96 lines
2.1 KiB
C++
96 lines
2.1 KiB
C++
/* qsort.cpp
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* Date: 2024-06-22
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* Author: Aryadev Chavali
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* Commentary: The inplace O(nlog(n)) sortitng algorithm. Originally designed
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* in C, hence the lack of iostream.
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*/
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#include <cstdio>
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#include <cstdlib>
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#define ARR_SIZE(XS) (sizeof(XS) / sizeof((XS)[0]))
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#define SWAP(A, B) \
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do \
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{ \
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(A) ^= (B); \
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(B) ^= (A); \
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(A) ^= (B); \
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} while (0)
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#define TEST_SIZE 10
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void print_arr(int *arr, int size)
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{
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printf("{\n");
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for (int i = 0; i < size; ++i)
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printf(" %d,\n", arr[i]);
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printf("}\n");
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}
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void quicksort(int *arr, int arr_size)
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{
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#ifdef TRACE
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printf("before, arr=");
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print_arr(arr, arr_size);
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printf("\n");
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#endif
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// TODO Optimisation: for small arr_size, a different sort like the bubble
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// sort may be better, instead of initialising new stack frames.
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if (arr_size < 2)
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return;
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// TODO Optimisation: Use some heuristics to figure out a better pivot than
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// some constant choice of element.
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int pivot = 0;
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int pivot_value = arr[0];
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for (int i = 1; i < arr_size; ++i)
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{
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if (arr[i] <= pivot_value)
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{
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if (i != pivot)
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SWAP(arr[i], arr[pivot]);
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++pivot;
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}
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}
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#ifdef TRACE
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printf("after, arr=");
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print_arr(arr, arr_size);
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printf("\n");
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#endif
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// TODO Optimisation: Use a stack to do this instead of recursion.
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//
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// This algorithm is similar to a pre order traversal/dfs, in this case on an
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// array where left and right partition are children nodes to the original
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// array node. Also with recursion there's a real risk for very large
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// ARR_SIZE of running out of space in the call stack.
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if (pivot != 0)
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quicksort(arr, pivot);
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if (pivot < arr_size - 1)
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quicksort(arr + pivot + 1, arr_size - pivot - 1);
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}
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int main(void)
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{
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// randomly generate some numbers
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int *arr = new int[TEST_SIZE];
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for (size_t i = 0; i < TEST_SIZE; ++i)
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{
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arr[i] = rand() % (2L << 30);
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}
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printf("Before: ");
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print_arr(arr, TEST_SIZE);
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printf("\n");
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quicksort(arr, TEST_SIZE);
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printf("After: ");
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print_arr(arr, TEST_SIZE);
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printf("\n");
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delete[] arr;
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return 0;
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}
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