.GetIntervalProvenanceRange(j, tokens - j),
            "#undef: excess tokens at end of directive"_port_en_US);
      } else {
        definitions_.erase(nameToken);
      }
    }
  } else if (dirName == "ifdef" || dirName == "ifndef") {
    bool doThen{false};
    if (nameToken.empty()) {
      prescanner.Say(
          dir.GetIntervalProvenanceRange(dirOffset, tokens - dirOffset),
          "#%s: missing name"_err_en_US, dirName);
    } else {
      if (dir.IsAnythingLeft(++j)) {
        prescanner.Warn(common::UsageWarning::Portability,
            dir.GetIntervalProvenanceRange(j, tokens - j),
            "#%s: excess tokens at end of directive"_port_en_US, dirName);
      }
      doThen = IsNameDefined(nameToken) == (dirName == "ifdef");
    }
    if (doThen) {
      ifStack_.push(CanDeadElseAppear::Yes);
    } else {
      SkipDisabledConditionalCode(dirName, IsElseActive::Yes, prescanner,
          dir.GetTokenProvenance(dirOffset));
    }
  } else if (dirName == "if") {
    if (IsIfPredicateTrue(dir, j, tokens - j, prescanner)) {
      ifStack_.push(CanDeadElseAppear::Yes);
    } else {
      SkipDisabledConditionalCode(dirName, IsElseActive::Yes, prescanner,
          dir.GetTokenProvenanceRange(dirOffset));
    }
  } else if (dirName == "else") {
    if (dir.IsAnythingLeft(j)) {
      prescanner.Warn(common::UsageWarning::Portability,
          dir.GetIntervalProvenanceRange(j, tokens - j),
          "#else: excess tokens at end of directive"_port_en_US);
    }
    if (ifStack_.empty()) {
      prescanner.Say(dir.GetTokenProvenanceRange(dirOffset),
          "#else: not nested within #if, #ifdef, or #ifndef"_err_en_US);
    } else if (ifStack_.top() != CanDeadElseAppear::Yes) {
      prescanner.Say(dir.GetTokenProvenanceRange(dirOffset),
          "#else: already appeared within this #if, #ifdef, or #ifndef"_err_en_US);
    } else {
      ifStack_.pop();
      SkipDisabledConditionalCode("else", IsElseActive::No, prescanner,
          dir.GetTokenProvenanceRange(dirOffset));
    }
  } else if (dirName == "elif") {
    if (ifStack_.empty()) {
      prescanner.Say(dir.GetTokenProvenanceRange(dirOffset),
          "#elif: not nested within #if, #ifdef, or #ifndef"_err_en_US);
    } else if (ifStack_.top() != CanDeadElseAppear::Yes) {
      prescanner.Say(dir.GetTokenProvenanceRange(dirOffset),
          "#elif: #else previously appeared within this #if, #ifdef, or #ifndef"_err_en_US);
    } else {
      ifStack_.pop();
      SkipDisabledConditionalCode("elif", IsElseActive::No, prescanner,
          dir.GetTokenProvenanceRange(dirOffset));
    }
  } else if (dirName == "endif") {
    if (dir.IsAnythingLeft(j)) {
      prescanner.Warn(common::UsageWarning::Portability,
          dir.GetIntervalProvenanceRange(j, tokens - j),
          "#endif: excess tokens at end of directive"_port_en_US);
    } else if (ifStack_.empty()) {
      prescanner.Say(dir.GetTokenProvenanceRange(dirOffset),
          "#endif: no #if, #ifdef, or #ifndef"_err_en_US);
    } else {
      ifStack_.pop();
    }
  } else if (dirName == "error") {
    prescanner.Say(
        dir.GetIntervalProvenanceRange(dirOffset, tokens - dirOffset),
        "%s"_err_en_US, dir.ToString());
  } else if (dirName == "warning") {
    prescanner.Say(
        dir.GetIntervalProvenanceRange(dirOffset, tokens - dirOffset),
        "%s"_warn_en_US, dir.ToString());
