From b6e02f4224975a6425f62095bc35478e8866db77 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Wed, 24 May 2017 13:26:13 +0200
Subject: Various improvements to char_scanner

---
 libbutl/char-scanner.cxx | 68 +++++++++++++++++++-----------------------------
 libbutl/char-scanner.hxx | 38 ++++++++++++++++++++-------
 libbutl/char-scanner.ixx | 32 +++++++++++++++++++++++
 3 files changed, 87 insertions(+), 51 deletions(-)
 create mode 100644 libbutl/char-scanner.ixx

diff --git a/libbutl/char-scanner.cxx b/libbutl/char-scanner.cxx
index cbc2503..42a72cc 100644
--- a/libbutl/char-scanner.cxx
+++ b/libbutl/char-scanner.cxx
@@ -14,44 +14,44 @@ namespace butl
   peek () -> xchar
   {
     if (unget_)
-      return buf_;
-    else
-    {
-      if (eos_)
-        return xchar (xchar::traits_type::eof (), line, column);
-      else
-      {
-        xchar::int_type v (is_.peek ());
+      return ungetc_;
 
-        if (v == xchar::traits_type::eof ())
-          eos_ = true;
-        else if (crlf_ && v == 0x0D)
-        {
-          is_.get ();
-          xchar::int_type v1 (is_.peek ());
+    if (unpeek_)
+      return unpeekc_;
 
-          if (v1 != '\n')
-          {
-            unget_ = true;
-            buf_ = '\n';
-          }
+    if (eos_)
+      return xchar (xchar::traits_type::eof (), line, column);
 
-          v = '\n';
-        }
+    xchar::int_type v (is_.peek ());
 
-        return xchar (v, line, column);
+    if (v == xchar::traits_type::eof ())
+      eos_ = true;
+    else if (crlf_ && v == 0x0D)
+    {
+      is_.get ();
+      xchar::int_type v1 (is_.peek ());
+
+      if (v1 != '\n')
+      {
+        // We need to make sure subsequent calls to peek() return newline.
+        //
+        unpeek_ = true;
+        unpeekc_ = xchar ('\n', line, column);
       }
+
+      v = '\n';
     }
+
+    return xchar (v, line, column);
   }
 
-  auto char_scanner::
-  get () -> xchar
+  void char_scanner::
+  get (const xchar& c)
   {
     if (unget_)
-    {
       unget_ = false;
-      return buf_;
-    }
+    else if (unpeek_)
+      unpeek_ = false;
     else
     {
       // When is_.get () returns eof, the failbit is also set (stupid,
@@ -60,8 +60,6 @@ namespace butl
       // eof. But we can only call peek() on eof once; any subsequent
       // calls will spoil the failbit (even more stupid).
       //
-      xchar c (peek ());
-
       if (!eos (c))
       {
         is_.get ();
@@ -74,18 +72,6 @@ namespace butl
         else
           column++;
       }
-
-      return c;
     }
   }
-
-  void char_scanner::
-  unget (const xchar& c)
-  {
-    // Because iostream::unget cannot work once eos is reached,
-    // we have to provide our own implementation.
-    //
-    buf_ = c;
-    unget_ = true;
-  }
 }
diff --git a/libbutl/char-scanner.hxx b/libbutl/char-scanner.hxx
index 71f8313..e71f286 100644
--- a/libbutl/char-scanner.hxx
+++ b/libbutl/char-scanner.hxx
@@ -33,8 +33,12 @@ namespace butl
     //
   public:
 
-    // Extended character. It includes line/column information
-    // and is capable of representing EOF.
+    // Extended character. It includes line/column information and is capable
+    // of representing EOF.
+    //
+    // Note that implicit conversion of EOF to char_type results in NUL
+    // character (which means in most cases it is safe to compare xchar to
+    // char without checking for EOF).
     //
     class xchar
     {
@@ -47,7 +51,12 @@ namespace butl
       std::uint64_t line;
       std::uint64_t column;
 
-      operator char_type () const {return static_cast<char_type> (value);}
+      operator char_type () const
+      {
+        return value != traits_type::eof ()
+          ? static_cast<char_type> (value)
+          : char_type (0);
+      }
 
       xchar (int_type v, std::uint64_t l = 0, std::uint64_t c = 0)
           : value (v), line (l), column (c) {}
@@ -57,6 +66,9 @@ namespace butl
     get ();
 
     void
+    get (const xchar& peeked); // Get previously peeked character (faster).
+
+    void
     unget (const xchar&);
 
     // Note that if there is an "ungot" character, peek() will return
@@ -71,20 +83,26 @@ namespace butl
     static bool
     eos (const xchar& c) {return c.value == xchar::traits_type::eof ();}
 
-    // Line and column of the furthest seen (either via get() or
-    // peek()) character.
+    // Line and column of the next character to be extracted from the stream
+    // by peek() or get().
     //
-    std::uint64_t line {1};
-    std::uint64_t column {1};
+    std::uint64_t line = 1;
+    std::uint64_t column = 1;
 
   protected:
     std::istream& is_;
+
     bool crlf_;
+    bool eos_ = false;
+
+    bool unget_ = false;
+    bool unpeek_ = false;
 
-    bool unget_ {false};
-    xchar buf_ = '\0';
-    bool eos_ {false};
+    xchar ungetc_ = '\0';
+    xchar unpeekc_ = '\0';
   };
 }
 
+#include <libbutl/char-scanner.ixx>
+
 #endif // LIBBUTL_CHAR_SCANNER_HXX
diff --git a/libbutl/char-scanner.ixx b/libbutl/char-scanner.ixx
new file mode 100644
index 0000000..2d96207
--- /dev/null
+++ b/libbutl/char-scanner.ixx
@@ -0,0 +1,32 @@
+// file      : libbutl/char-scanner.ixx -*- C++ -*-
+// copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+namespace butl
+{
+  inline auto char_scanner::
+  get () -> xchar
+  {
+    if (unget_)
+    {
+      unget_ = false;
+      return ungetc_;
+    }
+    else
+    {
+      xchar c (peek ());
+      get (c);
+      return c;
+    }
+  }
+
+  inline void char_scanner::
+  unget (const xchar& c)
+  {
+    // Because iostream::unget cannot work once eos is reached, we have to
+    // provide our own implementation.
+    //
+    unget_ = true;
+    ungetc_ = c;
+  }
+}
-- 
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