// file      : libbuild2/depdb.hxx -*- C++ -*-
// license   : MIT; see accompanying LICENSE file

#ifndef LIBBUILD2_DEPDB_HXX
#define LIBBUILD2_DEPDB_HXX

#include <cstring> // strlen()

#include <libbuild2/types.hxx>
#include <libbuild2/utility.hxx>

#include <libbuild2/export.hxx>

namespace build2
{
  // Auxiliary dependency database (those .d files). Prints the diagnostics
  // and fails on system and IO errors.
  //
  // This is a strange beast: a line-oriented, streaming database that can, at
  // some point, be switched from reading to (over)writing. The idea is to
  // store auxiliary/ad-hoc dependency information in the "invalidation"
  // order. That is, if an earlier line is out of date, then all the
  // subsequent ones are out of date as well.
  //
  // As an example, consider a dependency database for foo.o which is built
  // from foo.cxx by the cxx.compile rule. The first line could be the rule
  // name itself (perhaps with the version). If a different rule is now
  // building foo.o, then any dep info that was saved by cxx.compile is
  // probably useless. Next we can have the command line options that were
  // used to build foo.o. Then could come the source file name followed by the
  // extracted header dependencies. If the compile options or the source file
  // name have changed, then the header dependencies are likely to have
  // changed as well.
  //
  // As an example, here is what our foo.o.d could look like (the first line
  // is the database format version and the last '\0' character is the end
  // marker):
  //
  // 1
  // cxx.compile 1
  // g++-4.8 -I/tmp/foo -O3
  // /tmp/foo/foo.cxx
  // /tmp/foo/foo.hxx
  // /usr/include/string.h
  // /usr/include/stdlib.h
  // /tmp/foo/bar.hxx
  // ^@
  //
  // A race is possible between updating the database and the target. For
  // example, we may detect a line mismatch that renders the target out-of-
  // date (say, compile options in the above example). We update the database
  // but before getting a chance to update the target, we get interrupted. On
  // a subsequent re-run, because the database has been updated, we will miss
  // the "target requires update" condition.
  //
  // If we assume that an update of the database also means an update of the
  // target, then this "interrupted update" situation can be easily detected
  // by comparing the database and target modification timestamps. This is
  // also used to handle the dry-run mode where we essentially do the
  // interruption ourselves.
  //
  struct LIBBUILD2_SYMEXPORT depdb_base
  {
    explicit
    depdb_base (const path&, timestamp);

    ~depdb_base ();

    enum class state {read, read_eof, write} state_;

    union
    {
      ifdstream is_; // read, read_eof
      ofdstream os_; // write
    };

    butl::fdbuf* buf_; // Current buffer (for tellg()/tellp()).
  };

  class LIBBUILD2_SYMEXPORT depdb: private depdb_base
  {
  public:
    using path_type = build2::path;

    // The modification time of the database only makes sense while reading
    // (in the write mode it will be set to timestamp_unknown).
    //
    // If touch is set to true, update the database modification time in
    // close() even if otherwise no modifications are necessary (i.e., the
    // database is in the read mode and is at eof).
    //
    path_type path;
    timestamp mtime;
    bool      touch;

    // Open the database for reading. Note that if the file does not exist,
    // has wrong format version, or is corrupt, then the database will be
    // immediately switched to writing.
    //
    // The failure commonly happens when the user tries to stash the target in
    // a non-existent subdirectory but forgets to add the corresponding fsdir{}
    // prerequisite. That's why the issued diagnostics may provide the
    // corresponding hint.
    //
    explicit
    depdb (path_type);

    // Close the database. If this function is not called, then the database
    // may be left in the old/currupt state. Note that in the read mode this
    // function will "chop off" lines that haven't been read.
    //
    // Make sure to also call check_mtime() after updating the target to
    // perform the target/database modification times sanity checks.
    //
    void
    close ();

    // Flush any unwritten data to disk. This is primarily useful when reusing
    // a (partially written) database as an input to external programs (e.g.,
    // as a module map).
    //
    void
    flush ();

