# The Hashline Technique

Hashline is AnchorCli's unique approach to safe, precise file editing.

## How It Works

Every line returned by file tools is prefixed with a content-derived hash anchor:

```
  function hello() {
    return "world";
  }
```

The format is `lineNumber:hash|content` where:
- `lineNumber` is the current line number in the file
- `hash` is an Adler-8 checksum derived from the line content and position
- `content` is the actual line text

## Editing with Anchors

When editing, you reference these `line:hash` anchors instead of reproducing old content. Before any mutation, both the line number **and** hash are validated — stale anchors are rejected immediately.

### Example

To replace lines 2-3 from the example above:

```json
{
  "tool": "replace_lines",
  "path": "example.js",
  "startAnchor": "2:f1",
  "endAnchor": "3:0e",
  "newLines": ["    return \"hello world\";"]
}
```

## Benefits

This eliminates:

- **Whitespace/indentation reproduction errors** — No need to match exact spacing
- **Silent edits to the wrong line** — Hash validation ensures you're editing the right content
- **Entire-file rewrites** — Change just one line without touching the rest
- **Line drift in batch operations** — BatchEdit validates all anchors upfront and applies changes bottom-to-top

## Key Principles

1. **Never include anchors in your content** — When using `replace_lines`, `insert_after`, or similar tools, the `newLines` parameter should contain raw source code only, WITHOUT the `lineNumber:hash|` prefix
2. **Always validate before editing** — Re-read files to get fresh anchors if your previous anchors fail validation
3. **Use BatchEdit for multiple changes** — This validates all anchors upfront and applies operations atomically

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