init backend

dotnet/AipsCore/Utility/Text/Charset.cs

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+namespace AipsCore.Utility.Text;
+
+public static class Charset
+{
+    public static char[] GetNumericCharset()
+    {
+        var charset = new List<char>();
+
+        // 0–9
+        for (char c = '0'; c <= '9'; c++)
+            charset.Add(c);
+        
+        return charset.ToArray();
+    }
+
+    public static char[] GetLettersCharset()
+    {
+        
+        var charset = new List<char>();
+
+        // a–z
+        for (char c = 'a'; c <= 'z'; c++)
+            charset.Add(c);
+
+        // A–Z
+        for (char c = 'A'; c <= 'Z'; c++)
+            charset.Add(c);
+        
+        return charset.ToArray();
+    }
+    
+    public static char[] GetAlphanumericCharset()
+    {
+        var lettersCharset = GetLettersCharset();
+        var numericCharset = GetNumericCharset();
+
+        char[] alphanumericCharset = [..numericCharset, ..lettersCharset];
+
+        return alphanumericCharset;
+    }
+}