Add helpers to truncate UTF-8 at code points

This will help to avoid truncating a UTF-8 string in the middle of a
code point, producing an invalid UTF-8 result.

server/src/main/java/com/genymobile/scrcpy/StringUtils.java

@@ -0,0 +1,23 @@
+package com.genymobile.scrcpy;
+
+public final class StringUtils {
+    private StringUtils() {
+        // not instantiable
+    }
+
+    @SuppressWarnings("checkstyle:MagicNumber")
+    public static int getUtf8TruncationIndex(byte[] utf8, int maxLength) {
+        int len = utf8.length;
+        if (len <= maxLength) {
+            return len;
+        }
+        len = maxLength;
+        // see UTF-8 encoding <https://en.wikipedia.org/wiki/UTF-8#Description>
+        while ((utf8[len] & 0x80) != 0 && (utf8[len] & 0xc0) != 0xc0) {
+            // the next byte is not the start of a new UTF-8 codepoint
+            // so if we would cut there, the character would be truncated
+            len--;
+        }
+        return len;
+    }
+}

server/src/test/java/com/genymobile/scrcpy/StringUtilsTest.java

@@ -0,0 +1,44 @@
+package com.genymobile.scrcpy;
+
+import junit.framework.Assert;
+
+import org.junit.Test;
+
+import java.nio.charset.StandardCharsets;
+
+public class StringUtilsTest {
+
+    @Test
+    @SuppressWarnings("checkstyle:MagicNumber")
+    public void testUtf8Trucate() {
+        String s = "aÉbÔc";
+        byte[] utf8 = s.getBytes(StandardCharsets.UTF_8);
+        Assert.assertEquals(7, utf8.length);
+
+        int count;
+
+        count = StringUtils.getUtf8TruncationIndex(utf8, 1);
+        Assert.assertEquals(1, count);
+
+        count = StringUtils.getUtf8TruncationIndex(utf8, 2);
+        Assert.assertEquals(1, count); // É is 2 bytes-wide
+
+        count = StringUtils.getUtf8TruncationIndex(utf8, 3);
+        Assert.assertEquals(3, count);
+
+        count = StringUtils.getUtf8TruncationIndex(utf8, 4);
+        Assert.assertEquals(4, count);
+
+        count = StringUtils.getUtf8TruncationIndex(utf8, 5);
+        Assert.assertEquals(4, count); // Ô is 2 bytes-wide
+
+        count = StringUtils.getUtf8TruncationIndex(utf8, 6);
+        Assert.assertEquals(6, count);
+
+        count = StringUtils.getUtf8TruncationIndex(utf8, 7);
+        Assert.assertEquals(7, count);
+
+        count = StringUtils.getUtf8TruncationIndex(utf8, 8);
+        Assert.assertEquals(7, count); // no more chars
+    }
+}