76 lines
2.1 KiB
Java
76 lines
2.1 KiB
Java
|
import java.util.Random;
|
||
|
|
import java.util.function.DoubleUnaryOperator;
|
||
|
|
import java.util.function.IntUnaryOperator;
|
||
|
|
|
||
|
|
// https://en.wikipedia.org/wiki/Ternary_search
|
||
|
|
public class TernarySearch {
|
||
|
|
|
||
|
|
// finds maximum of strictly increasing and then strictly decreasing function
|
||
|
|
public static double ternarySearch(DoubleUnaryOperator f, double lo, double hi) {
|
||
|
|
for (int step = 0; step < 1000; step++) {
|
||
|
|
double m1 = lo + (hi - lo) / 3;
|
||
|
|
double m2 = hi - (hi - lo) / 3;
|
||
|
|
if (f.applyAsDouble(m1) < f.applyAsDouble(m2))
|
||
|
|
lo = m1;
|
||
|
|
else
|
||
|
|
hi = m2;
|
||
|
|
}
|
||
|
|
return (lo + hi) / 2;
|
||
|
|
}
|
||
|
|
|
||
|
|
// finds maximum of strictly increasing and then strictly decreasing function
|
||
|
|
public static int ternarySearch(IntUnaryOperator f, int fromInclusive, int toInclusive) {
|
||
|
|
int lo = fromInclusive;
|
||
|
|
int hi = toInclusive;
|
||
|
|
while (hi > lo + 2) {
|
||
|
|
int m1 = lo + (hi - lo) / 3;
|
||
|
|
int m2 = hi - (hi - lo) / 3;
|
||
|
|
if (f.applyAsInt(m1) < f.applyAsInt(m2))
|
||
|
|
lo = m1;
|
||
|
|
else
|
||
|
|
hi = m2;
|
||
|
|
}
|
||
|
|
int res = lo;
|
||
|
|
for (int i = lo + 1; i <= hi; i++)
|
||
|
|
if (f.applyAsInt(res) < f.applyAsInt(i))
|
||
|
|
res = i;
|
||
|
|
return res;
|
||
|
|
}
|
||
|
|
|
||
|
|
public static int ternarySearch2(IntUnaryOperator f, int fromInclusive, int toInclusive) {
|
||
|
|
int lo = fromInclusive - 1;
|
||
|
|
int hi = toInclusive;
|
||
|
|
while (hi - lo > 1) {
|
||
|
|
int mid = (lo + hi) >>> 1;
|
||
|
|
if (f.applyAsInt(mid) < f.applyAsInt(mid + 1)) {
|
||
|
|
lo = mid;
|
||
|
|
} else {
|
||
|
|
hi = mid;
|
||
|
|
}
|
||
|
|
}
|
||
|
|
return hi;
|
||
|
|
}
|
||
|
|
|
||
|
|
// random tests
|
||
|
|
public static void main(String[] args) {
|
||
|
|
System.out.println(ternarySearch((DoubleUnaryOperator) x -> -(x - 2) * (x - 2), -10, 10));
|
||
|
|
|
||
|
|
Random rnd = new Random(1);
|
||
|
|
for (int step = 0; step < 1000; step++) {
|
||
|
|
int n = rnd.nextInt(20) + 1;
|
||
|
|
int p = rnd.nextInt(n);
|
||
|
|
int[] a = new int[n];
|
||
|
|
final int range = 10;
|
||
|
|
a[p] = rnd.nextInt(range);
|
||
|
|
for (int i = p - 1; i >= 0; i--)
|
||
|
|
a[i] = a[i + 1] - rnd.nextInt(range) - 1;
|
||
|
|
for (int i = p + 1; i < n; i++)
|
||
|
|
a[i] = a[i - 1] - rnd.nextInt(range) - 1;
|
||
|
|
int res1 = ternarySearch((IntUnaryOperator) i -> a[i], 0, a.length - 1);
|
||
|
|
int res2 = ternarySearch2(i -> a[i], 0, a.length - 1);
|
||
|
|
if (p != res1 || p != res2)
|
||
|
|
throw new RuntimeException();
|
||
|
|
}
|
||
|
|
}
|
||
|
|
}
|