# Copilot Instructions for `o8_pics_size`

## Build, test, and lint commands

- Build: `cargo build`
- Run app: `cargo run`
- Run all tests: `cargo test`
- Run a single test: `cargo test <test_name>`
- Formatting check: `cargo fmt --check`
- Lint (if available in your toolchain): `cargo clippy -- -D warnings`

## High-level architecture

The application is a single-binary Rust CLI (`src/main.rs`) built as a staged pipeline:

1. Pick an input Excel file (`.xls`/`.xlsx`) via `rfd::FileDialog`.
2. Read workbook/sheet data with `calamine`.
3. Use terminal UI selectors (`dialoguer::Select`) to choose:
   - sheet
   - `Cikkszám` column
   - `Url` column
   - `img_sequence` column
4. Build input rows from sheet data (header row is row 0, data starts from row 1).
5. Download images from URLs using blocking `reqwest`.
6. Decode image bytes with `image` to extract dimensions; compute size from downloaded byte length.
7. Export results to a new workbook with `rust_xlsxwriter`, saved as `result_[uuid].xlsx` next to the input file.

Core helper responsibilities are split into functions in `main.rs`:
- selection helpers: `prompt_sheet_selection`, `prompt_column_selection`
- sheet parsing helpers: `extract_headers`, `collect_input_rows`, `cell_string`
- image metadata fetch: `fetch_image_metadata`
- output generation: `build_output_path`, `write_results_excel`

## Key conventions in this repository

- **Fail-fast error handling:** most failures print a clear `eprintln!` message and exit with status 1.
- **Column naming is contract-like:** output headers are currently written as:
  - `Cikkszám`
  - `Sqeuence` (keep this spelling unless intentionally changed)
  - `Url`
  - `Width (px)`
  - `Height (px)`
  - `Size (KB)`
- **Interactive flow is TUI-driven:** keep sheet/column selection cursor-based (`dialoguer::Select`), not numeric text prompts.
- **Output path pattern is fixed:** write result files as `result_[uuid].xlsx` in the same directory as the source workbook.