Function parseFitArgs

src/compute-engine/library/statistics.ts:1000–1032

* Parse the regression argument list: an optional trailing variable symbol, * an optional trailing integer degree (for `PolynomialFit`), and the data as * either two collections or one collection of pairs.

(
  ops: ReadonlyArray<Expression>,
  wantDegree: boolean
)

      // `SlidingWindow("abcd", 2)` is `["ab","bc","cd"]` (ruling D9(b),
      // 2026-08-16; see `innerRun` in `library/collections.ts`). Spelled as a
      // BOUNDED type variable (`S where S: string`), never the ground type
      // `string`: an `unknown`- or `any`-typed operand refutes no arm, so a
      // ground `string` parameter would win most-specific-wins on every
      // untyped operand.
      signature:
        '((S, integer, integer?) -> list<string> where S: string) & ((collection, integer, integer?) -> list<list>)',
      examples: [
        'SlidingWindow([1, 2, 3, 4], 2)  // Returns [[1,2], [2,3], [3,4]]',
        'SlidingWindow("abcd", 2)  // Returns ["ab", "bc", "cd"]',
      ],
      evaluate: ([xs, winArg, stepArg], { engine: ce }) => {
        if (!xs.isFiniteCollection) return undefined;
        // Small finite sources materialize eagerly (all existing semantics);
        // larger — or unknown-length — sources stay symbolic and are served
        // lazily by the `collection` handlers below (Tycho item 52).
        const size = xs.count;
        if (size === undefined || size > MAX_SIZE_EAGER_COLLECTION)
          return undefined;
        const windowSize = toInteger(winArg);
        const stepSize = stepArg ? toInteger(stepArg) : 1;
        if (
          windowSize === null ||
          windowSize <= 0 ||
          stepSize === null ||
          stepSize <= 0
        )
          return undefined;

        const data = Array.from(xs.each()) as Expression[];
        const result: Expression[] = [];

        // Each window is emitted through `innerRun`, which makes it a STRING
        // when the source is a string. Joining a window's grapheme clusters
        // re-runs segmentation, and two adjacent clusters can merge — but only
        // when the source itself contained a lone combining mark, the only way
        // a cluster can begin with a character that attaches to what precedes