EonaCat.DoxaApi/DoxaApi/Exporter/OpenApiExporter.cs

using System.Text.Json;
using System.Text.Json.Nodes;
using EonaCat.DoxaApi.Models;

namespace EonaCat.DoxaApi.Interop
{
    public static class OpenApiExporter
    {
        public static JsonObject Export(ApiDocument doc)
        {
            var root = new JsonObject
            {
                ["openapi"] = "3.0.3",
                ["info"] = BuildInfo(doc.Info),
            };

            if (doc.Servers.Count > 0)
            {
                var servers = new JsonArray();
                foreach (var s in doc.Servers)
                {
                    servers.Add(new JsonObject { ["url"] = s });
                }

                root["servers"] = servers;
            }

            var paths = new JsonObject();
            foreach (var group in doc.Groups)
            {
                foreach (var endpoint in group.Endpoints)
                {
                    var openApiPath = ToOpenApiPath(endpoint.Path);
                    if (!paths.ContainsKey(openApiPath))
                    {
                        paths[openApiPath] = new JsonObject();
                    }

                    var pathItem = (JsonObject)paths[openApiPath]!;
                    pathItem[endpoint.Method.ToLowerInvariant()] = BuildOperation(endpoint, group.Name);
                }
            }
            root["paths"] = paths;

            if (doc.Schemas.Count > 0)
            {
                var schemas = new JsonObject();
                foreach (var (name, schema) in doc.Schemas)
                {
                    schemas[name] = SchemaToOpenApi(schema);
                }

                root["components"] = new JsonObject { ["schemas"] = schemas };
            }

            return root;
        }

        private static JsonObject BuildInfo(ApiInfo info)
        {
            var obj = new JsonObject
            {
                ["title"] = info.Title,
                ["version"] = info.Version
            };
            if (info.Description is not null)
            {
                obj["description"] = info.Description;
            }

            return obj;
        }

        private static JsonObject BuildOperation(ApiEndpoint endpoint, string groupName)
        {
            var op = new JsonObject();

            var tags = new JsonArray();
            foreach (var t in (endpoint.Tags.Count > 0 ? endpoint.Tags : new List<string> { groupName }))
            {
                tags.Add(t);
            }

            op["tags"] = tags;

            op["operationId"] = endpoint.OperationId;

            if (endpoint.Summary is not null)
            {
                op["summary"] = endpoint.Summary;
            }

            if (endpoint.Description is not null)
            {
                op["description"] = endpoint.Description;
            }

            if (endpoint.Deprecated)
            {
                op["deprecated"] = true;
            }

            if (endpoint.Parameters.Count > 0)
            {
                var parameters = new JsonArray();
                foreach (var p in endpoint.Parameters)
                {
                    var param = new JsonObject
                    {
                        ["name"] = p.Name,
                        ["in"] = p.In,
                        ["required"] = p.Required,
                        ["schema"] = SchemaToOpenApi(p.Schema)
                    };
                    if (p.Description is not null)
                    {
                        param["description"] = p.Description;
                    }

                    parameters.Add(param);
                }
                op["parameters"] = parameters;
            }

            if (endpoint.RequestBody is not null)
            {
                var rb = endpoint.RequestBody;
                var content = new JsonObject
                {
                    [rb.ContentType] = new JsonObject { ["schema"] = SchemaToOpenApi(rb.Schema) }
                };
                if (rb.Example is not null)
                {
                    ((JsonObject)content[rb.ContentType]!)["example"] =
                        JsonNode.Parse(rb.Example) ?? JsonValue.Create(rb.Example)!;
                }
                op["requestBody"] = new JsonObject
                {
                    ["required"] = rb.Required,
                    ["content"] = content
                };
            }

            var responses = new JsonObject();
            foreach (var r in endpoint.Responses)
            {
                var resp = new JsonObject();
                resp["description"] = r.Description ?? HttpStatusDescription(r.StatusCode);

                if (r.Schema is not null && r.Schema.Type != "void")
                {
                    resp["content"] = new JsonObject
                    {
                        ["application/json"] = new JsonObject
                        {
                            ["schema"] = SchemaToOpenApi(r.Schema)
                        }
                    };
                }
                responses[r.StatusCode] = resp;
            }
            op["responses"] = responses;

            return op;
        }

        private static JsonObject SchemaToOpenApi(SchemaModel schema)
        {

            if (schema.RefName is not null)
            {
                return new JsonObject { ["$ref"] = $"#/components/schemas/{schema.RefName}" };
            }

            var obj = new JsonObject();

            switch (schema.Type)
            {
                case "void":
                    return new JsonObject { ["type"] = "object" };

                case "enum":
                    obj["type"] = "string";
                    if (schema.EnumValues?.Count > 0)
                    {
                        var enums = new JsonArray();
                        foreach (var v in schema.EnumValues)
                        {
                            enums.Add(v);
                        }

                        obj["enum"] = enums;
                    }
                    break;

                case "array":
                    obj["type"] = "array";
                    if (schema.Items is not null)
                    {
                        obj["items"] = SchemaToOpenApi(schema.Items);
                    }

                    break;

                case "object":
                    obj["type"] = "object";
                    if (schema.Properties?.Count > 0)
                    {
                        var props = new JsonObject();
                        foreach (var (name, propSchema) in schema.Properties)
                        {
                            props[name] = SchemaToOpenApi(propSchema);
                        }

                        obj["properties"] = props;
                    }
                    if (schema.Required?.Count > 0)
                    {
                        var req = new JsonArray();
                        foreach (var r in schema.Required)
                        {
                            req.Add(r);
                        }

                        obj["required"] = req;
                    }

                    if (schema.Items is not null && schema.Properties is null)
                    {
                        obj["additionalProperties"] = SchemaToOpenApi(schema.Items);
                    }

                    break;

                default:
                    obj["type"] = schema.Type;
                    if (schema.Format is not null)
                    {
                        obj["format"] = schema.Format;
                    }

                    break;
            }

            if (schema.Nullable)
            {
                obj["nullable"] = true;
            }

            return obj;
        }

        private static string ToOpenApiPath(string path)

            => path.StartsWith('/') ? path : "/" + path;

        private static string HttpStatusDescription(string code) => code switch
        {
            "200" => "OK",
            "201" => "Created",
            "204" => "No Content",
            "400" => "Bad Request",
            "401" => "Unauthorized",
            "403" => "Forbidden",
            "404" => "Not Found",
            "409" => "Conflict",
            "422" => "Unprocessable Entity",
            "500" => "Internal Server Error",
            _ => "Response"
        };
    }
}