89 lines · c
1//===-- Implementation header for qsort utilities ---------------*- C++ -*-===//2//3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.4// See https://llvm.org/LICENSE.txt for license information.5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception6//7//===----------------------------------------------------------------------===//8 9#ifndef LLVM_LIBC_SRC_STDLIB_QSORT_PIVOT_H10#define LLVM_LIBC_SRC_STDLIB_QSORT_PIVOT_H11 12#include "src/__support/macros/attributes.h"13 14#include <stddef.h> // For size_t15 16namespace LIBC_NAMESPACE_DECL {17namespace internal {18 19// Recursively select a pseudomedian if above this threshold.20constexpr size_t PSEUDO_MEDIAN_REC_THRESHOLD = 64;21 22// Selects a pivot from `array`. Algorithm taken from glidesort by Orson Peters.23//24// This chooses a pivot by sampling an adaptive amount of points, approximating25// the quality of a median of sqrt(n) elements.26template <typename A, typename F>27LIBC_INLINE size_t choose_pivot(const A &array, const F &is_less) {28 const size_t len = array.len();29 30 if (len < 8) {31 return 0;32 }33 34 const size_t len_div_8 = len / 8;35 36 const size_t a = 0; // [0, floor(n/8))37 const size_t b = len_div_8 * 4; // [4*floor(n/8), 5*floor(n/8))38 const size_t c = len_div_8 * 7; // [7*floor(n/8), 8*floor(n/8))39 40 if (len < PSEUDO_MEDIAN_REC_THRESHOLD)41 return median3(array, a, b, c, is_less);42 else43 return median3_rec(array, a, b, c, len_div_8, is_less);44}45 46// Calculates an approximate median of 3 elements from sections a, b, c, or47// recursively from an approximation of each, if they're large enough. By48// dividing the size of each section by 8 when recursing we have logarithmic49// recursion depth and overall sample from f(n) = 3*f(n/8) -> f(n) =50// O(n^(log(3)/log(8))) ~= O(n^0.528) elements.51template <typename A, typename F>52LIBC_INLINE size_t median3_rec(const A &array, size_t a, size_t b, size_t c,53 size_t n, const F &is_less) {54 if (n * 8 >= PSEUDO_MEDIAN_REC_THRESHOLD) {55 const size_t n8 = n / 8;56 a = median3_rec(array, a, a + (n8 * 4), a + (n8 * 7), n8, is_less);57 b = median3_rec(array, b, b + (n8 * 4), b + (n8 * 7), n8, is_less);58 c = median3_rec(array, c, c + (n8 * 4), c + (n8 * 7), n8, is_less);59 }60 return median3(array, a, b, c, is_less);61}62 63/// Calculates the median of 3 elements.64template <typename A, typename F>65LIBC_INLINE size_t median3(const A &array, size_t a, size_t b, size_t c,66 const F &is_less) {67 const void *a_ptr = array.get(a);68 const void *b_ptr = array.get(b);69 const void *c_ptr = array.get(c);70 71 const bool x = is_less(a_ptr, b_ptr);72 const bool y = is_less(a_ptr, c_ptr);73 if (x == y) {74 // If x=y=0 then b, c <= a. In this case we want to return max(b, c).75 // If x=y=1 then a < b, c. In this case we want to return min(b, c).76 // By toggling the outcome of b < c using XOR x we get this behavior.77 const bool z = is_less(b_ptr, c_ptr);78 return z ^ x ? c : b;79 } else {80 // Either c <= a < b or b <= a < c, thus a is our median.81 return a;82 }83}84 85} // namespace internal86} // namespace LIBC_NAMESPACE_DECL87 88#endif // LLVM_LIBC_SRC_STDLIB_QSORT_PIVOT_H89