  } else if (dirName == "comment" || dirName == "note") {
    prescanner.Say(
        dir.GetIntervalProvenanceRange(dirOffset, tokens - dirOffset),
        "%s"_en_US, dir.ToString());
  } else if (dirName == "include") {
    if (j == tokens) {
      prescanner.Say(
          dir.GetIntervalProvenanceRange(dirOffset, tokens - dirOffset),
          "#include: missing name of file to include"_err_en_US);
      return;
    }
    std::optional<std::string> prependPath;
    TokenSequence path{dir, j, tokens - j};
    std::string include{path.TokenAt(0).ToString()};
    if (include != "<" && include.substr(0, 1) != "\"" &&
        include.substr(0, 1) != "'") {
      path = ReplaceMacros(path, prescanner);
      include = path.empty() ? ""s : path.TokenAt(0).ToString();
    }
    auto pathTokens{path.SizeInTokens()};
    std::size_t k{0};
    if (include == "<") { // #include <foo>
      k = 1;
      if (k >= pathTokens) {
        prescanner.Say(dir.GetIntervalProvenanceRange(j, pathTokens),
            "#include: file name missing"_err_en_US);
        return;
      }
      while (k < pathTokens && path.TokenAt(k) != ">") {
        ++k;
      }
      if (k >= pathTokens) {
        prescanner.Warn(common::UsageWarning::Portability,
            dir.GetIntervalProvenanceRange(j, tokens - j),
            "#include: expected '>' at end of included file"_port_en_US);
      }
      TokenSequence braced{path, 1, k - 1};
      include = braced.ToString();
    } else if ((include.substr(0, 1) == "\"" || include.substr(0, 1) == "'") &&
        include.front() == include.back()) {
      // #include "foo" and #include 'foo'
      include = include.substr(1, include.size() - 2);
      // Start search in directory of file containing the directive
      auto prov{dir.GetTokenProvenanceRange(dirOffset).start()};
      if (const auto *currentFile{allSources_.GetSourceFile(prov)}) {
        prependPath = DirectoryName(currentFile->path());
      }
    } else {
      prescanner.Say(dir.GetTokenProvenanceRange(j < tokens ? j : tokens - 1),
          "#include %s: expected name of file to include"_err_en_US,
          path.ToString());
      return;
    }
    if (include.empty()) {
      prescanner.Say(dir.GetTokenProvenanceRange(dirOffset),
          "#include %s: empty include file name"_err_en_US, path.ToString());
      return;
    }
    k = path.SkipBlanks(k + 1);
    if (k < pathTokens && path.TokenAt(k).ToString() != "!") {
      prescanner.Warn(common::UsageWarning::Portability,
          dir.GetIntervalProvenanceRange(j, tokens - j),
          "#include: extra stuff ignored after file name"_port_en_US);
    }
    std::string buf;
    llvm::raw_string_ostream error{buf};
    if (const SourceFile *
        included{allSources_.Open(include, error, std::move(prependPath))}) {
      if (included->bytes() > 0) {
        ProvenanceRange fileRange{
            allSources_.AddIncludedFile(*included, dir.GetProvenanceRange())};
        Prescanner{prescanner, *this, /*isNestedInIncludeDirective=*/true}
            .set_encoding(included->encoding())
            .Prescan(fileRange);
      }
    } else {
      prescanner.Say(dir.GetTokenProvenanceRange(j), "#include: %s"_err_en_US,
          error.str());
    }
  } else {
    prescanner.Say(dir.GetTokenProvenanceRange(dirOffset),
        "#%s: unknown or unimplemented directive"_err_en_US, dirName);
  }
}

