Initial release

engine/plugins/solvers/dpm3m.lua

@@ -0,0 +1,45 @@
+-- dpm3m.lua: DPM++ 3M solver (Adams-Bashforth 3, 1 NFE, stateful)
+-- Uses two previous velocities for 3rd order correction.
+
+solver = {
+    name        = "dpm3m",
+    display     = "DPM++ 3M",
+    description = "3rd order Adams-Bashforth multistep (1 NFE)",
+    nfe         = 1,
+    order       = 3,
+    needs_model = false,
+    stateful    = true,
+    stochastic  = false,
+}
+
+local prev_vt = nil
+local prev_prev_vt = nil
+
+function step(xt, vt, t_curr, t_prev, n)
+    if (step_index or 0) == 0 then prev_vt = nil; prev_prev_vt = nil end
+    local dt = t_curr - t_prev
+
+    if prev_vt and prev_prev_vt then
+        -- AB3: v_eff = (23/12)*vt - (16/12)*prev - (5/12)*prev_prev
+        for i = 0, n - 1 do
+            local v_eff = (23/12) * vt[i] - (16/12) * prev_vt[i] + (5/12) * prev_prev_vt[i]
+            xt[i] = xt[i] - v_eff * dt
+        end
+    elseif prev_vt then
+        -- AB2 fallback
+        for i = 0, n - 1 do
+            local v_eff = 1.5 * vt[i] - 0.5 * prev_vt[i]
+            xt[i] = xt[i] - v_eff * dt
+        end
+    else
+        -- Euler fallback
+        for i = 0, n - 1 do
+            xt[i] = xt[i] - vt[i] * dt
+        end
+    end
+
+    -- Shift history
+    prev_prev_vt = prev_vt
+    prev_vt = {}
+    for i = 0, n - 1 do prev_vt[i] = vt[i] end
+end