From f81037083cd86fef4d1309b0adfefca335abffa8 Mon Sep 17 00:00:00 2001
From: siddharth <s@ricketyspace.net>
Date: Sun, 27 Jun 2021 21:13:39 -0400
Subject: challenge: do challenge 21

---
 challenge/c21.go | 16 +++++++++++++
 cryptopals.go    |  2 ++
 lib/rng.go       | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 87 insertions(+)
 create mode 100644 challenge/c21.go
 create mode 100644 lib/rng.go

diff --git a/challenge/c21.go b/challenge/c21.go
new file mode 100644
index 0000000..3cc3dd3
--- /dev/null
+++ b/challenge/c21.go
@@ -0,0 +1,16 @@
+// Copyright © 2021 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package challenge
+
+import (
+	"fmt"
+
+	"ricketyspace.net/cryptopals/lib"
+)
+
+func C21() {
+	for {
+		fmt.Printf("%v\n", lib.MTExtract())
+	}
+}
diff --git a/cryptopals.go b/cryptopals.go
index 103bfbb..98b6759 100644
--- a/cryptopals.go
+++ b/cryptopals.go
@@ -59,5 +59,7 @@ func main() {
 		challenge.C19()
 	case 20:
 		challenge.C20()
+	case 21:
+		challenge.C21()
 	}
 }
diff --git a/lib/rng.go b/lib/rng.go
new file mode 100644
index 0000000..7e7611e
--- /dev/null
+++ b/lib/rng.go
@@ -0,0 +1,69 @@
+// Copyright © 2021 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package lib
+
+// MT19937 coefficients.
+var mtCoefW uint32 = 32
+var mtCoefN uint32 = 624
+var mtCoefM uint32 = 397
+var mtCoefR uint32 = 31
+var mtCoefA uint32 = 0x9908B0DF
+var mtCoefU uint32 = 11
+var mtCoefD uint32 = 0xFFFFFFFF
+var mtCoefS uint32 = 7
+var mtCoefB uint32 = 0x9D2C5680
+var mtCoefT uint32 = 15
+var mtCoefC uint32 = 0xEFC60000
+var mtCoefL uint32 = 18
+
+var mtF uint32 = 1812433253
+
+// Stores state of MT19937 generator.
+var mtGenSt []uint32 = make([]uint32, mtCoefN)
+var mtIndex uint32 = mtCoefN + 1
+
+var mtLowerMask uint32 = (1 << mtCoefR) - 1
+var mtUpperMask uint32 = 0xFFFFFFFF & (^mtLowerMask)
+
+func MTSeed(seed uint32) {
+	mtIndex = mtCoefN
+	mtGenSt[0] = seed
+	for i := uint32(1); i < mtCoefN; i++ {
+		mtGenSt[i] = (0xFFFFFFFF &
+			(mtF*(mtGenSt[i-1]^(mtGenSt[i-1]>>(mtCoefW-2))) + i))
+	}
+}
+
+func MTExtract() uint32 {
+	if mtIndex >= mtCoefN {
+		if mtIndex > mtCoefN {
+			MTSeed(5489)
+		}
+		mtTwist()
+	}
+
+	y := mtGenSt[mtIndex]
+	y = y ^ ((y >> mtCoefU) & mtCoefD)
+	y = y ^ ((y << mtCoefS) & mtCoefB)
+	y = y ^ ((y << mtCoefT) & mtCoefC)
+	y = y ^ (y >> mtCoefL)
+
+	mtIndex = mtIndex + 1
+
+	r := 0xFFFFFFFF & y
+	return r
+}
+
+func mtTwist() {
+	for i := uint32(0); i < mtCoefN-1; i++ {
+		x := (mtGenSt[i] & mtUpperMask) +
+			(mtGenSt[(i+1)%mtCoefN] & mtLowerMask)
+		xA := x >> 1
+		if x%2 != 0 { // lowest bit of x is 1
+			xA = xA ^ mtCoefA
+		}
+		mtGenSt[i] = mtGenSt[(i+mtCoefM)%mtCoefN] ^ xA
+	}
+	mtIndex = 0
+}
-- 
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