App Graph Calculator — Android

App Graph Calculator — Android
Interactive graph construction and calculations on it.


## Working with the graph
- **Vertices** — tap an empty space for a new vertex; drag a vertex to move it (in any mode).
- **Edges** — touch two vertices → enter weight (can be fractional/negative). Reconnecting updates the weight.
- **Delete** — tap a vertex (deletes its edges as well) or an edge.
- **Oriented** — switches the type; in unoriented the pairs u→v / v→u merge.
- **Weights** — shows/hides weights. **Example** — loads a graph with 7 vertices and 11 edges.

## Set of graphs
All graphs are stored in SQLite (`graphs.db`) and are saved **automatically** with every change
(after a 0.8 s pause), when moving a vertex and when exiting the application.

- The **Name** of the current graph is above the canvas — tap it to rename it.
- **Graphs** — list of all graphs (most recently modified first) with number of vertices/edges and modification time.
Tap a row to open it; **⋮** gives Open / Rename / Duplicate / Delete.
- **＋ New** — empty graph with a suggested name ("Graph 1", "Graph 2"…).
- **Example** — creates a *new* graph with the example (the current one is not overwritten).
- **Clear** — deletes the vertices and edges of the current graph (the graph itself remains).
- When starting, the last used graph is opened. When deleting the current one, the next one is opened.

Each entry also keeps the size of the canvas it was drawn in, so the graph scales correctly.
when rotating the screen or on another device.

## Import / Export
The **⇅** button next to the column name opens the menu. Files are selected via the system selector
(Downloads, Google Drive, memory card…), so **no permissions** are needed in the manifest.

| Action | File | Content |

| Export graph | `.json` | current graph: name, type, coordinates, weights |
| Export the list of ribs | `.csv` | `from,to,weight` — opens in Excel / LibreOffice |
| Export as image | `.png` | the graph as seen, incl. the colored result of the algorithm |
| Backup | `.json` | all columns in one file |
| Import | `.json` `.csv` `.txt` | adds the columns as new (existing ones are not changed) |

When importing with a matching name, a number is added: "Roads (2)". The first imported column opens.
Limitations: up to 5 MB per file and up to 1000 vertices per graph.

**JSON** (a single graph or array of graphs is also accepted; `from`/`to` can be a label or index;
without `x`/`y` the vertices are arranged in a circle):
```json
{"format": "graphcalc", "version": 1, "graphs": [
{"name": "Roads", "directed": false, "canvas": {"w": 1080, "h": 1400},
"nodes": [{"label": "Sofia", "x": 150, "y": 300}, {"label": "Plovdiv", "x": 500, "y": 700}],
"edges": [{"from": "Sofia", "to": "Plovdiv", "weight": 150}]}]}
```

**Edge List** (CSV or TXT; one edge per row, vertices arranged in a circle):
```
# directed ← as desired; otherwise undirected
from,to,weight ← header row optional
Sofia, Plovdiv, 150
Plovdiv;Burgas;250.5 ← ";" allows decimal point
Varna Ruse 200 ← separator space
A -> B 3 ← "->" makes the graph directed
Shumen ← isolated peak
```
No weight → 1. Lines with `#` are comments. Loops (edge ​​to itself) are skipped.

## Languages
The button in the top left changes the language in the app, regardless of the device language:
English, Bulgarian, German, French, Spanish. The selection is saved in SharedPreferences;
On first startup, the device language is used (if supported), otherwise English.