namespace DodoSSH.Client.App.Platform;

/// <summary>
/// Compares two RPM version strings the way <c>rpm</c> itself does.
/// </summary>
/// <remarks>
/// <para>
/// <b>Why this exists rather than a SemVer comparison.</b> <c>AndroidUpdateChannel</c> settles the same
/// question and its reasoning carries over exactly: the comparison has to be the one the installer will
/// make, because anything else can offer an update the platform then refuses. On the phone that number is
/// Android's <c>versionCode</c>; here it is the RPM version, and the authority that reads it is
/// <c>rpm</c> — so this is <c>rpmvercmp</c> and not an approximation of it.
/// </para>
/// <para>
/// It matters in a specific direction. The nightly channel is rolling and CI replaces it on every push, so
/// the published version normally only rises — but a revert, a force-push or a re-run of an older commit
/// can lower it, and a client comparing "is this different from mine" would then offer a downgrade that
/// PackageKit declines. The user sees an update that fails, every six hours, forever.
/// </para>
/// <para>
/// <b>The algorithm is rpm's, including the parts that look like bugs.</b> Segments of digits and segments
/// of letters are compared separately and never against each other; everything that is neither is a
/// separator, so <c>1.0_1</c> and <c>1.0.1</c> are equal. Leading zeros are stripped, so <c>1.01</c> and
/// <c>1.1</c> are equal too. <c>~</c> sorts before anything including the end of the string, which is what
/// makes <c>0.0.1~alpha.0.143</c> older than <c>0.0.1</c> and is the whole reason
/// <c>scripts/ci-linux-rpm.sh</c> translates a SemVer prerelease to one. <c>^</c> sorts after everything
/// except the end of the string.
/// </para>
/// <para>
/// Verified against <c>rpm</c>'s own answers rather than against a reading of the source: the cases in
/// <c>RpmVersionTests</c> were generated by running <c>rpm.vercmp</c> in a container, so a disagreement is
/// a disagreement with the program that will actually decide.
/// </para>
/// </remarks>
internal static class RpmVersion
{
    /// <summary>
    /// Orders two RPM version strings.
    /// </summary>
    /// <param name="left">The left version.</param>
    /// <param name="right">The right version.</param>
    /// <returns>-1, 0 or 1, as <c>rpmvercmp</c> would answer.</returns>
    /// <remarks>
    /// Null and empty are accepted and treated as equal to each other and older than anything else, which
    /// <c>rpm</c>'s own Lua binding refuses outright. The refusal is right for a packaging tool and wrong
    /// here: this compares a version read off a feed, and a feed that answered with an empty string should
    /// leave the client on the build it has rather than throwing on a background timer.
    /// </remarks>
    internal static int Compare(string? left, string? right)
    {
        var one = left ?? string.Empty;
        var two = right ?? string.Empty;

        if (string.Equals(one, two, StringComparison.Ordinal))
        {
            return 0;
        }

        int i = 0, j = 0;

        while (i < one.Length || j < two.Length)
        {
            // Separators. Anything that is not alphanumeric, ~ or ^ divides segments and is skipped on
            // both sides independently — which is why 1.0_1 and 1.0.1 compare equal.
            Skip(one, ref i);
            Skip(two, ref j);

            if (CompareMarkers(one, i, two, j) is { } marker)
            {
                if (marker != 0)
                {
                    return marker;
                }

                // Both sides carried the same marker, so step over it and keep going.
                i++;
                j++;

                continue;
            }

            if (i >= one.Length || j >= two.Length)
            {
                break;
            }

            if (CompareSegments(one, ref i, two, ref j) is { } ordering)
            {
                return ordering;
            }
        }

        // Both ran out together, so every segment matched and the only differences were separators.
        if (i >= one.Length && j >= two.Length)
        {
            return 0;
        }

        // Whichever still has characters left has more version, and more version is newer.
        return i < one.Length ? 1 : -1;
    }

    /// <summary>Advances past anything that divides segments rather than taking part in them.</summary>
    private static void Skip(string value, ref int at)
    {
        while (at < value.Length && !IsVersionCharacter(value[at]))
        {
            at++;
        }
    }

    /// <summary>
    /// Orders the two sides when either is at a <c>~</c> or a <c>^</c>.
    /// </summary>
    /// <returns>
    /// Null when neither side is at a marker, so the caller should compare segments; 0 when both are at
    /// the same one, so the caller should step over it; otherwise the ordering.
    /// </returns>
    /// <remarks>
    /// <c>~</c> sorts before everything including the end of the string, which is what carries the whole
    /// prerelease ordering. <c>^</c> sorts after everything <em>except</em> the end of the string, so
    /// 1.0^post is newer than 1.0 and 1.2.3 is older than 1.2.3^ — the end-of-string checks coming first
    /// is the difference between the two halves of that sentence.
    /// </remarks>
    private static int? CompareMarkers(string one, int i, string two, int j)
    {
        var oneTilde = i < one.Length && one[i] == '~';
        var twoTilde = j < two.Length && two[j] == '~';

        if (oneTilde || twoTilde)
        {
            return oneTilde && twoTilde ? 0 : oneTilde ? -1 : 1;
        }

        var oneCaret = i < one.Length && one[i] == '^';
        var twoCaret = j < two.Length && two[j] == '^';

        if (!oneCaret && !twoCaret)
        {
            return null;
        }

        if (i >= one.Length)
        {
            return -1;
        }

        if (j >= two.Length)
        {
            return 1;
        }

        return oneCaret && twoCaret ? 0 : oneCaret ? -1 : 1;
    }

    /// <summary>
    /// Takes one segment from each side and orders them, advancing past both.
    /// </summary>
    /// <returns>The ordering, or null if the two segments were equal and the walk should continue.</returns>
    private static int? CompareSegments(string one, ref int i, string two, ref int j)
    {
        var numeric = char.IsAsciiDigit(one[i]);

        var start1 = i;
        var start2 = j;

        Take(one, ref i, numeric);
        Take(two, ref j, numeric);

        var segment1 = one.AsSpan(start1, i - start1);
        var segment2 = two.AsSpan(start2, j - start2);

        // The two sides are at segments of different kinds — digits against letters. rpm calls the
        // numeric one newer, always, on the reasoning that a version that has started counting is further
        // along than one still spelling. segment1 is never empty: its kind was chosen from its own first
        // character.
        if (segment2.IsEmpty)
        {
            return numeric ? 1 : -1;
        }

        if (numeric)
        {
            segment1 = segment1.TrimStart('0');
            segment2 = segment2.TrimStart('0');

            // Longer, once the zeros are gone, is larger. Compared before the characters are, because a
            // string comparison would call "9" larger than "10".
            if (segment1.Length != segment2.Length)
            {
                return segment1.Length > segment2.Length ? 1 : -1;
            }
        }

        var ordering = segment1.CompareTo(segment2, StringComparison.Ordinal);

        return ordering == 0 ? null : ordering > 0 ? 1 : -1;
    }

    /// <summary>Advances over one run of digits, or one run of letters.</summary>
    private static void Take(string value, ref int at, bool numeric)
    {
        while (at < value.Length
               && (numeric ? char.IsAsciiDigit(value[at]) : char.IsAsciiLetter(value[at])))
        {
            at++;
        }
    }

    /// <summary>Whether this character takes part in the comparison rather than dividing it.</summary>
    private static bool IsVersionCharacter(char value) =>
        char.IsAsciiLetterOrDigit(value) || value == '~' || value == '^';
}