Expand the city dialog text for nationality

Explain the rules and give the result of the calculation. The rule sentence is
not fully correct but it's easier to understand than the actual mechanism.
Hopefully helps with longturn#269.

client/text.cpp

@@ -28,6 +28,7 @@
 #include "clientutils.h"
 #include "combat.h"
 #include "culture.h"
+#include "effects.h"
 #include "fc_types.h" // LINE_BREAK
 #include "game.h"
 #include "government.h"
@@ -1388,42 +1389,64 @@ QString text_happiness_buildings(const struct city *pcity)
 }
 
 /**
-   Describing nationality effects that affect happiness.
+ * Describing nationality effects that affect happiness.
  */
-const QString text_happiness_nationality(const struct city *pcity)
+QString text_happiness_nationality(const struct city *pcity)
 {
-  QString str;
-  int enemies = 0;
+  if (auto effects = get_effects(EFT_ENEMY_CITIZEN_UNHAPPY_PCT);
+      !game.info.citizen_nationality || effect_list_size(effects) == 0) {
+    // Disabled in the ruleset
+    effect_list_destroy(effects);
+    return QString();
+  }
 
-  str = _("Nationality: ") + qendl();
+  auto str = QStringLiteral("<p>");
+  str +=
+      _("The presence of enemy citizens can create additional unhappiness.");
+  str += QStringLiteral(" ");
 
-  if (game.info.citizen_nationality) {
-    if (get_city_bonus(pcity, EFT_ENEMY_CITIZEN_UNHAPPY_PCT) > 0) {
-      struct player *owner = city_owner(pcity);
+  int pct = get_city_bonus(pcity, EFT_ENEMY_CITIZEN_UNHAPPY_PCT);
+  if (pct == 0) {
+    str += _("However, it is not the case in this city.");
+    return str + QStringLiteral("</p>");
+  }
 
-      citizens_foreign_iterate(pcity, pslot, nationality)
-      {
-        if (pplayers_at_war(owner, player_slot_get_player(pslot))) {
-          enemies += nationality;
-        }
-      }
-      citizens_foreign_iterate_end;
+  // This is not exactly correct, but gives a first idea.
+  int num = std::ceil(100. / pct);
+  str += QString(PL_("For every %1 citizen of an enemy nation, one citizen "
+                     "becomes unhappy.",
+                     "For every %1 citizens of an enemy nation, one citizen "
+                     "becomes unhappy.",
+                     num))
+             .arg(num);
+  str += QStringLiteral("</p><p>");
 
-      if (enemies > 0) {
-        str += QString(PL_("%1 enemy nationalist", "%1 enemy nationalists",
-                           enemies))
-                   .arg(QString::number(enemies));
-      }
+  int enemies = 0;
+  const auto owner = city_owner(pcity);
+  citizens_foreign_iterate(pcity, pslot, nationality)
+  {
+    if (pplayers_at_war(owner, player_slot_get_player(pslot))) {
+      enemies += nationality;
     }
+  }
+  citizens_foreign_iterate_end;
 
-    if (enemies == 0) {
-      str += QString(_("None."));
-    }
+  if (enemies == 0) {
+    str += _("There is <b>no enemy citizen</b> in this city.");
   } else {
-    str += QString(_("Disabled."));
+    auto unhappy = enemies * pct / 100;
+    // TRANS: Pluralized on "enemy citizen"
+    str +=
+        QString(PL_("There is %1 enemy citizen in this city, resulting in "
+                    "<b>%2 additional unhappy.</b>",
+                    "There are %1 enemy citizens in this city, resulting "
+                    "in <b>%2 additional unhappy.</b>",
+                    enemies))
+            .arg(enemies)
+            .arg(unhappy);
   }
 
-  return str.trimmed();
+  return str + QStringLiteral("</p>");
 }
 
 /**

client/text.h

@@ -45,7 +45,7 @@ const QString act_sel_action_tool_tip(const struct action *paction,
                                       const struct act_prob prob);
 
 QString text_happiness_buildings(const struct city *pcity);
-const QString text_happiness_nationality(const struct city *pcity);
+QString text_happiness_nationality(const struct city *pcity);
 QString text_happiness_cities(const struct city *pcity);
 QString text_happiness_luxuries(const struct city *pcity);
 const QString text_happiness_units(const struct city *pcity);