Initial release

engine/plugins/solvers/rfsolver.lua

@@ -0,0 +1,40 @@
+-- rfsolver.lua: RF-Solver (2 NFE) — Rectified Flow specific
+-- Exploits the RF ODE structure for higher accuracy than generic midpoint.
+
+solver = {
+    name        = "rfsolver",
+    display     = "RF-Solver (2 NFE)",
+    description = "Rectified-flow-aware midpoint solver",
+    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_curr < 1e-8 then
+        for i = 0, n-1 do xt[i] = xt[i] - vt[i] * dt end
+        return
+    end
+
+    -- Save v_t
+    local v_t = {}
+    for i = 0, n-1 do v_t[i] = vt[i] end
+
+    -- Half-step to midpoint
+    local half_dt = dt * 0.5
+    local t_mid = t_curr - half_dt
+    for i = 0, n-1 do xt[i] = xt[i] - v_t[i] * half_dt end
+
+    -- Evaluate at midpoint
+    model_fn(xt, t_mid)
+
+    -- RF-specific: reconstruct via x_0 prediction from midpoint
+    for i = 0, n-1 do
+        local x_0_mid = xt[i] - t_mid * vt_buf[i]
+        xt[i] = x_0_mid + t_prev * vt_buf[i]
+    end
+end