Architecture

Two projects and a line between them. SpaceSharp.Core targets plain net10.0 and cannot reference Avalonia, Velopack or Win32 (the compiler enforces it: none of them exist for that target). SpaceSharp is the app on top. The rule when adding code: if it would work the same on every platform, it goes in Core; anything that draws, opens a window, talks to the shell, the trash, the registry or the NTFS file table stays in the app.

The pieces

scan ─────▶ FsNode tree ─────▶ Squarify + Grouping ─────▶ TreemapControl (render, zoom, input)
  │              │                                                │
  │              ├──▶ FileFilter (dims the map)                   ├──▶ MainWindow partials: FilterPanel, Insights, Changes, Scans, Tooltip, StatusBar, PalettePicker
  │              ├──▶ DriveInsights (the Insights cards)
  │              ├──▶ ScanCompare (baselines + gone items)        
  │              └──▶ ScanFile (.sscan save and load)
  └── FolderWalker (every platform) / MftScanner + ElevatedScan (Windows, NTFS)

The tree (Core/Models/FsNode.cs). One node per file and folder: name, kind, size, size on disk, file count, last write time, attributes, and after a comparison the baseline sizes. Folders sum their children once in FinishDirectory. Memory matters at a million nodes, so fields are compact and the free-space box is a node like any other.

Scanning. FolderWalker is the System.IO scan every platform has, parallel, with exclusions and hard-link detection behind IPlatformFiles. On Windows a whole NTFS drive goes through MftScanner, which reads the master file table as one stream, verifies every record, and builds the tree from it; ElevatedScan runs it in a second elevated copy of the exe and streams the tree back over a pipe. While a walk runs, LiveTree keeps a thread-safe ledger of finished folders and stand-ins so the map can draw itself.

Layout (Core/Layout/Squarify.cs, Grouping.cs). The squarified treemap algorithm (Bruls, Huizing and van Wijk) on a UI-free LayoutRect. Grouping decides which small children become one "312 files" box according to Density.

Rendering and zoom (Controls/TreemapControl*.cs). One custom Avalonia control in six partial files (fields, camera, layout, render, input, live). It draws every box itself in two render passes rather than creating a control per box, which is why tens of thousands of boxes stay fast. Zoom re-lays out the visible folder rather than scaling a picture; the camera animates between layouts, and the live map tweens boxes between snapshots by path.

Filter (Core/Services/FileFilter.cs, MainWindow.FilterPanel.cs). Text is parsed into a FilterSpec; the tree is evaluated once per change and a folder stays lit when anything inside it matches. The drawer and the rail both write the same text.

Insights (Core/Services/DriveInsights.cs, MainWindow.Insights.cs). Pure, tested functions: bytes by type, by month, the cleanup suggestions (name patterns in CachePatterns), change totals, the free-space projection. The window only arranges cards.

Changes (Core/Services/ScanCompare.cs, MainWindow.Changes.cs). ScanCompare.Apply walks two trees matching names within each folder, writes baseline sizes onto the current nodes and returns a ScanComparison holding what is gone per folder. The view is a virtualized table over that.

Saved scans (Core/Services/ScanFile.cs). A gzip-compressed binary of the tree with a header (root, time, method, free space). Autosaves live per root, with a .prev copy for comparison.

Strings (Core/Resources/Strings*.resx, Core/Util/Strings.cs). Every user-visible string is a key; LocalizationTests fails the build when code asks for one the English file lacks, and a key missing from a translation falls back to English. Eighteen files, discovered by culture code at startup.

Settings and looks (Core/Util/AppSettings.cs, UiLooks.cs, App.axaml, Themes/Styles.axaml). Settings are a JSON file loaded once; every control style references colors through DynamicResource, so a theme or look change is a dictionary swap.

Where to add things

To add	Touch
A setting	A property with a default in AppSettings (and ResetToDefaults), a row in SettingsWindow.Build(), its effect in MainWindow.ApplySettings() or ApplyLook(), the Settings reference
A shortcut	MainWindow.OnKeyDown, the list in ShortcutsWindow.cs, the README table, the Keyboard shortcuts page
UI text	A key in Strings.resx and the same key in all seventeen translation files
A built-in palette	BuildSchemes() in Util/Palette.cs with twelve folder colors and a PaletteAbout_ string
A file type	FileCategories in Core/Util/FileCategory.cs; the filter's words in FileFilter.TypeWords
A cleanup pattern	CachePatterns in DriveInsights.cs and a Cleanup_ string pair
An icon	A line in tools/make-icons.py's table (Lucide), which regenerates Themes/Icons.axaml
A translation	One more Strings.<culture>.resx with a Meta_MachineTranslated entry; it appears in the language list on its own (Translating)

DEVELOPING.md in the repository goes deeper on each of these and is kept in step with the code.