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Markdown::Merge provides intelligent Markdown file merging using tree_haver backends. It can be used standalone or through parser-specific wrappers.

Direct usage (with auto-detected or specified backend):

require "markdown/merge"

# Auto-detect available backend (commonmarker or markly)
merger = Markdown::Merge::SmartMerger.new(template_content, dest_content)
result = merger.merge

# Or specify a backend explicitly
merger = Markdown::Merge::SmartMerger.new(template_content, dest_content, backend: :markly)

Via parser-specific wrappers (for hard dependencies and backend-specific defaults):

• commonmarker-merge - Uses Comrak (Rust) via Commonmarker
• markly-merge - Uses libcmark-gfm (C) via Markly

• Multiple Backends: Supports Commonmarker and Markly through tree_haver's unified API
• Type Normalization: Canonical node types (:heading, :paragraph, etc.) work across all backends
• Extensible: Register custom backends via NodeTypeNormalizer.register_backend
• Structure-Aware: Understands headings, paragraphs, lists, code blocks, tables, and other block elements
• Freeze Block Support: Respects freeze markers (default: markdown-merge:freeze / markdown-merge:unfreeze) for template merge control - customizable to match your project's conventions
• Inner-Merge Code Blocks: Optionally merge fenced code blocks using language-specific mergers (Ruby → prism-merge, YAML → psych-merge, JSON → json-merge, TOML → toml-merge)
• Table Match Refiner: Fuzzy matching algorithm for tables with similar but not identical headers
• Full Provenance: Tracks origin of every node
• Customizable:
  • backend - select :commonmarker, :markly, or :auto
  • signature_generator - callable custom signature generators
  • preference - setting of :template, :destination, or a Hash for per-node-type preferences
  • add_template_only_nodes - setting to retain sections that do not exist in destination
  • freeze_token - customize freeze block markers (default: "markdown-merge")
  • inner_merge_code_blocks - enable language-aware code block merging
  • match_refiner - fuzzy matching for unmatched nodes (e.g., TableMatchRefiner)

Signatures computed by default for common Markdown block elements:

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

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	Daily download rank
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

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

bundle add markdown-merge

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

gem install markdown-merge

The SmartMerger class is the main entry point for merging Markdown files:

require "markdown/merge"

merger = Markdown::Merge::SmartMerger.new(
  template_content,
  dest_content,

  # Backend selection (default: :auto)
  # :auto - auto-detect available backend (tries commonmarker first, then markly)
  # :commonmarker - use Commonmarker (comrak Rust parser)
  # :markly - use Markly (cmark-gfm C library)
  backend: :auto,

  # Which version to prefer when nodes match but differ
  # :destination (default) - keep destination content (preserves customizations)
  # :template - use template content (applies updates)
  preference: :destination,

  # Whether to add template-only nodes to the result
  # false (default) - only include sections that exist in destination
  # true - include all template sections
  add_template_only_nodes: false,

  # Token for freeze block markers
  # Default: "markdown-merge"
  # Looks for: <!-- markdown-merge:freeze --> / <!-- markdown-merge:unfreeze -->
  freeze_token: "markdown-merge",

  # Enable inner-merge for fenced code blocks
  # false (default) - use standard conflict resolution for code blocks
  # true - merge code block contents using language-specific mergers
  # CodeBlockMerger instance - use custom CodeBlockMerger
  inner_merge_code_blocks: false,

  # Match refiner for fuzzy matching of unmatched nodes
  # nil (default) - exact matching only
  # TableMatchRefiner.new - enable fuzzy table matching
  match_refiner: nil,

  # Custom signature generator (optional)
  # Receives a node (wrapped with canonical merge_type), returns a signature array or nil
  # Return the node itself to fall through to default signature
  signature_generator: nil,

  # Backend-specific options (passed through to parser)
  # For commonmarker: options: {}
  # For markly: flags: Markly::DEFAULT, extensions: [:table]
)

Important: When matching nodes by text content (such as for anchor patterns in PartialTemplateMerger), the .text method returns plain text without markdown formatting.

This means:

• Markdown: ### The `*-merge` Gem Family
• .text returns: "The *-merge Gem Family\n"

The backticks around *-merge are stripped because they are inline formatting, not content. This is true for both Commonmarker and Markly backends.

