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73 lines
1 KiB
C++
73 lines
1 KiB
C++
/*
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Quick select algorithm
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Copyright 2024 Ahmet Inan <inan@aicodix.de>
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*/
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#pragma once
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namespace DSP {
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namespace QUICK {
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template <typename TYPE>
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static inline void swap(TYPE *a, int i, int j)
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{
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TYPE t = a[i];
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a[i] = a[j];
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a[j] = t;
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}
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template <typename TYPE>
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static inline void sort(TYPE *a, int i, int j, int k)
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{
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if (a[i] > a[j])
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swap(a, i, j);
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if (a[i] > a[k])
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swap(a, i, k);
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if (a[j] > a[k])
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swap(a, j, k);
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}
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template <typename TYPE>
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static void partition(TYPE *a, int &l, int &h)
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{
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int m = l + (h - l) / 2;
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sort(a, l, m, h);
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TYPE pivot = a[m];
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for (int i = l; i <= h;)
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if (a[i] < pivot)
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swap(a, i++, l++);
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else if (a[i] > pivot)
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swap(a, i, h--);
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else
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++i;
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}
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template <typename TYPE>
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static void select(TYPE *a, int l, int h, int k)
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{
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while (l < h) {
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int lt = l, gt = h;
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partition(a, lt, gt);
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if (k < lt)
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h = lt - 1;
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else if (k > gt)
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l = gt + 1;
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else
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break;
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}
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}
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}
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template <typename TYPE>
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TYPE quick_select(TYPE *a, int k, int n)
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{
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assert(n && k < n);
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QUICK::select(a, 0, n-1, k);
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return a[k];
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}
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}
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