"""Educational currency converter using fixed example rates."""
import math

RATES = {
    "USD": {"EUR": 0.92, "GBP": 0.81, "USD": 1},
    "EUR": {"USD": 1.09, "GBP": 0.83, "EUR": 1},
    "GBP": {"USD": 1.24, "EUR": 1.20, "GBP": 1},
}


def convert_currency(amount, from_currency, to_currency, rates):
    if from_currency not in rates or to_currency not in rates[from_currency]:
        raise ValueError("Unsupported currency or conversion.")
    if not math.isfinite(amount) or amount < 0:
        raise ValueError("Amount must be a finite number of 0 or more.")
    result = amount * rates[from_currency][to_currency]
    if not math.isfinite(result):
        raise ValueError("Amount is too large to convert.")
    return result


def read_currency(prompt):
    while True:
        currency = input(prompt).strip().upper()
        if currency in RATES:
            return currency
        print("Invalid currency. Please enter USD, EUR or GBP.")


def main():
    print("Welcome to Harlow College Currency Converter")
    print("Educational example rates; these are not live market rates.")
    from_currency = read_currency("Convert from (USD, EUR, GBP): ")
    to_currency = read_currency("Convert to (USD, EUR, GBP): ")
    while True:
        try:
            amount = float(input(f"Enter the amount in {from_currency}: "))
            converted = convert_currency(amount, from_currency, to_currency, RATES)
        except ValueError:
            print("Invalid amount. Enter a finite number of 0 or more, small enough to convert.")
            continue
        break
    print(f"{amount} {from_currency} is equal to {converted:.2f} {to_currency}")


if __name__ == "__main__":
    main()