from transformers import Tool

def simple_calculator(expression):
    # Find the index of the operator
    operator_index = -1
    for i, char in enumerate(expression):
        if char in ('+', '-', '*', '/'):
            operator_index = i
            break

    # Ensure that an operator was found
    if operator_index == -1:
        return "Invalid expression"

    # Split the expression into the operator and operands
    operator = expression[operator_index]
    operand1 = float(expression[:operator_index])
    operand2 = float(expression[operator_index + 1:])

    # Perform the calculation based on the operator
    if operator == '+':
        result = operand1 + operand2
    elif operator == '-':
        result = operand1 - operand2
    elif operator == '*':
        result = operand1 * operand2
    elif operator == '/':
        result = operand1 / operand2
    else:
        raise ValueError("Input string is not a valid mathematical expression.")

    # Convert the result to a string and return it
    return str(result)

# Test the function
expression = "1+3"
result = simple_calculator(expression)


class SimpleCalculatorTool(Tool):
    name = "simple_calculator"
    description = (
        "This is a tool that returns the result of a simple mathematical expression written in a string."
    )

    inputs = ["text"]
    outputs = ["text"]

    def __call__(self, input_str: str):
        return simple_calculator(input_str)