    // Perform target/database modification times sanity check.
    //
    // Note that it would also be good to compare the target timestamp against
    // the newest prerequisite. However, obtaining this information would cost
    // extra (see execute_prerequisites()). So maybe later, if we get a case
    // where this is a problem (in a sense, the database is a buffer between
    // prerequisites and the target).
    //
    void
    check_mtime (const path_type& target, timestamp end = timestamp_unknown);

    static void
    check_mtime (timestamp start,
                 const path_type& db,
                 const path_type& target,
                 timestamp end);

    // Return true if mtime checks are enabled.
    //
    static bool
    mtime_check ();

    // Read the next line. If the result is not NULL, then it is a pointer to
    // the next line in the database (which you are free to move from). If you
    // then call write(), this line will be overwritten.
    //
    // If the result is NULL, then it means no next line is unavailable. This
    // can be due to several reasons:
    //
    // - eof reached (you can detect this by calling more() before read())
    // - database is already in the write mode
    // - the next line (and the rest of the database are corrupt)
    //
    string*
    read () {return state_ == state::write ? nullptr : read_ ();}

    // Return true if the database is in the read mode and there is at least
    // one more line available. Note that there is no guarantee that the line
    // is not corrupt. In other words, read() can still return NULL, it just
    // won't be because of eof.
    //
    bool
    more () const {return state_ == state::read;}

    bool
    reading () const {return state_ != state::write;}

    bool
    writing () const {return state_ == state::write;}

    // Skip to the end of the database and return true if it is valid.
    // Otherwise, return false, in which case the database must be
    // overwritten. Note that this function expects the database to be in the
    // read state.
    //
    bool
    skip ();

    // Write the next line. If nl is false then don't write the newline yet.
    // Note that this switches the database into the write mode and no further
    // reading will be possible.
    //
    void
    write (const string& l, bool nl = true) {write (l.c_str (), l.size (), nl);}

    void
    write (const path_type& p, bool nl = true) {write (p.string (), nl);}

    void
    write (const char* s, bool nl = true) {write (s, std::strlen (s), nl);}

    void
    write (const char*, size_t, bool nl = true);

    void
    write (char, bool nl = true);

    // Mark the previously read line as to be overwritte.
    //
    void
    write () {if (state_ != state::write) change ();}

    // Read the next line and compare it to the expected value. If it matches,
    // return NULL. Otherwise, overwrite it and return the old value (which
    // could also be NULL). This strange-sounding result semantics is used to
    // detect the "there is a value but it does not match" case for tracing:
    //
    // if (string* o = d.expect (...))
    //   l4 ([&]{trace << "X mismatch forcing update of " << t;});
    //
    string*
    expect (const string& v)
    {
      string* l (read ());
      if (l == nullptr || *l != v)
      {
        write (v);
        return l;
      }

      return nullptr;
    }

    string*
    expect (const path_type& v)
    {
      string* l (read ());
      if (l == nullptr ||
          path_type::traits_type::compare (*l, v.string ()) != 0)
      {
        write (v);
        return l;
      }

      return nullptr;
    }

    string*
    expect (const char* v)
    {
      string* l (read ());
      if (l == nullptr || *l != v)
      {
        write (v);
        return l;
      }

      return nullptr;
    }

    // Could be supported if required.
    //
    depdb (depdb&&) = delete;
    depdb (const depdb&) = delete;

    depdb& operator= (depdb&&) = delete;
    depdb& operator= (const depdb&) = delete;

  private:
    depdb (path_type&&, timestamp);

    void
    change (bool truncate = true);

    string*
    read_ ();

    void
    check_mtime_ (const path_type&, timestamp);

    static void
    check_mtime_ (timestamp, const path_type&, const path_type&, timestamp);

  private:
    uint64_t  pos_;   // Start of the last returned line.
    string    line_;  // Current line.
    timestamp start_; // Sequence start (mtime check).
  };
}

#include <libbuild2/depdb.ixx>

#endif // LIBBUILD2_DEPDB_HXX