☯️ Commonmarker::Merge

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👣 How will this project approach the September 2025 hostile takeover of RubyGems? 🚑️

I've summarized my thoughts in this blog post.

🌻 Synopsis

commonmarker-merge is a thin wrapper around markdown-merge that provides:

• Hard dependency on Commonmarker - Ensures the Comrak (Rust) parser is installed
• Commonmarker-specific defaults - Freeze token: "commonmarker-merge", inner_merge_code_blocks: false
• CommonMarker parse options - Pass options via the options: parameter

Features (via markdown-merge)

• Smart element matching - Headings, paragraphs, lists, code blocks, and other block elements are matched by their structural signatures
• Fuzzy table matching - Tables are matched using a multi-factor scoring algorithm that considers header similarity, first column (row labels), content overlap, and position
• Freeze blocks - Mark sections with HTML comments to preserve them during merges
• Configurable merge strategies - Choose whether template or destination wins for conflicts, or use a Hash for per-node-type preferences with node_splitter (see ast-merge docs)
• Conservative removal mode - remove_template_missing_nodes: true removes top-level destination-only structural blocks while preserving standalone HTML comment-only fragments, link reference definitions, freeze blocks, and stable separator blank lines around preserved standalone fragments
• Type normalization - Canonical node types work across all markdown backends
• Full CommonMarker support - Works with all CommonMark and GitHub Flavored Markdown extensions

Removal Mode Scope

remove_template_missing_nodes: true in commonmarker-merge follows the shared markdown-merge full-document contract:

• removes top-level destination-only structural blocks
• preserves standalone HTML comment-only fragments, link reference definitions, and freeze blocks
• preserves one separator blank line when removed structural content collapses around a kept standalone HTML comment fragment
• does not yet define generic inline-comment promotion or recursive/nested section-removal semantics

Section-local replace_mode / partial-template behavior still follows its own conservative Markdown rules and should not be assumed to inherit the same recursive removal contract as full-document smart merge.

💡 Info you can shake a stick at

Tokens to Remember
Works with MRI Ruby 4
Support & Community
Source
Documentation
Compliance
Style
Maintainer 🎖️
... 💖	🧊 🐙 🛖 🧪

Compatibility

Compatible with MRI Ruby 4.0.0+, and concordant releases of JRuby, and TruffleRuby. CI workflows and Appraisals are generated for MRI Ruby 4.0.0+. This test floor is configured by ruby.test_minimum in .kettle-jem.yml and may be higher than the gem's runtime compatibility floor when legacy Rubies are not practical for the current toolchain.

The amazing test matrix is powered by the kettle-dev stack.

How kettle-dev manages complexity in tests

Gem	Source	Role	Total downloads
appraisal2	GitHub	multi-dependency Appraisal matrix generation
appraisal2-rubocop	GitHub	RuboCop Appraisal generator integration
kettle-dev	GitHub	development, release, and CI workflow tooling
kettle-jem	GitHub	Appraisals & CI workflow templates
kettle-soup-cover	GitHub	SimpleCov coverage policy and reporting
kettle-test	GitHub	standard test runner and coverage harness
rubocop-lts	GitHub	Ruby-version-aware linting
turbo_tests2	GitHub	parallel test execution

✨ Installation

Install the gem and add to the application's Gemfile by executing:

bundle add commonmarker-merge

If bundler is not being used to manage dependencies, install the gem by executing:

gem install commonmarker-merge

⚙️ Configuration

Freeze Blocks

Freeze blocks prevent sections from being modified during merges. They are marked with HTML comments that are invisible when the Markdown is rendered:

<!-- commonmarker-merge:freeze -->
## This Section Is Protected

Any content here will be preserved exactly as-is during merges.
The merge tool will not modify, replace, or remove this content.

<!-- commonmarker-merge:unfreeze -->

You can add an optional reason to document why a section is frozen:

<!-- commonmarker-merge:freeze Manual TOC - do not auto-generate -->
## Table of Contents
- [Installation](#installation)
- [Usage](#usage)
<!-- commonmarker-merge:unfreeze -->

Custom Freeze Token

Use a custom freeze token if you need to avoid conflicts with other tools:

merger = Commonmarker::Merge::SmartMerger.new(
  template,
  destination,
  freeze_token: "my-project",
)
# Now looks for: <!-- my-project:freeze --> and <!-- my-project:unfreeze -->

Merge Preferences

Control how conflicts between template and destination are resolved:

# Preserve destination customizations (default)
merger = Commonmarker::Merge::SmartMerger.new(
  template,
  destination,
  preference: :destination,
)

# Apply template updates (overwrite destination)
merger = Commonmarker::Merge::SmartMerger.new(
  template,
  destination,
  preference: :template,
)

# Add new sections from template that don't exist in destination
merger = Commonmarker::Merge::SmartMerger.new(
  template,
  destination,
  add_template_only_nodes: true,
)

Debug Logging

Enable debug logging to see merge decisions:

export COMMONMARKER_MERGE_DEBUG=1

Table Match Refiner

When tables don't match by exact signature (identical headers), the TableMatchRefiner uses fuzzy matching to pair tables that have:

