Add fire-control lock-on lead targeting overlay

Content.Client/_Mono/FireControl/UI/FireControlLeadSolver.cs

@@ -0,0 +1,78 @@
+// SPDX-FileCopyrightText: 2026 HardLight contributors
+//
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+using System.Numerics;
+
+namespace Content.Client._Mono.FireControl.UI;
+
+/// <summary>
+/// Client-side helper that mirrors the firing-solution math used by
+/// <c>ShipTargetingSystem.FireWeapons</c>. Given a gun position, a target
+/// position+velocity and a projectile speed, it returns the world-space
+/// point where the gunner should aim so the projectile intercepts the
+/// target.
+/// </summary>
+public static class FireControlLeadSolver
+{
+    /// <summary>
+    /// Compute the predicted intercept point in world space.
+    /// </summary>
+    /// <param name="gunWorldPos">World position of the firing weapon.</param>
+    /// <param name="targetWorldPos">Current world position of the target.</param>
+    /// <param name="targetVel">Target's world linear velocity (m/s).</param>
+    /// <param name="ourVel">Firing platform's world linear velocity (m/s).</param>
+    /// <param name="projectileSpeed">Projectile muzzle speed (m/s).</param>
+    /// <param name="lead">Outputs the lead displacement applied to the target (= relVel * hitTime).</param>
+    /// <param name="hitTime">Predicted travel time, in seconds.</param>
+    /// <returns>True if a real intercept exists; false otherwise (target uncatchable).</returns>
+    public static bool TrySolve(
+        Vector2 gunWorldPos,
+        Vector2 targetWorldPos,
+        Vector2 targetVel,
+        Vector2 ourVel,
+        float projectileSpeed,
+        out Vector2 interceptWorldPos,
+        out Vector2 lead,
+        out float hitTime)
+    {
+        interceptWorldPos = targetWorldPos;
+        lead = Vector2.Zero;
+        hitTime = 0f;
+
+        if (projectileSpeed <= 0f)
+            return false;
+
+        var gunToDest = targetWorldPos - gunWorldPos;
+        if (gunToDest.LengthSquared() < 0.0001f)
+            return false;
+
+        var dir = Vector2.Normalize(gunToDest);
+        var relVel = targetVel - ourVel;
+
+        // Decompose relative velocity into along-axis (normVel) and cross-axis (tgVel).
+        var normVel = dir * Vector2.Dot(relVel, dir);
+        var tgVel = relVel - normVel;
+
+        // Tangential component too fast to ever catch (>= so the sqrt below stays > 0).
+        var projSpeedSq = projectileSpeed * projectileSpeed;
+        var tgVelLenSq = tgVel.LengthSquared();
+        if (tgVelLenSq >= projSpeedSq)
+            return false;
+
+        var normTarget = dir * MathF.Sqrt(projSpeedSq - tgVelLenSq);
+
+        // Target receding faster than we can chase.
+        if (Vector2.Dot(normTarget, normVel) > 0f && normVel.Length() > normTarget.Length())
+            return false;
+
+        var approachVel = (normTarget - normVel).Length();
+        if (approachVel < 0.001f)
+            return false;
+
+        hitTime = gunToDest.Length() / approachVel;
+        lead = relVel * hitTime;
+        interceptWorldPos = targetWorldPos + lead;
+        return true;
+    }
+}