refactor(engine): extract density filter from bar/column/dot/pie labels

The 'none' and 'endpoints' density branches were byte-identical across
four label modules (~3 lines each). Consolidate into
`_shared/density-filter.ts` exposing `filterByDensity<T>(marks, density)`.

Line chart labels are intentionally left alone: their 'none' returns a
Map, and 'endpoints' semantically collapses to 'auto' (end-of-line
label per series), so there's no density filter to extract.

No behavior change: preserves the `marks.length > 1` guard for
single-element arrays and short-circuits 'none' before candidate
construction.

Author: rileyhilliard

packages/engine/src/charts/_shared/__tests__/density-filter.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, it } from 'vitest';
+import { filterByDensity } from '../density-filter';
+
+describe('filterByDensity', () => {
+  const marks = ['a', 'b', 'c', 'd'];
+
+  it("returns [] for 'none'", () => {
+    expect(filterByDensity(marks, 'none')).toEqual([]);
+  });
+
+  it("returns first + last for 'endpoints'", () => {
+    expect(filterByDensity(marks, 'endpoints')).toEqual(['a', 'd']);
+  });
+
+  it("returns marks unchanged for 'all'", () => {
+    expect(filterByDensity(marks, 'all')).toBe(marks);
+  });
+
+  it("returns marks unchanged for 'auto'", () => {
+    expect(filterByDensity(marks, 'auto')).toBe(marks);
+  });
+
+  it("returns single-element array unchanged for 'endpoints'", () => {
+    const single = ['only'];
+    expect(filterByDensity(single, 'endpoints')).toBe(single);
+  });
+
+  it("returns empty array unchanged for 'endpoints'", () => {
+    const empty: string[] = [];
+    expect(filterByDensity(empty, 'endpoints')).toBe(empty);
+  });
+});

packages/engine/src/charts/_shared/density-filter.ts

@@ -0,0 +1,26 @@
+/**
+ * Shared density filter for data labels.
+ *
+ * Maps a `LabelDensity` setting to the subset of marks eligible for
+ * labeling. The `'auto'` branch is a pass-through — auto resolution
+ * happens upstream in the per-chart label modules (typically via
+ * collision detection on the full candidate set).
+ */
+
+import type { LabelDensity } from '@opendata-ai/openchart-core';
+
+/**
+ * Filter a mark array by label density.
+ *
+ * - `'none'` returns `[]` (no labels)
+ * - `'endpoints'` returns first + last marks when `marks.length > 1`,
+ *   otherwise the input unchanged (preserves single-element arrays as-is)
+ * - `'all'` and `'auto'` return the input unchanged
+ */
+export function filterByDensity<T>(marks: T[], density: LabelDensity): T[] {
+  if (density === 'none') return [];
+  if (density === 'endpoints' && marks.length > 1) {
+    return [marks[0], marks[marks.length - 1]];
+  }
+  return marks;
+}

packages/engine/src/charts/bar/labels.ts

@@ -23,6 +23,7 @@ import {
   getRepresentativeColor,
   resolveCollisions,
 } from '@opendata-ai/openchart-core';
+import { filterByDensity } from '../_shared/density-filter';
 import { formatLabelValue } from '../_shared/format-label-value';
 
 // ---------------------------------------------------------------------------
@@ -93,12 +94,7 @@ export function computeBarLabels(
   labelPrefix?: string,
   valueField?: string,
 ): ResolvedLabel[] {
-  // 'none': no labels at all
-  if (density === 'none') return [];
-
-  // Filter marks for 'endpoints' density
-  const targetMarks =
-    density === 'endpoints' && marks.length > 1 ? [marks[0], marks[marks.length - 1]] : marks;
+  const targetMarks = filterByDensity(marks, density);
 
   const candidates: LabelCandidate[] = [];
   // Track whether each candidate fits within its stacked segment.

packages/engine/src/charts/column/labels.ts

@@ -23,6 +23,7 @@ import {
   getRepresentativeColor,
   resolveCollisions,
 } from '@opendata-ai/openchart-core';
+import { filterByDensity } from '../_shared/density-filter';
 import { formatLabelValue } from '../_shared/format-label-value';
 
 // ---------------------------------------------------------------------------
@@ -50,12 +51,7 @@ export function computeColumnLabels(
   labelPrefix?: string,
   valueField?: string,
 ): ResolvedLabel[] {
-  // 'none': no labels at all
-  if (density === 'none') return [];
-
-  // Filter marks for 'endpoints' density
-  const targetMarks =
-    density === 'endpoints' && marks.length > 1 ? [marks[0], marks[marks.length - 1]] : marks;
+  const targetMarks = filterByDensity(marks, density);
 
   const formatter = buildD3Formatter(labelFormat);
 

packages/engine/src/charts/dot/labels.ts

@@ -23,6 +23,7 @@ import {
   getRepresentativeColor,
   resolveCollisions,
 } from '@opendata-ai/openchart-core';
+import { filterByDensity } from '../_shared/density-filter';
 import { formatLabelValue } from '../_shared/format-label-value';
 
 // ---------------------------------------------------------------------------
@@ -50,12 +51,7 @@ export function computeDotLabels(
   labelFormat?: string,
   valueField?: string,
 ): ResolvedLabel[] {
-  // 'none': no labels at all
-  if (density === 'none') return [];
-
-  // Filter marks for 'endpoints' density
-  const targetMarks =
-    density === 'endpoints' && marks.length > 1 ? [marks[0], marks[marks.length - 1]] : marks;
+  const targetMarks = filterByDensity(marks, density);
 
   const formatter = buildD3Formatter(labelFormat);
   const candidates: LabelCandidate[] = [];

packages/engine/src/charts/pie/labels.ts

@@ -20,6 +20,7 @@ import type {
   ResolvedLabel,
 } from '@opendata-ai/openchart-core';
 import { estimateTextWidth, resolveCollisions } from '@opendata-ai/openchart-core';
+import { filterByDensity } from '../_shared/density-filter';
 
 // ---------------------------------------------------------------------------
 // Constants
@@ -48,16 +49,13 @@ export function computePieLabels(
 ): ResolvedLabel[] {
   if (marks.length === 0) return [];
 
-  // 'none': no labels at all
-  if (density === 'none') return [];
-
   // Get the pie center from the first mark's center property
+  // (read before filtering — 'endpoints' still needs the original center)
   const centerX = marks[0].center.x;
   const centerY = marks[0].center.y;
 
-  // Filter marks for 'endpoints' density
-  const targetMarks =
-    density === 'endpoints' && marks.length > 1 ? [marks[0], marks[marks.length - 1]] : marks;
+  const targetMarks = filterByDensity(marks, density);
+  if (targetMarks.length === 0) return [];
 
   const candidates: LabelCandidate[] = [];
   const targetMarkIndices: number[] = [];