brainy/docs/vfs/PROJECTION_STRATEGY_API.md

Projection Strategy API

Creating Custom Semantic Dimensions

Projection strategies allow you to create custom ways to organize and access files in Semantic VFS. This guide shows you how to build your own.

---

What is a Projection Strategy?

A projection strategy maps a semantic dimension (like "priority" or "language") to actual file entities using Brainy queries.

Example:

/by-priority/high → Files with metadata.priority = 'high'
/by-language/typescript → Files with .ts extension

---

Interface Definition

Every projection must implement the ProjectionStrategy interface:

export interface ProjectionStrategy {
  /**
   * Unique name for this dimension
   * Used in paths like: /by-{name}/...
   */
  readonly name: string

  /**
   * Convert dimension value to Brainy FindParams
   * This is for documentation/debugging (not always used)
   *
   * @param value - The dimension value (e.g., 'high' for priority)
   * @param subpath - Optional file filter within dimension
   */
  toQuery(value: any, subpath?: string): FindParams

  /**
   * Resolve dimension value to entity IDs
   * This is the MAIN method that does the work
   *
   * @param brain - Brainy instance (use brain.find, brain.similar, etc.)
   * @param vfs - VirtualFileSystem instance
   * @param value - The dimension value to resolve
   * @returns Array of entity IDs matching this dimension
   */
  resolve(brain: Brainy, vfs: VirtualFileSystem, value: any): Promise<string[]>

  /**
   * OPTIONAL: List all items in this dimension
   * Used for directory listings like: readdir('/by-priority')
   *
   * @param brain - Brainy instance
   * @param vfs - VirtualFileSystem instance
   * @param limit - Max results to return
   */
  list?(brain: Brainy, vfs: VirtualFileSystem, limit?: number): Promise<VFSEntity[]>
}

---

Quick Start: Priority Projection

Let's build a projection that organizes files by priority (high, medium, low):

Step 1: Create the Strategy Class

import { BaseProjectionStrategy } from '@soulcraft/brainy/vfs/semantic'
import { Brainy } from '@soulcraft/brainy'
import { VirtualFileSystem, VFSEntity } from '@soulcraft/brainy/vfs'

export class PriorityProjection extends BaseProjectionStrategy {
  readonly name = 'priority'

  /**
   * Convert priority value to FindParams
   */
  toQuery(priority: string, subpath?: string) {
    const query = {
      where: {
        vfsType: 'file',
        priority: priority  // Match metadata.priority field
      },
      limit: 1000
    }

    // Filter by filename if subpath provided
    if (subpath) {
      query.where = {
        ...query.where,
        anyOf: [
          { name: subpath },
          { path: { endsWith: subpath } }
        ]
      }
    }

    return query
  }

  /**
   * Resolve priority to entity IDs
   */
  async resolve(brain: Brainy, vfs: VirtualFileSystem, priority: string): Promise<string[]> {
    // Query Brainy for files with this priority
    const results = await brain.find({
      where: {
        vfsType: 'file',
        priority: priority
      },
      limit: 1000
    })

    // Extract entity IDs using helper from base class
    return this.extractIds(results)
  }

  /**
   * List all files that have priority metadata
   */
  async list(brain: Brainy, vfs: VirtualFileSystem, limit = 100): Promise<VFSEntity[]> {
    const results = await brain.find({
      where: {
        vfsType: 'file',
        priority: { exists: true }
      },
      limit
    })

    return results.map(r => r.entity as VFSEntity)
  }
}

Step 2: Register the Strategy

import { Brainy } from '@soulcraft/brainy'
import { PriorityProjection } from './PriorityProjection'

const brain = new Brainy()
await brain.init()

const vfs = brain.vfs()
await vfs.init()

// Register custom projection
// TODO: This will be exposed as public API
// For now, access via internal property
vfs['projectionRegistry'].register(new PriorityProjection())

Step 3: Use It!

