From 76a7e2000d526b2a982a922f08a3a5dcb6bc0c3c Mon Sep 17 00:00:00 2001
From: juan <juancloudcomputing@protonmail.com>
Date: Wed, 16 Nov 2022 11:25:42 +0800
Subject: [PATCH] Add Quick-Sort

---
 semester3/pset5/7-3-Quick-Sort.c | 109 +++++++++++++++++++++++--------
 1 file changed, 80 insertions(+), 29 deletions(-)

diff --git a/semester3/pset5/7-3-Quick-Sort.c b/semester3/pset5/7-3-Quick-Sort.c
index 618b1b4..3ddd678 100644
--- a/semester3/pset5/7-3-Quick-Sort.c
+++ b/semester3/pset5/7-3-Quick-Sort.c
@@ -1,56 +1,107 @@
 #include <stdio.h>
-#define N 10
 #define CUTOFF 3
+#define SIZE 10
 
-void print(int *arr) {
-  for (int i = 0; i < N; i++) {
+void swap(int *l, int *r) {
+  int tmp = *l;
+  *l = *r;
+  *r = tmp;
+}
+
+void print(int arr[]) {
+  for (int i = 0; i < SIZE; i++) {
     printf("%d,", arr[i]);
   }
+  printf("\n");
 }
 
-void swap(int *arr, int l, int r) {
-  if (l != r) {
-    int tmp = arr[l];
-    arr[l] = arr[r];
-    arr[r] = tmp;
+int med3(int arr[], int l, int r) {
+  // remember, it's (l + r)
+  int center = (l + r) / 2;
+  // printf("l:%d\t r:%d\t center:%d\n", l, r, center);
+  if (arr[l] > arr[center]) {
+    swap(&arr[l], &arr[center]);
   }
+  if (arr[l] > arr[r]) {
+    swap(&arr[l], &arr[r]);
+  }
+  if (arr[center] > arr[r]) {
+    swap(&arr[center], &arr[r]);
+  }
+  // arr[l] <= arr[center] <= arr[r]
+  swap(&arr[center], &arr[r - 1]); // Hide pivot
+
+  return r - 1; // Return pivot
 }
 
-int partition(int *arr, int l, int r) {
-  // both edges included
-  int index = l;
-  int pivot = arr[l];
-  for (int i = l + 1; i <= r; i++) {
-    if (arr[i] < pivot) {
-      swap(arr, i, index);
-      index++;
+void insert(int arr[], int l, int r) {
+  // actually bubble sort is easier to implement
+  printf("insert(%d,%d):", l, r - l + 1);
+  // rememver using <=, because r is visited
+  for (int i = l; i <= r; i++) {
+    for (int j = l; j <= r - 1; j++) {
+      if (arr[j] > arr[j + 1]) {
+        swap(&arr[j], &arr[j + 1]);
+      }
     }
   }
-
-  swap(arr, index, l);
-  return index;
+  print(arr);
 }
 
-void quickSort(int *arr, int l, int r) {
+void Qsort(int arr[], int l, int r, int pivotIndex) {
+  int i, j;
 
-  int pi;
-  if (l < r) {
-    if (r - l < CUTOFF) {
-      printf("insert(%d,%d):", l, r - l + 1);
+  if (l + CUTOFF <= r) {
+    if (pivotIndex == -1) {
+      pivotIndex = med3(arr, l, r);
+      i = l;
+      j = r - 1;
     } else {
-      printf("Qsort(%d,%d):", l, r);
+      i = l - 1;
+      j = r;
     }
+    while (1) {
+      while (arr[++i] < arr[pivotIndex]) {
+      }
+      while (arr[--j] > arr[pivotIndex]) {
+      }
+      if (i < j && i != pivotIndex && j != pivotIndex) {
+        swap(&arr[i], &arr[j]);
+      } else {
+        break;
+      }
+    }
+    // printf("i:%d\n", i);
+    swap(&arr[i], &arr[pivotIndex]); // swap back arr[pivotIndex]
 
-    pi = partition(arr, l, r);
-
+    printf("Qsort(%d,%d):", l, r);
     print(arr);
+    // from here on, use med3-generated arr[pivotIndex]
+    if (i != r) {
+      Qsort(arr, l, i - 1, -1);
+    } else {
+      Qsort(arr, l, i, -1);
+    }
+    Qsort(arr, i + 1, r, -1);
 
-    quickSort(arr, l, pi - 1);
-    quickSort(arr, pi + 1, r);
+  } else {
+    insert(arr, l, r);
   }
 }
 
 int main(void) {
   int arr[] = {49, 38, 65, 97, 76, 13, 27, 50, 2, 8};
+
+  int pivotIndex; // pivot == 8, pivot == 4, other 2 cases pivot <= 3
+  scanf("%d", &pivotIndex);
+
+  swap(&arr[pivotIndex], &arr[SIZE - 1]);
+  // make sure arr[0] is smaller, arr[r] is larger.
+
+
+  // remember the pivot index is changed to SIZE - 1
+  Qsort(arr, 0, SIZE - 1, SIZE - 1);
+  // Qsort(arr, 0, SIZE - 1, pivotIndex);
+
   return 0;
 }