namespace AcDream.Core.CharGen;

/// <summary>
/// Pure port of retail's skill-credit spend calculation
/// (<c>CharGenState::UpdateRemainingSkillCredits @ 0x005C37C0</c>): walk
/// every skill slot, add <c>NormalCost</c> for Trained or <c>PrimaryCost</c>
/// for Specialized (never both), and subtract the total from the heritage's
/// <c>SkillCredits</c> budget. No DAT/DatReaderWriter dependency — callers
/// (CC3's Runtime owner) pass in the heritage's already-projected
/// <see cref="ChargenSkillCost"/> lookup.
/// </summary>
public static class ChargenSkillCreditMath
{
    /// <summary>
    /// Total credits spent across every Trained/Specialized skill in
    /// <paramref name="advancement"/>. A skill with no cost entry for the
    /// active heritage (i.e. the heritage doesn't offer it) is skipped —
    /// retail's own UI can never reach that state, so this is defensive
    /// rather than a documented retail behavior.
    /// </summary>
    public static int ComputeSpent(
        ChargenSkillAdvancementSet advancement,
        IReadOnlyDictionary<uint, ChargenSkillCost> costsBySkillId)
    {
        ArgumentNullException.ThrowIfNull(advancement);
        ArgumentNullException.ThrowIfNull(costsBySkillId);

        int spent = 0;
        for (uint skillId = 1; skillId < ChargenSkillAdvancementSet.SlotCount; skillId++)
        {
            ChargenSkillAdvancementClass cls = advancement[skillId];
            if (cls != ChargenSkillAdvancementClass.Trained
                && cls != ChargenSkillAdvancementClass.Specialized)
            {
                continue;
            }

            if (!costsBySkillId.TryGetValue(skillId, out ChargenSkillCost cost))
                continue;

            spent += cls == ChargenSkillAdvancementClass.Specialized
                ? cost.PrimaryCost
                : cost.NormalCost;
        }
        return spent;
    }

    /// <summary>
    /// <c>totalSkillCredits - ComputeSpent(...)</c> — retail's
    /// <c>remainingSkillCredits</c>. Retail's Finish gate (<c>DoFinish @
    /// 0x004E91F2</c>-adjacent) only checks <c>remainingAtrbCredits &gt; 0</c>
    /// for attributes; skill credits are NOT required to hit exactly zero
    /// (unspent skill credits are simply lost on creation) — callers should
    /// not port an "exact spend" gate for skills the way
    /// <see cref="ChargenAttributeMath.IsFullySpent"/> does for attributes.
    /// </summary>
    public static int RemainingCredits(
        uint totalSkillCredits,
        ChargenSkillAdvancementSet advancement,
        IReadOnlyDictionary<uint, ChargenSkillCost> costsBySkillId) =>
        checked((int)totalSkillCredits) - ComputeSpent(advancement, costsBySkillId);
}