Show text line in error messages

Upsilon/Diagnostics.cs

@@ -83,7 +83,14 @@ namespace Upsilon
 
         public override string ToString()
         {
-            return $"{Message} at {Span.Start}\n{Diagnostics.ScriptString.GetSpan(Span)}";
+            var linePos = Diagnostics.ScriptString.GetLinePosition(Span.Start);
+            return $"{Message} at ({linePos.Line}, {linePos.Pos})\n{Diagnostics.ScriptString.GetSpan(Span)}";
+        }
+
+        public string GetDiagnosticPosition()
+        {
+            var linePos = Diagnostics.ScriptString.GetLinePosition(Span.Start);
+            return $"({linePos.Line},{linePos.Pos})";
         }
 
         public string BeforeError(int i = 5)

Upsilon/Text/SourceText.cs

@@ -5,10 +5,20 @@ namespace Upsilon.Text
     public class SourceText
     {
         private readonly string _text;
+        private readonly SourceTextLine[] _lines;
 
         public SourceText(string text)
         {
             _text = text;
+            var lines = text.Split('\n');
+            _lines = new SourceTextLine[lines.Length];
+            var linePos = 0;
+            for (var index = 0; index < lines.Length; index++)
+            {
+                var line = lines[index];
+                _lines[index] = new SourceTextLine(linePos, line.Length);
+                linePos += line.Length;
+            }
         }
 
         public string GetSpan(int start, int length)
@@ -24,6 +34,32 @@ namespace Upsilon.Text
             return _text.Substring(start, length);
         }
 
+        public (int Line, int Pos) GetLinePosition(int spanPos)
+        {
+            var min = 0;
+            var max = _lines.Length - 1;
+            while (min <= max)
+            {
+                var mid = (min + max) / 2 ;
+                var midLine = _lines[mid];
+                if (midLine.Start <= spanPos && midLine.End > spanPos)
+                {
+                    var pos = spanPos - midLine.Start;
+                    return (mid, pos);
+                }
+                if (spanPos >= midLine.End)
+                {
+                    min = mid + 1;
+                }
+                else if (spanPos < midLine.Start)
+                {
+                    max = mid - 1;
+                }
+            }
+            var newPos = spanPos - _lines[min].Start;
+            return (min, newPos);
+        }
+
         public string GetSpan(TextSpan span)
         {
             return GetSpan(span.Start, span.Length);

Upsilon/Text/SourceTextLine.cs

@@ -0,0 +1,15 @@
+namespace Upsilon.Text
+{
+    public struct SourceTextLine
+    {
+        public SourceTextLine(int start, int lineLength)
+        {
+            Start = start;
+            LineLength = lineLength;
+        }
+
+        public int Start { get; }
+        public int LineLength { get; }
+        public int End => Start + LineLength;
+    }
+}

Ycicle/Program.cs

@@ -65,6 +65,7 @@ namespace Ycicle
             Console.WriteLine(message.Message);
 
             Console.ForegroundColor = ConsoleColor.Gray;
+            Console.Write(message.GetDiagnosticPosition() + " ");
             Console.Write(message.BeforeError());
 
             Console.ForegroundColor = ConsoleColor.Red;