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we ain't come this far to omit sorting

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Ahmet Inan 2024-03-11 10:42:28 +01:00
commit cf60e38b9d
5 changed files with 57 additions and 4 deletions
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quick_select.hh
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@ -1,86 +0,0 @@
/*
Quick select algorithm
Copyright 2024 Ahmet Inan <inan@aicodix.de>
*/
#pragma once
namespace DSP {
namespace QUICK {
template <typename TYPE>
static inline void swap(TYPE *a, int i, int j)
{
TYPE t = a[i];
a[i] = a[j];
a[j] = t;
}
template <typename TYPE>
static inline void median(TYPE *A, int a, int b, int c, int d, int e)
{
if (A[c] < A[a])
swap(A, a, c);
if (A[d] < A[b])
swap(A, b, d);
if (A[d] < A[c]) {
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>
static void partition(TYPE *a, int &l, int &h)
{
int half = (h - l) / 2;
int quarter = (h - l) / 4;
int middle = l + half;
median(a, l, l + quarter, middle, middle + quarter, h);
TYPE pivot = a[middle];
for (int i = l; i <= h;)
if (a[i] < pivot)
swap(a, i++, l++);
else if (a[i] > pivot)
swap(a, i, h--);
else
++i;
}
template <typename TYPE>
static void select(TYPE *a, int l, int h, int k)
{
while (l < h) {
int lt = l, gt = h;
partition(a, lt, gt);
if (k < lt)
h = lt - 1;
else if (k > gt)
l = gt + 1;
else
break;
}
}
}
template <typename TYPE>
TYPE quick_select(TYPE *a, int k, int n)
{
assert(n && k < n);
QUICK::select(a, 0, n-1, k);
return a[k];
}
}