Complete structuretocsv implementation with proper tests and documentation

- Fixed bug handling union types (type as list) in converter
- Updated tests to follow proper pattern: compare with reference files in test/struct
- Generated all 10 CSV reference files for test schemas
- Updated README.md with s2csv command documentation
- All 10 tests now passing with reference comparisons

Co-authored-by: clemensv <542030+clemensv@users.noreply.github.com>

Author: 2 people

README.md

@@ -80,6 +80,7 @@ Generate code from JSON Structure:
 
 Direct JSON Structure conversions:
 
+- [`avrotize s2csv`](#convert-json-structure-to-csv-schema) - Convert JSON Structure schema to CSV schema.
 - [`avrotize s2x`](#convert-json-structure-to-xml-schema-xsd) - Convert JSON Structure to XML Schema (XSD).
 
 Other commands:
@@ -910,6 +911,27 @@ Conversion notes:
 - The tool generates Markdown documentation from the Avrotize Schema. Each record type in the Avrotize Schema is converted to a Markdown section.
 - The fields of the record are documented in a table in the Markdown section. Nested records are documented in nested sections in the Markdown file.
 
+### Convert JSON Structure to CSV Schema
+
+```bash
+avrotize s2csv <path_to_structure_schema_file> [--out <path_to_csv_schema_file>]
+```
+
+Parameters:
+
+- `<path_to_structure_schema_file>`: The path to the JSON Structure schema file to be converted. If omitted, the file is read from stdin.
+- `--out`: The path to the CSV schema file to write the conversion result to. If omitted, the output is directed to stdout.
+
+Conversion notes:
+
+- The tool converts JSON Structure schemas to CSV Schema format.
+- All JSON Structure Core types are supported including primitives (string, number, boolean, null, integer), extended types (int8-128, uint8-128, float/double, decimal, date, datetime, time, duration, uuid, uri, binary), and compound types (object, array, set, map, tuple, choice).
+- Compound types (arrays, objects, maps) are represented as strings in CSV schema, as CSV format doesn't have native support for complex nested structures.
+- Required/optional properties are preserved with the `nullable` flag.
+- Validation constraints (maxLength, minLength, pattern, minimum, maximum, precision, scale) are preserved in the CSV schema.
+- Enum and const keywords are supported and preserved in the output.
+- JSON Structure-specific features like `$ref`, `$extends`, definitions, and namespaces are resolved during conversion.
+
 ### Convert JSON Structure to Protocol Buffers
 
 ```bash

avrotize/structuretocsv.py

@@ -258,6 +258,16 @@ def convert_structure_type_to_csv_type(self, field_schema: Union[Dict, str, List
                 return 'string'
 
             struct_type = field_schema['type']
+            
+            # Handle union types when type is a list within the dict
+            if isinstance(struct_type, list):
+                non_null_types = [t for t in struct_type if t != 'null']
+                if len(non_null_types) == 1:
+                    # Simple nullable type
+                    return self.map_primitive_type_to_csv(non_null_types[0])
+                else:
+                    # Complex union - use string as fallback
+                    return "string"
 
             # Handle compound types
             if struct_type == 'array':

test/struct/choice-types-csv-ref.json

@@ -0,0 +1,18 @@
+{
+  "fields": [
+    {
+      "name": "taggedChoice",
+      "type": "string"
+    },
+    {
+      "name": "inlineChoice",
+      "type": "string",
+      "nullable": true
+    },
+    {
+      "name": "nullableString",
+      "type": "string",
+      "nullable": true
+    }
+  ]
+}

test/struct/edge-cases-csv-ref.json

@@ -0,0 +1,45 @@
+{
+  "fields": [
+    {
+      "name": "emptyString",
+      "type": "string",
+      "nullable": true,
+      "maxLength": 0
+    },
+    {
+      "name": "veryLongString",
+      "type": "string",
+      "nullable": true,
+      "maxLength": 1000000
+    },
+    {
+      "name": "preciseDecimal",
+      "type": "number",
+      "nullable": true,
+      "precision": 38,
+      "scale": 18
+    },
+    {
+      "name": "largeInteger",
+      "type": "integer",
+      "nullable": true,
+      "minimum": -9223372036854775808,
+      "maximum": 9223372036854775807
+    },
+    {
+      "name": "emptyArray",
+      "type": "string",
+      "nullable": true
+    },
+    {
+      "name": "nullableEverything",
+      "type": "string",
+      "nullable": true
+    },
+    {
+      "name": "deeplyNested",
+      "type": "string",
+      "nullable": true
+    }
+  ]
+}