acdream/src/AcDream.App/Rendering/RetailChaseCamera.cs

using System;
using System.Numerics;
using AcDream.Core.Rendering;

namespace AcDream.App.Rendering;

/// <summary>
/// Retail-faithful chase camera. Ports the chase-cam behavior from the
/// 2013 acclient (<c>CameraManager</c> + <c>CameraSet</c>, decomp at
/// <c>docs/research/named-retail/acclient_2013_pseudo_c.txt:95505</c>):
/// exponential damping toward a target pose, 5-frame velocity-averaged
/// slope-aligned heading frame, mouse-input low-pass filter.
///
/// <para>
/// Sits behind <see cref="CameraDiagnostics.UseRetailChaseCamera"/>
/// next to the legacy <see cref="ChaseCamera"/>; both update every
/// frame so toggling the flag swaps cameras instantly. Visible behavior
/// vs legacy: lag-then-catch-up on turn/stop, tilt-with-terrain on
/// hills, jump-feedback without the legacy <c>_trackedZ</c> hack.
/// </para>
///
/// <para>
/// Spec: <c>docs/superpowers/specs/2026-05-18-retail-chase-camera-design.md</c>.
/// </para>
/// </summary>
public sealed class RetailChaseCamera : ICamera
{
    // ICamera surface.
    public Vector3   Position   { get; private set; }
    public float     Aspect     { get; set; } = 16f / 9f;
    public float     FovY       { get; set; } = MathF.PI / 3f;
    public Matrix4x4 View       { get; private set; } = Matrix4x4.Identity;
    public Matrix4x4 Projection =>
        Matrix4x4.CreatePerspectiveFieldOfView(FovY, Aspect, 1f, 5000f);

    // ── Public tunables (per-instance) ──────────────────────────────

    /// <summary>Length of the viewer_offset vector. Retail default ≈ 2.61.</summary>
    public float Distance { get; set; } = 2.61f;

    /// <summary>Angle of the camera above the heading-frame XY plane. Retail default ≈ 0.291 rad (16.7°).</summary>
    public float Pitch { get; set; } = 0.291f;

    /// <summary>
    /// Yaw offset added on top of player yaw when slope-align is off
    /// or velocity is too small to derive a heading. Used by hold-RMB
    /// orbit to swing the camera around the player without rotating
    /// the character.
    /// </summary>
    public float YawOffset { get; set; } = 0f;

    /// <summary>Height of look-at anchor above the player's feet (m). Retail default 1.5.</summary>
    public float PivotHeight { get; set; } = 1.5f;

    /// <summary>Computed translucency for the player mesh (0 = opaque, 1 = invisible). Read by GameWindow.</summary>
    public float PlayerTranslucency { get; private set; }

    /// <summary>Clamp bounds carried over from legacy ChaseCamera.</summary>
    public const float DistanceMin = 2f;
    public const float DistanceMax = 40f;
    public const float PitchMin    = -0.7f;
    public const float PitchMax    = 1.4f;

    // ── Damped state ────────────────────────────────────────────────

    private readonly Vector3[] _velocityRing = new Vector3[5];
    private int     _velocityCount;
    private Vector3 _dampedEye;
    private Vector3 _dampedForward = new(1f, 0f, 0f);
    private bool    _initialised;

    // Mouse-filter state — shared by FilterMouseDelta entrypoint.
    private float _lastMouseDeltaX;
    private float _lastMouseDeltaY;
    private float _lastFilterTimeSec;

    // ── Per-frame entry point ────────────────────────────────────────

    /// <summary>
    /// Advance the camera one frame. Caller passes the player's current
    /// pose + velocity (in world space) + the frame's <c>dt</c> in
    /// seconds. After this returns, <see cref="Position"/>,
    /// <see cref="View"/>, and <see cref="PlayerTranslucency"/> reflect
    /// the new state.
    /// </summary>
    public void Update(Vector3 playerPosition, float playerYaw, Vector3 playerVelocity, float dt)
    {
        // 1. Push velocity into 5-frame ring, get average.
        PushVelocity(_velocityRing, ref _velocityCount, playerVelocity);
        Vector3 avgVel = AverageVelocity(_velocityRing, _velocityCount);

        // 2. Heading vector — slope-aligned when fast enough, flat fallback otherwise.
        Vector3 heading = ComputeHeading(avgVel, playerYaw + YawOffset, CameraDiagnostics.AlignToSlope);

        // 3. Orthonormal heading-frame basis.
        var (forward, _, up) = BuildBasis(heading);

