# HG changeset patch
# User Pauli Nieminen <suokkos@gmail.com>
# Date 1404227086 -10800
#      Tue Jul 01 18:04:46 2014 +0300
# Node ID 423bfcb6b3ce1f241cb972efb4c7d9354c60086f
# Parent  5f9dfe6b0789dccf3a60bf8ffb4c4e015e97c211
Make rngSource implementation thread safe

It is relatively simple to make default math/rand implementation
thread safe allowing multiple go routiness to use same random number
generator if there is no requirement for deterministic number sequance.

diff -r 5f9dfe6b0789 -r 423bfcb6b3ce src/pkg/math/rand/rng.go
--- a/src/pkg/math/rand/rng.go	Tue Jul 01 17:42:32 2014 +0300
+++ b/src/pkg/math/rand/rng.go	Tue Jul 01 18:04:46 2014 +0300
@@ -4,6 +4,8 @@
 
 package rand
 
+import "sync/atomic"
+
 /*
  * Uniform distribution
  *
@@ -183,8 +185,7 @@
 )
 
 type rngSource struct {
-	tap  int         // index into vec
-	feed int         // index into vec
+	feed int32       // index into vec
 	vec  [_LEN]int64 // current feedback register
 }
 
@@ -201,8 +202,7 @@
 
 // Seed uses the provided seed value to initialize the generator to a deterministic state.
 func (rng *rngSource) Seed(seed int64) {
-	rng.tap = 0
-	rng.feed = _LEN - _TAP
+	atomic.StoreInt32(&rng.feed, _LEN - _TAP)
 
 	seed = seed % _M
 	if seed < 0 {
@@ -230,17 +230,25 @@
 
 // Int63 returns a non-negative pseudo-random 63-bit integer as an int64.
 func (rng *rngSource) Int63() int64 {
-	rng.tap--
-	if rng.tap < 0 {
-		rng.tap += _LEN
+	feed := atomic.AddInt32(&rng.feed, -1)
+
+	if feed < 0 {
+		feed += _LEN
+		if feed < 0 {
+			panic("Too many parallel calls to rngSource.Int63")
+		}
+	}
+	if feed == 0 {
+		atomic.AddInt32(&rng.feed, _LEN)
 	}
 
-	rng.feed--
-	if rng.feed < 0 {
-		rng.feed += _LEN
+	tap := feed + _TAP
+
+	if tap >= _LEN {
+		tap -= _LEN
 	}
 
-	x := (rng.vec[rng.feed] + rng.vec[rng.tap]) & _MASK
-	rng.vec[rng.feed] = x
+	x := (rng.vec[feed] + rng.vec[tap]) & _MASK
+	rng.vec[feed] = x
 	return x
 }