private static void Gcd(ref BitsBuffer left, ref BitsBuffer right) { Debug.Assert(left.GetLength() >= 2); Debug.Assert(right.GetLength() >= 2); Debug.Assert(left.GetLength() >= right.GetLength()); // Executes Lehmer's gcd algorithm, but uses the most // significant bits to work with 64-bit (not 32-bit) values. // Furthermore we're using an optimized version due to Jebelean. // http://cacr.uwaterloo.ca/hac/about/chap14.pdf (see 14.4.2) // ftp://ftp.risc.uni-linz.ac.at/pub/techreports/1992/92-69.ps.gz while (right.GetLength() > 2) { ulong x, y; ExtractDigits(ref left, ref right, out x, out y); uint a = 1U, b = 0U; uint c = 0U, d = 1U; int iteration = 0; // Lehmer's guessing while (y != 0) { ulong q, r, s, t; // odd iteration q = x / y; if (q > 0xFFFFFFFF) break; r = a + q * c; s = b + q * d; t = x - q * y; if (r > 0x7FFFFFFF || s > 0x7FFFFFFF) break; if (t < s || t + r > y - c) break; a = (uint)r; b = (uint)s; x = t; ++iteration; if (x == b) break; // even iteration q = y / x; if (q > 0xFFFFFFFF) break; r = d + q * b; s = c + q * a; t = y - q * x; if (r > 0x7FFFFFFF || s > 0x7FFFFFFF) break; if (t < s || t + r > x - b) break; d = (uint)r; c = (uint)s; y = t; ++iteration; if (y == c) break; } if (b == 0) { // Euclid's step left.Reduce(ref right); BitsBuffer temp = left; left = right; right = temp; } else { // Lehmer's step LehmerCore(ref left, ref right, a, b, c, d); if (iteration % 2 == 1) { // ensure left is larger than right BitsBuffer temp = left; left = right; right = temp; } } } if (right.GetLength() > 0) { // Euclid's step left.Reduce(ref right); uint[] xBits = right.GetBits(); uint[] yBits = left.GetBits(); ulong x = ((ulong)xBits[1] << 32) | xBits[0]; ulong y = ((ulong)yBits[1] << 32) | yBits[0]; left.Overwrite(Gcd(x, y)); right.Overwrite(0); } }
private static void Gcd(ref BitsBuffer left, ref BitsBuffer right) { Debug.Assert(left.GetLength() >= 2); Debug.Assert(right.GetLength() >= 2); Debug.Assert(left.GetLength() >= right.GetLength()); // Executes Lehmer's gcd algorithm, but uses the most // significant bits to work with 64-bit (not 32-bit) values. // Furthermore we're using an optimized version due to Jebelean. // http://cacr.uwaterloo.ca/hac/about/chap14.pdf (see 14.4.2) // ftp://ftp.risc.uni-linz.ac.at/pub/techreports/1992/92-69.ps.gz while (right.GetLength() > 2) { ulong x, y; ExtractDigits(ref left, ref right, out x, out y); uint a = 1U, b = 0U; uint c = 0U, d = 1U; int iteration = 0; // Lehmer's guessing while (y != 0) { ulong q, r, s, t; // Odd iteration q = x / y; if (q > 0xFFFFFFFF) { break; } r = a + q * c; s = b + q * d; t = x - q * y; if (r > 0x7FFFFFFF || s > 0x7FFFFFFF) { break; } if (t < s || t + r > y - c) { break; } a = (uint)r; b = (uint)s; x = t; ++iteration; if (x == b) { break; } // Even iteration q = y / x; if (q > 0xFFFFFFFF) { break; } r = d + q * b; s = c + q * a; t = y - q * x; if (r > 0x7FFFFFFF || s > 0x7FFFFFFF) { break; } if (t < s || t + r > x - b) { break; } d = (uint)r; c = (uint)s; y = t; ++iteration; if (y == c) { break; } } if (b == 0) { // Euclid's step left.Reduce(ref right); BitsBuffer temp = left; left = right; right = temp; } else { // Lehmer's step LehmerCore(ref left, ref right, a, b, c, d); if (iteration % 2 == 1) { // Ensure left is larger than right BitsBuffer temp = left; left = right; right = temp; } } } if (right.GetLength() > 0) { // Euclid's step left.Reduce(ref right); uint[] xBits = right.GetBits(); uint[] yBits = left.GetBits(); ulong x = ((ulong)xBits[1] << 32) | xBits[0]; ulong y = ((ulong)yBits[1] << 32) | yBits[0]; left.Overwrite(Gcd(x, y)); right.Overwrite(0); } }