        // 4. Target pose.
        Vector3 pivotWorld = playerPosition + new Vector3(0f, 0f, PivotHeight);
        float   horizontal = Distance * MathF.Cos(Pitch);
        float   vertical   = Distance * MathF.Sin(Pitch);
        // viewer_offset = -horizontal along forward + vertical along up.
        Vector3 targetEye     = pivotWorld + forward * (-horizontal) + up * vertical;
        Vector3 targetForward = Vector3.Normalize(pivotWorld - targetEye);

        // 5. Exponential damping (independent translation + rotation rates).
        if (!_initialised)
        {
            _dampedEye     = targetEye;
            _dampedForward = targetForward;
            _initialised   = true;
        }
        else
        {
            float tAlpha = ComputeDampingAlpha(CameraDiagnostics.TranslationStiffness, dt);
            float rAlpha = ComputeDampingAlpha(CameraDiagnostics.RotationStiffness,    dt);
            _dampedEye     = Vector3.Lerp(_dampedEye, targetEye, tAlpha);
            _dampedForward = Vector3.Normalize(Vector3.Lerp(_dampedForward, targetForward, rAlpha));
        }

        // 6. Publish renderer surface.
        Position = _dampedEye;
        View     = Matrix4x4.CreateLookAt(_dampedEye, _dampedEye + _dampedForward, new Vector3(0f, 0f, 1f));

        // 7. Auto-fade translucency.
        float d = Vector3.Distance(_dampedEye, pivotWorld);
        PlayerTranslucency = ComputeTranslucency(d);
    }

    /// <summary>
    /// Adjust the camera distance (zoom) by a delta, clamped to
    /// <see cref="DistanceMin"/>..<see cref="DistanceMax"/>. Mirrors
    /// legacy <c>ChaseCamera.AdjustDistance</c>.
    /// </summary>
    public void AdjustDistance(float delta) =>
        Distance = Math.Clamp(Distance + delta, DistanceMin, DistanceMax);

    /// <summary>
    /// Adjust the camera pitch by a delta (radians), clamped to
    /// <see cref="PitchMin"/>..<see cref="PitchMax"/>. Mirrors legacy
    /// <c>ChaseCamera.AdjustPitch</c>.
    /// </summary>
    public void AdjustPitch(float delta) =>
        Pitch = Math.Clamp(Pitch + delta, PitchMin, PitchMax);

    /// <summary>
    /// Public entry point for the mouse-input low-pass filter. Calls
    /// <see cref="FilterMouseAxis"/> on each axis with shared state.
    /// </summary>
    public (float outX, float outY) FilterMouseDelta(float rawX, float rawY, float weight, float nowSec)
    {
        // X first — advances the shared timestamp.
        float x = FilterMouseAxis(rawX, weight, nowSec,
            ref _lastMouseDeltaX, ref _lastFilterTimeSec, CameraDiagnostics.MouseLowPassWindowSec);
        // Y uses a throwaway timestamp so the within-window check still uses the original delta
        // (X already advanced _lastFilterTimeSec to nowSec; if Y reused it, the within-window
        // check would be 0 < windowSec which is always true — which is what we want here, since
        // both axes are sampled simultaneously and should both blend.).
        float yTimeShadow = _lastFilterTimeSec - 1f;  // force within-window path for the Y axis
        float y = FilterMouseAxis(rawY, weight, nowSec,
            ref _lastMouseDeltaY, ref yTimeShadow, CameraDiagnostics.MouseLowPassWindowSec);
        return (x, y);
    }

    // Math primitives — pure, internal-static for unit-testability.

    /// <summary>
    /// Pick the heading vector that drives the camera basis. Slope-
    /// aligned when velocity is non-trivial and the toggle is on; flat
    /// fallback otherwise. Matches retail's <c>target_status &amp;
    /// ALIGN_WITH_PLANE</c> path with the contact-plane branch
    /// collapsed into the flat fallback.
    /// </summary>
    internal static Vector3 ComputeHeading(Vector3 avgVelocity, float yaw, bool alignToSlope)
    {
        if (alignToSlope && avgVelocity.LengthSquared() > 1e-4f)
            return Vector3.Normalize(avgVelocity);

        return new Vector3(MathF.Cos(yaw), MathF.Sin(yaw), 0f);
    }

    /// <summary>
    /// Build an orthonormal basis with <c>forward = heading</c>. World
    /// up is <c>(0, 0, 1)</c>; if <c>heading</c> is near-parallel to it
    /// the right axis falls back to world <c>+X</c> so the cross
    /// product doesn't collapse.
    /// </summary>
    internal static (Vector3 forward, Vector3 right, Vector3 up) BuildBasis(Vector3 heading)
    {
        Vector3 forward = Vector3.Normalize(heading);
        Vector3 worldUp = new(0f, 0f, 1f);

