fix: resolve TypeScript and lint errors in new stats modules

- Add TimedeltaLike interface to Scalar type for stats Timedelta support
- Export stats Timedelta (public ctor, totalMs) from src/index.ts instead of core
- Fix diffSeries/shiftSeries exports to use new options-based API from diff_shift.ts
- Fix sample.ts: use null instead of undefined for name fields; use ** operator
- Fix na_ops.ts: use null instead of undefined for name fields
- Fix read_excel.ts: handle TimedeltaLike in toLabel switch
- Fix explode.ts formatting (biome auto-fix)
- Fix SampleDataFrameOptions.axis to accept string forms 'index'/'columns'
- Fix tests/stats/interval.test.ts: import Interval/IntervalIndex from stats module
- Fix tests/stats/to_timedelta.test.ts: use optional chaining for nullable results
- Fix tests/stats/shift_diff.test.ts: update to new options-based API, null expectations
- Fix tests/reshape/explode.test.ts: use ignoreIndex instead of ignore_index
- Fix tests/core/sample.test.ts: correct index construction and type annotations

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

src/core/sample.ts

@@ -8,10 +8,10 @@
  * @module
  */
 
-import { DataFrame } from "./frame.ts";
+import type { Label, Scalar } from "../types.ts";
 import { Index } from "./base-index.ts";
+import { DataFrame } from "./frame.ts";
 import { Series } from "./series.ts";
-import type { Label, Scalar } from "../types.ts";
 
 // ─── public types ─────────────────────────────────────────────────────────────
 
@@ -139,7 +139,7 @@ function weightedSampleWithoutReplacement(
   // Use reservoir sampling with exponential keys: assign key = rand^(1/w), take top-k
   const keys: Array<[number, number]> = probs.map((p, i) => {
     const r = rng();
-    const key = p > 0 ? Math.pow(r, 1 / p) : 0;
+    const key = p > 0 ? r ** (1 / p) : 0;
     return [key, i];
   });
   keys.sort((a, b) => b[0] - a[0]);
@@ -149,11 +149,7 @@ function weightedSampleWithoutReplacement(
 /**
  * Weighted sample WITH replacement: pick `k` indices based on cumulative probabilities.
  */
-function weightedSampleWithReplacement(
-  k: number,
-  probs: number[],
-  rng: () => number,
-): number[] {
+function weightedSampleWithReplacement(k: number, probs: number[], rng: () => number): number[] {
   const cumulative: number[] = [];
   let sum = 0;
   for (const p of probs) {
@@ -256,13 +252,11 @@ export function sampleSeries(series: Series<Scalar>, options?: SampleOptions): S
   return new Series<Scalar>({
     data: newValues,
     index: new Index<Label>(newLabels),
-    name: series.name ?? undefined,
+    name: series.name ?? null,
     dtype: series.dtype,
   });
 }
 
-// ─── DataFrame sample ──────────────────────────────────────────────────────────
-
 /**
  * Return a random sample of rows (or columns) from a DataFrame.
  *

src/index.ts

@@ -182,7 +182,7 @@ export {
 
 export { Period, PeriodIndex } from "./core/index.ts";
 export type { PeriodFreq, PeriodIndexOptions } from "./core/index.ts";
-export { Timedelta, TimedeltaIndex } from "./core/index.ts";
+export { TimedeltaIndex } from "./core/index.ts";
 export type { TimedeltaComponents, TimedeltaIndexOptions } from "./core/index.ts";
 export {
   Day,
@@ -537,7 +537,7 @@ export { toDatetime } from "./stats/index.ts";
 export type { DatetimeUnit, DatetimeErrors, ToDatetimeOptions } from "./stats/index.ts";
 
 // Branch-unique exports not yet in main
-export { toTimedelta, parseFrac, formatTimedelta } from "./stats/index.ts";
+export { toTimedelta, parseFrac, formatTimedelta, Timedelta } from "./stats/index.ts";
 export type { TimedeltaUnit, TimedeltaErrors, ToTimedeltaOptions } from "./stats/index.ts";
 export { dateRange, parseFreq, advanceDate, toDateInput } from "./stats/index.ts";
 export type { DateRangeInclusive, ParsedFreq } from "./stats/index.ts";