• Similar headers (e.g., "Value" vs "Values")
• Similar first column content (row labels)
• Similar overall structure and content

# Enable table fuzzy matching with custom threshold
merger = Commonmarker::Merge::SmartMerger.new(
  template,
  destination,
  match_refiners: [
    Commonmarker::Merge::TableMatchRefiner.new(threshold: 0.6),
  ],
)

TableMatchAlgorithm Weights

The TableMatchAlgorithm uses a multi-factor scoring system with configurable weights:

Factor	Default Weight	Description
header_match	0.25	Percentage of matching header cells (Levenshtein similarity)
first_column	0.20	Percentage of matching first column cells
row_content	0.25	Average match percentage for rows with matching first column
total_cells	0.15	Overall cell matching percentage
position	0.15	Position distance (closer tables score higher)

# Custom weights for specific use cases
refiner = Commonmarker::Merge::TableMatchRefiner.new(
  threshold: 0.5,
  algorithm_options: {
    weights: {
      header_match: 0.4,  # Prioritize header matching
      first_column: 0.2,
      row_content: 0.2,
      total_cells: 0.1,
      position: 0.1,
    },
  },
)

🔧 Basic Usage

Merging Two Markdown Files

require "commonmarker/merge"

template_content = File.read("template.md")
dest_content = File.read("destination.md")

merger = Commonmarker::Merge::SmartMerger.new(template_content, dest_content)
result = merger.merge

if result.success?
  File.write("destination.md", result.content)
  puts "Merged successfully!"
  puts "  - Nodes added: #{result.nodes_added}"
  puts "  - Nodes modified: #{result.nodes_modified}"
  puts "  - Frozen blocks preserved: #{result.frozen_count}"
else
  puts "Merge had conflicts:"
  result.conflicts.each do |conflict|
    puts "  - #{conflict[:location]}: #{conflict[:reason]}"
  end
end

Analyzing a Markdown File

require "commonmarker/merge"

source = File.read("README.md")
analysis = Commonmarker::Merge::FileAnalysis.new(source)

# Iterate over all block elements
analysis.statements.each do |node|
  case node
  when Commonmarker::Merge::FreezeNode
    puts "Freeze block: lines #{node.start_line}-#{node.end_line}"
    puts "  Reason: #{node.reason}" if node.reason
  else
    sig = analysis.generate_signature(node)
    puts "#{node.type}: #{sig.inspect}"
  end
end

# Get just the freeze blocks
analysis.freeze_blocks.each do |freeze_node|
  puts "Protected: #{freeze_node.content[0..50]}..."
end

Custom Signature Generator

Override how elements are matched between files:

# Match headings only by level, ignoring content
custom_sig = ->(node) {
  if node.respond_to?(:type) && node.type == :heading
    [:heading, node.header_level]  # Match any h1 to any h1, etc.
  else
    node  # Fall through to default signature
  end
}

merger = Commonmarker::Merge::SmartMerger.new(
  template,
  destination,
  signature_generator: custom_sig,
)

Fuzzy Table Matching

When merging documents with tables that have been renamed or restructured, use the TableMatchRefiner to find the best matches:

require "commonmarker/merge"

template = <<~MD
  # API Reference

  | Endpoint | Method | Description |
  |----------|--------|-------------|
  | /users   | GET    | List users  |
  | /users   | POST   | Create user |
MD

destination = <<~MD
  # API Reference

  | API Endpoint | HTTP Method | Descriptions |
  |--------------|-------------|--------------|
  | /users       | GET         | List users   |
  | /posts       | GET         | List posts   |
MD

# Default merge won't match the tables (headers differ)
# Use TableMatchRefiner to enable fuzzy matching
merger = Commonmarker::Merge::SmartMerger.new(
  template,
  destination,
  match_refiners: [
    Commonmarker::Merge::TableMatchRefiner.new(threshold: 0.5),
  ],
)
result = merger.merge

# Tables are now matched despite header differences
# ("Endpoint" ~ "API Endpoint", "Method" ~ "HTTP Method", etc.)

🔐 Security

See SECURITY.md.

🤝 Contributing

If you need some ideas of where to help, you could work on adding more code coverage, or if it is already 💯 (see below) check issues or PRs, or use the gem and think about how it could be better.

We so if you make changes, remember to update it.

See CONTRIBUTING.md for more detailed instructions.

Code Coverage

Coverage service badges

📌 Versioning

This library follows for its public API where practical. For most applications, prefer the Pessimistic Version Constraint with two digits of precision.

For example:

spec.add_dependency("commonmarker-merge", "~> 7.0")

📌 Is "Platform Support" part of the public API? More details inside.

Dropping support for a platform can be a breaking change for affected users. If a release changes supported platforms, it should be called out clearly in the changelog and versioned with that impact in mind.

To get a better understanding of how SemVer is intended to work over a project's lifetime, read this article from the creator of SemVer:

• "Major Version Numbers are Not Sacred"

See CHANGELOG.md for a list of releases.

📄 License

The gem is available under the following licenses: AGPL-3.0-only, PolyForm-Small-Business-1.0.0. See LICENSE.md for details.

If none of the available licenses suit your use case, please contact us to discuss a custom commercial license.