void Preprocessor::PrintMacros(llvm::raw_ostream &out) const {
  // std::set is ordered. Use that to print the macros in an
  // alphabetical order.
  std::set<std::string> macroNames;
  for (const auto &[name, _] : definitions_) {
    macroNames.insert(name.ToString());
  }

  for (const std::string &name : macroNames) {
    out << "#define " << name;
    definitions_.at(name).Print(out, name.c_str());
    out << '\n';
  }
}

CharBlock Preprocessor::SaveTokenAsName(const CharBlock &t) {
  names_.push_back(t.ToString());
  return {names_.back().data(), names_.back().size()};
}

bool Preprocessor::IsNameDefined(const CharBlock &token) {
  return definitions_.find(token) != definitions_.end();
}

bool Preprocessor::IsNameDefinedEmpty(const CharBlock &token) {
  if (auto it{definitions_.find(token)}; it != definitions_.end()) {
    const Definition &def{it->second};
    return !def.isFunctionLike() && def.replacement().SizeInChars() == 0;
  } else {
    return false;
  }
}

bool Preprocessor::IsFunctionLikeDefinition(const CharBlock &token) {
  auto it{definitions_.find(token)};
  return it != definitions_.end() && it->second.isFunctionLike();
}

static std::string GetDirectiveName(
    const TokenSequence &line, std::size_t *rest) {
  std::size_t tokens{line.SizeInTokens()};
  std::size_t j{line.SkipBlanks(0)};
  if (j == tokens || line.TokenAt(j).ToString() != "#") {
    *rest = tokens;
    return "";
  }
  j = line.SkipBlanks(j + 1);
  if (j == tokens) {
    *rest = tokens;
    return "";
  }
  *rest = line.SkipBlanks(j + 1);
  return ToLowerCaseLetters(line.TokenAt(j).ToString());
}

void Preprocessor::SkipDisabledConditionalCode(const std::string &dirName,
    IsElseActive isElseActive, Prescanner &prescanner,
    ProvenanceRange provenanceRange) {
  int nesting{0};
  while (!prescanner.IsAtEnd()) {
    if (!prescanner.IsNextLinePreprocessorDirective()) {
      prescanner.NextLine();
      continue;
    }
    TokenSequence line{prescanner.TokenizePreprocessorDirective()};
    std::size_t rest{0};
    std::string dn{GetDirectiveName(line, &rest)};
    if (dn == "ifdef" || dn == "ifndef" || dn == "if") {
      ++nesting;
    } else if (dn == "endif") {
      if (nesting-- == 0) {
        return;
      }
    } else if (isElseActive == IsElseActive::Yes && nesting == 0) {
      if (dn == "else") {
        ifStack_.push(CanDeadElseAppear::No);
        return;
      }
      if (dn == "elif" &&
          IsIfPredicateTrue(
              line, rest, line.SizeInTokens() - rest, prescanner)) {
        ifStack_.push(CanDeadElseAppear::Yes);
        return;
      }
    }
  }
  prescanner.Say(provenanceRange, "#%s: missing #endif"_err_en_US, dirName);
}

// Precedence level codes used here to accommodate mixed Fortran and C:
// 15: parentheses and constants, logical !, bitwise ~
// 14: unary + and -
// 13: **
// 12: *, /, % (modulus)
// 11: + and -
// 10: << and >>
//  9: bitwise &
//  8: bitwise ^
//  7: bitwise |
//  6: relations (.EQ., ==, &c.)
//  5: .NOT.
//  4: .AND., &&
//  3: .OR., ||
//  2: .EQV. and .NEQV. / .XOR.
//  1: ? :
//  0: ,
static std::int64_t ExpressionValue(const TokenSequence &token,
    int minimumPrecedence, std::size_t *atToken,
    std::optional<Message> *error) {
  enum Operator {
    PARENS,
    CONST,
    NOTZERO, // !
    COMPLEMENT, // ~
    UPLUS,
    UMINUS,
    POWER,
    TIMES,
    DIVIDE,
    MODULUS,
    ADD,
    SUBTRACT,
    LEFTSHIFT,
    RIGHTSHIFT,
    BITAND,
    BITXOR,
    BITOR,
    LT,
    LE,
    EQ,
    NE,
    GE,
    GT,
    NOT,
    AND,
    OR,
    EQV,
    NEQV,
    SELECT,
    COMMA
  };
  static const int precedence[]{
      15, 15, 15, 15, // (), 6, !, ~
      14, 14, // unary +, -
      13, 12, 12, 12, 11, 11, 10, 10, // **, *, /, %, +, -, <<, >>
      9, 8, 7, // &, ^, |
      6, 6, 6, 6, 6, 6, // relations .LT. to .GT.
      5, 4, 3, 2, 2, // .NOT., .AND., .OR., .EQV., .NEQV.
      1, 0 // ?: and ,
  };
  static const int operandPrecedence[]{0, -1, 15, 15, 15, 15, 13, 12, 12, 12,
      11, 11, 11, 11, 9, 8, 7, 7, 7, 7, 7, 7, 7, 6, 4, 3, 3, 3, 1, 0};