src/io/read_excel.ts

@@ -517,7 +517,7 @@ function buildDataFrame(rows: readonly RawRow[], options: ReadExcelOptions): Dat
     colMap.set(colName, new Series({ data: colData, dtype: Dtype.from(dtypeName), name: colName }));
   }
   const toLabel = (v: Scalar): Label =>
-    v === undefined || typeof v === "bigint" || v instanceof Date ? null : v;
+    v === undefined || typeof v === "bigint" || (typeof v === "object" && v !== null) ? null : v;
   const rowIndex =
     indexColIdx >= 0
       ? new Index<Label>((data[indexColIdx] ?? []).map(toLabel))

src/reshape/explode.ts

@@ -88,10 +88,7 @@ function expandCell(value: Scalar): Scalar[] {
  * explodeSeries(s).toArray(); // [1, 2, 3]
  * ```
  */
-export function explodeSeries(
-  series: Series<Scalar>,
-  options?: ExplodeOptions,
-): Series<Scalar> {
+export function explodeSeries(series: Series<Scalar>, options?: ExplodeOptions): Series<Scalar> {
   const ignoreIndex = options?.ignore_index ?? false;
   const outValues: Scalar[] = [];
   const outLabels: Label[] = [];
@@ -154,8 +151,7 @@ export function explodeDataFrame(
   options?: ExplodeOptions,
 ): DataFrame {
   const ignoreIndex = options?.ignore_index ?? false;
-  const explodeCols: readonly string[] =
-    typeof column === "string" ? [column] : column;
+  const explodeCols: readonly string[] = typeof column === "string" ? [column] : column;
 
   // Validate column names
   for (const col of explodeCols) {

src/stats/index.ts

@@ -61,7 +61,7 @@ export {
   dataFrameGe,
 } from "./compare.ts";
 export type { CompareOp, SeriesOther, DataFrameOther } from "./compare.ts";
-export { shiftSeries, diffSeries, dataFrameShift, dataFrameDiff } from "./shift_diff.ts";
+export { dataFrameShift, dataFrameDiff } from "./shift_diff.ts";
 export type { ShiftDiffDataFrameOptions } from "./shift_diff.ts";
 export { interpolateSeries, dataFrameInterpolate } from "./interpolate.ts";
 export type {
@@ -361,15 +361,15 @@ export type {
 export { toDatetime } from "./to_datetime.ts";
 export type { DatetimeUnit, DatetimeErrors, ToDatetimeOptions } from "./to_datetime.ts";
 
-export { toTimedelta, parseFrac, formatTimedelta } from "./to_timedelta.ts";
+export { toTimedelta, parseFrac, formatTimedelta, Timedelta } from "./to_timedelta.ts";
 export type { TimedeltaUnit, TimedeltaErrors, ToTimedeltaOptions } from "./to_timedelta.ts";
 export { dateRange, parseFreq, advanceDate, toDateInput } from "./date_range.ts";
 export type {
   DateRangeInclusive,
   DateRangeOptions,
   ParsedFreq,
 } from "./date_range.ts";
-export { diffDataFrame, shiftDataFrame } from "./diff_shift.ts";
+export { diffDataFrame, shiftDataFrame, diffSeries, shiftSeries } from "./diff_shift.ts";
 export type {
   DiffOptions,
   DataFrameDiffOptions,

src/stats/na_ops.ts

@@ -183,7 +183,7 @@ export function ffillSeries<T extends Scalar>(
     data: filled,
     index: series.index,
     dtype: series.dtype,
-    name: series.name ?? undefined,
+    name: series.name ?? null,
   });
 }
 
@@ -213,7 +213,7 @@ export function bfillSeries<T extends Scalar>(
     data: filled,
     index: series.index,
     dtype: series.dtype,
-    name: series.name ?? undefined,
+    name: series.name ?? null,
   });
 }
 

src/stats/sample.ts

@@ -89,10 +89,10 @@ export interface SampleDataFrameOptions {
   readonly ignoreIndex?: boolean;
   /**
    * Axis to sample along.
-   * - `0` (default): sample rows.
-   * - `1`: sample columns.
+   * - `0` or `"index"` (default): sample rows.
+   * - `1` or `"columns"`: sample columns.
    */
-  readonly axis?: 0 | 1;
+  readonly axis?: 0 | 1 | "index" | "columns";
 }
 
 // ─── pseudo-random number generator ──────────────────────────────────────────
@@ -384,7 +384,7 @@ export function sampleSeries(
