AnchorCli/Hashline/HashlineValidator.cs

namespace AnchorCli.Hashline;

/// <summary>
/// Validates and resolves Hashline anchors of the form "{lineNumber}:{hash}" against
/// the current state of a file's lines.
///
/// An anchor is valid if and only if BOTH conditions hold:
///   1. The referenced line number exists in the file.
///   2. The hash at that line number matches what the encoder would produce today.
///
/// This dual-check makes stale edits fail loudly rather than silently corrupting a
/// different line that happens to share the same hash.
/// </summary>
internal static class HashlineValidator
{
    /// <summary>
    /// Parses a "{lineNumber}:{hash}" anchor string.
    /// </summary>
    /// <returns>True and the parsed values if the format is valid; false otherwise.</returns>
    public static bool TryParseAnchor(string anchor, out int lineNumber, out string hash)
    {
        lineNumber = 0;
        hash = string.Empty;

        int colonIndex = anchor.IndexOf(':');
        if (colonIndex <= 0 || colonIndex == anchor.Length - 1)
            return false;

        ReadOnlySpan<char> numPart = anchor.AsSpan(0, colonIndex);
        ReadOnlySpan<char> hashPart = anchor.AsSpan(colonIndex + 1);

        if (!int.TryParse(numPart, out lineNumber) || lineNumber < 1)
            return false;

        hash = hashPart.ToString();
        return hash.Length == 2;
    }

    /// <summary>
    /// Resolves an anchor to a 0-based index into <paramref name="lines"/>,
    /// verifying both the line number and the content hash.
    /// </summary>
    /// <param name="anchor">Anchor string, e.g. "5:a3".</param>
    /// <param name="lines">Current file lines (without newlines).</param>
    /// <param name="zeroBasedIndex">Resolved 0-based index on success.</param>
    /// <param name="error">Human-readable error message on failure.</param>
    /// <returns>True if the anchor is valid and current; false otherwise.</returns>
    public static bool TryResolve(
        string anchor,
        string[] lines,
        out int zeroBasedIndex,
        out string error)
    {
        zeroBasedIndex = -1;
        error = string.Empty;

        if (!TryParseAnchor(anchor, out int lineNumber, out string expectedHash))
        {
            error = $"Anchor '{anchor}' is not in 'lineNumber:hash' format (e.g. '5:a3').";
            return false;
        }

        if (lineNumber < 1 || lineNumber > lines.Length)
        {
            error = $"Anchor '{anchor}': line {lineNumber} is out of range " +
                    $"(file has {lines.Length} line(s)).";
            return false;
        }

        string actualHash = HashlineEncoder.ComputeHash(lines[lineNumber - 1].AsSpan(), lineNumber);
        if (!string.Equals(actualHash, expectedHash, StringComparison.OrdinalIgnoreCase))
        {
            error = $"Anchor '{anchor}': hash mismatch at line {lineNumber} " +
                    $"(expected '{expectedHash}', got '{actualHash}'). " +
                    $"The file has changed — re-read before editing.";
            return false;
        }

        zeroBasedIndex = lineNumber - 1;
        return true;
    }

    /// <summary>
    /// Validates both a start and end anchor, and ensures start &lt;= end.
    /// </summary>
    /// <param name="startAnchor">The starting anchor string.</param>
    /// <param name="endAnchor">The ending anchor string.</param>
    /// <param name="lines">Current file lines (without newlines).</param>
    /// <param name="startIndex">Resolved 0-based start index on success.</param>
    /// <param name="endIndex">Resolved 0-based end index on success.</param>
    /// <param name="error">Human-readable error message on failure.</param>
    /// <returns>True if the range is valid; false otherwise.</returns>
    public static bool TryResolveRange(
        string startAnchor,
        string endAnchor,
        string[] lines,
        out int startIndex,
        out int endIndex,
        out string error)
    {
        startIndex = -1;
        endIndex = -1;
        error = string.Empty;

        if (!TryResolve(startAnchor, lines, out startIndex, out error))
            return false;

        if (!TryResolve(endAnchor, lines, out endIndex, out error))
            return false;

        if (startIndex > endIndex)
        {
            error = $"Range error: start anchor '{startAnchor}' (line {startIndex + 1}) " +
                    $"is after end anchor '{endAnchor}' (line {endIndex + 1}).";
            return false;
        }

        return true;
    }
}