namespace AcDream.Core.CharGen;

/// <summary>
/// Pure port of retail's shade→palette-index resolution
/// (<c>PalSet::GetPaletteID @ 0x005AC570</c>, invoked from
/// <c>gmCG3DView::Update @ 0x004EE9D0</c> for the skin/hair subpalette
/// build and from <c>ClothingTable::BuildObjDesc @ 0x005A7900</c> for every
/// clothing-slot dye choice). The decompiled body is genuinely FPU-elided —
/// the <c>_ftol2()</c> truncating-cast operand is lost to the decompiler,
/// and can only be read as "some product of <paramref name="count"/>-ish and
/// <paramref name="shade"/>-ish operands" from the surrounding x87 stack
/// traffic — but the decomp's own control-flow SHAPE is still verifiable
/// independent of that lost operand: a two-sided FPU compare at
/// <c>0x005AC5A0</c> gating on <c>&gt;= 0.0</c>, consistent with a
/// <c>[0,1]</c> shade bounds check before the cast. What resolves the
/// elided operand is ACE's <c>ACE.DatLoader.FileTypes.PaletteSet.GetPaletteID</c>,
/// which carries the explicit comment "Taken from acclient.c
/// (PalSet::GetPaletteID)" against the exact formula below. That is TWO
/// sources (decomp control flow + ACE's cited port), not three: the
/// <c>PaletteSet.cs</c> file present in the vendored ACViewer checkout is
/// ACE's own file, not an independent reimplementation, and ACViewer's
/// <c>ClothingTableList.xaml.cs:97</c> UI slider computes a DIFFERENT
/// expression for a DIFFERENT problem (mapping a shade back to a slider tick
/// position against <c>Shades.Maximum</c>, i.e. <c>count-1</c>, not
/// <c>count</c>) — neither corroborates this formula and both are dropped
/// from the evidence chain here.
/// </summary>
public static class ChargenPalSetMath
{
    /// <summary>
    /// Resolves a shade fraction to an index into a palette-id list of the
    /// given <paramref name="count"/>. Returns -1 (retail's
    /// <c>INVALID_DID</c> outcome) when <paramref name="count"/> is
    /// non-positive or <paramref name="shade"/> falls outside
    /// <c>[0.0, 1.0]</c> — including retail's own <c>-1.0</c> "unset"
    /// sentinel (<c>CharGenState::Reset @ 0x005C68A0</c>), which is
    /// deliberately out of range so an untouched shade resolves to
    /// "nothing," matching retail. Callers should treat -1 as "skip this
    /// subpalette contribution" rather than emit a placeholder id.
    /// </summary>
    public static int GetPaletteIndex(int count, double shade)
    {
        if (count <= 0 || shade < 0.0 || shade > 1.0)
            return -1;

        // Truncating cast, exactly as ACE's cited port and the decomp's
        // _ftol2() (which truncates toward zero on x86, matching a plain
        // C-style (int) cast here since count > 0 and 0 <= shade <= 1 keep
        // the product non-negative).
        int index = (int)((count - 0.000001) * shade);
        if (index < 0)
            index = 0;
        if (index > count - 1)
            index = count - 1;
        return index;
    }
}