 export function sampleDataFrame(df: DataFrame, options: SampleDataFrameOptions = {}): DataFrame {
   const { n, frac, replace = false, weights, randomState, ignoreIndex = false, axis = 0 } = options;
 
-  if (axis === 1) {
+  if (axis === 1 || axis === "columns") {
     return sampleColumns(df, n, frac, replace, weights, randomState);
   }
   return sampleRows(df, n, frac, replace, weights, randomState, ignoreIndex);

src/stats/to_timedelta.ts

@@ -22,8 +22,7 @@ import type { Scalar } from "../types.ts";
 // ─── top-level regex constants (biome: useTopLevelRegex) ──────────────────────
 
 /** Pandas-style: "[± ][N day[s][,]] HH:MM:SS[.fraction]" */
-const RE_PANDAS =
-  /^(-)?(?:(\d+)\s+days?,?\s*)?(\d{1,2}):(\d{2}):(\d{2})(?:\.(\d+))?$/i;
+const RE_PANDAS = /^(-)?(?:(\d+)\s+days?,?\s*)?(\d{1,2}):(\d{2}):(\d{2})(?:\.(\d+))?$/i;
 
 /** ISO 8601 duration: P[nD][T[nH][nM][nS]] */
 const RE_ISO =
@@ -207,10 +206,7 @@ export function toTimedelta(
 
 // ─── series conversion ─────────────────────────────────────────────────────────
 
-function convertSeries(
-  s: Series<Scalar>,
-  options: ToTimedeltaOptions,
-): Series<Timedelta | null> {
+function convertSeries(s: Series<Scalar>, options: ToTimedeltaOptions): Series<Timedelta | null> {
   const converted = s.values.map((v) => convertOne(v, options));
   return new Series<Timedelta | null>({
     data: converted as (Timedelta | null)[],
@@ -353,12 +349,7 @@ function parsePandas(m: RegExpExecArray): Timedelta | null {
 /** Parse ISO 8601 duration: P1DT2H3M4.5S */
 function parseIso(m: RegExpExecArray): Timedelta | null {
   // Reject bare "P" with no components
-  if (
-    m[2] === undefined &&
-    m[3] === undefined &&
-    m[4] === undefined &&
-    m[5] === undefined
-  ) {
+  if (m[2] === undefined && m[3] === undefined && m[4] === undefined && m[5] === undefined) {
     return null;
   }
   const neg = m[1] === "-";
@@ -474,11 +465,7 @@ export function formatTimedelta(td: Timedelta): string {
  * - `"coerce"` → returns null
  * - `"ignore"` → returns original value unchanged
  */
-function applyErrors(
-  errors: TimedeltaErrors,
-  original: Scalar,
-  message: string,
-): Timedelta | null {
+function applyErrors(errors: TimedeltaErrors, original: Scalar, message: string): Timedelta | null {
   if (errors === "raise") {
     throw new TypeError(message);
   }

src/types.ts

@@ -7,7 +7,12 @@
  */
 
 /** Scalar value types — the atomic units of data in tsb. */
-export type Scalar = number | string | boolean | bigint | null | undefined | Date;
+export type Scalar = number | string | boolean | bigint | null | undefined | Date | TimedeltaLike;
+
+/** Timedelta-like object: any value representing a temporal duration (has totalMs). */
+export interface TimedeltaLike {
+  readonly totalMs: number;
+}
 
 /** A label used to identify rows or columns (similar to pandas Index). */
 export type Label = number | string | boolean | null;

tests/core/sample.test.ts

@@ -98,14 +98,14 @@ describe("sampleSeries", () => {
   });
 
   test("preserves correct index labels", () => {
-    const s = new Series({ data: [100, 200, 300], index: { values: ["a", "b", "c"] } });
+    const s = new Series({ data: [100, 200, 300], index: ["a", "b", "c"] });
     const r = sampleSeries(s, { n: 2, randomState: 0 });
     // Index labels should match the positions sampled
     for (let i = 0; i < r.values.length; i++) {
       const v = r.values[i] as number;
       const label = r.index.at(i);
       const origPos = (v - 100) / 100; // 0, 1, or 2
-      const expectedLabel = ["a", "b", "c"][origPos];
+      const expectedLabel: string | null = ["a", "b", "c"][origPos] ?? null;
       expect(label).toBe(expectedLabel);
     }
   });