obra/finding-duplicate-functions

Finding Duplicate-Intent Functions

Overview

LLM-generated codebases accumulate semantic duplicates: functions that serve the same purpose but were implemented independently. Classical copy-paste detectors (jscpd) find syntactic duplicates but miss "same intent, different implementation."

This skill uses a two-phase approach: classical extraction followed by LLM-powered intent clustering.

When to Use

• Codebase has grown organically with multiple contributors (human or LLM)
• You suspect utility functions have been reimplemented multiple times
• Before major refactoring to identify consolidation opportunities
• After jscpd has been run and syntactic duplicates are already handled

Quick Reference

| Phase | Tool | Model | Output | |-------|------|-------|--------| | 1. Extract | scripts/extract-functions.sh | - | catalog.json | | 2. Categorize | scripts/categorize-prompt.md | haiku | categorized.json | | 3. Split | scripts/prepare-category-analysis.sh | - | categories/*.json | | 4. Detect | scripts/find-duplicates-prompt.md | opus | duplicates/*.json | | 5. Report | scripts/generate-report.sh | - | report.md |

Process

digraph duplicate_detection {
  rankdir=TB;
  node [shape=box];

  extract [label="1. Extract function catalog\n./scripts/extract-functions.sh"];
  categorize [label="2. Categorize by domain\n(haiku subagent)"];
  split [label="3. Split into categories\n./scripts/prepare-category-analysis.sh"];
  detect [label="4. Find duplicates per category\n(opus subagent per category)"];
  report [label="5. Generate report\n./scripts/generate-report.sh"];
  review [label="6. Human review & consolidate"];

  extract -> categorize -> split -> detect -> report -> review;
}

Phase 1: Extract Function Catalog

./scripts/extract-functions.sh src/ -o catalog.json

Options:

• -o FILE: Output file (default: stdout)
• -c N: Lines of context to capture (default: 15)
• -t GLOB: File types (default: *.ts,*.tsx,*.js,*.jsx)
• --include-tests: Include test files (excluded by default)

Test files (*.test.*, *.spec.*, __tests__/**) are excluded by default since test utilities are less likely to be consolidation candidates.

Phase 2: Categorize by Domain

Dispatch a haiku subagent using the prompt in scripts/categorize-prompt.md.

Insert the contents of catalog.json where indicated in the prompt template. Save output as categorized.json.

Phase 3: Split into Categories

./scripts/prepare-category-analysis.sh categorized.json ./categories

Creates one JSON file per category. Only categories with 3+ functions are worth analyzing.

Phase 4: Find Duplicates (Per Category)

For each category file in ./categories/, dispatch an opus subagent using the prompt in scripts/find-duplicates-prompt.md.

Save each output as ./duplicates/{category}.json.

Phase 5: Generate Report

./scripts/generate-report.sh ./duplicates ./duplicates-report.md

Produces a prioritized markdown report grouped by confidence level.

Phase 6: Human Review

Review the report. For HIGH confidence duplicates:

1. Verify the recommended survivor has tests
2. Update callers to use the survivor
3. Delete the duplicates
4. Run tests

High-Risk Duplicate Zones

Focus extraction on these areas first - they accumulate duplicates fastest:

| Zone | Common Duplicates | |------|-------------------| | utils/, helpers/, lib/ | General utilities reimplemented | | Validation code | Same checks written multiple ways | | Error formatting | Error-to-string conversions | | Path manipulation | Joining, resolving, normalizing paths | | String formatting | Case conversion, truncation, escaping | | Date formatting | Same formats implemented repeatedly | | API response shaping | Similar transformations for different endpoints |

Common Mistakes

Extracting too much: Focus on exported functions and public methods. Internal helpers are less likely to be duplicated across files.

Skipping the categorization step: Going straight to duplicate detection on the full catalog produces noise. Categories focus the comparison.

Using haiku for duplicate detection: Haiku is cost-effective for categorization but misses subtle semantic duplicates. Use Opus for the actual duplicate analysis.

Consolidating without tests: Before deleting duplicates, ensure the survivor has tests covering all use cases of the deleted functions.