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-rw-r--r--libdimension/polynomial.c50
1 files changed, 20 insertions, 30 deletions
diff --git a/libdimension/polynomial.c b/libdimension/polynomial.c
index e968832..d78361c 100644
--- a/libdimension/polynomial.c
+++ b/libdimension/polynomial.c
@@ -76,47 +76,38 @@ dmnsn_real_degree(double poly[], size_t degree)
return 0;
}
-/* Improve a root with Newton's method */
-static inline double
-dmnsn_improve_root(double poly[], size_t degree, double x)
-{
- double dx;
- do {
- /* Calculate the value of the polynomial and its derivative at once */
- double p = poly[degree], dp = 0.0;
- for (ssize_t i = degree - 1; i >= 0; --i) {
- dp = dp*x + p;
- p = p*x + poly[i];
- }
-
- dx = p/dp;
- x -= dx;
- } while (fabs(dx/x) > dmnsn_epsilon);
-
- return x;
-}
-
-/* Use the method of bisection to find a root in a range that contains exactly
- one root, counting multiplicity */
+/* Use the false position method to find a root in a range that contains exactly
+ one root */
static inline double
dmnsn_bisect_root(double poly[], size_t degree, double min, double max)
{
double evmin = dmnsn_evaluate_polynomial(poly, degree, min);
double evmax = dmnsn_evaluate_polynomial(poly, degree, max);
+ double mid = 0.0, evmid;
+ int lastsign = -1;
- while (max - min > dmnsn_epsilon) {
- double mid = (min + max)/2.0;
- double evmid = dmnsn_evaluate_polynomial(poly, degree, mid);
- if (dmnsn_signbit(evmid) == dmnsn_signbit(evmax)) {
+ do {
+ mid = (min*evmax - max*evmin)/(evmax - evmin);
+ evmid = dmnsn_evaluate_polynomial(poly, degree, mid);
+ int sign = dmnsn_signbit(evmid);
+ if (sign == dmnsn_signbit(evmax)) {
max = mid;
evmax = evmid;
+ if (sign == lastsign) {
+ evmin /= 2.0;
+ }
} else {
min = mid;
evmin = evmid;
+ if (sign == lastsign) {
+ evmax /= 2.0;
+ }
}
- }
- return (min + max)/2.0;
+ lastsign = sign;
+ } while (fabs(evmid) > fabs(mid)*dmnsn_epsilon);
+
+ return mid;
}
/* Use synthetic division to eliminate the root `r' from poly[] */
@@ -158,7 +149,7 @@ dmnsn_descartes_rule(double poly[], size_t degree)
}
/* Get the (n k) binomial coefficient */
-static double
+static inline double
dmnsn_binom(size_t n, size_t k)
{
if (k > n - k) {
@@ -270,7 +261,6 @@ dmnsn_solve_polynomial(double poly[], size_t degree, double x[])
/* Bisect within the found range */
double r = dmnsn_bisect_root(p, degree, ranges[j][0], ranges[j][1]);
- r = dmnsn_improve_root(p, degree, r);
/* Use synthetic division to eliminate the root `r' */
dmnsn_eliminate_root(p, &degree, r);