// Write files with priority metadata
await vfs.writeFile('/src/critical-fix.ts', code, {
  metadata: { priority: 'high' }
})

await vfs.writeFile('/src/nice-to-have.ts', code, {
  metadata: { priority: 'low' }
})

// Access by priority
const highPriority = await vfs.readdir('/by-priority/high')
console.log(highPriority)  // ['critical-fix.ts']

const lowPriority = await vfs.readdir('/by-priority/low')
console.log(lowPriority)   // ['nice-to-have.ts']

---

Base Class Helpers

BaseProjectionStrategy provides utility methods:

extractIds(results: Result[]): string[]

Extracts entity IDs from Brainy query results:

const results = await brain.find({ where: { ... } })
return this.extractIds(results)  // ['id1', 'id2', ...]

filterFiles(brain: Brainy, ids: string[]): Promise<string[]>

Filters to only file entities (removes directories):

const allIds = await this.traverseGraph(...)
return await this.filterFiles(brain, allIds)  // Only files

---

Advanced Examples

Example 1: Language Projection

Organize files by programming language:

export class LanguageProjection extends BaseProjectionStrategy {
  readonly name = 'language'

  // Map extensions to languages
  private languageMap = {
    ts: 'typescript',
    js: 'javascript',
    py: 'python',
    go: 'go',
    rs: 'rust'
  }

  toQuery(language: string, subpath?: string) {
    // Find extension for this language
    const ext = Object.entries(this.languageMap)
      .find(([_, lang]) => lang === language)?.[0]

    return {
      where: {
        vfsType: 'file',
        extension: ext
      },
      limit: 1000
    }
  }

  async resolve(brain: Brainy, vfs: VirtualFileSystem, language: string): Promise<string[]> {
    const ext = Object.entries(this.languageMap)
      .find(([_, lang]) => lang === language)?.[0]

    if (!ext) return []

    const results = await brain.find({
      where: {
        vfsType: 'file',
        extension: ext
      },
      limit: 5000
    })

    return this.extractIds(results)
  }

  async list(brain: Brainy, vfs: VirtualFileSystem, limit = 100): Promise<VFSEntity[]> {
    // Return sample files from each language
    const results = await brain.find({
      where: { vfsType: 'file' },
      limit
    })

    return results.map(r => r.entity as VFSEntity)
  }
}

// Usage:
// /by-language/typescript  → All .ts files
// /by-language/python      → All .py files

Example 2: Size Projection

Organize files by size category:

export class SizeProjection extends BaseProjectionStrategy {
  readonly name = 'size'

  // Size categories in bytes
  private readonly categories = {
    tiny: [0, 1024],              // < 1 KB
    small: [1024, 102400],        // 1-100 KB
    medium: [102400, 1048576],    // 100 KB - 1 MB
    large: [1048576, Infinity]    // > 1 MB
  }

  toQuery(category: string, subpath?: string) {
    const [min, max] = this.categories[category] || [0, Infinity]

    return {
      where: {
        vfsType: 'file',
        size: {
          greaterEqual: min,
          lessThan: max
        }
      },
      limit: 1000
    }
  }

  async resolve(brain: Brainy, vfs: VirtualFileSystem, category: string): Promise<string[]> {
    const [min, max] = this.categories[category]
    if (!min && min !== 0) return []

    const results = await brain.find({
      where: {
        vfsType: 'file',
        size: {
          greaterEqual: min,
          lessThan: max
        }
      },
      limit: 1000
    })

    return this.extractIds(results)
  }

  async list(brain: Brainy, vfs: VirtualFileSystem, limit = 100): Promise<VFSEntity[]> {
    // Return files sorted by size
    const results = await brain.find({
      where: { vfsType: 'file' },
      limit
    })

    return results
      .map(r => r.entity as VFSEntity)
      .sort((a, b) => (b.metadata.size || 0) - (a.metadata.size || 0))
  }
}

// Usage:
// /by-size/tiny    → Files < 1 KB
// /by-size/large   → Files > 1 MB

Example 3: Status Projection (Custom Logic)

