* Use a symbol table to represent identifiers and attribute names

efficiently.  The symbol table ensures that there is only one copy
  of each symbol, thus allowing symbols to be compared efficiently
  using a pointer equality test.

src/libexpr/symbol-table.hh

@@ -0,0 +1,75 @@
+#ifndef __SYMBOL_TABLE_H
+#define __SYMBOL_TABLE_H
+
+#include <map>
+
+#include "types.hh"
+
+namespace nix {
+
+/* Symbol table used by the parser and evaluator to represent and look
+   up identifiers and attribute sets efficiently.
+   SymbolTable::create() converts a string into a symbol.  Symbols
+   have the property that they can be compared efficiently (using a
+   pointer equality test), because the symbol table stores only one
+   copy of each string. */
+
+class Symbol
+{
+private:
+    const string * s; // pointer into SymbolTable
+    Symbol(const string * s) : s(s) { };
+    friend class SymbolTable;
+
+public:
+    bool operator == (const Symbol & s2) const
+    {
+        return s == s2.s;
+    }
+    
+    bool operator != (const Symbol & s2) const
+    {
+        return s != s2.s;
+    }
+    
+    bool operator < (const Symbol & s2) const
+    {
+        return s < s2.s;
+    }
+
+    operator const string & () const
+    {
+        return *s;
+    }
+
+    bool empty() const
+    {
+        return s->empty();
+    }
+
+    friend std::ostream & operator << (std::ostream & str, const Symbol & sym);
+};
+
+inline std::ostream & operator << (std::ostream & str, const Symbol & sym)
+{
+    str << *sym.s;
+    return str;
+}
+
+class SymbolTable
+{
+private:
+    typedef std::set<string> Symbols;
+    Symbols symbols;
+
+public:
+    Symbol create(const string & s)
+    {
+        std::pair<Symbols::iterator, bool> res = symbols.insert(s);
+        return Symbol(&*res.first);
+    }
+};
+
+}
+
+#endif /* !__SYMBOL_TABLE_H */