```ts
export function splitLength(mm: number): string {
  if (!Number.isInteger(mm) || mm < 0) {
    throw new Error("Invalid length: must be a non-negative integer.");
  }

  const LADDER = [
    { unit: "m", divisor: 1000 },
    { unit: "cm", divisor: 10 },
    { unit: "mm" }
  ];

  return LADDER
    .map(({ unit, divisor }) => {
      const count = Math.floor(mm / divisor);
      if (count > 0) {
        return `${count}${unit}`;
      }
      return "";
    })
    .filter(Boolean)
    .join(" ");
}
```