perf: byte to hex string conversion optimizations

android/src/main/java/com/capacitorjs/community/plugins/bluetoothle/Conversion.kt

@@ -1,13 +1,28 @@
 package com.capacitorjs.community.plugins.bluetoothle
 
+// Create a LUT for high performance ByteArray conversion
+val HEX_LOOKUP_TABLE = IntArray(256) {
+    val hexChars = "0123456789ABCDEF"
+    val h: Int = (hexChars[(it shr 4)].code shl 8)
+    val l: Int = hexChars[(it and 0x0F)].code
+    (h or l)
+}
 
-fun bytesToString(bytes: ByteArray): String {
-    val stringBuilder = StringBuilder(bytes.size)
-    for (byte in bytes) {
-        // byte to hex string
-        stringBuilder.append(String.format("%02X ", byte))
+// Custom implementation of ByteArray.toHexString until stdlib stabilizes
+private fun ByteArray.toHexString(): String {
+    val result = CharArray(this.size * 2);
+    var i = 0;
+    for (byte in this) {
+        val hx = HEX_LOOKUP_TABLE[byte.toInt() and 0xFF]
+        result[i]   = (hx shr 8).toChar()
+        result[i+1] = (hx and 0xFF).toChar()
+        i+=2
     }
-    return stringBuilder.toString()
+    return result.concatToString()
+}
+
+fun bytesToString(bytes: ByteArray): String {
+    return bytes.toHexString()
 }
 
 fun stringToBytes(value: String): ByteArray {

android/src/test/java/com/capacitorjs/community/plugins/bluetoothle/ConversionKtTest.kt

@@ -9,7 +9,7 @@ class ConversionKtTest : TestCase() {
     fun testBytesToString() {
         val input = byteArrayOfInts(0xA1, 0x2E, 0x38, 0xD4, 0x89, 0xC3)
         val output = bytesToString(input)
-        assertEquals("A1 2E 38 D4 89 C3 ", output)
+        assertEquals("A12E38D489C3", output)
     }
 
     fun testEmptyBytesToString() {

ios/Plugin/Conversion.swift

@@ -15,12 +15,36 @@ func descriptorValueToString(_ value: Any) -> String {
     return ""
 }
 
-func dataToString(_ data: Data) -> String {
-    var valueString = ""
-    for byte in data {
-        valueString += String(format: "%02hhx ", byte)
+extension Data {
+    func toHexString() -> String {
+        let hexChars = Array("0123456789abcdef".utf8)
+        if #available(iOS 14, *) {
+            return String(unsafeUninitializedCapacity: self.count*2) { (ptr) -> Int in
+                var strp = ptr.baseAddress!
+                for byte in self {
+                    strp[0] = hexChars[Int(byte >> 4)]
+                    strp[1] = hexChars[Int(byte & 0xF)]
+                    strp += 2
+                }
+                return 2 * self.count
+            }
+        } else {
+            // Fallback implementation for iOS < 14, a bit slower
+            var result = ""
+            result.reserveCapacity(self.count * 2)
+            for byte in self {
+                let high = Int(byte >> 4)
+                let low  = Int(byte & 0xF)
+                result.append(Character(UnicodeScalar(hexChars[high])))
+                result.append(Character(UnicodeScalar(hexChars[low])))
+            }
+            return result
+        }
     }
-    return valueString
+}
+
+func dataToString(_ data: Data) -> String {
+    return data.toHexString()
 }
 
 func stringToData(_ dataString: String) -> Data {

ios/PluginTests/ConversionTests.swift

@@ -8,7 +8,7 @@ class ConversionTests: XCTestCase {
     func testDataToString() throws {
         let input = Data([0xA1, 0x2E, 0x38, 0xD4, 0x89, 0xC3])
         let output = dataToString(input)
-        XCTAssertEqual(output, "a1 2e 38 d4 89 c3 ")
+        XCTAssertEqual(output, "a12e38d489c3")
     }
 
     func testStringToData() throws {
@@ -44,11 +44,11 @@ class ConversionTests: XCTestCase {
     }
 
     func testDescriptorValueToString() throws {
-        XCTAssertEqual(descriptorValueToString("Hello"), "48 65 6c 6c 6f ")
-        XCTAssertEqual(descriptorValueToString(Data([0, 5, 255])), "00 05 ff ")
-        XCTAssertEqual(descriptorValueToString(UInt16(258)), "02 01 ")
-        XCTAssertEqual(descriptorValueToString(UInt16(1)), "01 00 ")
-        XCTAssertEqual(descriptorValueToString(NSNumber(1)), "01 00 ")
+        XCTAssertEqual(descriptorValueToString("Hello"), "48656c6c6f")
+        XCTAssertEqual(descriptorValueToString(Data([0, 5, 255])), "0005ff")
+        XCTAssertEqual(descriptorValueToString(UInt16(258)), "0201")
+        XCTAssertEqual(descriptorValueToString(UInt16(1)), "0100")
+        XCTAssertEqual(descriptorValueToString(NSNumber(1)), "0100")
         XCTAssertEqual(descriptorValueToString(0), "")
     }
 

src/conversion.spec.ts

@@ -114,6 +114,15 @@ describe('hexStringToDataView', () => {
     expect(result.getUint8(2)).toEqual(200);
   });
 
+  it('should work without spaces', () => {
+    const value = '0005C8';
+    const result = hexStringToDataView(value);
+    expect(result.byteLength).toEqual(3);
+    expect(result.getUint8(0)).toEqual(0);
+    expect(result.getUint8(1)).toEqual(5);
+    expect(result.getUint8(2)).toEqual(200);
+  });
+
   it('should convert an empty hex string to a DataView', () => {
     const value = '';
     const result = hexStringToDataView(value);

src/conversion.ts

@@ -35,13 +35,28 @@ export function numberToUUID(value: number): string {
   return `0000${value.toString(16).padStart(4, '0')}-0000-1000-8000-00805f9b34fb`;
 }
 
-export function hexStringToDataView(value: string): DataView {
-  const numbers: number[] = value
-    .trim()
-    .split(' ')
-    .filter((e) => e !== '')
-    .map((s) => parseInt(s, 16));
-  return numbersToDataView(numbers);
+/**
+ * Convert a string of hex into a DataView of raw bytes.
+ * Note: characters other than [0-9a-fA-F] are ignored
+ * @param hex string of values, e.g. "00 01 02" or "000102"
+ * @return DataView of raw bytes
+ */
+export function hexStringToDataView(hex: string): DataView {
+  const bin = [];
+  let i,
+    c,
+    isEmpty = 1,
+    buffer = 0;
+  for (i = 0; i < hex.length; i++) {
+    c = hex.charCodeAt(i);
+    if ((c > 47 && c < 58) || (c > 64 && c < 71) || (c > 96 && c < 103)) {
+      buffer = (buffer << 4) ^ ((c > 64 ? c + 9 : c) & 15);
+      if ((isEmpty ^= 1)) {
+        bin.push(buffer & 0xff);
+      }
+    }
+  }
+  return numbersToDataView(bin);
 }
 
 export function dataViewToHexString(value: DataView): string {