# `Dllb.MetaAST.Diff`
[🔗](https://github.com/Oeditus/dllb_ex/blob/v0.9.0/lib/dllb/meta_ast/diff.ex#L1)

AST-level diff between two versions of a MetaAST tree.

Unlike text diffs (which report line changes), this module understands tree
structure and reports semantic changes: added functions, removed imports,
modified bodies, renamed containers.

Primary use case: incremental re-indexing. When a file changes, only the
entities that actually changed need re-embedding and re-insertion into dllb.

## Usage

    old_ast = Metastatic.parse!(old_source)
    new_ast = Metastatic.parse!(new_source)

    diff = Dllb.MetaAST.Diff.diff_trees(old_ast, new_ast)

    # Entities needing re-indexing
    diff.stale   # [{:function_def, "process/2"}, ...]

    # Entities to remove from index
    diff.removed # [{:function_def, "old_helper/1"}]

# `ast_change`

```elixir
@type ast_change() :: %{
  kind: change_kind(),
  node_type: atom(),
  name: String.t(),
  line: non_neg_integer() | nil
}
```

# `change_kind`

```elixir
@type change_kind() :: :added | :removed | :modified | :renamed
```

# `diff_summary`

```elixir
@type diff_summary() :: %{
  changes: [ast_change()],
  added: non_neg_integer(),
  removed: non_neg_integer(),
  modified: non_neg_integer(),
  renamed: non_neg_integer()
}
```

# `diff_trees`

```elixir
@spec diff_trees(tuple(), tuple()) :: diff_summary()
```

Compare two MetaAST trees and produce a summary of structural changes.

Comparison is done at the "named entity" level — functions, containers,
imports — matching the granularity at which dllb stores AST documents.

# `empty?`

```elixir
@spec empty?(diff_summary()) :: boolean()
```

Returns true if the diff contains no changes.

# `removed_entities`

```elixir
@spec removed_entities(diff_summary()) :: [String.t()]
```

Returns names of entities that should be removed from the index.

# `stale_entities`

```elixir
@spec stale_entities(diff_summary()) :: [String.t()]
```

Returns names of entities that need re-indexing (added + modified + renamed).

---

*Consult [api-reference.md](api-reference.md) for complete listing*