import { z } from "zod";

export class ChiSchemaError extends Error {
	constructor(message: string) {
		super(message);
		this.name = "ChiSchemaError";
	}
}

export type EditableField =
	| { type: "enum"; key: string; values: readonly string[]; defaultValue: string }
	| { type: "string"; key: string; defaultValue: string };

export interface SchemaInfo {
	empty: boolean;
	fields: readonly EditableField[];
}

function zodTypeName(value: unknown): string | undefined {
	const def = (value as { _def?: { typeName?: string; type?: string } } | undefined)?._def;
	return def?.typeName ?? def?.type;
}

export function inspectSchema(schema: z.ZodObject<any>): SchemaInfo {
	const fields = Object.entries(schema.shape);
	if (fields.length === 0) return { empty: true, fields: [] };

	return {
		empty: false,
		fields: fields.flatMap<EditableField>(([key, field]) => {
			// Do not use instanceof here: Chi modules are separate Pi packages and
			// may load their own copy of zod. Structural checks keep the contract
			// valid across package boundaries while retaining strict field shapes.
			if (zodTypeName(field) !== "ZodDefault") {
				throw new ChiSchemaError("invalid schema field " + key + ": expected a field with a default");
			}

			const inner = (field as { _def: { innerType: unknown } })._def.innerType;
			const innerType = zodTypeName(inner);
			if (innerType !== "ZodString" && innerType !== "ZodEnum") return [];

			const defaultValue = (field as { parse(value: undefined): unknown }).parse(undefined);
			if (typeof defaultValue !== "string") {
				throw new ChiSchemaError("invalid schema field " + key + ": editable default must be a string");
			}

			if (innerType === "ZodString") return [{ type: "string", key, defaultValue }];

			const options = (inner as { options?: unknown }).options;
			if (!Array.isArray(options) || !options.every((value) => typeof value === "string")) {
				throw new ChiSchemaError("invalid schema field " + key + ": enum options must be strings");
			}

			return [{
				type: "enum",
				key,
				values: options,
				defaultValue,
			}];
		}),
	};
}