Initial release

engine/plugins/solvers/dpm2m.lua

@@ -0,0 +1,39 @@
+-- dpm2m.lua: DPM++ 2M solver (Adams-Bashforth 2, 1 NFE, stateful)
+-- Uses previous step's velocity for 2nd order correction.
+-- First step falls back to Euler.
+
+solver = {
+    name        = "dpm2m",
+    display     = "DPM++ 2M",
+    description = "2nd order Adams-Bashforth multistep (1 NFE)",
+    nfe         = 1,
+    order       = 2,
+    needs_model = false,
+    stateful    = true,
+    stochastic  = false,
+}
+
+-- Persistent state across steps (Lua tables survive between calls)
+local prev_vt = nil
+
+function step(xt, vt, t_curr, t_prev, n)
+    if (step_index or 0) == 0 then prev_vt = nil end
+    local dt = t_curr - t_prev
+
+    if prev_vt then
+        -- AB2: v_eff = 1.5 * vt - 0.5 * prev_vt
+        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
+        -- First step: Euler
+        for i = 0, n - 1 do
+            xt[i] = xt[i] - vt[i] * dt
+        end
+    end
+
+    -- Save velocity for next step
+    prev_vt = {}
+    for i = 0, n - 1 do prev_vt[i] = vt[i] end
+end