initial commit

Hashline/HashlineEncoder.cs

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+namespace AnchorCli.Hashline;
+
+/// <summary>
+/// Encodes file lines with Hashline anchors in the format: {lineNumber}:{hash}|{content}
+///
+/// Hash algorithm: Adler-8 with a position seed.
+///   raw  = ((sum_of_bytes % 251) ^ (count_of_bytes % 251)) & 0xFF
+///   final = raw ^ (lineNumber & 0xFF)
+///
+/// The position seed ensures duplicate lines produce different tags,
+/// preventing ambiguous anchor resolution on large files.
+/// </summary>
+internal static class HashlineEncoder
+{
+    /// <summary>
+    /// Computes the 2-char hex Hashline tag for a single line.
+    /// </summary>
+    /// <param name="line">Raw line content (without newline).</param>
+    /// <param name="lineNumber">1-indexed line number within the file.</param>
+    public static string ComputeHash(ReadOnlySpan<char> line, int lineNumber)
+    {
+        int sum = 0;
+        int count = line.Length;
+
+        foreach (char c in line)
+        {
+            // Use the lower byte of each character — sufficient for source code
+            sum += c & 0xFF;
+        }
+
+        int raw = ((sum % 251) ^ (count % 251)) & 0xFF;
+        int final = raw ^ (lineNumber & 0xFF);
+
+        return final.ToString("x2");
+    }
+
+    /// <summary>
+    /// Encodes all lines with Hashline prefixes.
+    /// </summary>
+    /// <param name="lines">All lines of the file (without trailing newlines).</param>
+    /// <returns>A single string with each line prefixed as "{n}:{hash}|{content}\n".</returns>
+    public static string Encode(string[] lines) => Encode(lines, startLine: 1, endLine: lines.Length);
+
+    /// <summary>
+    /// Encodes a window of lines with Hashline prefixes.
+    /// Line numbers in the output reflect their true position in the file.
+    /// </summary>
+    /// <param name="lines">All lines of the file.</param>
+    /// <param name="startLine">1-indexed first line to include (inclusive).</param>
+    /// <param name="endLine">1-indexed last line to include (inclusive). Clamped to file length.</param>
+    public static string Encode(string[] lines, int startLine, int endLine)
+    {
+        if (lines.Length == 0) return string.Empty;
+
+        startLine = Math.Max(1, startLine);
+        endLine = Math.Min(lines.Length, endLine <= 0 ? lines.Length : endLine);
+
+        var sb = new System.Text.StringBuilder();
+
+        for (int i = startLine; i <= endLine; i++)
+        {
+            string line = lines[i - 1];
+            string hash = ComputeHash(line.AsSpan(), i);
+            sb.Append(i).Append(':').Append(hash).Append('|').AppendLine(line);
+        }
+
+        return sb.ToString();
+    }
+
+    /// <summary>
+    /// Computes a short file-level fingerprint: XOR of all per-line hashes (as bytes).
+    /// Useful for cheap full-file staleness checks.
+    /// </summary>
+    public static string FileFingerprint(string[] lines)
+    {
+        int fp = 0;
+        for (int i = 0; i < lines.Length; i++)
+        {
+            string hash = ComputeHash(lines[i].AsSpan(), i + 1);
+            fp ^= Convert.ToInt32(hash, 16);
+        }
+        return fp.ToString("x2");
+    }
+}