  static std::map<std::string, enum Operator> opNameMap;
  if (opNameMap.empty()) {
    opNameMap["("] = PARENS;
    opNameMap["!"] = NOTZERO;
    opNameMap["~"] = COMPLEMENT;
    opNameMap["**"] = POWER;
    opNameMap["*"] = TIMES;
    opNameMap["/"] = DIVIDE;
    opNameMap["%"] = MODULUS;
    opNameMap["+"] = ADD;
    opNameMap["-"] = SUBTRACT;
    opNameMap["<<"] = LEFTSHIFT;
    opNameMap[">>"] = RIGHTSHIFT;
    opNameMap["&"] = BITAND;
    opNameMap["^"] = BITXOR;
    opNameMap["|"] = BITOR;
    opNameMap[".lt."] = opNameMap["<"] = LT;
    opNameMap[".le."] = opNameMap["<="] = LE;
    opNameMap[".eq."] = opNameMap["=="] = EQ;
    opNameMap[".ne."] = opNameMap["/="] = opNameMap["!="] = NE;
    opNameMap[".ge."] = opNameMap[">="] = GE;
    opNameMap[".gt."] = opNameMap[">"] = GT;
    opNameMap[".not."] = NOT;
    opNameMap[".and."] = opNameMap[".a."] = opNameMap["&&"] = AND;
    opNameMap[".or."] = opNameMap[".o."] = opNameMap["||"] = OR;
    opNameMap[".eqv."] = EQV;
    opNameMap[".neqv."] = opNameMap[".xor."] = opNameMap[".x."] = NEQV;
    opNameMap["?"] = SELECT;
    opNameMap[","] = COMMA;
  }

  std::size_t tokens{token.SizeInTokens()};
  CHECK(tokens > 0);
  if (*atToken >= tokens) {
    *error =
        Message{token.GetProvenanceRange(), "incomplete expression"_err_en_US};
    return 0;
  }

  // Parse and evaluate a primary or a unary operator and its operand.
  std::size_t opAt{*atToken};
  std::string t{token.TokenAt(opAt).ToString()};
  enum Operator op;
  std::int64_t left{0};
  if (t == "(") {
    op = PARENS;
  } else if (IsDecimalDigit(t[0])) {
    op = CONST;
    std::size_t consumed{0};
    left = std::stoll(t, &consumed, 0 /*base to be detected*/);
    if (consumed < t.size()) {
      *error = Message{token.GetTokenProvenanceRange(opAt),
          "Uninterpretable numeric constant '%s'"_err_en_US, t};
      return 0;
    }
  } else if (IsLegalIdentifierStart(t[0])) {
    // undefined macro name -> zero
    // TODO: BOZ constants?
    op = CONST;
  } else if (t == "+") {
    op = UPLUS;
  } else if (t == "-") {
    op = UMINUS;
  } else if (t == "." && *atToken + 2 < tokens &&
      ToLowerCaseLetters(token.TokenAt(*atToken + 1).ToString()) == "not" &&
      token.TokenAt(*atToken + 2).ToString() == ".") {
    op = NOT;
    *atToken += 2;
  } else {
    auto it{opNameMap.find(t)};
    if (it != opNameMap.end()) {
      op = it->second;
    } else {
      *error = Message{token.GetTokenProvenanceRange(opAt),
          "operand expected in expression"_err_en_US};
      return 0;
    }
  }
  if (precedence[op] < minimumPrecedence) {
    *error = Message{token.GetTokenProvenanceRange(opAt),
        "operator precedence error"_err_en_US};
    return 0;
  }
  ++*atToken;
  if (op != CONST) {
    left = ExpressionValue(token, operandPrecedence[op], atToken, error);
    if (*error) {
      return 0;
    }
    switch (op) {
    case PARENS:
      if (*atToken < tokens && token.TokenAt(*atToken).OnlyNonBlank() == ')') {
        ++*atToken;
        break;
      }
      if (*atToken >= tokens) {
        *error = Message{token.GetProvenanceRange(),
            "')' missing from expression"_err_en_US};
      } else {
        *error = Message{
            token.GetTokenProvenanceRange(*atToken), "expected ')'"_err_en_US};
      }
      return 0;
    case NOTZERO:
      left = !left;
      break;
    case COMPLEMENT:
      left = ~left;
      break;
    case UPLUS:
      break;
    case UMINUS:
      left = -left;
      break;
    case NOT:
      left = -!left;
      break;
    default:
      CRASH_NO_CASE;
    }
  }

