From 9b3196d8a009a79c2ca6e0763e741c61d8eb6256 Mon Sep 17 00:00:00 2001
From: Christian Ulrich <christian@ulrich.earth>
Date: Wed, 18 Nov 2020 17:47:15 +0100
Subject: [PATCH] try random ports for randokm port mapping

---
 port_prediction.nim | 30 +++++++++++++-----------------
 1 file changed, 13 insertions(+), 17 deletions(-)

diff --git a/port_prediction.nim b/port_prediction.nim
index 8369bc1..905f79d 100644
--- a/port_prediction.nim
+++ b/port_prediction.nim
@@ -1,5 +1,6 @@
 import algorithm
 import net
+import random
 import sequtils
 import unittest
 
@@ -82,15 +83,17 @@ proc predictPortRange*(localPort: Port, probedPorts: seq[Port]): seq[Port] =
           result.add(Port(maxPort.subtractOffset(i)))
         return
   # assume symmetric NAT with random port mapping
-  let portRange = maxPort - minPort
-  let first = if portRange > RandomPortCount:
-    minPort
+  randomize()
+  let first = if minPort >= 1024 + 5000:
+    minPort - 5000
   else:
-    let notCovered = RandomPortCount - portRange
-    max(minPort - notCovered shr 1, 1024)
-  let last = first + RandomPortCount
-  for i in first .. last:
-    result.add(Port(i))
+    1024
+  let last = if maxPort <= uint16.high - 5000:
+    maxPort + 5000
+  else:
+    uint16.high
+  for _ in 1 .. RandomPortCount:
+    result.add(Port(rand(first .. last)))
 
 suite "port prediction tests":
   test "single port":
@@ -153,13 +156,6 @@ suite "port prediction tests":
     let predicted = predictPortRange(Port(1234), @[Port(1039), Port(1030)])
     check(predicted == @[Port(65533)])
 
-  test "random mapping, distance > RandomPortCount":
+  test "random mapping":
     let predicted = predictPortRange(Port(1234), @[Port(3546), Port(7624)])
-    check(predicted == toSeq(countup(3546'u16, 3546'u16 + RandomPortCount)).map(toPort))
-
-  test "random mapping, distance < RandomPortCount":
-    let centerPort = 30000'u16
-    let minPort = centerPort - RandomPortCount.uint16 shr 1 + 1
-    let maxPort = centerPort + RandomPortCount.uint16 shr 1 - 1
-    let predicted = predictPortRange(Port(centerPort), @[Port(minPort), Port(maxPort)])
-    check(predicted == toSeq(countup(minPort - 1, maxPort + 1)).map(toPort))
+    check(predicted.len == RandomPortCount)