# Fit Markdown with Pruning & BM25

**Fit Markdown** is a specialized **filtered** version of your page’s markdown, focusing on the most relevant content. By default, Crawl4AI converts the entire HTML into a broad **raw_markdown**. With fit markdown, we apply a **content filter** algorithm (e.g., **Pruning** or **BM25**) to remove or rank low-value sections—such as repetitive sidebars, shallow text blocks, or irrelevancies—leaving a concise textual “core.”

---

## 1. How “Fit Markdown” Works

### 1.1 The `content_filter`

In **`CrawlerRunConfig`**, you can specify a **`content_filter`** to shape how content is pruned or ranked before final markdown generation. A filter’s logic is applied **before** or **during** the HTML→Markdown process, producing:

- **`result.markdown.raw_markdown`** (unfiltered)
- **`result.markdown.fit_markdown`** (filtered or “fit” version)
- **`result.markdown.fit_html`** (the corresponding HTML snippet that produced `fit_markdown`)


### 1.2 Common Filters

1. **PruningContentFilter** – Scores each node by text density, link density, and tag importance, discarding those below a threshold.  
2. **BM25ContentFilter** – Focuses on textual relevance using BM25 ranking, especially useful if you have a specific user query (e.g., “machine learning” or “food nutrition”).

---

## 2. PruningContentFilter

**Pruning** discards less relevant nodes based on **text density, link density, and tag importance**. It’s a heuristic-based approach—if certain sections appear too “thin” or too “spammy,” they’re pruned.

### 2.1 Usage Example

```python
import asyncio
from crawl4ai import AsyncWebCrawler, CrawlerRunConfig
from crawl4ai.content_filter_strategy import PruningContentFilter
from crawl4ai.markdown_generation_strategy import DefaultMarkdownGenerator

async def main():
    # Step 1: Create a pruning filter
    prune_filter = PruningContentFilter(
        # Lower → more content retained, higher → more content pruned
        threshold=0.45,           
        # "fixed" or "dynamic"
        threshold_type="dynamic",  
        # Ignore nodes with <5 words
        min_word_threshold=5      
    )

    # Step 2: Insert it into a Markdown Generator
    md_generator = DefaultMarkdownGenerator(content_filter=prune_filter)
    
    # Step 3: Pass it to CrawlerRunConfig
    config = CrawlerRunConfig(
        markdown_generator=md_generator
    )

    async with AsyncWebCrawler() as crawler:
        result = await crawler.arun(
            url="https://news.ycombinator.com", 
            config=config
        )
        
        if result.success:
            # 'fit_markdown' is your pruned content, focusing on "denser" text
            print("Raw Markdown length:", len(result.markdown.raw_markdown))
            print("Fit Markdown length:", len(result.markdown.fit_markdown))
        else:
            print("Error:", result.error_message)

if __name__ == "__main__":
    asyncio.run(main())
```

### 2.2 Key Parameters

- **`min_word_threshold`** (int): If a block has fewer words than this, it’s pruned.  
- **`threshold_type`** (str):
  - `"fixed"` → each node must exceed `threshold` (0–1).  
  - `"dynamic"` → node scoring adjusts according to tag type, text/link density, etc.  
- **`threshold`** (float, default ~0.48): The base or “anchor” cutoff.  

**Algorithmic Factors**:

- **Text density** – Encourages blocks that have a higher ratio of text to overall content.  
- **Link density** – Penalizes sections that are mostly links.  
- **Tag importance** – e.g., an `<article>` or `<p>` might be more important than a `<div>`.  
- **Structural context** – If a node is deeply nested or in a suspected sidebar, it might be deprioritized.

---

## 3. BM25ContentFilter

**BM25** is a classical text ranking algorithm often used in search engines. If you have a **user query** or rely on page metadata to derive a query, BM25 can identify which text chunks best match that query.