        Vector3 right;
        if (MathF.Abs(forward.Z) > 0.99f)
        {
            // Near-vertical forward — use world +X as the secondary axis.
            right = Vector3.Normalize(Vector3.Cross(forward, new Vector3(1f, 0f, 0f)));
        }
        else
        {
            right = Vector3.Normalize(Vector3.Cross(forward, worldUp));
        }
        Vector3 up = Vector3.Cross(right, forward);  // already unit (forward + right orthonormal)
        return (forward, right, up);
    }

    /// <summary>
    /// FIFO-push a velocity sample into the 5-entry ring. Returns the
    /// updated ring (mutates the input array; the return is for fluent
    /// usage in tests). <paramref name="count"/> grows from 0 toward 5
    /// and stays at 5 once the ring is full.
    /// </summary>
    internal static Vector3[] PushVelocity(Vector3[] ring, ref int count, Vector3 sample)
    {
        if (ring.Length != 5)
            throw new ArgumentException("velocity ring must have 5 entries", nameof(ring));

        // Shift left by 1 (oldest is overwritten), append new sample at the tail.
        for (int i = 0; i < 4; i++) ring[i] = ring[i + 1];
        ring[4] = sample;
        if (count < 5) count++;
        return ring;
    }

    /// <summary>
    /// Average the <paramref name="count"/> most-recent entries of the
    /// ring (entries <c>[ring.Length-count .. ring.Length)</c>). Returns
    /// <see cref="Vector3.Zero"/> when count is zero.
    /// </summary>
    internal static Vector3 AverageVelocity(Vector3[] ring, int count)
    {
        if (count == 0) return Vector3.Zero;
        Vector3 sum = Vector3.Zero;
        int start = ring.Length - count;
        for (int i = start; i < ring.Length; i++) sum += ring[i];
        return sum / count;
    }

    /// <summary>
    /// Exponential-damping rate per frame.
    /// <c>alpha = clamp(stiffness * dt * 10, 0, 1)</c>. At
    /// <c>stiffness=0.45</c>, <c>dt=1/60</c> → <c>~0.075</c>
    /// (~150 ms half-life). Matches retail's
    /// <c>x_1 = stiffness * dt * 10</c> formulation.
    /// </summary>
    internal static float ComputeDampingAlpha(float stiffness, float dt)
    {
        float a = stiffness * dt * 10f;
        if (a <= 0f) return 0f;
        if (a >= 1f) return 1f;
        return a;
    }

    /// <summary>
    /// Low-pass filter for a single mouse axis. Mirrors retail's
    /// <c>CameraSet::FilterMouseInput</c>: if last sample was within
    /// <paramref name="windowSec"/>, blend output with the average of
    /// (previous, raw); otherwise pass-through. Final output =
    /// <c>raw * (1 - weight) + blended * weight</c>. Updates
    /// <paramref name="lastDelta"/> and <paramref name="lastTimeSec"/>
    /// to the new state.
    /// </summary>
    internal static float FilterMouseAxis(
        float raw,
        float weight,
        float nowSec,
        ref float lastDelta,
        ref float lastTimeSec,
        float windowSec)
    {
        float avg;
        if (nowSec - lastTimeSec < windowSec)
            avg = (lastDelta + raw) * 0.5f;
        else
            avg = raw;

        float output    = raw * (1f - weight) + avg * weight;
        lastDelta       = output;
        lastTimeSec     = nowSec;
        return output;
    }

    /// <summary>
    /// Player-mesh translucency as a function of camera-to-pivot
    /// distance. <c>0</c> = fully opaque, <c>1</c> = fully transparent.
    /// Opaque at and beyond 0.45 m; fully transparent at and within
    /// 0.20 m; linear ramp between. Matches retail's <c>CameraSet::
    /// UpdateCamera</c> distance check (decomp :97703–97725).
    /// </summary>
    internal static float ComputeTranslucency(float distance)
    {
        const float Far  = 0.45f;
        const float Near = 0.20f;

        if (distance >= Far)  return 0f;
        if (distance <= Near) return 1f;
        // Linear: t = 1 - (Near - distance) / (Near - Far)
        return 1f - (Near - distance) / (Near - Far);
    }
}