Refactor: extract getHighlightTheme helper function

Consolidate repeated theme resolution logic into a single exported function.
Three functions now use this helper instead of duplicating the fallback chain.

- Add getHighlightTheme() with syntax-highlighting > highlight-style > default
- Simplify readHighlightingTheme, hasAdaptiveTheme, hasTextHighlighting
- Add 4 unit tests for getHighlightTheme (18 total)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: cderv

src/quarto-core/text-highlighting.ts

@@ -22,6 +22,15 @@ export interface ThemeDescriptor {
 const kDarkSuffix = "dark";
 const kLightSuffix = "light";
 
+// Resolve highlight theme from syntax-highlighting (new) or highlight-style (deprecated)
+export function getHighlightTheme(
+  pandoc: FormatPandoc,
+): string | Record<string, string> {
+  return pandoc[kSyntaxHighlighting] ||
+    pandoc[kHighlightStyle] ||
+    kDefaultHighlightStyle;
+}
+
 export function textHighlightThemePath(
   inputDir: string,
   theme: string | Record<string, string>,
@@ -69,39 +78,23 @@ export function readHighlightingTheme(
   pandoc: FormatPandoc,
   style: "dark" | "light" | "default",
 ): ThemeDescriptor | undefined {
-  // Check both syntax-highlighting (new) and highlight-style (deprecated alias)
-  const theme = pandoc[kSyntaxHighlighting] ||
-    pandoc[kHighlightStyle] ||
-    kDefaultHighlightStyle;
-  if (theme) {
-    const themeRaw = readTheme(inputDir, theme, style);
-    if (themeRaw) {
-      return {
-        json: JSON.parse(themeRaw),
-        isAdaptive: isAdaptiveTheme(theme),
-      };
-    } else {
-      return undefined;
-    }
-  } else {
-    return undefined;
+  const theme = getHighlightTheme(pandoc);
+  const themeRaw = readTheme(inputDir, theme, style);
+  if (themeRaw) {
+    return {
+      json: JSON.parse(themeRaw),
+      isAdaptive: isAdaptiveTheme(theme),
+    };
   }
+  return undefined;
 }
 
 export function hasAdaptiveTheme(pandoc: FormatPandoc) {
-  // Check both syntax-highlighting (new) and highlight-style (deprecated alias)
-  const theme = pandoc[kSyntaxHighlighting] ||
-    pandoc[kHighlightStyle] ||
-    kDefaultHighlightStyle;
-  return theme && isAdaptiveTheme(theme);
+  return isAdaptiveTheme(getHighlightTheme(pandoc));
 }
 
 export function hasTextHighlighting(pandoc: FormatPandoc): boolean {
-  // Check both syntax-highlighting (new) and highlight-style (deprecated alias)
-  const theme = pandoc[kSyntaxHighlighting] ||
-    pandoc[kHighlightStyle] ||
-    kDefaultHighlightStyle;
-  return theme !== "none";
+  return getHighlightTheme(pandoc) !== "none";
 }
 
 export function isAdaptiveTheme(theme: string | Record<string, string>) {

tests/unit/text-highlighting.test.ts

@@ -7,12 +7,44 @@
 import { unitTest } from "../test.ts";
 import { assertEquals } from "testing/asserts";
 import {
+  getHighlightTheme,
   hasAdaptiveTheme,
   hasTextHighlighting,
   isAdaptiveTheme,
 } from "../../src/quarto-core/text-highlighting.ts";
 import { FormatPandoc } from "../../src/config/types.ts";
 
+// getHighlightTheme tests
+
+// deno-lint-ignore require-await
+unitTest("getHighlightTheme - returns default when no options set", async () => {
+  const pandoc: FormatPandoc = {};
+  assertEquals(getHighlightTheme(pandoc), "arrow");
+});
+
+// deno-lint-ignore require-await
+unitTest("getHighlightTheme - returns syntax-highlighting when set", async () => {
+  const pandoc: FormatPandoc = { "syntax-highlighting": "github" };
+  assertEquals(getHighlightTheme(pandoc), "github");
+});
+
+// deno-lint-ignore require-await
+unitTest("getHighlightTheme - returns highlight-style as fallback", async () => {
+  const pandoc: FormatPandoc = { "highlight-style": "monokai" };
+  assertEquals(getHighlightTheme(pandoc), "monokai");
+});
+
+// deno-lint-ignore require-await
+unitTest("getHighlightTheme - syntax-highlighting takes precedence", async () => {
+  const pandoc: FormatPandoc = {
+    "syntax-highlighting": "github",
+    "highlight-style": "monokai",
+  };
+  assertEquals(getHighlightTheme(pandoc), "github");
+});
+
+// hasTextHighlighting tests
+
 // deno-lint-ignore require-await
 unitTest("text-highlighting - returns true when no options set (default applies)", async () => {
   const pandoc: FormatPandoc = {};