diff --git a/src/lib.rs b/src/lib.rs
index d0dde13..5f2cd79 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -20,18 +20,32 @@ pub fn eval_mem(bf_str: &&str, mem: &mut [u8], pointer: &mut usize) -> Result<()
         pos += 1;
 
         if char == '>' {
+            if (*pointer) + 1 == 30000 {
+                return Err("Attempt to increase pointer exceeding memory size".into());
+            }
             *pointer += 1;
             continue
         }
         if char == '<' {
+            if *pointer == 0 {
+                return Err("Attempt to reduce pointer to less than 1".into());
+            }
             *pointer -= 1;
             continue
         }
         if char == '+' {
+            if mem[*pointer] == 255 {
+                mem[*pointer] = 0;
+                continue
+            }
             mem[*pointer] += 1;
             continue
         }
         if char == '-' {
+            if mem[*pointer] == 0 {
+                mem[*pointer] = 255;
+                continue
+            }
             mem[*pointer] -= 1;
             continue
         }
@@ -72,6 +86,10 @@ pub fn eval_mem(bf_str: &&str, mem: &mut [u8], pointer: &mut usize) -> Result<()
         }
     }
 
+    if loop_stack.len() != 0 {
+        return Err(format!("Unclosed loop at position {}", loop_stack.last().unwrap()));
+    }
+
     Ok(())
 }