Rule System

/**
 * A function that transforms a tree, can be sync or async
 */
type Rule = (tree: Tree, context: SchematicContext) => 
  Tree | Observable<Tree> | Rule | Promise<void | Tree | Rule> | void;

/**
 * A function that creates a tree from context
 */
type Source = (context: SchematicContext) => Tree | Observable<Tree>;

/**
 * Factory function that creates rules from options
 */
type RuleFactory<T> = (options: T) => Rule;

/**
 * Function that operates on individual files
 */
type FileOperator = (entry: FileEntry) => FileEntry | null;

/**
 * Creates a source that returns the given tree
 */
function source(tree: Tree): Source;

/**
 * Creates a source that returns an empty tree
 */
function empty(): Source;

/**
 * Returns a rule that does nothing (identity function)
 */
function noop(): Rule;

/**
 * Chains multiple rules into a single rule that executes sequentially
 */
function chain(rules: Iterable<Rule> | AsyncIterable<Rule>): Rule;

/**
 * Applies multiple rules to a source and returns the transformed source
 */
function apply(source: Source, rules: Rule[]): Source;

/**
 * Merges input tree with the source tree using specified strategy
 */
function mergeWith(source: Source, strategy?: MergeStrategy): Rule;

import { 
  Rule, 
  chain, 
  apply, 
  mergeWith, 
  empty, 
  noop,
  Tree,
  SchematicContext 
} from "@angular-devkit/schematics";

// Simple rule that creates a file
function createReadme(): Rule {
  return (tree: Tree, context: SchematicContext) => {
    tree.create('/README.md', '# My Project\n\nThis is a generated project.');
    return tree;
  };
}

// Chain multiple rules together
function setupProject(): Rule {
  return chain([
    createReadme(),
    createPackageJson(),
    createGitignore()
  ]);
}

// Apply rules to a source
function generateFromTemplate(templateUrl: string, options: any): Rule {
  return mergeWith(
    apply(url(templateUrl), [
      template(options),
      move('/src')
    ])
  );
}

/**
 * Filters tree files based on predicate function
 */
function filter(predicate: FilePredicate<boolean>): Rule;

/**
 * Applies file operator to each file in the tree
 */
function forEach(operator: FileOperator): Rule;

/**
 * Conditionally applies file operator based on predicate
 */
function when(predicate: FilePredicate<boolean>, operator: FileOperator): FileOperator;

/**
 * Executes rule on a branch and merges back to original tree
 */
function branchAndMerge(rule: Rule, strategy?: MergeStrategy): Rule;

/**
 * Partitions tree, applies different rules to each partition, then merges
 */
function partitionApplyMerge(
  predicate: FilePredicate<boolean>, 
  ruleYes: Rule, 
  ruleNo?: Rule
): Rule;

/**
 * Composes multiple file operators into a single operator
 */
function composeFileOperators(operators: FileOperator[]): FileOperator;

/**
 * Applies rules to a subtree at the specified path
 */
function applyToSubtree(path: string, rules: Rule[]): Rule;

import { 
  filter, 
  forEach, 
  when, 
  branchAndMerge,
  partitionApplyMerge 
} from "@angular-devkit/schematics";

// Filter only TypeScript files
const onlyTsFiles = filter((path) => path.endsWith('.ts'));

// Transform all files with uppercase content
const uppercaseContent = forEach((entry) => {
  return {
    ...entry,
    content: Buffer.from(entry.content.toString().toUpperCase())
  };
});

// Conditionally modify files
const addHeader = when(
  (path) => path.endsWith('.ts'),
  (entry) => ({
    ...entry,
    content: Buffer.from(`// Generated file\n${entry.content.toString()}`)
  })
);

// Work on a branch to avoid conflicts
const safeTransform = branchAndMerge(
  chain([
    filter((path) => path.includes('/src/')),
    forEach(addHeader)
  ])
);

/**
 * Moves files or directories from one path to another
 * @param from - Source path pattern or specific path
 * @param to - Destination path (optional, defaults to root)
 */
function move(from: string, to?: string): Rule;

import { move } from "@angular-devkit/schematics";

// Move all files from /temp to /src
const moveToSrc = move('/temp', '/src');