Organize files by review status with custom logic:

export class StatusProjection extends BaseProjectionStrategy {
  readonly name = 'status'

  toQuery(status: string, subpath?: string) {
    return {
      where: {
        vfsType: 'file',
        reviewStatus: status
      },
      limit: 1000
    }
  }

  async resolve(brain: Brainy, vfs: VirtualFileSystem, status: string): Promise<string[]> {
    // Custom logic: "needs-review" means modified in last 24h without review
    if (status === 'needs-review') {
      const oneDayAgo = Date.now() - (24 * 60 * 60 * 1000)

      const results = await brain.find({
        where: {
          vfsType: 'file',
          modified: { greaterEqual: oneDayAgo },
          reviewStatus: { missing: true }  // No review status set
        },
        limit: 1000
      })

      return this.extractIds(results)
    }

    // Standard status query
    const results = await brain.find({
      where: {
        vfsType: 'file',
        reviewStatus: status
      },
      limit: 1000
    })

    return this.extractIds(results)
  }

  async list(brain: Brainy, vfs: VirtualFileSystem, limit = 100): Promise<VFSEntity[]> {
    // Return files with any review status
    const results = await brain.find({
      where: {
        vfsType: 'file',
        anyOf: [
          { reviewStatus: { exists: true } },
          { modified: { greaterEqual: Date.now() - 86400000 } }
        ]
      },
      limit
    })

    return results.map(r => r.entity as VFSEntity)
  }
}

// Usage:
// /by-status/needs-review  → Files modified in last 24h without review
// /by-status/approved      → Approved files
// /by-status/rejected      → Rejected files

---

Using Brainy Field Operators (BFO)

Projection strategies use Brainy Field Operators (BFO), not MongoDB-style operators:

Comparison Operators

// ❌ MongoDB style (WRONG)
{ size: { $gte: 1000, $lte: 5000 } }

// ✅ BFO style (CORRECT)
{ size: { greaterEqual: 1000, lessEqual: 5000 } }

Logical Operators

// ❌ MongoDB style (WRONG)
{ $or: [{ name: 'foo' }, { name: 'bar' }] }

// ✅ BFO style (CORRECT)
{ anyOf: [{ name: 'foo' }, { name: 'bar' }] }

Existence Operators

// ❌ MongoDB style (WRONG)
{ tags: { $exists: true } }

// ✅ BFO style (CORRECT)
{ tags: { exists: true } }

String Operators

// ❌ MongoDB style (WRONG)
{ path: { $regex: /\.ts$/ } }

// ✅ BFO style (CORRECT)
{ path: { endsWith: '.ts' } }

Full BFO Operator Reference

// Comparison
{ field: value }                          // Exact match
{ field: { greaterThan: 10 } }           // >
{ field: { greaterEqual: 10 } }          // >=
{ field: { lessThan: 10 } }              // <
{ field: { lessEqual: 10 } }             // <=
{ field: { not: value } }                // !=

// Logical
{ anyOf: [{ a: 1 }, { b: 2 }] }          // OR
{ allOf: [{ a: 1 }, { b: 2 }] }          // AND

// Existence
{ field: { exists: true } }              // Field exists
{ field: { missing: true } }             // Field doesn't exist

// String
{ field: { startsWith: 'prefix' } }      // Starts with
{ field: { endsWith: 'suffix' } }        // Ends with
{ field: { matches: 'pattern' } }        // Regex match

// Array
{ array: { contains: 'item' } }          // Array contains item
{ array: { hasAll: ['a', 'b'] } }        // Has all items
{ array: { oneOf: ['a', 'b', 'c'] } }    // Value in list

---

Performance Guidelines

1. Use Indexes

All metadata fields are automatically indexed. Use direct equality or range queries for best performance:

// ✅ Fast: Direct index lookup (O(log n))
{ priority: 'high' }
{ size: { greaterEqual: 1000 } }

// ⚠️ Slower: Must scan results
{ path: { matches: /complex-regex/ } }

2. Limit Results

Always set reasonable limits:

async resolve(brain, vfs, value) {
  const results = await brain.find({
    where: { ... },
    limit: 1000  // Prevent unbounded queries
  })
  return this.extractIds(results)
}

3. Avoid Post-Filtering When Possible

If you need post-filtering, consider flattening data:

// ❌ Slow: Fetch 5000, filter in memory
const all = await brain.find({ where: { type: 'file' }, limit: 5000 })
return all.filter(item => item.metadata.nested.value === target)

// ✅ Fast: Flatten during write, query directly
// Store: metadata.nested_value = target
const results = await brain.find({
  where: { nested_value: target },
  limit: 1000
})

4. Cache Expensive Operations

Use the projection's resolve cache:

// Automatic caching in SemanticPathResolver
// Results cached for 5 minutes by default
// No manual caching needed!

