fix m_zero_offset init and update

src/sparrowshell.cpp

@@ -72,8 +72,10 @@ void SparrowShell::update()
 }
 
 void SparrowShell::ShellBuffer::push(std::string s){
-    if (m_buffer.size() >= m_size)
+    if (m_buffer.size() >= m_size){
         m_buffer[m_zero_offset++] = s;
+        m_zero_offset %= m_size;
+    }
     else
         m_buffer.push_back(s);
 }

src/sparrowshell.h

@@ -36,11 +36,11 @@ private:
     class ShellBuffer{
     private:
         std::vector<std::string> m_buffer;
-        int m_zero_offset;
+        int m_zero_offset = 0;
         unsigned int m_size;
     public:
         ShellBuffer(int size):m_size(size){}
-        std::string& operator[](int i){return m_buffer[m_zero_offset+i%m_size];}
+        std::string& operator[](int i){return m_buffer[(m_zero_offset+i)%m_size];}
         void push(std::string);
         unsigned int size(){return m_size;}
     };