Title menu background: an endless battle at a bridge over an underground river

- dungeon creatures against knights, balanced reinforcements for the weaker side
- towers on both banks; heavy units go for them, destroyed towers are rebuilt after 25 s
- the camera slowly circles the bridge; battle sounds stay muted; the demo is never saved

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/game.rs

@@ -445,6 +445,8 @@ pub struct Game {
     pub gate_on: bool, // free game: the humans' gate exists
     #[serde(skip)]
     pub springs: usize, // claimed mana springs (recounted)
+    #[serde(skip)]
+    pub demo: Option<crate::demo::Demo>, // the endless bridge battle behind the title menu
     #[serde(default)]
     pub dwarf_caves: Vec<crate::dwarves::DwarfCave>,
     #[serde(default)]
@@ -603,6 +605,7 @@ impl Game {
             gate_on: true,
             springs: 0,
             freed: 0,
+            demo: None,
             dwarf_caves: Vec::new(),
             dwarf_t: 30.0,
             rune_free: 0,
@@ -964,7 +967,25 @@ impl Game {
         self.fx.retain(|f| f.t < f.max);
     }
 
+    pub(crate) fn update_units(&mut self, dt: f32) {
+        for i in 0..self.ents.len() {
+            if !self.ents[i].alive || self.ents[i].held {
+                continue;
+            }
+            let mut e = std::mem::replace(&mut self.ents[i], Ent::dummy());
+            let (x0, y0) = (e.x, e.y);
+            self.update_ent(&mut e, dt);
+            self.turn(&mut e, x0, y0);
+            self.ents[i] = e;
+        }
+        self.ents.retain(|e| e.alive);
+    }
+
     fn step(&mut self, dt: f32) {
+        if self.demo.is_some() {
+            self.demo_step(dt);
+            return;
+        }
         self.time += dt;
         self.recount();
         self.mana = (self.mana + (1.0 + SPRING_MANA * self.springs as f32) * dt).min(self.mana_max());
@@ -1014,17 +1035,7 @@ impl Game {
             self.reinforce_corners();
         }
         self.update_contested();
-        for i in 0..self.ents.len() {
-            if !self.ents[i].alive || self.ents[i].held {
-                continue;
-            }
-            let mut e = std::mem::replace(&mut self.ents[i], Ent::dummy());
-            let (x0, y0) = (e.x, e.y);
-            self.update_ent(&mut e, dt);
-            self.turn(&mut e, x0, y0);
-            self.ents[i] = e;
-        }
-        self.ents.retain(|e| e.alive);
+        self.update_units(dt);
         self.update_towers(dt);
         if !self.rivals.is_empty() {
             self.update_rivals(dt);
@@ -1611,6 +1622,8 @@ impl Game {
                 self.sfx(Sfx::Convert, e.x, e.y);
                 self.msg("msg.creature_freed", vec![format!("@{}", st(e.kind).name)]);
             }
+        } else if self.demo.is_some() {
+            self.demo_ai(e, dt);
         } else if e.hero && e.captive != 0 {
             self.captive_update(e, dt);
         } else if e.rival() {
@@ -1934,7 +1947,7 @@ impl Game {
         cap.saturating_sub(used)
     }
 
-    fn kill(&mut self, j: usize) {
+    pub(crate) fn kill(&mut self, j: usize) {
         let e = &self.ents[j];
         if e.hero && e.captive == 0 && self.prison_space() > 0 {
             // knocked out instead of killed: imps carry the hero to the torture chamber
@@ -4064,7 +4077,7 @@ impl Game {
         }
     }
 
-    fn update_towers(&mut self, dt: f32) {
+    pub(crate) fn update_towers(&mut self, dt: f32) {
         for k in 0..self.towers.len() {
             if k >= self.towers.len() {
                 break;
@@ -4227,7 +4240,7 @@ impl Game {
     }
 
     // a hero attacks a tower; false = tower gone or unreachable
-    fn attack_tower(&mut self, e: &mut Ent, id: u32, dt: f32) -> bool {
+    pub(crate) fn attack_tower(&mut self, e: &mut Ent, id: u32, dt: f32) -> bool {
         let Some(k) = self.towers.iter().position(|t| t.id == id) else { return false };
         let (tx, ty) = self.towers[k].pos();
         let hit_h = self.towers[k].top() * 0.6;