starting moduli.c port from OpenSSH

dh/const.go

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+package dh
+
+const (
+	// QSizeMinimum Specifies the number of the most significant bit (0 to M).
+	// WARNING: internally, usually 1 to N.
+	QSizeMinimum = 511
+
+	// Prime sieving constants
+	// Assuming 8 bit bytes and 32 bit words.
+	ShiftBit = 3
+	ShiftByte = 2
+	ShiftWord = ShiftBit + ShiftByte
+	ShiftMegabyte = 20
+	ShiftMegaWord = ShiftMegabyte - ShiftBit
+
+	// Memory limits.
+	// LargeMinimum is 8 megabytes
+	LargeMinimum = uint32(8) // Originally an 8UL in moduli.c
+	// LargeMaximum is 127MB.
+	LargeMaximum = uint32(127)
+	// The largest sieve prime has to be < 2**32 on 32-bit systems.
+	SmallMaximum = uint32(0xffffffff) // 4294967295
+	// Can sieve all primes less than 2**32, as 65537**2 > 2**32-1.
+	TinyNumber = uint32(1) << 16
+	// Ensure enough bit space for testing 2*q.
+	TestMaximum = uint32(1) << 16
+	TestMinimum = QSizeMinimum + 1 // (uint32(1) << (ShiftWord - TestPower))
+	TestPower = 3 // 2**n, n < ShiftWord
+)
+
+var (
+)
+
+// Bit* functions operate on 32-bit words
+func BitClear(a []uint32, n uint32) (i uint32) {
+
+	i = a[n >> ShiftWord] &= ~(uint32(1) << (n & 31))
+
+	return
+}