```ts
/**
 * Parses a semantic version string.
 */
export function parseSemver(version: string): {
  major: number;
  minor: number;
  patch: number;
  prerelease: string | null;
  build: string | null;
} {
  const match = version.match(/^(\d+)\.(\d+)\.(\d+)(?:-(.+))?(?:\+(.+))?$/);
  
  if (match === null) {
    throw new Error(`Invalid semver: ${version}`);
  }
  
  const [, major, minor, patch, prerelease, build] = match;
  
  return {
    major: parseInt(major),
    minor: parseInt(minor),
    patch: parseInt(patch),
    prerelease,
    build,
  };
}
```