```typescript

interface RunLengthEncodeResult {
  characters: string;
}

export function runLengthEncode(input: string): string {
  if (typeof input !== "string") {
    throw new Error("Input must be a string");
  }
  
  if (input === "") {
    return "";
  }

  let result = '';
  let currentChar = input[0];
  let count = 1;

  for (let i = 1; i < input.length; i++) {
    if (input[i] === currentChar) {
      count++;
    } else {
      result += `${currentChar}${count}`;
      currentChar = input[i];
      count = 1;
    }
  }

  result += `${currentChar}${count}`;

  return result;
}
```