refactor: extract mpv render metrics runtime apply helper

src/core/services/mpv-render-metrics-service.test.ts

@@ -0,0 +1,37 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { MpvSubtitleRenderMetrics } from "../../types";
+import { applyMpvSubtitleRenderMetricsPatchService } from "./mpv-render-metrics-service";
+
+const BASE: MpvSubtitleRenderMetrics = {
+  subPos: 100,
+  subFontSize: 38,
+  subScale: 1,
+  subMarginY: 34,
+  subMarginX: 19,
+  subFont: "sans-serif",
+  subSpacing: 0,
+  subBold: false,
+  subItalic: false,
+  subBorderSize: 2.5,
+  subShadowOffset: 0,
+  subAssOverride: "yes",
+  subScaleByWindow: true,
+  subUseMargins: true,
+  osdHeight: 720,
+  osdDimensions: null,
+};
+
+test("applyMpvSubtitleRenderMetricsPatchService returns unchanged on empty patch", () => {
+  const { next, changed } = applyMpvSubtitleRenderMetricsPatchService(BASE, {});
+  assert.equal(changed, false);
+  assert.deepEqual(next, BASE);
+});
+
+test("applyMpvSubtitleRenderMetricsPatchService reports changed when patch modifies value", () => {
+  const { next, changed } = applyMpvSubtitleRenderMetricsPatchService(BASE, {
+    subPos: 95,
+  });
+  assert.equal(changed, true);
+  assert.equal(next.subPos, 95);
+});

src/core/services/mpv-render-metrics-service.ts

@@ -55,3 +55,28 @@ export function updateMpvSubtitleRenderMetricsService(
     osdDimensions: nextOsdDimensions,
   };
 }
+
+export function applyMpvSubtitleRenderMetricsPatchService(
+  current: MpvSubtitleRenderMetrics,
+  patch: Partial<MpvSubtitleRenderMetrics>,
+): { next: MpvSubtitleRenderMetrics; changed: boolean } {
+  const next = updateMpvSubtitleRenderMetricsService(current, patch);
+  const changed =
+    next.subPos !== current.subPos ||
+    next.subFontSize !== current.subFontSize ||
+    next.subScale !== current.subScale ||
+    next.subMarginY !== current.subMarginY ||
+    next.subMarginX !== current.subMarginX ||
+    next.subFont !== current.subFont ||
+    next.subSpacing !== current.subSpacing ||
+    next.subBold !== current.subBold ||
+    next.subItalic !== current.subItalic ||
+    next.subBorderSize !== current.subBorderSize ||
+    next.subShadowOffset !== current.subShadowOffset ||
+    next.subAssOverride !== current.subAssOverride ||
+    next.subScaleByWindow !== current.subScaleByWindow ||
+    next.subUseMargins !== current.subUseMargins ||
+    next.osdHeight !== current.osdHeight ||
+    JSON.stringify(next.osdDimensions) !== JSON.stringify(current.osdDimensions);
+  return { next, changed };
+}