whisper.android : address ARM's big.LITTLE arch by checking cpu info (#1254)

Addresses https://github.com/ggerganov/whisper.cpp/issues/1248

examples/whisper.android/app/src/main/java/com/whispercppdemo/whisper/LibWhisper.kt

@@ -18,7 +18,9 @@ class WhisperContext private constructor(private var ptr: Long) {
 
     suspend fun transcribeData(data: FloatArray): String = withContext(scope.coroutineContext) {
         require(ptr != 0L)
-        WhisperLib.fullTranscribe(ptr, data)
+        val numThreads = WhisperCpuConfig.preferredThreadCount
+        Log.d(LOG_TAG, "Selecting $numThreads threads")
+        WhisperLib.fullTranscribe(ptr, numThreads, data)
         val textCount = WhisperLib.getTextSegmentCount(ptr)
         return@withContext buildString {
             for (i in 0 until textCount) {
@@ -126,7 +128,7 @@ private class WhisperLib {
         external fun initContextFromAsset(assetManager: AssetManager, assetPath: String): Long
         external fun initContext(modelPath: String): Long
         external fun freeContext(contextPtr: Long)
-        external fun fullTranscribe(contextPtr: Long, audioData: FloatArray)
+        external fun fullTranscribe(contextPtr: Long, numThreads: Int, audioData: FloatArray)
         external fun getTextSegmentCount(contextPtr: Long): Int
         external fun getTextSegment(contextPtr: Long, index: Int): String
         external fun getSystemInfo(): String

examples/whisper.android/app/src/main/java/com/whispercppdemo/whisper/WhisperCpuConfig.kt

@@ -0,0 +1,73 @@
+package com.whispercppdemo.whisper
+
+import android.util.Log
+import java.io.BufferedReader
+import java.io.FileReader
+
+object WhisperCpuConfig {
+    val preferredThreadCount: Int
+        // Always use at least 2 threads:
+        get() = CpuInfo.getHighPerfCpuCount().coerceAtLeast(2)
+}
+
+private class CpuInfo(private val lines: List<String>) {
+    private fun getHighPerfCpuCount(): Int = try {
+        getHighPerfCpuCountByFrequencies()
+    } catch (e: Exception) {
+        Log.d(LOG_TAG, "Couldn't read CPU frequencies", e)
+        getHighPerfCpuCountByVariant()
+    }
+
+    private fun getHighPerfCpuCountByFrequencies(): Int =
+        getCpuValues(property = "processor") { getMaxCpuFrequency(it.toInt()) }
+            .also { Log.d(LOG_TAG, "Binned cpu frequencies (frequency, count): ${it.binnedValues()}") }
+            .countDroppingMin()
+
+    private fun getHighPerfCpuCountByVariant(): Int =
+        getCpuValues(property = "CPU variant") { it.substringAfter("0x").toInt(radix = 16) }
+            .also { Log.d(LOG_TAG, "Binned cpu variants (variant, count): ${it.binnedValues()}") }
+            .countKeepingMin()
+
+    private fun List<Int>.binnedValues() = groupingBy { it }.eachCount()
+
+    private fun getCpuValues(property: String, mapper: (String) -> Int) = lines
+        .asSequence()
+        .filter { it.startsWith(property) }
+        .map { mapper(it.substringAfter(':').trim()) }
+        .sorted()
+        .toList()
+
+
+    private fun List<Int>.countDroppingMin(): Int {
+        val min = min()
+        return count { it > min }
+    }
+
+    private fun List<Int>.countKeepingMin(): Int {
+        val min = min()
+        return count { it == min }
+    }
+
+    companion object {
+        private const val LOG_TAG = "WhisperCpuConfig"
+
+        fun getHighPerfCpuCount(): Int = try {
+            readCpuInfo().getHighPerfCpuCount()
+        } catch (e: Exception) {
+            Log.d(LOG_TAG, "Couldn't read CPU info", e)
+            // Our best guess -- just return the # of CPUs minus 4.
+            (Runtime.getRuntime().availableProcessors() - 4).coerceAtLeast(0)
+        }
+
+        private fun readCpuInfo() = CpuInfo(
+            BufferedReader(FileReader("/proc/cpuinfo"))
+                .useLines { it.toList() }
+        )
+
+        private fun getMaxCpuFrequency(cpuIndex: Int): Int {
+            val path = "/sys/devices/system/cpu/cpu${cpuIndex}/cpufreq/cpuinfo_max_freq"
+            val maxFreq = BufferedReader(FileReader(path)).use { it.readLine() }
+            return maxFreq.toInt()
+        }
+    }
+}