using UnityEngine;

namespace ProjectM.Client
{
    /// <summary>
    /// Pure HUD presentation math (unit-tested, no ECS / no UnityEngine.Object). Drives the low-health screen
    /// vignette + the transient hurt-flash so the damage feedback is deterministic and testable. Used by the
    /// client-only observe-only <see cref="HudSystem"/> in PresentationSystemGroup; the flash decay runs on the
    /// wall-frame <c>SystemAPI.Time.DeltaTime</c>, which is correct in a presentation system (the dt-trap only
    /// applies to plain simulation systems).
    /// </summary>
    public static class HudVisualMath
    {
        /// <summary>Below this health fraction the low-health vignette starts to ramp in.</summary>
        public const float LowHealthThreshold = 0.35f;

        /// <summary>Maximum steady vignette opacity at 0 HP.</summary>
        public const float MaxVignetteOpacity = 0.55f;

        /// <summary>Opacity boost added by a fresh hit (a hurt flash), decaying back to the steady vignette.</summary>
        public const float HurtFlashKick = 0.40f;

        /// <summary>Seconds for a full hurt-flash kick to decay to zero.</summary>
        public const float HurtFlashDuration = 0.40f;

        /// <summary>0 at/above the low-health threshold, ramping to 1 as the health fraction falls to 0.</summary>
        public static float VignetteIntensity(float healthFrac)
        {
            float f = Mathf.Clamp01(healthFrac);
            if (f >= LowHealthThreshold) return 0f;
            return Mathf.Clamp01((LowHealthThreshold - f) / LowHealthThreshold);
        }

        /// <summary>Steady low-health vignette opacity (no flash) from the current health fraction.</summary>
        public static float VignetteOpacity(float healthFrac) => VignetteIntensity(healthFrac) * MaxVignetteOpacity;

        /// <summary>Decay an active hurt-flash boost toward 0 over <paramref name="dt"/> seconds (clamped, never negative).</summary>
        public static float DecayFlash(float flash, float dt)
            => Mathf.Max(0f, flash - (HurtFlashKick / HurtFlashDuration) * Mathf.Max(0f, dt));

        /// <summary>Final overlay opacity: the steady low-health vignette plus the current flash boost, clamped to 1.</summary>
        public static float CombinedOpacity(float healthFrac, float flash)
            => Mathf.Clamp01(VignetteOpacity(healthFrac) + Mathf.Clamp01(flash));
    }
}