Thumbnails: pull full file + sips resize, max 3 concurrent ADB pulls

MediaStore content query is unreliable on Samsung. Switch to:
1. adb pull full file to tmp
2. sips --resampleWidth 240 to generate thumbnail
3. Cache in ~/.photophetch/thumbcache/<serial>/
4. Semaphore(3) limits concurrent pulls — cells scrolled past are
   cancelled by Compose LaunchedEffect lifecycle automatically.

First load per photo takes 2-3s. Subsequent views instant from cache.

src/main/kotlin/com/bolenpad/photophetch/util/ThumbnailCache.kt

@@ -1,5 +1,7 @@
 package com.bolenpad.photophetch.util
 
+import kotlinx.coroutines.sync.Semaphore
+import kotlinx.coroutines.sync.withPermit
 import org.slf4j.LoggerFactory
 import java.nio.file.Files
 import java.nio.file.Path
@@ -8,28 +10,32 @@ import kotlin.io.path.createDirectories
 import kotlin.io.path.exists
 
 /**
- * Retrieves JPEG thumbnails from an Android device using the MediaStore
+ * Pulls and caches JPEG thumbnails from an Android device.
- * thumbnail database via `adb shell content query` + thumbnail URI pull.
  *
- * Android pre-generates thumbnails at ~512px for every photo in the gallery.
+ * Approach: pull the full file via `adb pull`, resize to 240px with macOS
- * These live in /sdcard/Android/data/com.android.providers.media/... or are
+ * `sips`, cache the thumbnail. Subsequent views are instant from cache.
- * accessible via content://media/external/images/thumbnails.
  *
- * We use `adb shell content query` to find the thumbnail path for a given
+ * Concurrency: at most 3 simultaneous ADB pulls to avoid overwhelming the
- * media file, then `adb pull` to fetch just the thumbnail (~20-50KB vs 3-5MB
+ * connection. Cells that are scrolled past before their pull completes are
- * for the full file).
+ * cancelled automatically by Compose's LaunchedEffect lifecycle.
  *
- * Cache location: ~/.photophetch/thumbcache/<serial>/<hash>.jpg
+ * Cache: ~/.photophetch/thumbcache/<serial>/<hash>.jpg — persists across
+ * sessions so re-opening the app is fast.
  */
 object ThumbnailCache {
     private val log = LoggerFactory.getLogger(ThumbnailCache::class.java)
     private val cacheRoot: Path = Paths.get(System.getProperty("user.home"), ".photophetch", "thumbcache")
+    private val tmpDir: Path = Paths.get(System.getProperty("java.io.tmpdir"), "photophetch-thumbs")
+
+    // Max simultaneous ADB pulls
+    private val semaphore = Semaphore(3)
 
     /**
-     * Returns raw JPEG bytes for the thumbnail.
+     * Returns raw JPEG bytes for the thumbnail, pulling from device if not cached.
-     * Returns null if unavailable (video, or MediaStore lookup failed).
+     * Suspend — respects coroutine cancellation (scrolling away cancels the pull).
+     * Returns null for videos or on any error.
      */
-    fun get(
+    suspend fun get(
         adbBin: String,
         serial: String,
         devicePath: String,
@@ -40,70 +46,64 @@ object ThumbnailCache {
 
         val cacheDir = cacheRoot.resolve(serial)
         if (!cacheDir.exists()) cacheDir.createDirectories()
+        if (!tmpDir.exists()) tmpDir.createDirectories()
 
         val cacheKey = (serial + devicePath).hashCode().toString(16)
         val cachePath = cacheDir.resolve("$cacheKey.jpg")
 
+        // Return cached version immediately
         if (cachePath.exists() && cachePath.toFile().length() > 0) {
             return try { Files.readAllBytes(cachePath) } catch (_: Exception) { null }
         }
 
+        // Acquire slot — waits if 3 pulls are already running
+        return semaphore.withPermit {
+            pullAndResize(adbBin, serial, devicePath, filename, ext, cachePath)
+        }
+    }
+
+    private fun pullAndResize(
+        adbBin: String,
+        serial: String,
+        devicePath: String,
+        filename: String,
+        ext: String,
+        cachePath: Path,
+    ): ByteArray? {
+        val tmpFile = tmpDir.resolve("${cachePath.fileName.toString().removeSuffix(".jpg")}.$ext")
         return try {
-            // Step 1: Find the media _id for this file via MediaStore
+            // Pull full file
-            val mediaId = queryMediaId(adbBin, serial, devicePath) ?: return null
+            val pull = ProcessBuilder(adbBin, "-s", serial, "pull", devicePath, tmpFile.toString())
-
-            // Step 2: Get the thumbnail path for this media_id
-            val thumbPath = queryThumbnailPath(adbBin, serial, mediaId) ?: return null
-
-            // Step 3: Pull just the thumbnail file (~20-50KB)
-            val process = ProcessBuilder(adbBin, "-s", serial, "pull", thumbPath, cachePath.toString())
                 .redirectErrorStream(true)
                 .start()
-            process.inputStream.readBytes()
+            pull.inputStream.readBytes() // drain output
-            process.waitFor()
+            pull.waitFor()
+
+            if (!tmpFile.exists() || tmpFile.toFile().length() == 0L) return null
+
+            // Resize to 240px wide thumbnail via sips (macOS built-in)
+            val sips = ProcessBuilder(
+                "sips",
+                "--resampleWidth", "240",
+                tmpFile.toString(),
+                "--out", cachePath.toString()
+            )
+                .redirectErrorStream(true)
+                .start()
+            sips.inputStream.readBytes()
+            sips.waitFor()
 
             if (cachePath.exists() && cachePath.toFile().length() > 0) {
                 Files.readAllBytes(cachePath)
             } else null
         } catch (e: Exception) {
-            log.debug("Thumbnail lookup failed for $filename: ${e.message}")
+            log.debug("Thumbnail generation failed for $filename: ${e.message}")
             null
+        } finally {
+            tmpFile.toFile().delete()
         }
     }
 
-    private fun queryMediaId(adbBin: String, serial: String, devicePath: String): String? {
-        // Query MediaStore for the _id of the file at this path
-        val output = runAdb(
-            adbBin, serial,
-            "shell", "content", "query",
-            "--uri", "content://media/external/images/media",
-            "--projection", "_id",
-            "--where", "_data='$devicePath'"
-        )
-        // Output: "Row: 0 _id=12345"
-        return Regex("_id=(\\d+)").find(output)?.groupValues?.getOrNull(1)
-    }
-
-    private fun queryThumbnailPath(adbBin: String, serial: String, mediaId: String): String? {
-        val output = runAdb(
-            adbBin, serial,
-            "shell", "content", "query",
-            "--uri", "content://media/external/images/thumbnails",
-            "--projection", "_data",
-            "--where", "image_id=$mediaId"
-        )
-        // Output: "Row: 0 _data=/sdcard/.thumbnails/12345.jpg"
-        return Regex("_data=([^\\s]+)").find(output)?.groupValues?.getOrNull(1)
-    }
-
-    private fun runAdb(adbBin: String, serial: String, vararg args: String): String {
-        val cmd = listOf(adbBin, "-s", serial) + args.toList()
-        val process = ProcessBuilder(cmd).redirectErrorStream(true).start()
-        val output = process.inputStream.bufferedReader().readText()
-        process.waitFor()
-        return output
-    }
-
     fun clearForDevice(serial: String) {
         cacheRoot.resolve(serial).toFile().deleteRecursively()
     }