math/rand: Document origin of cooked pseudo-random numbers

The Source provided by math/rand relies on an array of cooked
pseudo-random 63bit integers for seeding. The origin of these
numbers is undocumented.

Add a standalone program in math/rand folder that generates
the 63bit integer array as well as a 64bit version supporting
extension of the Source to 64bit pseudo-random number
generation while maintaining the current sequence in the
lower 63bit.

The code is largely based on the initial implementation of the
random number generator in the go repository by Ken Thompson
(revision 399).

Change-Id: Ib4192aea8127595027116a0f5a7be53f11dc110b
Reviewed-on: https://go-review.googlesource.com/22230Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/rand/gen_cooked.go

+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build ignore
+
+// This program computes the value of rng_cooked in rng.go,
+// which is used for seeding all instances of rand.Source.
+// a 64bit and a 63bit version of the array is printed to
+// the standard output.
+
+package main
+
+import "fmt"
+
+const (
+	length = 607
+	tap    = 273
+	mask   = (1 << 63) - 1
+	a      = 48271
+	m      = (1 << 31) - 1
+	q      = 44488
+	r      = 3399
+)
+
+var (
+	rngVec          [length]int64
+	rngTap, rngFeed int
+)
+
+func seedrand(x int32) int32 {
+	hi := x / q
+	lo := x % q
+	x = a*lo - r*hi
+	if x < 0 {
+		x += m
+	}
+	return x
+}
+
+func srand(seed int32) {
+	rngTap = 0
+	rngFeed = length - tap
+	seed %= m
+	if seed < 0 {
+		seed += m
+	} else if seed == 0 {
+		seed = 89482311
+	}
+	x := seed
+	for i := -20; i < length; i++ {
+		x = seedrand(x)
+		if i >= 0 {
+			var u int64
+			u = int64(x) << 20
+			x = seedrand(x)
+			u ^= int64(x) << 10
+			x = seedrand(x)
+			u ^= int64(x)
+			rngVec[i] = u
+		}
+	}
+}
+
+func vrand() int64 {
+	rngTap--
+	if rngTap < 0 {
+		rngTap += length
+	}
+	rngFeed--
+	if rngFeed < 0 {
+		rngFeed += length
+	}
+	x := (rngVec[rngFeed] + rngVec[rngTap])
+	rngVec[rngFeed] = x
+	return x
+}
+
+func main() {
+	srand(1)
+	for i := uint64(0); i < 7.8e12; i++ {
+		vrand()
+	}
+	fmt.Printf("rngVec after 7.8e12 calls to vrand:\n%#v\n", rngVec)
+	for i := range rngVec {
+		rngVec[i] &= mask
+	}
+	fmt.Printf("lower 63bit of rngVec after 7.8e12 calls to vrand:\n%#v\n", rngVec)
+}