TheriapolisV3/Theriapolis.Core/Rules/Combat/DamageRoll.cs

using Theriapolis.Core.Rules.Stats;

namespace Theriapolis.Core.Rules.Combat;

/// <summary>
/// Parsed damage expression: <c>NdM+B</c> where N = dice count, M = die
/// sides, B = flat modifier (can be negative). Examples: "1d6", "2d8+2",
/// "1d4-1". <see cref="Roll"/> takes a function that returns 1..M for each
/// dice and aggregates with the flat modifier.
/// </summary>
public sealed record DamageRoll(int DiceCount, int DiceSides, int FlatMod, DamageType DamageType)
{
    /// <summary>
    /// Roll the damage dice. <paramref name="rollDie"/> takes the die size
    /// (e.g. 6) and returns 1..size. On crit, dice double per d20 rules
    /// (the flat modifier does NOT double).
    /// </summary>
    public int Roll(System.Func<int, int> rollDie, bool isCrit = false)
    {
        int diceToRoll = isCrit ? DiceCount * 2 : DiceCount;
        int total = FlatMod;
        for (int i = 0; i < diceToRoll; i++)
            total += rollDie(DiceSides);
        return System.Math.Max(0, total);
    }

    /// <summary>Theoretical maximum (every die rolls its top face) + flat mod.</summary>
    public int Max(bool isCrit = false)
    {
        int dice = isCrit ? DiceCount * 2 : DiceCount;
        return dice * DiceSides + FlatMod;
    }

    /// <summary>Theoretical minimum (every die rolls 1) + flat mod, clamped to 0.</summary>
    public int Min(bool isCrit = false)
    {
        int dice = isCrit ? DiceCount * 2 : DiceCount;
        return System.Math.Max(0, dice * 1 + FlatMod);
    }

    public override string ToString()
    {
        string mod = FlatMod == 0 ? "" : (FlatMod > 0 ? $"+{FlatMod}" : $"{FlatMod}");
        return $"{DiceCount}d{DiceSides}{mod} {DamageType.ToString().ToLowerInvariant()}";
    }

    /// <summary>
    /// Parses an expression like "1d6", "2d8+2", "1d4-1", "5" (flat 5),
    /// or "0" (no damage). Whitespace is allowed. Throws on malformed input.
    /// </summary>
    public static DamageRoll Parse(string expr, DamageType damageType)
    {
        if (string.IsNullOrWhiteSpace(expr))
            throw new System.ArgumentException("Damage expression is empty", nameof(expr));

        string s = expr.Replace(" ", "").ToLowerInvariant();
        int dIdx = s.IndexOf('d');
        if (dIdx < 0)
        {
            // No dice — pure flat (e.g. "5" or "-1").
            if (!int.TryParse(s, out int flat))
                throw new System.FormatException($"Cannot parse damage '{expr}' as flat int.");
            return new DamageRoll(0, 0, flat, damageType);
        }

        // Split into "<count>" "d" "<sides>[modifier]"
        string countStr = s.Substring(0, dIdx);
        if (countStr.Length == 0) countStr = "1";  // "d6" → 1d6
        if (!int.TryParse(countStr, out int diceCount))
            throw new System.FormatException($"Bad dice count in '{expr}'");

        string rest = s.Substring(dIdx + 1);
        int signIdx = -1;
        for (int i = 0; i < rest.Length; i++)
        {
            if (rest[i] == '+' || rest[i] == '-') { signIdx = i; break; }
        }

        int sides;
        int flatMod;
        if (signIdx < 0)
        {
            if (!int.TryParse(rest, out sides))
                throw new System.FormatException($"Bad dice sides in '{expr}'");
            flatMod = 0;
        }
        else
        {
            if (!int.TryParse(rest.Substring(0, signIdx), out sides))
                throw new System.FormatException($"Bad dice sides in '{expr}'");
            if (!int.TryParse(rest.Substring(signIdx), out flatMod))
                throw new System.FormatException($"Bad flat mod in '{expr}'");
        }

        if (diceCount < 0 || sides < 0)
            throw new System.FormatException($"Negative dice count or sides in '{expr}'");

        return new DamageRoll(diceCount, sides, flatMod, damageType);
    }
}