Introduce error types (#709)

* typing errors, extra tests

* linting

---------

Co-authored-by: Piotr Idzik <65706193+vil02@users.noreply.github.com>

src/data_structures/postfix_evaluation.rs

@@ -1,4 +1,12 @@
-pub fn evaluate_postfix(expression: &str) -> Result<i32, &'static str> {
+#[derive(Debug, PartialEq)]
+pub enum PostfixError {
+    DivisionByZero,
+    InvalidOperator,
+    InsufficientOperands,
+    InvalidExpression,
+}
+
+pub fn evaluate_postfix(expression: &str) -> Result<i32, PostfixError> {
     let mut stack: Vec<i32> = Vec::new();
 
     for token in expression.split_whitespace() {
@@ -15,23 +23,22 @@ pub fn evaluate_postfix(expression: &str) -> Result<i32, &'static str> {
                     "*" => stack.push(a * b),
                     "/" => {
                         if b == 0 {
-                            return Err("Division by zero");
+                            return Err(PostfixError::DivisionByZero);
                         }
                         stack.push(a / b);
                     }
-                    _ => return Err("Invalid operator"),
+                    _ => return Err(PostfixError::InvalidOperator),
                 }
             } else {
-                return Err("Insufficient operands");
+                return Err(PostfixError::InsufficientOperands);
             }
         }
     }
-
     // The final result should be the only element on the stack.
     if stack.len() == 1 {
         Ok(stack[0])
     } else {
-        Err("Invalid expression")
+        Err(PostfixError::InvalidExpression)
     }
 }
 
@@ -48,11 +55,34 @@ mod tests {
 
     #[test]
     fn test_insufficient_operands() {
-        assert_eq!(evaluate_postfix("+"), Err("Insufficient operands"));
+        assert_eq!(
+            evaluate_postfix("+"),
+            Err(PostfixError::InsufficientOperands)
+        );
     }
 
     #[test]
     fn test_division_by_zero() {
-        assert_eq!(evaluate_postfix("5 0 /"), Err("Division by zero"));
+        assert_eq!(evaluate_postfix("5 0 /"), Err(PostfixError::DivisionByZero));
+    }
+
+    #[test]
+    fn test_invalid_operator() {
+        assert_eq!(
+            evaluate_postfix("2 3 #"),
+            Err(PostfixError::InvalidOperator)
+        );
+    }
+
+    #[test]
+    fn test_invalid_expression() {
+        assert_eq!(
+            evaluate_postfix("2 3"),
+            Err(PostfixError::InvalidExpression)
+        );
+        assert_eq!(
+            evaluate_postfix("2 3 4 +"),
+            Err(PostfixError::InvalidExpression)
+        );
     }
 }

src/data_structures/queue.rs

@@ -47,7 +47,7 @@ mod tests {
     fn test_enqueue() {
         let mut queue: Queue<u8> = Queue::new();
         queue.enqueue(64);
-        assert!(!queue.is_empty());
+        assert!(!queue.is_empty(), "Queue should not be empty after enqueue");
     }
 
     #[test]
@@ -56,7 +56,11 @@ mod tests {
         queue.enqueue(32);
         queue.enqueue(64);
         let retrieved_dequeue = queue.dequeue();
-        assert_eq!(retrieved_dequeue, Some(32));
+        assert_eq!(
+            retrieved_dequeue,
+            Some(32),
+            "Dequeue should return the first element"
+        );
     }
 
     #[test]
@@ -65,14 +69,30 @@ mod tests {
         queue.enqueue(8);
         queue.enqueue(16);
         let retrieved_peek = queue.peek_front();
-        assert_eq!(retrieved_peek, Some(&8));
+        assert_eq!(
+            retrieved_peek,
+            Some(&8),
+            "Peek should return a reference to the first element"
+        );
     }
 
     #[test]
     fn test_size() {
         let mut queue: Queue<u8> = Queue::new();
         queue.enqueue(8);
         queue.enqueue(16);
-        assert_eq!(2, queue.len());
+        assert_eq!(
+            2,
+            queue.len(),
+            "Queue length should be equal to the number of enqueued elements"
+        );
+    }
+
+    #[test]
+    fn test_is_empty() {
+        let mut queue: Queue<u8> = Queue::new();
+        assert!(queue.is_empty(), "Newly created queue should be empty");
+        queue.enqueue(8);
+        assert!(!queue.is_empty(), "Queue should not be empty after enqueue");
     }
 }

src/data_structures/range_minimum_query.rs

@@ -10,6 +10,17 @@
 */
 
 use std::cmp::PartialOrd;
+use std::fmt;
+
+/// Custom error for invalid range
+#[derive(Debug, PartialEq)]
+pub struct RangeError;
+
+impl fmt::Display for RangeError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "Invalid range")
+    }
+}
 
 pub struct RangeMinimumQuery<T: PartialOrd + Copy> {
     // the current version makes a copy of the input array, but this could be changed
@@ -26,9 +37,9 @@ impl<T: PartialOrd + Copy> RangeMinimumQuery<T> {
         }
     }
 
-    pub fn get_range_min(&self, start: usize, end: usize) -> Result<T, &str> {
+    pub fn get_range_min(&self, start: usize, end: usize) -> Result<T, RangeError> {
         if start >= end || start >= self.array.len() || end > self.array.len() {
-            return Err("invalid range");
+            return Err(RangeError);
         }
         let loglen = (end - start).ilog2() as usize;
         let idx: usize = end - (1 << loglen);