  // Parse and evaluate binary operators and their second operands, if present.
  while (*atToken < tokens) {
    int advance{1};
    t = token.TokenAt(*atToken).ToString();
    if (t == "." && *atToken + 2 < tokens &&
        token.TokenAt(*atToken + 2).ToString() == ".") {
      t += ToLowerCaseLetters(token.TokenAt(*atToken + 1).ToString()) + '.';
      advance = 3;
    }
    auto it{opNameMap.find(t)};
    if (it == opNameMap.end()) {
      break;
    }
    op = it->second;
    if (op < POWER || precedence[op] < minimumPrecedence) {
      break;
    }
    opAt = *atToken;
    *atToken += advance;

    std::int64_t right{
        ExpressionValue(token, operandPrecedence[op], atToken, error)};
    if (*error) {
      return 0;
    }

    switch (op) {
    case POWER:
      if (left == 0) {
        if (right < 0) {
          *error = Message{token.GetTokenProvenanceRange(opAt),
              "0 ** negative power"_err_en_US};
        }
      } else if (left != 1 && right != 1) {
        if (right <= 0) {
          left = !right;
        } else {
          std::int64_t power{1};
          for (; right > 0; --right) {
            if ((power * left) / left != power) {
              *error = Message{token.GetTokenProvenanceRange(opAt),
                  "overflow in exponentation"_err_en_US};
              left = 1;
            }
            power *= left;
          }
          left = power;
        }
      }
      break;
    case TIMES:
      if (left != 0 && right != 0 && ((left * right) / left) != right) {
        *error = Message{token.GetTokenProvenanceRange(opAt),
            "overflow in multiplication"_err_en_US};
      }
      left = left * right;
      break;
    case DIVIDE:
      if (right == 0) {
        *error = Message{
            token.GetTokenProvenanceRange(opAt), "division by zero"_err_en_US};
        left = 0;
      } else {
        left = left / right;
      }
      break;
    case MODULUS:
      if (right == 0) {
        *error = Message{
            token.GetTokenProvenanceRange(opAt), "modulus by zero"_err_en_US};
        left = 0;
      } else {
        left = left % right;
      }
      break;
    case ADD:
      if ((left < 0) == (right < 0) && (left < 0) != (left + right < 0)) {
        *error = Message{token.GetTokenProvenanceRange(opAt),
            "overflow in addition"_err_en_US};
      }
      left = left + right;
      break;
    case SUBTRACT:
      if ((left < 0) != (right < 0) && (left < 0) == (left - right < 0)) {
        *error = Message{token.GetTokenProvenanceRange(opAt),
            "overflow in subtraction"_err_en_US};
      }
      left = left - right;
      break;
    case LEFTSHIFT:
      if (right < 0 || right > 64) {
        *error = Message{token.GetTokenProvenanceRange(opAt),
            "bad left shift count"_err_en_US};
      }
      left = right >= 64 ? 0 : left << right;
      break;
    case RIGHTSHIFT:
      if (right < 0 || right > 64) {
        *error = Message{token.GetTokenProvenanceRange(opAt),
            "bad right shift count"_err_en_US};
      }
      left = right >= 64 ? 0 : left >> right;
      break;
    case BITAND:
      left = left & right;
      break;
    case BITXOR:
      left = left ^ right;
      break;
    case BITOR:
      left = left | right;
      break;
    case AND:
      left = left && right;
      break;
    case OR:
      left = left || right;
      break;
    case LT:
      left = -(left < right);
      break;
    case LE:
      left = -(left <= right);
      break;
    case EQ:
      left = -(left == right);
      break;
    case NE:
      left = -(left != right);
      break;
    case GE:
      left = -(left >= right);
      break;
    case GT:
      left = -(left > right);
      break;
    case EQV:
      left = -(!left == !right);
      break;
    case NEQV:
      left = -(!left != !right);
      break;
    case SELECT:
      if (*atToken >= tokens || token.TokenAt(*atToken).ToString() != ":") {
        *error = Message{token.GetTokenProvenanceRange(opAt),
            "':' required in selection expression"_err_en_US};
        return 0;
      } else {
        ++*atToken;
        std::int64_t third{
            ExpressionValue(token, operandPrecedence[op], atToken, error)};
        left = left != 0 ? right : third;
      }
      break;
    case COMMA:
      left = right;
      break;
    default:
      CRASH_NO_CASE;
    }
  }
  return left;
}

