Initial development for variant descriptions (#65)

Add command to print description of game rules, e.g.:
`rules chess`

src/uci.cpp

@@ -197,6 +197,17 @@ namespace {
         Options["VariantPath"] = token;
   }
 
+  // rules() prints the rules of the given variant.
+
+  void rules(istringstream& is) {
+
+    string token;
+    if (is >> token && variants.find(token) != variants.end())
+        sync_cout << variants.find(token) << sync_endl;
+    else
+        sync_cout << "No such variant: " << token << sync_endl;
+  }
+
 } // namespace
 
 
@@ -276,6 +287,7 @@ void UCI::loop(int argc, char* argv[]) {
       else if (token == "d")     sync_cout << pos << sync_endl;
       else if (token == "eval")  sync_cout << Eval::trace(pos) << sync_endl;
       else if (token == "load")  { load(is); argc = 1; } // continue reading stdin
+      else if (token == "rules") rules(is);
       else
           sync_cout << "Unknown command: " << cmd << sync_endl;
 

src/variant.cpp

@@ -23,6 +23,8 @@
 
 #include "parser.h"
 #include "piece.h"
+#include "position.h"
+#include "thread.h"
 #include "variant.h"
 
 using std::string;
@@ -962,3 +964,37 @@ std::vector<std::string> VariantMap::get_keys() {
       keys.push_back(element.first);
   return keys;
 }
+
+std::string value_to_result(Value value) {
+    return  value == VALUE_MATE  ? "win"
+          : value == -VALUE_MATE ? "loss"
+                                 : "draw";
+}
+
+std::ostream& operator<<(std::ostream& os, const VariantMap::iterator& variantIterator) {
+    std::string variantName = variantIterator->first;
+    variantName[0] = toupper(variantName[0]);
+    const Variant* v = variantIterator->second;
+    StateInfo st;
+    Position pos;
+    pos.set(v, v->startFen, v->chess960, &st, Threads.main());
+
+    os << variantName << " is a chess variant played on a board of " << v->maxFile + 1 << " x " << v->maxRank + 1 << " squares." << std::endl;
+
+    // diagram
+    os << pos << std::endl << std::endl;
+
+    os << "# Game end" << std::endl << std::endl;
+    // checkmate
+    if (v->checking && v->pieceTypes.find(KING) != v->pieceTypes.end())
+        os << "Checkmating the opponent's king leads to a "
+           << value_to_result(-v->checkmateValue)
+           << " for the the player who delivered the checkmate." << std::endl << std::endl;
+
+    // stalemate
+    os << "Stalemate is adjudicated as a " << value_to_result(v->stalemateValue) << " for player who can not move." << std::endl;
+
+    // TODO: implement more options
+
+    return os;
+}

src/variant.h

@@ -146,4 +146,6 @@ class VariantMap : public std::map<std::string, const Variant*> {
 
 extern VariantMap variants;
 
+extern std::ostream& operator<<(std::ostream& os, const VariantMap::iterator& variantIterator);
+
 #endif // #ifndef VARIANT_H_INCLUDED

src/xboard.cpp

@@ -83,6 +83,17 @@ namespace {
     moveList.pop_back();
   }
 
+  // rules() prints the rules of the given variant.
+
+  void rules(std::istringstream& is) {
+
+    std::string token;
+    if (is >> token && variants.find(token) != variants.end())
+        sync_cout << variants.find(token) << sync_endl;
+    else
+        sync_cout << "Error (unkown command): " << token << sync_endl;
+  }
+
 } // namespace
 
 namespace XBoard {
@@ -277,6 +288,8 @@ void StateMachine::process_command(Position& pos, std::string token, std::istrin
       sync_cout << pos << sync_endl;
   else if (token == "eval")
       sync_cout << Eval::trace(pos) << sync_endl;
+  else if (token == "rules")
+      rules(is);
   // Move strings and unknown commands
   else
   {