Currency Converter

Phase 1 — Rate source & normalization

• Fetch daily official FX from BNR (RSS/XML) and parse safely
• Normalize to an internal map {currency: RON_per_unit} and store the fetch timestamp
• Persist a tiny local cache (e.g., JSON) to reuse the latest successful rates

Functional result: After one fetch, rates exist locally (including RON) with a timestamp; re-running can load from cache if no network.

Phase 2 — Conversion engine

• Implement the conversion function using RON as the pivot
• Support all currencies published by BNR plus RON; round outputs to 4 decimals
• Guard against missing currencies and zero/negative amounts

Functional result: Given an amount and two currencies, the function returns a correct numeric result using the latest rates.

Phase 3 — GUI (Tkinter)

• Main window with: amount entry, “From” and “To” dropdowns, “Convert” button, result label
• Include a “Refresh rates” button; Display “Last update: ”
• Keep UI responsive while fetching

Functional result: User selects amount/currencies, clicks Convert, and sees an accurate result; Refresh updates rates and the timestamp without freezing the UI.

Phase 4 — Auto-update & minimal errors

• Auto-fetch on startup and once/day (or on app open if cache is older than 24h)
• Minimal errors only: if network fails, fall back to cached rates with a clear banner; reject non-numeric amounts politely

Functional result: With internet, the app auto-updates and converts; offline, it uses cached rates and still converts, warning the user about stale data.