fss-lang

File Style Sheets — a CSS-like language for describing how files should be rendered in a tree or panel.

Features

• CSS-like syntax powered by css-tree for robust parsing
• Type selectors: file, folder
• Attribute selectors: [ext="ts"], [name="Dockerfile"], [lang="typescript"], [vcs-status="modified"]
• Attribute operators: =, ^=, $=, ~=, !=
• Pseudo-classes: :expanded, :selected, :hovered, :active, :drag-over, :focused
• :root pseudo-class: folder:root matches the top-level folder
• :is() grouping: file:is([ext="ts"], [ext="tsx"])
• Descendant combinator: folder[name=".github"] folder[name="workflows"]
• Compound extensions: file[ext="test.ts"] overrides file[ext="ts"] — specificity scales by segment count
• Language selector: file[lang="typescript"] — match by VS Code language ID
• @theme blocks: light, dark, high-contrast, high-contrast-light — like VS Code
• @sorting block for sort priority/grouping
• @table block for column visibility/width/order
• Layered resolution: global → project → nested .faraday overrides
• Sub-O(N) matching via rule indexing by name, extension, language, and type
• Style caching for amortized O(1) per unique file signature

Install

npm install fss-lang

Quick Start

import { parseStylesheet, resolveStyle, createFsNode, StateFlags } from 'fss-lang'

// Parse an FSS stylesheet
const sheet = parseStylesheet(`
  file { icon: url(default-file.svg); color: white; }
  folder { icon: url(default-folder.svg); }
  folder:expanded { icon: url(folder-open.svg); }

  file[ext="ts"] { icon: url(ts.svg); color: blue; }
  file[ext="test.ts"] { icon: url(test.svg); badge: "T"; }
  file[name="Dockerfile"] { icon: url(docker.svg); }

  file[lang="typescript"] { color: blue; }
  file:is([ext="ts"], [ext="tsx"]) { color: blue; }
  file[vcs-status="modified"] { color: orange; }

  folder:root { icon: url(project-root.svg); }

  folder[name=".github"] folder[name="workflows"] {
    icon: url(gh-workflow.svg);
  }

  @sorting {
    file[executable] { priority: 10; }
  }

  @table {
    column(size) { visible: false; }
    column(vcs-status) { width: 30; order: 2; }
  }
`)

// Create a file node
const node = createFsNode({
  type: 'file',
  name: 'index.ts',
  path: '/src/index.ts',
  lang: 'typescript',
  meta: { 'vcs-status': 'modified' },
})

// Resolve style
const style = resolveStyle(sheet, node, 'dark')
// → { icon: 'url(ts.svg)', color: '#58a6ff' }

Cached Resolution

For large trees (10k+ nodes), use CachedResolver to avoid recomputing styles for nodes with identical signatures:

import { CachedResolver, parseStylesheet } from 'fss-lang'

const sheet = parseStylesheet(`...`)
const resolver = new CachedResolver(sheet, 'dark')

// Many .ts files share the same style — computed once, cached forever
const style = resolver.resolveStyle(node)

// Switch theme — cache is invalidated automatically
resolver.setTheme('light')

Layer System

Support global config with per-folder overrides (e.g. .faraday/style.fss):

import { LayeredResolver, createLayer, LayerPriority } from 'fss-lang'

const resolver = new LayeredResolver()

// Global defaults
resolver.addLayer(createLayer(`
  file { icon: url(file.svg); }
`, '/', LayerPriority.GLOBAL))

// Project-specific overrides
resolver.addLayer(createLayer(`
  file[ext="ts"] { icon: url(custom-ts.svg); }
`, '/my-project/', LayerPriority.PROJECT))

// Nested folder overrides (deeper = higher priority)
resolver.addLayer(createLayer(`
  file { icon: url(special.svg); }
`, '/my-project/packages/core/', LayerPriority.nestedPriority(1)))

const style = resolver.resolveStyle(node)

Grammar

Node Styling

file { icon: url(file.svg); }
folder { icon: url(folder.svg); }
file[ext="ts"] { icon: url(ts.svg); }
file[name="Dockerfile"] { icon: url(docker.svg); }
file[ext$="d.ts"] { badge: "DT"; }
folder:expanded { icon: url(open.svg); }
file:is([ext="ts"], [ext="tsx"]) { color: blue; }
file[lang="typescript"] { icon: url(ts.svg); }
folder[name=".github"] folder[name="workflows"] { icon: url(gh.svg); }
file[inVcsRepo] { badge: "V"; }
folder:root { icon: url(project-root.svg); }

Themes

@theme dark {
  file { color: #ccc; }
  file[ext="ts"] { color: #58a6ff; }
}

@theme light {
  file { color: #333; }
  file[ext="ts"] { color: #0366d6; }
}

@theme high-contrast {
  file[ext="ts"] { color: #79c0ff; font-weight: bold; }
}

@theme high-contrast-light {
  file[ext="ts"] { color: #0969da; font-weight: bold; }
}

Supported kinds: light, dark, high-contrast, high-contrast-light (matching VS Code). Rules outside @theme apply in all themes. Theme-scoped rules merge on top.

Sorting

@sorting {
  file[executable] { priority: 10; }
  folder { group-first: true; }
}

Table Configuration

@table {
  column(size) { visible: false; }
  column(vcs-status) { width: 30; order: 2; }
}

Performance

The engine uses rule indexing for sub-O(N) matching:

Technique	Benefit
Bucket indexing by name, ext, type	Only evaluate ~5–20 candidate rules instead of all
Bitmask state matching	Single integer AND for pseudo-class checks
Style key caching	Amortized O(1) for nodes with identical signatures
Pre-sorted rules	No sorting during match phase

| Candidate caching | WeakMap-based per-sheet cache for nodes sharing the same bucket key |

For a stylesheet with 10,000 rules and 5,000 files in the same directory, the engine evaluates only a small subset of candidate rules per node. Nodes sharing the same extension/type/name pattern reuse a cached candidate list, and CachedResolver further deduplicates resolution for nodes with identical style keys.

API

parseStylesheet(source: string): CompiledStylesheet

Parse FSS source into an indexed, compiled stylesheet.

resolveStyle(sheet: CompiledStylesheet, node: FsNode, theme?: ThemeKind): ResolvedStyle

Resolve all matching style declarations for a node. When theme is provided, both unscoped and matching-theme rules apply.

resolveSorting(sheet: CompiledStylesheet, node: FsNode, theme?: ThemeKind): Record<string, FssValue>

Resolve sorting declarations for a node.

createFsNode(opts): FsNode

Create a node with auto-computed extensions.

CachedResolver

Style resolver with per-signature caching. Accepts optional theme in constructor. Call setTheme(theme) to switch — cache is auto-invalidated.

LayeredResolver

Multi-layer resolver with scope-based priority. Call setTheme(theme) to switch — cache is auto-invalidated.

createLayer(source, scopePath, priority): StyleLayer

Create a scoped style layer from FSS source.

License

MIT