Parser Migration Guide

This guide documents how to migrate a MakerNote parser to use TagRegistry + ArraySchema infrastructure.

Before You Start

Review the shared infrastructure guide: docs/architecture/parser-shared-infrastructure.md

Migration Steps

1. Analyze Current Parser

Identify:

• Decoder functions (const_decoder! macros or inline functions)
• Array-based tags (CameraSettings, ShotInfo, etc.)
• Lens database implementation (if any)
• Repetitive if-statement patterns in parse() method

2. Create Registry Module

Create src/parsers/tiff/makernotes/registries/<manufacturer>.rs:

use super::super::shared::{array_schemas::*, tag_registry::TagRegistry};
use super::super::<manufacturer>::*; // Import existing decoders

static ARRAY_SCHEMA: ArraySchema = ArraySchema {
    name: "SchemaName",
    indices: &[
        ArrayIndexDef::with_i16_decoder(1, "Field1", &DECODER1),
        ArrayIndexDef::raw(2, "Field2"),
    ],
};

pub fn <manufacturer>_registry() -> TagRegistry {
    TagRegistry::new()
        .register_array_schema(0x0001, &ARRAY_SCHEMA)
}

3. Migrate Lens Database (if applicable)

Convert HashMap to StaticLensDb:

static LENS_DATA: [(u16, &str); N] = [ /* existing data */ ];
static LENS_DB: StaticLensDb = StaticLensDb::new(&LENS_DATA);

pub fn get_lens_database() -> &'static impl LensDatabase {
    &LENS_DB
}

4. Update Parser Implementation

Replace repetitive if-statements with registry calls:

let registry = <manufacturer>_registry();

match entry.tag_id {
    ARRAY_TAG => {
        if let Some(array) = extract_i16_array(entry, data, byte_order) {
            registry.decode_array_i16(ARRAY_TAG, &array, "Prefix", tags);
        }
    }
}

5. Run Tests

cargo test -p oxidex <manufacturer>

Verify all existing tests pass.

6. Measure Reduction

wc -l src/parsers/tiff/makernotes/<manufacturer>.rs  # Before
wc -l src/parsers/tiff/makernotes/<manufacturer>.rs  # After

7. Commit

git commit -m "refactor(parsers): migrate <Manufacturer> parser to TagRegistry + ArraySchema

- Reduce parser from X to Y lines (Z line reduction)
- All existing tests pass"

Pilot Migration Results

Parser	Before	After	Reduction	%
Canon	1,345	1,165	180 lines	13%
Nikon	792	642	150 lines	19%
Sony	1,113	943	170 lines	15%
Apple	558	458	100 lines	18%
Google	566	461	105 lines	19%

Common Patterns

Pattern 1: Array with Lens Lookup

pub fn process_file_info_with_lens(
    array: &[i16],
    prefix: &str,
    lens_db: &impl LensDatabase,
    tags: &mut HashMap<String, String>,
) {
    SCHEMA.process_i16_array(array, prefix, tags);

    if let Some(&lens_id) = array.get(LENS_INDEX) {
        if let Some(name) = lens_db.lookup(lens_id as u16) {
            tags.insert(format!("{}:LensID", prefix), name.to_string());
        }
    }
}

Pattern 2: Multiple Array Schemas

static SCHEMA_1: ArraySchema = ArraySchema { /* ... */ };
static SCHEMA_2: ArraySchema = ArraySchema { /* ... */ };

pub fn registry() -> TagRegistry {
    TagRegistry::new()
        .register_array_schema(0x0001, &SCHEMA_1)
        .register_array_schema(0x0002, &SCHEMA_2)
}

Pattern 3: Shared Decoders

Re-export decoders from original parser:

// In registries/<manufacturer>.rs
use super::super::<manufacturer>::{DECODER1, DECODER2};

Keep decoder definitions in original file for now; future refactoring can move them.