add PowerSign momentum to ARS, antithetic by default

ars.lua

@@ -24,9 +24,15 @@ local util = require "util"
 local argsort = util.argsort
 local calc_mean_dev = util.calc_mean_dev
 local normalize_sums = util.normalize_sums
+local sign = util.sign
 
 local Ars = Base:extend()
 
+local exp_lut = {}
+exp_lut[-1] = exp(-1)
+exp_lut[0] = exp(0)
+exp_lut[1] = exp(1)
+
 local function collect_best_indices(scored, top, antithetic)
     -- select one (the best) reward of each pos/neg pair.
     local best_rewards
@@ -60,7 +66,8 @@ local function kinda_lipschitz(dir, pos, neg, mid)
     return max(l0, l1) / (2 * dev)
 end
 
-function Ars:init(dims, popsize, poptop, base_rate, sigma, antithetic)
+function Ars:init(dims, popsize, poptop, base_rate, sigma, antithetic,
+                  momentum)
     self.dims = dims
     self.popsize = popsize or 4 + (3 * floor(log(dims)))
     base_rate = base_rate or 3/5 * (3 + log(dims)) / (dims * sqrt(dims))
@@ -68,13 +75,15 @@ function Ars:init(dims, popsize, poptop, base_rate, sigma, antithetic)
     self.sigma_rate = base_rate
     self.covar_rate = base_rate
     self.sigma = sigma or 1
-    self.antithetic = antithetic and true or false
+    self.antithetic = antithetic == nil and true or antithetic
+    self.momentum = momentum or 0
 
     self.poptop = poptop or popsize
     assert(self.poptop <= popsize)
     if self.antithetic then self.popsize = self.popsize * 2 end
 
     self._params = zeros(self.dims)
+    if self.momentum > 0 then self.accum = zeros(self.dims) end
 
     self.evals = 0
 end
@@ -189,6 +198,13 @@ function Ars:tell(scored, unperturbed_score)
         end
     end
 
+    if self.momentum > 0 then
+        for i, v in ipairs(step) do
+            self.accum[i] = self.momentum * self.accum[i] + v
+            step[i] = v * exp_lut[sign(v) * sign(self.accum[i])]
+        end
+    end
+
     for i, v in ipairs(self._params) do
         self._params[i] = v + self.param_rate * step[i]
     end

config.lua

@@ -58,6 +58,7 @@ local defaults = {
                      -- if you don't specify them individually.
     param_decay = 0.0,
     sigma_decay = 0.0, -- for SNES, xNES.
+    momentum = 0.0, -- for ARS.
 }
 
 local presets = require("presets")

main.lua

@@ -508,7 +508,8 @@ local function init()
         end
     elseif cfg.es == 'ars' then
         es = ars.Ars(network.n_param, cfg.epoch_trials, cfg.epoch_top_trials,
-                     cfg.base_rate, cfg.deviation, cfg.negate_trials)
+                     cfg.base_rate, cfg.deviation, cfg.negate_trials,
+                     cfg.momentum)
     else
         error("Unknown evolution strategy specified: " + tostring(cfg.es))
     end

util.lua

@@ -15,6 +15,11 @@ local select = select
 local sort = table.sort
 local sqrt = math.sqrt
 
+local function sign(x)
+    -- remember that 0 is truthy in Lua.
+    return x == 0 and 0 or x > 0 and 1 or -1
+end
+
 local function signbyte(x)
     if x >= 128 then x = 256 - x end
     return x
@@ -245,6 +250,7 @@ local function weighted_mann_whitney(s0, s1, w0, w1)
 end
 
 return {
+    sign=sign,
     signbyte=signbyte,
     boolean_xor=boolean_xor,
     log2=log2,