### 3.1 Usage Example

```python
import asyncio
from crawl4ai import AsyncWebCrawler, CrawlerRunConfig
from crawl4ai.content_filter_strategy import BM25ContentFilter
from crawl4ai.markdown_generation_strategy import DefaultMarkdownGenerator

async def main():
    # 1) A BM25 filter with a user query
    bm25_filter = BM25ContentFilter(
        user_query="startup fundraising tips",
        # Adjust for stricter or looser results
        bm25_threshold=1.2  
    )

    # 2) Insert into a Markdown Generator
    md_generator = DefaultMarkdownGenerator(content_filter=bm25_filter)
    
    # 3) Pass to crawler config
    config = CrawlerRunConfig(
        markdown_generator=md_generator
    )

    async with AsyncWebCrawler() as crawler:
        result = await crawler.arun(
            url="https://news.ycombinator.com", 
            config=config
        )
        if result.success:
            print("Fit Markdown (BM25 query-based):")
            print(result.markdown.fit_markdown)
        else:
            print("Error:", result.error_message)

if __name__ == "__main__":
    asyncio.run(main())
```

### 3.2 Parameters

- **`user_query`** (str, optional): E.g. `"machine learning"`. If blank, the filter tries to glean a query from page metadata.  
- **`bm25_threshold`** (float, default 1.0):  
  - Higher → fewer chunks but more relevant.  
  - Lower → more inclusive.  

> In more advanced scenarios, you might see parameters like `use_stemming`, `case_sensitive`, or `priority_tags` to refine how text is tokenized or weighted.

---

## 4. Accessing the “Fit” Output

After the crawl, your “fit” content is found in **`result.markdown.fit_markdown`**. 

```python
fit_md = result.markdown.fit_markdown
fit_html = result.markdown.fit_html
```

If the content filter is **BM25**, you might see additional logic or references in `fit_markdown` that highlight relevant segments. If it’s **Pruning**, the text is typically well-cleaned but not necessarily matched to a query.

---

## 5. Code Patterns Recap

### 5.1 Pruning

```python
prune_filter = PruningContentFilter(
    threshold=0.5,
    threshold_type="fixed",
    min_word_threshold=10
)
md_generator = DefaultMarkdownGenerator(content_filter=prune_filter)
config = CrawlerRunConfig(markdown_generator=md_generator)
```

### 5.2 BM25

```python
bm25_filter = BM25ContentFilter(
    user_query="health benefits fruit",
    bm25_threshold=1.2
)
md_generator = DefaultMarkdownGenerator(content_filter=bm25_filter)
config = CrawlerRunConfig(markdown_generator=md_generator)
```

---

## 6. Combining with “word_count_threshold” & Exclusions

Remember you can also specify:

```python
config = CrawlerRunConfig(
    word_count_threshold=10,
    excluded_tags=["nav", "footer", "header"],
    exclude_external_links=True,
    markdown_generator=DefaultMarkdownGenerator(
        content_filter=PruningContentFilter(threshold=0.5)
    )
)
```

Thus, **multi-level** filtering occurs:

1. The crawler’s `excluded_tags` are removed from the HTML first.  
2. The content filter (Pruning, BM25, or custom) prunes or ranks the remaining text blocks.  
3. The final “fit” content is generated in `result.markdown.fit_markdown`.

---

## 7. Custom Filters

If you need a different approach (like a specialized ML model or site-specific heuristics), you can create a new class inheriting from `RelevantContentFilter` and implement `filter_content(html)`. Then inject it into your **markdown generator**:

```python
from crawl4ai.content_filter_strategy import RelevantContentFilter

class MyCustomFilter(RelevantContentFilter):
    def filter_content(self, html, min_word_threshold=None):
        # parse HTML, implement custom logic
        return [block for block in ... if ... some condition...]

```

**Steps**:

1. Subclass `RelevantContentFilter`.  
2. Implement `filter_content(...)`.  
3. Use it in your `DefaultMarkdownGenerator(content_filter=MyCustomFilter(...))`.

---

## 8. Final Thoughts

**Fit Markdown** is a crucial feature for:

- **Summaries**: Quickly get the important text from a cluttered page.  
- **Search**: Combine with **BM25** to produce content relevant to a query.  
- **AI Pipelines**: Filter out boilerplate so LLM-based extraction or summarization runs on denser text.

**Key Points**:
- **PruningContentFilter**: Great if you just want the “meatiest” text without a user query.  
- **BM25ContentFilter**: Perfect for query-based extraction or searching.  
- Combine with **`excluded_tags`, `exclude_external_links`, `word_count_threshold`** to refine your final “fit” text.  
- Fit markdown ends up in **`result.markdown.fit_markdown`**; eventually **`result.markdown.fit_markdown`** in future versions.

With these tools, you can **zero in** on the text that truly matters, ignoring spammy or boilerplate content, and produce a concise, relevant “fit markdown” for your AI or data pipelines. Happy pruning and searching!

- Last Updated: 2025-01-01