# Extraction Result Error Structure Analysis

## Overview
This document analyzes how errors and diagnostics are organized in the pdftract extraction result structure.

## ExtractionResult Structure

The main `ExtractionResult` struct (defined in `crates/pdftract-core/src/extract.rs`) contains:

```rust
pub struct ExtractionResult {
    pub fingerprint: String,
    pub pages: Vec<PageResult>,
    pub metadata: ExtractionMetadata,
    pub signatures: Vec<SignatureJson>,
    pub form_fields: Vec<FormFieldJson>,
    pub links: Vec<LinkJson>,
    pub attachments: Vec<AttachmentJson>,
    pub threads: Vec<ThreadJson>,
    pub javascript_actions: Vec<JavascriptActionJson>,
}
```

## Error Reporting Locations

### 1. Document-Level Diagnostics

**Location**: `ExtractionResult.metadata.diagnostics: Vec<String>`

**Purpose**: Contains warnings and errors that affect the entire extraction process.

**Examples**:
- Tagged PDF detected (StructTree traversal deferred)
- Xref repaired via forward scan
- Coverage warnings
- Page range errors

### 2. Per-Page Errors

**Location**: `PageResult.error: Option<String>`

**Purpose**: Contains error messages when extraction fails for a specific page.

**Structure**:
```rust
pub struct PageResult {
    pub index: usize,
    pub page_number: u32,
    pub page_label: Option<String>,
    pub width: Option<f32>,
    pub height: Option<f32>,
    pub rotation: Option<u16>,
    pub page_type: Option<String>,
    pub spans: Vec<SpanJson>,
    pub blocks: Vec<BlockJson>,
    pub tables: Vec<TableJson>,
    pub annotations: Vec<AnnotationJson>,
    pub error: Option<String>,  // ← Per-page error
}
```

### 3. Metadata Error Count

**Location**: `ExtractionMetadata.error_count: usize`

**Purpose**: Tracks the total number of pages that failed to extract.

## Internal Diagnostic Structure

Before being converted to strings, diagnostics use the rich `Diagnostic` structure:

```rust
pub struct Diagnostic {
    pub code: DiagCode,                    // Error type code
    pub byte_offset: Option<u64>,          // Location in file
    pub object_ref: Option<ObjRef>,         // PDF object reference
    pub message: Cow<'static, str>,        // Human-readable message
}
```

## Diagnostic Code Categories

The `DiagCode` enum categorizes errors with prefixes:

| Category | Prefix | Description |
|----------|--------|-------------|
| Structure | `STRUCT_*` | PDF structure errors (parser/object/document) |
| Stream | `STREAM_*` | Stream decoder errors |
| XREF | `XREF_*` | Cross-reference table errors |
| Encryption | `ENCRYPTION_*` | Encryption-related errors |
| Page | `PAGE_*` | Page-level errors |
| Font | `FONT_*` | Font pipeline errors |
| Graphics State | `GSTATE_*` | Graphics state errors |
| Layout | `LAYOUT_*` | Layout and reading order errors |
| OCR | `OCR_*` | OCR pipeline errors |
| Remote | `REMOTE_*` | Remote source errors |
| MCP | `MCP_*` | MCP server errors |
| Cache | `CACHE_*` | Cache errors |
| Security | `SECURITY_*` | Security-related diagnostics |

## Severity Levels

Diagnostics have four severity levels:

1. **Info** — Does not affect output validity
   - Examples: `XREF_REPAIRED`, `TAGGED_PDF_STRUCT_TREE_DEFERRED`

2. **Warning** — Output is usable but degraded
   - Examples: `STRUCT_INVALID_NAME`, `FONT_GLYPH_UNMAPPED`, `STREAM_DECODE_ERROR`

3. **Error** — Output for this region/page is invalid; other regions OK
   - Examples: `STREAM_BOMB`, `PAGE_OUT_OF_RANGE`, `REMOTE_FETCH_INTERRUPTED`

4. **Fatal** — Extraction aborted, no usable output
   - Examples: `ENCRYPTION_UNSUPPORTED`, `REMOTE_TLS_FAILED`, `REMOTE_DNS_FAILED`

## Error Flow

1. **Emission**: During extraction, `Diagnostic` structs are emitted via the `emit!()` macro
2. **Collection**: Diagnostics are collected in `Vec<Diagnostic>` 
3. **Conversion**: Before returning `ExtractionResult`, diagnostics are converted to strings:
   ```rust
   let diagnostics: Vec<String> = coverage_result
       .diagnostics
       .iter()
       .map(|d| d.message.as_ref().to_string())
       .collect();
   ```
4. **Exposure**: Final string messages are exposed in `ExtractionMetadata.diagnostics`

## Key Files

- `crates/pdftract-core/src/extract.rs` - Main extraction pipeline, defines `ExtractionResult`
- `crates/pdftract-core/src/diagnostics.rs` - Diagnostic system, defines `Diagnostic` and `DiagCode`
- `src/graphics_state/diagnostics.rs` - Legacy diagnostic types (graphics state only)

## Example Diagnostic

```rust
// Creating a diagnostic
Diagnostic::with_static(
    DiagCode::StreamDecodeError,
    12345,  // byte offset
    "zlib stream truncated mid-inflation"
)

// Converts to string in metadata:
// "STREAM_DECODE_ERROR: zlib stream truncated mid-inflation (byte offset 12345)"
```

## Acceptance Criteria Status

✅ **Error array structure understood**
- Document-level diagnostics in `metadata.diagnostics`
- Per-page errors in `PageResult.error`

✅ **Error fields documented**
- Internal `Diagnostic` struct with code, offset, object_ref, message
- Final string representation in extraction result

✅ **Relationship between errors and extraction phases clear**
- Each diagnostic has a phase origin (e.g., "1.1", "1.5", "3.1")
- DiagCode category prefixes indicate which layer emitted the error
- Severity levels indicate impact on extraction result