final_grade_calculator.py

import os
import sys

from flask import Flask, jsonify, request, send_from_directory

app = Flask(__name__)


def calculate_needed_grade(current_pct: float, final_weight_pct: float, desired_pct: float) -> float:
    """Calculate the final exam score required to reach a desired course grade.

    Args:
        current_pct: Current grade percentage before the final.
        final_weight_pct: Final exam weight as a percentage of the total grade.
        desired_pct: Desired overall course grade percentage.

    Returns:
        The percentage score needed on the final exam.

    Formula:
        desired = current * (1 - w) + final_score * w
        => final_score = (desired - current * (1 - w)) / w
    """
    w = final_weight_pct / 100
    needed = (desired_pct - current_pct * (1 - w)) / w
    return needed


@app.route("/")
def index():
    return send_from_directory(os.path.dirname(os.path.abspath(__file__)), "index.html")


@app.route("/calculate", methods=["POST"])
def calculate():
    data = request.get_json(force=True)
    try:
        current = float(data["current"])
        weight  = float(data["weight"])
        desired = float(data["desired"])
    except (KeyError, TypeError, ValueError):
        return jsonify({"error": "Invalid input."}), 400

    if not (0 <= current <= 200):
        return jsonify({"error": "Current grade must be between 0 and 200."}), 400
    if not (1 <= weight <= 100):
        return jsonify({"error": "Final exam weight must be between 1 and 100."}), 400
    if not (0 <= desired <= 200):
        return jsonify({"error": "Desired grade must be between 0 and 200."}), 400

    needed = calculate_needed_grade(current, weight, desired)
    return jsonify({"needed": needed})


def get_float(prompt: str, low: float, high: float) -> float:
    """Prompt the user for a validated float within the given range."""
    while True:
        try:
            value = float(input(prompt))
        except ValueError:
            print("  Please enter a number.")
            continue

        if not (low <= value <= high):
            print(f"  Please enter a value between {low} and {high}.")
            continue

        return value


def main():
    print("=== Final Grade Calculator ===\n")

    current = get_float("Current grade (%): ", 0, 200)
    weight  = get_float("Final exam weight (%): ", 1, 100)
    desired = get_float("Desired final grade (%): ", 0, 200)

    needed = calculate_needed_grade(current, weight, desired)

    print(f"\nYou need {needed:.2f}% on the final to finish with {desired:.2f}%.", end="")

    if needed < 0:
        print("\nNote: you've already secured your desired grade regardless of the final.")
    else:
        print()


if __name__ == "__main__":
    if "--web" in sys.argv:
        print("Starting web server at http://127.0.0.1:5000")
        app.run(debug=False)
    else:
        main()