import { parse } from "@babel/parser";
type Node = { type: string; start: number; end: number; [key: string]: unknown };
const node = (value: unknown): value is Node =>
  !!value && typeof value === "object" && typeof (value as Node).type === "string";

/** One requested insertion into ordinary authored TypeScript, not regeneration.
 * Ambiguous/custom wiring is left alone with an actionable error. */
export function addImplementation(
  text: string,
  factory: "createRuntime" | "serveMigration",
  key: string,
  importPath: string,
  migrationOnly = false,
): string {
  const ast = parse(text, { sourceType: "module", plugins: ["typescript"] });
  const objects: Node[] = [];
  const names = new Set<string>();
  const visit = (value: unknown) => {
    if (Array.isArray(value)) {
      value.forEach(visit);
      return;
    }
    if (!node(value)) return;
    if (value.type === "Identifier") names.add(String(value.name));
    if (
      value.type === "CallExpression" &&
      node(value.callee) &&
      value.callee.type === "Identifier" &&
      value.callee.name === factory &&
      Array.isArray(value.arguments) &&
      value.arguments.length === 1 &&
      node(value.arguments[0]) &&
      value.arguments[0].type === "ObjectExpression"
    )
      objects.push(value.arguments[0]);
    Object.values(value).forEach(visit);
  };
  visit(ast);
  if (objects.length !== 1)
    throw new Error(
      `Cannot safely add implementation: expected one ${factory}({...}) literal. Wire the handler in your authored server code instead.`,
    );
  const object = objects[0];
  if (
    (object.properties as Node[]).some((p) => node(p.key) && !p.computed && (p.key.name === key || p.key.value === key))
  )
    throw new Error(`Implementation already exists for ${key}`);
  let alias = "qxImplementation";
  for (let index = 1; names.has(alias); index++) alias = `qxImplementation${index}`;
  const value = migrationOnly ? 'async () => { throw new Error("Migration-only export"); }' : alias;
  const insertion = object.start + 1;
  const edited = text.slice(0, insertion) + `\n  ${JSON.stringify(key)}: ${value},\n` + text.slice(insertion);
  return (migrationOnly ? "" : `import {handler as ${alias}} from ${JSON.stringify(importPath)};\n`) + edited;
}