feat(generate): introduce stateful context

in order to move towards a more functional and modular architecture
move state that previously was helt on Generator class to a context

the idea behind this is to pass the context object around between
internal units as well as plugin callbacks and all shared state
would be managed on the context object

ref #562

packages/codegen/src/context.ts

@@ -0,0 +1,75 @@
+import type {
+  InterfaceDeclaration,
+  Statement,
+  TypeAliasDeclaration,
+  TypeReferenceNode,
+} from "typescript";
+import type { Opts } from ".";
+import { OpenAPIDocument } from "./openApi3-x";
+import _ from "lodash";
+
+export type OnlyMode = "readOnly" | "writeOnly";
+export type OnlyModes = Record<OnlyMode, boolean>;
+
+export type OazapftsContext = {
+  readonly inputSpec: OpenAPIDocument;
+  readonly opts: Opts;
+  readonly isConverted: boolean;
+
+  spec: OpenAPIDocument;
+
+  // see `preprocessComponents` for the definition of a discriminating schema
+  discriminatingSchemas: Set<string>;
+
+  aliases: (TypeAliasDeclaration | InterfaceDeclaration)[];
+
+  enumAliases: Statement[];
+  enumRefs: Record<string, { values: string; type: TypeReferenceNode }>;
+
+  // Collect the types of all referenced schemas so we can export them later
+  // Referenced schemas can be pointing at the following versions:
+  // - "base": The regular type/interface e.g. ExampleSchema
+  // - "readOnly": The readOnly version e.g. ExampleSchemaRead
+  // - "writeOnly": The writeOnly version e.g. ExampleSchemaWrite
+  refs: Record<
+    string,
+    {
+      base: TypeReferenceNode;
+      readOnly?: TypeReferenceNode;
+      writeOnly?: TypeReferenceNode;
+    }
+  >;
+
+  // Maps a referenced schema to its readOnly/writeOnly status
+  // This field should be used exclusively within the `checkSchemaOnlyMode` method
+  refsOnlyMode: Map<string, OnlyModes>;
+
+  // Keep track of already used type aliases
+  typeAliases: Record<string, number>;
+};
+
+export function createContext(
+  spec: OazapftsContext["spec"],
+  opts: OazapftsContext["opts"],
+  isConverted: OazapftsContext["isConverted"] = false,
+): OazapftsContext {
+  return resetContext({ inputSpec: spec, opts, isConverted });
+}
+
+type ReadonlyContextAttrs = "inputSpec" | "opts" | "isConverted";
+type ResettableContext = Pick<OazapftsContext, ReadonlyContextAttrs> &
+  Partial<Omit<OazapftsContext, ReadonlyContextAttrs>>;
+
+export function resetContext(ctx: ResettableContext): OazapftsContext {
+  return {
+    ...ctx,
+    discriminatingSchemas: new Set(),
+    aliases: [],
+    enumAliases: [],
+    enumRefs: {},
+    refs: {},
+    refsOnlyMode: new Map(),
+    typeAliases: {},
+    spec: _.cloneDeep(ctx.inputSpec),
+  };
+}