Updater: pick the release asset that matches this build

A release now carries four APKs rather than one, so the updater has to choose
the asset built for the device it is running on instead of taking the first it
finds. Matches on the variant suffix in the asset name and falls through to the
next release rather than giving up when one has no usable asset.

android/armsx3-ui/app/src/main/java/com/armsx2/update/UpdaterEntry.kt

@@ -2,6 +2,7 @@ package com.armsx2.update
 
 import android.content.Context
 import android.content.Intent
+import android.os.Build
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.Row
@@ -284,7 +285,7 @@ private suspend fun checkForUpdate(includeNightly: Boolean, checkFailedPrefix: S
             // never prompt it — and never offer it a stable (that would be a versionCode downgrade).
             if (BuildConfig.VERSION_CODE > NIGHTLY_VC_THRESHOLD) return@withContext UpdateState.UpToDate
             val obj = JSONObject(httpGet(LATEST_URL))
-            val apkUrl = firstApkAsset(obj) ?: return@withContext UpdateState.UpToDate
+            val apkUrl = pickApkAsset(obj) ?: return@withContext UpdateState.UpToDate
             val tag = obj.getString("tag_name")
             return@withContext if (isNewer(tag, BuildConfig.VERSION_NAME))
                 UpdateState.Available(tag, obj.optString("body", ""), apkUrl)
@@ -302,7 +303,7 @@ private suspend fun checkForUpdate(includeNightly: Boolean, checkFailedPrefix: S
         for (i in 0 until arr.length()) {
             val rel = arr.getJSONObject(i)
             if (rel.optBoolean("draft", false)) continue
-            val apkUrl = firstApkAsset(rel) ?: continue
+            val apkUrl = pickApkAsset(rel) ?: continue
             val tag = rel.getString("tag_name")
             val isNightlyRel = rel.optBoolean("prerelease", false) || tag.startsWith("nightly-", ignoreCase = true)
             val newer = if (isNightlyRel) {
@@ -318,15 +319,83 @@ private suspend fun checkForUpdate(includeNightly: Boolean, checkFailedPrefix: S
     }
 }
 
-/** First `.apk` asset download URL in a release JSON object, or null if it has none. */
-private fun firstApkAsset(release: JSONObject): String? {
+/**
+ * Does this CPU have the ARMv8.2 features the a13/a15 builds are compiled with?
+ *
+ * Those two are built -march=armv8.2-a+dotprod+fp16, which is a hard floor: the instructions can
+ * appear anywhere in the core and a device without them takes SIGILL rather than falling back.
+ * Handing such a device the wrong APK is a crash on launch with no explanation, so this is
+ * checked rather than inferred from the Android version -- a budget A53 part can and does ship
+ * with Android 13.
+ *
+ * /proc/cpuinfo is the portable read here: HWCAP needs NDK glue, and this runs once.
+ */
+private val hasArmv8_2Features: Boolean by lazy {
+    runCatching {
+        val features = File("/proc/cpuinfo").useLines { lines ->
+            lines.firstOrNull { it.startsWith("Features") }.orEmpty()
+        }
+        // asimddp = FEAT_DotProd, asimdhp = FEAT_FP16 (half-precision Advanced SIMD).
+        features.contains("asimddp") && features.contains("asimdhp")
+    }.getOrDefault(false)
+}
+
+/**
+ * Best `.apk` asset for THIS device, or null if the release has none.
+ *
+ * Releases carry four APKs that are alternatives, not an upgrade path (see
+ * android/build-variants.sh):
+ *
+ *   -a15-armv8.2-sdk35       API 35 + armv8.2   newest devices
+ *   -a13-armv8.2-sdk33       API 33 + armv8.2   the standard build
+ *   -a11-armv8.2-sdk30       API 30 + armv8.2
+ *   -legacy-armv8.1-sdk26    API 26 + armv8.1   fallback, no dotprod/FP16 required
+ *
+ * Picking the first asset the API happens to list would hand everyone the same file -- and
+ * alphabetically that is a11, which fails to install below Android 11 and faults outright on a
+ * pre-8.2 core. So walk the preference order and take the best one the device can actually run,
+ * falling back to any .apk so single-asset releases (everything before 0.7.2) still work.
+ *
+ * Both tests matter and neither implies the other. The SDK gate is what the installer would
+ * enforce anyway; the ISA gate is not enforced by anything, because a budget A53 part ships with
+ * a current Android and would take SIGILL on the first dotprod instruction. A device that passes
+ * the SDK test but fails the ISA test correctly falls all the way to legacy.
+ */
+private fun pickApkAsset(release: JSONObject): String? {
     val assets = release.optJSONArray("assets") ?: return null
-    for (i in 0 until assets.length()) {
-        val a = assets.getJSONObject(i)
-        if (a.getString("name").endsWith(".apk", ignoreCase = true))
-            return a.getString("browser_download_url")
+
+    val byName = buildMap {
+        for (i in 0 until assets.length()) {
+            val a = assets.getJSONObject(i)
+            val name = a.getString("name")
+            if (name.endsWith(".apk", ignoreCase = true)) put(name.lowercase(), a.getString("browser_download_url"))
+        }
     }
-    return null
+    if (byName.isEmpty()) return null
+
+    val sdk = Build.VERSION.SDK_INT
+    val preferred = buildList {
+        if (sdk >= 35 && hasArmv8_2Features) add("-a15-armv8.2-sdk35")
+        if (sdk >= 33 && hasArmv8_2Features) add("-a13-armv8.2-sdk33")
+        if (sdk >= 30 && hasArmv8_2Features) add("-a11-armv8.2-sdk30")
+        add("-legacy-armv8.1-sdk26")
+
+        // Pre-0.7.3 spellings, kept so an install from an older release still finds a variant
+        // by name instead of dropping to "first asset wins" below. Ordered after the current
+        // names so a release carrying both is read with the new scheme.
+        if (sdk >= 35 && hasArmv8_2Features) add("-a15")
+        if (sdk >= 33 && hasArmv8_2Features) add("-a13")
+        add("-generic")
+    }
+
+    for (suffix in preferred) {
+        byName.entries.firstOrNull { it.key.contains(suffix) }?.let { return it.value }
+    }
+
+    // No variant matched by name: an older single-APK release, or a naming change. Taking the
+    // only asset is right for the former; for the latter a wrong guess beats offering nothing,
+    // because the installer still refuses an APK whose minSdk this device fails.
+    return byName.values.firstOrNull()
 }
 
 /** "nightly-YYYYMMDD" -> YYYYMMDD as an int (0 if the tag isn't a dated nightly). */