// Move files with a specific pattern
const moveComponents = move('/components', '/src/app/components');

/**
 * Runs a schematic from the same collection
 */
function schematic<OptionT>(
  schematicName: string,
  options: OptionT,
  executionOptions?: Partial<ExecutionOptions>
): Rule;

/**
 * Runs a schematic from a different collection
 */
function externalSchematic<OptionT>(
  collectionName: string,
  schematicName: string,
  options: OptionT,
  executionOptions?: Partial<ExecutionOptions>
): Rule;

import { schematic, externalSchematic } from "@angular-devkit/schematics";

// Run another schematic from same collection
function generateWithComponent(options: any): Rule {
  return chain([
    schematic('component', {
      name: options.componentName,
      path: options.path
    }),
    // Additional rules...
  ]);
}

// Run schematic from external collection  
function generateWithAngularMaterial(): Rule {
  return externalSchematic('@angular/material', 'nav', {
    name: 'main-nav'
  });
}

/**
 * Creates a source from a URL string (file://, http://, etc.)
 */
function url(urlString: string): Source;

import { url, apply, template, move } from "@angular-devkit/schematics";

// Load templates from local file system
function generateFromLocalTemplate(options: any): Rule {
  return mergeWith(
    apply(url('./files'), [
      template({
        ...options,
        classify: (str: string) => str.charAt(0).toUpperCase() + str.slice(1)
      }),
      move(options.path || '/')
    ])
  );
}

// Load from HTTP URL (if supported by host)
function generateFromRemoteTemplate(options: any): Rule {
  return mergeWith(
    apply(url('https://example.com/templates'), [
      template(options)
    ])
  );
}

/**
 * Calls a source function and returns observable tree
 */
function callSource(source: Source, context: SchematicContext): Observable<Tree>;

/**
 * Calls a rule function and returns observable tree
 */
function callRule(
  rule: Rule,
  input: Tree | Observable<Tree>,
  context: SchematicContext
): Observable<Tree>;

/**
 * Creates a source with randomly generated files for testing
 */
function random(options: RandomOptions): Source;

interface RandomOptions {
  /** Number of files to generate */
  count?: number;
  /** File content options */
  content?: {
    /** Minimum content length */
    minLength?: number;
    /** Maximum content length */
    maxLength?: number;
  };
  /** Path generation options */
  path?: {
    /** Path depth range */
    depth?: [number, number];
    /** File extension options */
    extensions?: string[];
  };
}

/**
 * Thrown when a rule returns an invalid result
 */
class InvalidRuleResultException extends SchematicsException {
  constructor(result?: any);
}

/**
 * Thrown when a source returns an invalid result
 */
class InvalidSourceResultException extends SchematicsException {
  constructor(result?: any);
}

import {
  Rule,
  chain,
  apply,
  mergeWith,
  url,
  template,
  move,
  filter,
  forEach,
  when,
  branchAndMerge
} from "@angular-devkit/schematics";

function complexSchematic(options: ComplexOptions): Rule {
  return (tree: Tree, context: SchematicContext) => {
    // Conditional logic in rules
    const rules: Rule[] = [];
    
    if (options.generateComponent) {
      rules.push(
        mergeWith(
          apply(url('./component-files'), [
            template(options),
            move(options.componentPath)
          ])
        )
      );
    }
    
    if (options.generateService) {
      rules.push(
        mergeWith(
          apply(url('./service-files'), [
            template(options),
            move(options.servicePath)
          ])
        )
      );
    }
    
    // Process existing files
    rules.push(
      branchAndMerge(
        chain([
          filter((path) => path.endsWith('.ts')),
          forEach(
            when(
              (path) => path.includes('/src/'),
              (entry) => ({
                ...entry,
                content: Buffer.from(
                  addImports(entry.content.toString(), options.imports)
                )
              })
            )
          )
        ])
      )
    );
    
    return chain(rules)(tree, context);
  };
}

interface ExecutionOptions {
  interactive: boolean;
  dryRun: boolean;
  force: boolean;
  defaults: boolean;
}

type TaskId = symbol;

interface TaskConfigurationGenerator<T> {
  name: string;
  options: T;
}