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use median of five

Browse source 
Median5 algorithm taken from:

Fast Deterministic Selection
by Andrei Alexandrescu - 2016
This commit is contained in:
Ahmet Inan 2024-03-11 09:17:21 +01:00
commit c86f767cac
1 changed files with 23 additions and 10 deletions
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33
quick_select.hh
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@ -19,22 +19,35 @@ static inline void swap(TYPE *a, int i, int j)
} }
template <typename TYPE> template <typename TYPE>
static inline void sort(TYPE *a, int i, int j, int k) static inline void median(TYPE *A, int a, int b, int c, int d, int e)
{ {
if (a[i] > a[j]) if (A[c] < A[a])
swap(a, i, j); swap(A, a, c);
if (a[i] > a[k]) if (A[d] < A[b])
swap(a, i, k); swap(A, b, d);
if (a[j] > a[k]) if (A[d] < A[c]) {
swap(a, j, k); swap(A, c, d);
swap(A, a, b);
}
if (A[e] < A[b])
swap(A, b, e);
if (A[e] < A[c]) {
swap(A, c, e);
if (A[c] < A[a])
swap(A, a, c);
} else if (A[c] < A[b]) {
swap(A, b, c);
}
} }
template <typename TYPE> template <typename TYPE>
static void partition(TYPE *a, int &l, int &h) static void partition(TYPE *a, int &l, int &h)
{ {
int m = l + (h - l) / 2; int half = (h - l) / 2;
sort(a, l, m, h); int quarter = (h - l) / 4;
TYPE pivot = a[m]; int middle = l + half;
median(a, l, l + quarter, middle, middle + quarter, h);
TYPE pivot = a[middle];
for (int i = l; i <= h;) for (int i = l; i <= h;)
if (a[i] < pivot) if (a[i] < pivot)
swap(a, i++, l++); swap(a, i++, l++);