Add a pure, unit-tested buildArgs() module

Extracts yt-dlp argv construction out of download.ts into its own
electron-free module (src/main/buildArgs.ts) so it can be exercised by
vitest without spinning up Electron, plus a real-download integration
test (skipped by default; opt in with AEROFETCH_REAL_DOWNLOAD=1) that
spawns the bundled yt-dlp.exe/ffmpeg.exe against its argv. download.ts
isn't wired up to it yet — that lands with the next commit.

This was done in an earlier local session but never actually committed,
even though download.ts has imported from it since the Phase A commit
(bb5dd6c) — the build had been relying on an uncommitted local file.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main/buildArgs.ts

@@ -0,0 +1,156 @@
+/**
+ * Pure yt-dlp argv construction for a download.
+ *
+ * This module is deliberately free of any electron / node-runtime dependency
+ * (it only imports types + the BEST_FORMAT_ID const from the shared contract)
+ * so the argv it produces can be unit-tested without spinning up Electron.
+ * `buildArgs` takes the ffmpeg/yt-dlp `binDir` as a parameter rather than
+ * resolving it itself; the caller in download.ts passes `getBinDir()`.
+ */
+
+import {
+  BEST_FORMAT_ID,
+  type StartDownloadOptions,
+  type DownloadOptions
+} from '@shared/ipc'
+
+// --- Quality → yt-dlp selector mapping --------------------------------------
+
+function videoFormat(quality: string): string {
+  switch (quality) {
+    case '1080p':
+      return 'bv*[height<=1080]+ba/b[height<=1080]'
+    case '720p':
+      return 'bv*[height<=720]+ba/b[height<=720]'
+    case '480p':
+      return 'bv*[height<=480]+ba/b[height<=480]'
+    case '360p':
+      return 'bv*[height<=360]+ba/b[height<=360]'
+    default:
+      return 'bv*+ba/b' // Best available
+  }
+}
+
+/**
+ * The video format selector. A specific probed format_id wins; otherwise fall
+ * back to the height-based preset. Video-only formats get +bestaudio so the
+ * merge produces a file with sound (with a video-only fallback if no audio).
+ */
+function videoSelector(opts: StartDownloadOptions): string {
+  if (opts.formatId && opts.formatId !== BEST_FORMAT_ID) {
+    return opts.formatHasAudio
+      ? opts.formatId
+      : `${opts.formatId}+bestaudio/${opts.formatId}`
+  }
+  return videoFormat(opts.quality)
+}
+
+function audioQuality(quality: string): string {
+  switch (quality) {
+    case '320 kbps':
+      return '320K'
+    case '192 kbps':
+      return '192K'
+    case '128 kbps':
+      return '128K'
+    default:
+      return '0' // Best
+  }
+}
+
+// The progress line yt-dlp emits (one per --newline tick). Note the leading
+// `download:` is the progress-template TYPE selector and is consumed by yt-dlp
+// (it does NOT appear in the output). The literal `prog|` that follows is our
+// own marker, so we can tell progress lines apart from the after-move filepath
+// print on the same stdout stream.
+export const PROGRESS_TEMPLATE =
+  'download:prog|%(progress.status)s|%(progress.downloaded_bytes)s|' +
+  '%(progress.total_bytes)s|%(progress.total_bytes_estimate)s|' +
+  '%(progress.speed)s|%(progress.eta)s'
+
+// Crop embedded audio artwork to a centred square. The double-quote/single-quote
+// dance is deliberate: yt-dlp shlex-splits the `--ppa` value, consuming the outer
+// double quotes and leaving ffmpeg single-quoted crop expressions whose commas are
+// protected from ffmpeg's filtergraph separator. Side = min(width, height).
+export const CROP_SQUARE_PPA =
+  'EmbedThumbnail+ffmpeg_o:-c:v mjpeg -vf ' +
+  'crop="\'if(gt(ih,iw),iw,ih)\':\'if(gt(iw,ih),ih,iw)\'"'
+
+/** Subtitle / SponsorBlock / chapter / metadata flags shared by both kinds. */
+function postProcessArgs(opts: StartDownloadOptions, o: DownloadOptions): string[] {
+  const args: string[] = []
+
+  // Subtitles — video only; audio extraction has nowhere to embed them.
+  if (opts.kind === 'video' && o.embedSubtitles) {
+    args.push('--write-subs', '--embed-subs', '--sub-langs', o.subtitleLanguages || 'en')
+    if (o.autoSubtitles) args.push('--write-auto-subs')
+    // Normalise to srt first so embedding works across containers (mp4 → mov_text).
+    args.push('--convert-subs', 'srt')
+  }
+
+  // SponsorBlock.
+  if (o.sponsorBlock && o.sponsorBlockCategories.length > 0) {
+    const cats = o.sponsorBlockCategories.join(',')
+    if (o.sponsorBlockMode === 'remove') {
+      // Force keyframes at the cut points so segment boundaries are frame-accurate.
+      args.push('--sponsorblock-remove', cats, '--force-keyframes-at-cuts')
+    } else {
+      args.push('--sponsorblock-mark', cats)
+    }
+  }
+
+  if (o.embedChapters) args.push('--embed-chapters')
+  if (o.embedMetadata) args.push('--embed-metadata')
+
+  return args
+}
+
+export function buildArgs(
+  opts: StartDownloadOptions,
+  outputTemplate: string,
+  o: DownloadOptions,
+  binDir: string
+): string[] {
+  const args = [
+    '--newline',
+    '--no-color',
+    '--no-playlist',
+    // --print (below) implies --quiet, which would suppress progress; --progress
+    // forces the progress template to emit anyway.
+    '--progress',
+    '--ffmpeg-location',
+    binDir,
+    '-o',
+    outputTemplate,
+    '--progress-template',
+    PROGRESS_TEMPLATE,
+    // Print the final path after post-processing/move so we can open it later.
+    '--print',
+    'after_move:path|%(filepath)s',
+    '--no-simulate'
+  ]
+
+  args.push(...postProcessArgs(opts, o))
+
+  if (opts.kind === 'audio') {
+    args.push('-x', '--audio-format', o.audioFormat, '--audio-quality', audioQuality(opts.quality))
+    if (o.embedThumbnail) {
+      args.push('--embed-thumbnail')
+      if (o.cropThumbnail) args.push('--ppa', CROP_SQUARE_PPA)
+    }
+  } else {
+    args.push('-f', videoSelector(opts), '--merge-output-format', o.videoContainer)
+    // Codec preference is a sort tiebreaker AFTER resolution/fps, so it nudges the
+    // pick toward the chosen codec without overriding the requested quality.
+    if (o.preferredVideoCodec !== 'any') {
+      const token = o.preferredVideoCodec === 'av1' ? 'av01' : o.preferredVideoCodec
+      args.push('-S', `res,fps,vcodec:${token}`)
+    }
+    // mp4/mkv carry a cover image; webm has no reliable way to embed one.
+    if (o.embedThumbnail && o.videoContainer !== 'webm') args.push('--embed-thumbnail')
+  }
+
+  // `--` terminates option parsing so the URL can never be read as a flag.
+  args.push('--', opts.url)
+  return args
+}