add lagrange interpolation

1 files changed, 71 insertions, 0 deletions

diff --git a/include/mcl/lagrange.hpp b/include/mcl/lagrange.hpp
new file mode 100644
index 0000000..25b29a6
--- /dev/null
+++ b/include/mcl/lagrange.hpp
@@ -0,0 +1,71 @@
+#pragma once
+/**
+	@file
+	@brief Lagrange Interpolation
+	@author MITSUNARI Shigeo(@herumi)
+	@license modified new BSD license
+	http://opensource.org/licenses/BSD-3-Clause
+*/
+#include <vector>
+
+namespace mcl {
+
+/*
+	recover out = f(0) by { (x, y) | x = S[i], y = f(x) = vec[i] }
+	@retval 0 if succeed else -1
+*/
+template<class G, class F>
+void LagrangeInterpolation(G& out, const F *S, const G *vec, size_t k)
+{
+	/*
+		delta_{i,S}(0) = prod_{j != i} S[j] / (S[j] - S[i]) = a / b
+		where a = prod S[j], b = S[i] * prod_{j != i} (S[j] - S[i])
+	*/
+	if (k < 2) throw cybozu::Exception("LagrangeInterpolation:smalll k") << k;
+	std::vector<F> delta(k);
+	F a = S[0];
+	for (size_t i = 1; i < k; i++) {
+		a *= S[i];
+	}
+	if (a.isZero()) throw cybozu::Exception("LagrangeInterpolation:S has zero");
+	for (size_t i = 0; i < k; i++) {
+		F b = S[i];
+		for (size_t j = 0; j < k; j++) {
+			if (j != i) {
+				F v = S[j] - S[i];
+				if (v.isZero()) throw cybozu::Exception("LagrangeInterpolation:same S") << i << j;
+				b *= v;
+			}
+		}
+		delta[i] = a / b;
+	}
+
+	/*
+		f(0) = sum_i f(S[i]) delta_{i,S}(0)
+	*/
+	G r, t;
+	r.clear();
+	for (size_t i = 0; i < delta.size(); i++) {
+		G::mul(t, vec[i], delta[i]);
+		r += t;
+	}
+	out = r;
+}
+
+/*
+	out = f(x) = c[0] + c[1] * x + c[2] * x^2 + ... + c[cSize - 1] * x^(cSize - 1)
+	@retval 0 if succeed else -1
+*/
+template<class G, class T>
+void evaluatePolynomial(G& out, const G *c, size_t cSize, const T& x)
+{
+	if (cSize < 2) throw cybozu::Exception("evaluatePolynomial:small cSize") << cSize;
+	G y = c[cSize - 1];
+	for (int i = (int)cSize - 2; i >= 0; i--) {
+		G::mul(y, y, x);
+		G::add(y, y, c[i]);
+	}
+	out = y;
+}
+
+} // mcl