add argsort function

util.lua

@@ -8,6 +8,7 @@ local min = math.min
 local pairs = pairs
 local random = math.random
 local select = select
+local sort = table.sort
 local sqrt = math.sqrt
 
 local function signbyte(x)
@@ -153,6 +154,14 @@ local function rbool()
     return 0.5 >= random()
 end
 
+local function argsort(t, comp, out)
+    comp = comp or function(a, b) return a < b end
+    out = out or {}
+    for i=1, #t do out[i] = i end
+    sort(out, function(a, b) return comp(t[a], t[b]) end)
+    return out
+end
+
 return {
     signbyte=signbyte,
     boolean_xor=boolean_xor,
@@ -173,4 +182,5 @@ return {
     argmax2=argmax2,
     rchoice2=rchoice2,
     rbool=rbool,
+    argsort=argsort,
 }