/**
 * Retrieves the default values for a given Zod schema.
 * @param schema The Zod schema for which to retrieve the default values.
 * @param defaults A partial object containing default values for the schema.
 * @param discriminant The discriminant value for discriminated unions.
 * @returns An object containing the default values for the schema, or `undefined` if no defaults are found.
 */
export function getZodDefaults<T extends z.ZodType>(schema: T, defaults?: Partial<T['_output']>, discriminant?: string): T['_output'] | undefined {
  if (schema instanceof z.ZodDefault) {
    const def = schema.def.defaultValue
    return typeof def === 'function' ? def() : def
  }
  else if (schema instanceof z.ZodLiteral) return [...schema.values][0]
  else if (schema instanceof z.ZodDiscriminatedUnion) {
    if (!discriminant) throw new Error('Discriminant value is required for discriminated unions')
    const discriminator = schema._zod.def.discriminator
    for (const opt of schema.options) {
      const shape = (opt as z.ZodObject).def.shape
      const literals = (shape[discriminator] as z.ZodLiteral).values
      if (literals.has(discriminant)) return getZodDefaults(opt as any, defaults, discriminant)
    }
  }
  else if (schema instanceof z.ZodObject) {
    const shape = schema.def.shape
    const result: any = {}
    for (const key in shape) {
      const value = getZodDefaults(shape[key], typeof defaults === 'object' ? (defaults as any)[key] : defaults, discriminant)
      result[key] = defaults?.[key as keyof typeof defaults] ?? value
    }
    return result
  }
  else if (schema instanceof z.ZodIntersection) {
    const left = getZodDefaults(schema.def.left as any, defaults, discriminant)
    const right = getZodDefaults(schema.def.right as any, defaults, discriminant)
    return { ...left, ...right }
  }
  else if (schema instanceof z.ZodArray || schema instanceof z.ZodTuple) return []
  else if (schema instanceof z.ZodUnion) {
    for (const val of schema.options) {
      const value = getZodDefaults(val as any, defaults, discriminant)
      if (value !== undefined) return value
    }
    return undefined
  }
  else if (schema instanceof z.ZodRecord) return {}
  else if (schema instanceof z.ZodMap) return new Map()
  else if (schema instanceof z.ZodSet) return new Set()
  else return undefined
}