feat: 正则词法识别器 (#1)

Reviewed-on: https://git.bupt-hpc.cn/jackfiled/CanonSharp/pulls/1
Co-authored-by: jackfiled <xcrenchangjun@outlook.com>
Co-committed-by: jackfiled <xcrenchangjun@outlook.com>

CanonSharp.Common/Reader/SourceReader.cs

@@ -0,0 +1,78 @@
+using CanonSharp.Common.Abstractions;
+
+namespace CanonSharp.Common.Reader;
+
+public class SourceReader : ISourceReader
+{
+    private readonly StreamReader _reader;
+    private char? _lookAhead;
+
+    public SourceReader(string filename)
+    {
+        FileInfo source = new(filename);
+
+        if (!source.Exists)
+        {
+            throw new InvalidOperationException();
+        }
+
+        _reader = new StreamReader(filename);
+    }
+
+    public char Read()
+    {
+        if (_lookAhead.HasValue)
+        {
+            char result = _lookAhead.Value;
+            _lookAhead = null;
+            return result;
+        }
+
+        if (!TryFetchChar(out char c))
+        {
+            throw new InvalidOperationException();
+        }
+
+        return c;
+    }
+
+    public bool TryPeek(out char c)
+    {
+        if (_lookAhead.HasValue)
+        {
+            c = _lookAhead.Value;
+            return true;
+        }
+
+        if (!TryFetchChar(out c))
+        {
+            return false;
+        }
+
+        _lookAhead = c;
+        return true;
+    }
+
+    private readonly char[] _buffer = new char[1024];
+    private int _length;
+    private int _count;
+
+    private bool TryFetchChar(out char c)
+    {
+        if (_length == _count)
+        {
+            _length = _reader.Read(_buffer);
+            _count = 0;
+        }
+
+        if (_length == 0)
+        {
+            c = char.MinValue;
+            return false;
+        }
+
+        c = _buffer[_count];
+        _count += 1;
+        return true;
+    }
+}

CanonSharp.Common/Reader/StringReader.cs

@@ -0,0 +1,33 @@
+using CanonSharp.Common.Abstractions;
+
+namespace CanonSharp.Common.Reader;
+
+public class StringReader(string source) : ISourceReader
+{
+    private int _pos;
+
+    public char Read()
+    {
+        if (_pos >= source.Length)
+        {
+            throw new InvalidOperationException();
+        }
+
+        char result = source[_pos];
+        _pos += 1;
+        return result;
+
+    }
+
+    public bool TryPeek(out char c)
+    {
+        if (_pos < source.Length)
+        {
+            c = source[_pos];
+            return true;
+        }
+
+        c = char.MinValue;
+        return false;
+    }
+}