Initial release

engine/plugins/solvers/jkass_quality.lua

@@ -0,0 +1,40 @@
+-- jkass_quality.lua: JKASS Quality solver (2 NFE)
+-- Heun with derivative averaging.
+
+solver = {
+    name        = "jkass_quality",
+    display     = "JKASS Quality (2 NFE)",
+    description = "Heun predictor-corrector with derivative averaging",
+    accent      = "amber",
+    nfe         = 2,
+    order       = 2,
+    needs_model = true,
+    stateful    = false,
+    stochastic  = false,
+}
+
+function step(xt, vt, t_curr, t_prev, n, model_fn, vt_buf)
+    local dt = t_curr - t_prev
+
+    if t_prev <= 0 then
+        -- Fallback to Euler for final step
+        for i = 0, n - 1 do xt[i] = xt[i] - vt[i] * dt end
+        return
+    end
+
+    -- Save k1
+    local k1 = {}
+    for i = 0, n - 1 do k1[i] = vt[i] end
+
+    -- Euler predictor
+    for i = 0, n - 1 do xt[i] = xt[i] - vt[i] * dt end
+
+    -- Second evaluation
+    model_fn(xt, t_prev)
+
+    -- Correct: undo Euler, apply averaged derivative
+    for i = 0, n - 1 do
+        xt[i] = xt[i] + k1[i] * dt  -- undo Euler
+        xt[i] = xt[i] - 0.5 * (k1[i] + vt_buf[i]) * dt  -- Heun average
+    end
+end