Document invariants and add test instructions

README.md

@@ -66,3 +66,44 @@ print(ru_subdivs[0])
 
 - `cterdam/iso_3166-1_alpha-2`: Countries/regions with lang and hash
 - `cterdam/iata-metro`: IATA metro codes with lang and hash and coordinates
+
+
+## Invariants
+
+All entries in this dataset maintain the following invariants (verified by `test/test_invariants.py`):
+
+### Name Field
+- **Required**: Every entry has a `name` field
+- **Constraint**: `name` always equals one of `name_en`, `name_ru`, or `name_zh` (based on `lang`)
+- **Example**: If `lang: ru`, then `name == name_ru`
+
+### Lang Field
+- **Required**: Every entry has a `lang` field
+- **Values**: One of `en`, `ru`, or `zh`
+- **Determination**: Hardcoded by region (not inferred from text)
+  - **Slavic regions** (RU, UA, BY, KZ, etc.) → `lang: ru`
+  - **Sinophone regions** (CN, TW, HK, JP, etc.) → `lang: zh`
+  - **All others** → `lang: en`
+
+### Hash Field
+- **Required**: Every entry with a non-null `name` has a `hash` field
+- **Format**: 64-character hexadecimal string (SHA256)
+- **Constraint**: `hash == SHA256(name)`
+- **Usage**: First 6 hex digits used for sharding in source repositories
+
+## Verify Invariants
+
+Clone the dataset and run tests:
+
+\`\`\`bash
+git clone https://huggingface.co/datasets/cterdam/iso_3166-1_alpha-2
+cd iso_3166-1_alpha-2
+
+# Install dependencies
+pip install datasets
+
+# Run invariant tests
+pytest test/test_invariants.py -v
+\`\`\`
+
+Expected output: All tests pass, verifying dataset integrity.