implement stack-based conversation position

src/chat.rs

@@ -127,7 +127,16 @@ impl ChatDB {
 
     // returns false if the advanced pointer is out of bounds
     fn advance_pointer(&self, chat: &mut Chat) -> bool {
-        chat.position[0] += 1;
+        let index = chat.position.len() - 1;
+        chat.position[index] += 1;
+
+        while chat.position.len() > 0 {
+            if let None = self.at(chat.id, &chat.position) {
+                chat.position.pop();
+            }
+            let index = chat.position.len() - 1;
+            chat.position[index] += 1;
+        }
         return true;
     }
 
@@ -137,10 +146,18 @@ impl ChatDB {
     }
 
     fn at(&self, id: usize, position: &Vec<usize>) -> Option<&Value> {
-        if let Some(Value::Sequence(seq)) = self.0.get(id) {
+        if position.len() == 0 {
-            return Some(&seq[position[0]]); // TODO: panic
+            return None;
         }
-        return None;
+        let mut pointer: Option<&Value> = Some(&self.0[id]);
+        for index in position {
+            if let Some(Value::Sequence(seq)) = self.0.get(id) {
+                pointer = Some(&seq[*index]);
+            } else {
+                return None;
+            }
+        }
+        return pointer;
     }
 
     pub fn advance_chat(&self, chat: &mut Chat, event: &mut EventWriter<ChatEvent>) {