bool Preprocessor::IsIfPredicateTrue(const TokenSequence &directive,
    std::size_t first, std::size_t exprTokens, Prescanner &prescanner) {
  TokenSequence expr{directive, first, exprTokens};
  TokenSequence replaced{
      ReplaceMacros(expr, prescanner, nullptr, /*inIfExpression=*/true)};
  if (replaced.HasBlanks()) {
    replaced.RemoveBlanks();
  }
  if (replaced.empty()) {
    prescanner.Say(expr.GetProvenanceRange(), "empty expression"_err_en_US);
    return false;
  }
  std::size_t atToken{0};
  std::optional<Message> error;
  bool result{ExpressionValue(replaced, 0, &atToken, &error) != 0};
  if (error) {
    prescanner.Say(std::move(*error));
  } else if (atToken < replaced.SizeInTokens() &&
      replaced.TokenAt(atToken).ToString() != "!") {
    prescanner.Say(replaced.GetIntervalProvenanceRange(
                       atToken, replaced.SizeInTokens() - atToken),
        atToken == 0 ? "could not parse any expression"_err_en_US
                     : "excess characters after expression"_err_en_US);
  }
  return result;
}

void Preprocessor::LineDirective(
    const TokenSequence &dir, std::size_t j, Prescanner &prescanner) {
  std::size_t tokens{dir.SizeInTokens()};
  const std::string *linePath{nullptr};
  std::optional<int> lineNumber;
  SourceFile *sourceFile{nullptr};
  std::optional<SourcePosition> pos;
  for (; j < tokens; j = dir.SkipBlanks(j + 1)) {
    std::string tstr{dir.TokenAt(j).ToString()};
    Provenance provenance{dir.GetTokenProvenance(j)};
    if (!pos) {
      pos = allSources_.GetSourcePosition(provenance);
    }
    if (!sourceFile && pos) {
      sourceFile = const_cast<SourceFile *>(&*pos->sourceFile);
    }
    if (tstr.front() == '"' && tstr.back() == '"') {
      tstr = tstr.substr(1, tstr.size() - 2);
      if (!tstr.empty() && sourceFile) {
        linePath = &sourceFile->SavePath(std::move(tstr));
      }
    } else if (IsDecimalDigit(tstr[0])) {
      if (!lineNumber) { // ignore later column number
        int ln{0};
        for (char c : tstr) {
          if (IsDecimalDigit(c)) {
            int nln{10 * ln + c - '0'};
            if (nln / 10 == ln && nln % 10 == c - '0') {
              ln = nln;
              continue;
            }
          }
          prescanner.Say(provenance,
              "bad line number '%s' in #line directive"_err_en_US, tstr);
          return;
        }
        lineNumber = ln;
      }
    } else {
      prescanner.Say(
          provenance, "bad token '%s' in #line directive"_err_en_US, tstr);
      return;
    }
  }
  if (lineNumber && sourceFile) {
    CHECK(pos);
    if (!linePath) {
      linePath = &*pos->path;
    }
    sourceFile->LineDirective(pos->trueLineNumber + 1, *linePath, *lineNumber);
  }
}

} // namespace Fortran::parser