Anchor pattern examples:

# ❌ WRONG - backticks are stripped, so this won't match
anchor: { type: :heading, text: /`\*-merge` Gem Family/ }

# ✅ CORRECT - match the plain text content
anchor: { type: :heading, text: /\*-merge.*Gem Family/ }

# ✅ CORRECT - use beginning anchor for exact heading match
anchor: { type: :heading, text: /^The \*-merge Gem Family/ }

Other markdown formatting that is stripped from .text:

• Bold: **text** → text
• Italic: *text* or _text_ → text
• Code: `code` → code
• Links: [text](url) → text
• Images: ![alt](src) → alt

Note: Different parsers may have other idiosyncrasies. For example:

• Trailing newlines may or may not be present
• Whitespace normalization may differ
• Entity encoding may vary

Always test your patterns against actual parsed content when building merge recipes.

markdown-merge normalizes node types across backends so merge rules are portable:

# These are equivalent regardless of backend
# Markly's :header becomes :heading
# Markly's :hrule becomes :thematic_break
# etc.

# Register a custom backend's type mappings
Markdown::Merge::NodeTypeNormalizer.register_backend(:my_parser, {
  h1: :heading,
  h2: :heading,
  para: :paragraph,
  # ...
})

For convenience, parser-specific wrappers provide backend-specific defaults:

# commonmarker-merge (freeze_token: "commonmarker-merge", inner_merge_code_blocks: false)
require "commonmarker/merge"
merger = Commonmarker::Merge::SmartMerger.new(template, dest, options: {})

# markly-merge (freeze_token: "markly-merge", inner_merge_code_blocks: true)
require "markly/merge"
merger = Markly::Merge::SmartMerger.new(template, dest, flags: Markly::DEFAULT, extensions: [:table])

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

<!-- markdown-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.

<!-- markdown-merge:unfreeze -->

Add an optional frozen reason to document why:

<!-- markdown-merge:freeze Custom table - manually maintained -->
| Feature | Status |
|---------|--------|
| Custom  | ✅     |
<!-- markdown-merge:unfreeze -->

When enabled, fenced code blocks are merged using language-specific *-merge gems:

merger = SomeParser::Merge::SmartMerger.new(
  template,
  destination,
  inner_merge_code_blocks: true,
)

Supported languages and their mergers:

Example with a Ruby code block:

```ruby

# Template

class MyClass
  def new_method
    puts "from template"
  end
end
```

When merged(with:

```ruby

# Destination

class MyClass
  def existing_method
    puts "custom"
  end
end)
```

Result (with inner_merge_code_blocks: true):

```ruby
class MyClass
  def existing_method
    puts "custom"
  end

  def new_method
    puts "from template"
  end
end
```

When tables don't match by exact signature, the TableMatchRefiner uses fuzzy matching to pair tables with similar structure:

refiner = Markdown::Merge::TableMatchRefiner.new(
  threshold: 0.5,  # Minimum similarity (0.0-1.0)
  algorithm_options: {
    weights: {
      header_match: 0.25,  # Header cell similarity
      first_column: 0.20,  # Row label similarity
      row_content: 0.25,   # Row content overlap
      total_cells: 0.15,   # Overall cell matching
      position: 0.15,      # Position distance
    },
  },
)

merger = SomeParser::Merge::SmartMerger.new(
  template,
  destination,
  match_refiner: refiner,
)

Enable debug logging to see merge decisions:

export MARKDOWN_MERGE_DEBUG=1

Note: This gem provides base classes for implementers. End users should use commonmarker-merge or markly-merge instead.

Use a parser-specific implementation:

require "commonmarker/merge"

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

merger = Commonmarker::Merge::SmartMerger.new(template, destination)
result = merger.merge

File.write("merged.md", result.content)

require "markly/merge"

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

merger = Markly::Merge::SmartMerger.new(template, destination)
result = merger.merge

File.write("merged.md", result.to_markdown)

Creating a new parser-specific implementation:

require "markdown/merge"

module MyParser
  module Merge
    class FileAnalysis < Markdown::Merge::FileAnalysisBase
      def parse_document(source)
        # Parse source and return root document node
        MyParser.parse(source)
      end

      def next_sibling(node)
        # Return the next sibling of a node
        node.next_sibling
      end

      def compute_parser_signature(node)
        # Compute signature for parser-specific nodes
        # Or call super for default implementation
        super
      end
    end

    class SmartMerger < Markdown::Merge::SmartMergerBase
      def create_file_analysis(content, **options)
        FileAnalysis.new(content, **options)
      end

      def node_to_source(node, analysis)
        case node
        when Markdown::Merge::FreezeNode
          node.full_text
        else
          # Convert node back to source text
          node.to_markdown
        end
      end
    end
  end
end

Both implementations support freeze blocks for protecting customized sections:

# My Project

## Installation

<!-- markdown-merge:freeze Custom install instructions -->
This installation section has been customized and will be preserved
during template merges, regardless of what the template contains.
<!-- markdown-merge:unfreeze -->

## Usage

Standard usage section - can be updated from template.

Content between freeze markers is always preserved from the destination file, even when the template has different content for that section.

See SECURITY.md.

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.

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("markdown-merge", "~> 7.0")

See CHANGELOG.md for a list of releases.

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.