check-similarity

name	check-similarity
description	Detects duplicate TypeScript/JavaScript code using AST comparison for refactoring. Use when the user wants to find similar or duplicated functions, plan refactoring, or clean up redundant code in TS/JS projects.
user-invocable	true

similarity-ts: AI Assistant Guide

Purpose

Detects duplicate TypeScript/JavaScript code using AST comparison for refactoring.

AI用プロンプト / AI Prompt

日本語:

`similarity-ts .` でコードの意味的な類似が得られます。あなたはこれを実行し、ソースコードの重複を検知して、リファクタリング計画を立てます。細かいオプションは similarity-ts -h で確認してください。

English:

Run `similarity-ts .` to detect semantic code similarities. Execute this command, analyze the duplicate code patterns, and create a refactoring plan. Check `similarity-ts -h` for detailed options.

Installation

cargo install similarity-ts
# check options
similarity-ts --help

Key Options

• --threshold <0-1>: Similarity threshold (default: 0.8)
• --min-tokens <n>: Skip functions with <n AST nodes (recommended: 20-30)
• --print: Show actual code snippets
• --experimental-types: Enable type similarity detection (interfaces, type aliases)
• --filter-function <name>: Filter results by function name (substring match)
• --filter-function-body <text>: Filter results by function body content (substring match)

AI Refactoring Workflow

1. Broad Scan

Find all duplicates in codebase:

similarity-ts src/ --threshold 0.85 --min-tokens 25

2. Focused Analysis

Examine specific file pairs:

similarity-ts file1.ts file2.ts --threshold 0.8 --min-tokens 20 --print

3. Threshold Tuning

If no results, progressively lower:

similarity-ts file1.ts file2.ts --threshold 0.75 --min-tokens 20
similarity-ts file1.ts file2.ts --threshold 0.7 --min-tokens 20

Output Format

Function: functionName (file.ts:startLine-endLine)
Similar to: otherFunction (other.ts:startLine-endLine)
Similarity: 85%

Effective Thresholds

• 0.95+: Nearly identical (variable renames only)
• 0.85-0.95: Same algorithm, minor differences
• 0.75-0.85: Similar structure, different details
• 0.7-0.75: Related logic, worth investigating

Refactoring Strategy

1. Start with high threshold (0.9) to find obvious duplicates
2. Compare specific pairs when similarity found
3. Use --print to see actual code differences
4. Extract common logic into shared functions/modules
5. Re-run after refactoring to verify no new duplicates

Common Patterns to Refactor

• Data processing loops with different field names
• API handlers with similar request/response logic
• Validation functions with different rules
• State management with repeated patterns

Best Practices

• Use --min-tokens for accurate complexity filtering (20-30 tokens)
• Focus on files with 80%+ similarity first
• Check if similar functions are in same module (easier to refactor)
• Consider function size - larger duplicates have more impact
• Look for patterns across multiple files, not just pairs

Test Scenario: Following README Examples

Test the tool's functionality by running through README examples:

# 1. Quick Start - Zero configuration
similarity-ts

# 2. Analyze specific paths
similarity-ts src/ lib/

# 3. Custom threshold
similarity-ts . --threshold 0.9

# 4. Print code details
similarity-ts . --print

# 5. Type checking (experimental)
similarity-ts . --experimental-types

# 6. Specific file extensions
similarity-ts ./src --extensions ts,tsx

# 7. Examples directory test
similarity-ts examples/specs/duplicate-functions.ts --threshold 0.8 --min-tokens 20
similarity-ts examples/specs/duplicate-types.ts --experimental-types --threshold 0.8

Expected Results

1. Function Detection (duplicate-functions.ts):

   • calculateUserAge vs calculateCustomerAge (~95%)
   • findMaxValue vs getMaximumValue (~85%)
   • processUserData vs processCustomerData (~90%)

2. Type Detection (duplicate-types.ts):

   • User vs Customer (100%)
   • UserResponse vs CustomerResponse (~85%)
   • ApiResponse vs ServiceResponse (100%)