```ts
/** Count the hits a key stream scores on a direct-mapped cache. */

export function slotHits(keys: number[], slots: number): number {
  if (!Array.isArray(keys)) throw new Error('keys must be an array');
  if (!Number.isInteger(slots) || slots <= 0) throw new Error('slots must be a positive integer');

  const held: (number | null)[] = new Array(slots).fill(null);
  let hits = 0;

  for (const key of keys) {
    if (!Number.isInteger(key)) throw new Error('all keys must be integers');
    const slotIndex = key % slots;
    if (slotIndex < 0) throw new Error('key modulo slots must not result in a negative index');

    if (held[slotIndex] === key) {
      hits++;
    } else {
      held[slotIndex] = key;
    }
  }

  return hits;
}
```