---

Testing Projections

Unit Test Example

import { describe, it, expect, beforeAll } from 'vitest'
import { Brainy } from '@soulcraft/brainy'
import { PriorityProjection } from './PriorityProjection'

describe('PriorityProjection', () => {
  let brain: Brainy
  let vfs: any
  let projection: PriorityProjection

  beforeAll(async () => {
    brain = new Brainy()
    await brain.init()
    vfs = brain.vfs()
    await vfs.init()
    projection = new PriorityProjection()
  })

  it('should resolve high priority files', async () => {
    // Create test files
    await vfs.writeFile('/test1.ts', 'code', {
      metadata: { priority: 'high' }
    })
    await vfs.writeFile('/test2.ts', 'code', {
      metadata: { priority: 'low' }
    })

    // Resolve high priority
    const ids = await projection.resolve(brain, vfs, 'high')

    expect(ids).toHaveLength(1)

    const entity = await brain.get(ids[0])
    expect(entity.metadata.priority).toBe('high')
  })

  it('should list all files with priority', async () => {
    const entities = await projection.list(brain, vfs, 100)

    expect(entities.length).toBeGreaterThan(0)
    expect(entities.every(e => e.metadata.priority)).toBe(true)
  })
})

---

Best Practices

1. ✅ Name projections clearly

// ✅ Good
readonly name = 'priority'      // /by-priority/high
readonly name = 'language'      // /by-language/typescript

// ❌ Bad
readonly name = 'proj1'         // /by-proj1/??? unclear

2. ✅ Document expected metadata

/**
 * Priority Projection
 *
 * Requires metadata fields:
 * - priority: string ('high' | 'medium' | 'low')
 *
 * Usage:
 *   /by-priority/high
 */
export class PriorityProjection extends BaseProjectionStrategy {
  // ...
}

3. ✅ Handle missing data gracefully

async resolve(brain, vfs, value) {
  const results = await brain.find({
    where: { priority: value },
    limit: 1000
  })

  // Return empty array if no results, don't throw
  return this.extractIds(results)  // [] if empty
}

4. ✅ Validate input

async resolve(brain, vfs, priority: string) {
  // Validate priority value
  const valid = ['high', 'medium', 'low']
  if (!valid.includes(priority)) {
    return []  // Or throw error
  }

  // Continue with query...
}

---

Common Patterns

Pattern 1: Enum-Based Projection

For fixed sets of values (status, priority, type):

private readonly validValues = ['draft', 'review', 'approved']

async resolve(brain, vfs, status: string) {
  if (!this.validValues.includes(status)) return []
  // ... query
}

Pattern 2: Range-Based Projection

For numeric or time ranges:

private readonly ranges = {
  recent: Date.now() - 86400000,      // Last 24h
  week: Date.now() - 7 * 86400000,    // Last week
  month: Date.now() - 30 * 86400000   // Last month
}

async resolve(brain, vfs, period: string) {
  const since = this.ranges[period]
  if (!since) return []

  const results = await brain.find({
    where: {
      modified: { greaterEqual: since }
    }
  })
  return this.extractIds(results)
}

Pattern 3: Computed Projection

Combine multiple criteria:

async resolve(brain, vfs, value: string) {
  // "stale" = not modified in 30 days AND no recent access
  if (value === 'stale') {
    const thirtyDaysAgo = Date.now() - 30 * 86400000

    const results = await brain.find({
      where: {
        allOf: [
          { modified: { lessThan: thirtyDaysAgo } },
          { accessed: { lessThan: thirtyDaysAgo } }
        ]
      }
    })
    return this.extractIds(results)
  }

  // Regular query for other values...
}

---

Troubleshooting

Projection returns empty results

1. Check metadata exists: console.log(entity.metadata)
2. Verify query syntax: Use BFO operators, not MongoDB
3. Check limits: Increase limit if needed

Slow performance

1. Check if field is indexed: All metadata fields are auto-indexed
2. Avoid post-filtering: Flatten complex structures
3. Use appropriate limits: Don't fetch more than needed

Type errors

1. Import correct types: import { Brainy, VirtualFileSystem } from '@soulcraft/brainy'
2. Use as VFSEntity when mapping results
3. Check BaseProjectionStrategy import

---

See Also

• Semantic VFS Guide - Using semantic paths
• Performance Tuning - Optimization guide
• VFS Core API - Base VFS operations