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README.md

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+# Fifth
+
+A simple stack-based language called Fifth
+
+## Usage
+
+```shell
+python fifth.py
+```
+
+### Commands
+
+- `PUSH <n>` - push integer onto stack
+- `POP` - remove top element
+- `SWAP` - swap top two elements
+- `DUP` - duplicate top element
+- `+`, `-`, `*`, `/` - arithmetic operations
+- `EXIT` - quit
+
+### Example
+
+```
+stack is: []
+PUSH 10
+stack is: [10]
+PUSH 5
+stack is: [10, 5]
++
+stack is: [15]
+PUSH 45
+stack is: [15, 45]
+/
+stack is: [3]
+EXIT
+```
+
+## Tests
+
+```bash
+python -m pytest test_stack_calculator.py
+```
+
+## Requirements
+
+- Python 3.10+

fifth.py

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+import operator
+import readline
+
+class FifthStack:
+    def __init__(self):
+        self.stack = []
+        self.commands = {
+            'push': self.push,
+            'pop': self.pop,
+            'swap': self.swap,
+            'dup': self.dup,
+            '+': self.add,
+            '-': self.sub,
+            '*': self.mul,
+            '/': self.div,
+        }
+        self.binary_ops = {
+            '+': operator.add,
+            '-': operator.sub,
+            '*': operator.mul,
+            '/': operator.floordiv,
+        }
+    
+    def _require(self, number, message):
+        if len(self.stack) < number:
+            print(message)
+            return False
+        return True
+    
+    def _get_binary_op(self, command):
+        return self.binary_ops.get(command)
+    
+    def _binary_op(self, operator):
+        if not self._require(2, "two numbers required"):
+            return
+        a, b = self.stack.pop(), self.stack.pop()
+        try:
+            self.stack.append(operator(a, b))
+        except Exception as e:
+            print(f"ERROR: {e}")
+            self.stack.extend([a, b])
+    
+    def push(self, value):
+        try:
+            self.stack.append(int(value))
+        except ValueError:
+            print("ERROR: integer required")
+
+    def pop(self, _value):
+        if not self.stack:
+            print("ERROR: stack empty")
+            return
+        self.stack.pop()
+
+    def swap(self, _value):
+        if not self._require(2, 'two numbers required'):
+            return
+        self.stack[-1], self.stack[-2] = self.stack[-2], self.stack[-1]
+
+    def dup(self, _value):
+        if self._require(1, 'stack empty'):
+            self.stack.append(self.stack[-1])
+
+    def add(self, _value):
+        self._binary_op(self._get_binary_op('+'))
+
+    def sub(self, _value):
+        self._binary_op(self._get_binary_op('-'))
+
+    def mul(self, _value):
+        self._binary_op(self._get_binary_op('*'))
+
+    def div(self, _value):
+        self._binary_op(self._get_binary_op('/'))
+
+    def execute(self, command):
+        tokens = command.lower().strip().split()
+        if not tokens:
+            return
+
+        command, argument = tokens[0], tokens[1] if len(tokens) > 1 else None
+        function = self.commands.get(command)
+
+        try:
+            if function: function(argument)
+            else: print("ERROR: unknown command")
+        except:
+            print("ERROR: invalid command")
+
+def main():
+    fifth = FifthStack()
+    
+    while True:
+        print(f'stack is: {fifth.stack}')
+        command = input()
+        if command.lower() == 'exit':
+            break
+        fifth.execute(command)
+
+if __name__ == '__main__':
+    main()