Added hoare algorithm.

current/algorithms/hoare.c

@@ -0,0 +1,120 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+
+int hoare(int* arr, int i, int j, int idx) {
+  printf("I: %d, J: %d, IDX: %d\n", i, j, idx);
+  if (idx < i) {
+    return 0xdeadbeef;
+  }
+
+  if (idx > j) {
+    return 0xdeadbeef;
+  }
+
+  int v;
+  int x1,x2;
+  int t;
+  while (i < j - 1) {
+    v = arr[(i + j) / 2];
+    printf("Round: (%d,%d) - v: %d\n", i, j, v);
+    // sorting on three partitions
+
+    x1 = i;
+    x2 = j;
+
+    while (x1 < x2) {
+
+      while (x1 < x2 && arr[x1] <= v) { 
+        printf("x1: %d, arr[x1]: %d\n", x1, arr[x1]);
+        x1++;
+      }
+
+      while (x1 < x2 && arr[x2] >  v) { 
+        printf("x2: %d, arr[x2]: %d\n", x2, arr[x2]);
+        x2--;
+      }
+
+      printf("X1 %d to swap %d\n", x1, arr[x1]);
+      printf("X2 %d to swap %d\n", x2, arr[x2]);
+
+      // if x1 == x2, the change makes no difference
+
+      t = arr[x1];
+      arr[x1] = arr[x2];
+      arr[x2] = t;
+
+    }
+
+    if (arr[x1] > v) x1--; // we could cross == element by
+
+    x2 = x1;
+    x1 = i;
+
+    while (x1 < x2) {
+
+      while (x1 < x2 && arr[x1] < v) { 
+        printf("x1: %d, arr[x1]: %d\n", x1, arr[x1]);
+        x1++;
+      }
+
+      while (x1 < x2 && arr[x2] == v) { 
+        printf("x2: %d, arr[x2]: %d\n", x2, arr[x2]);
+        x2--;
+      }
+
+      printf("X1 %d to swap %d\n", x1, arr[x1]);
+      printf("X2 %d to swap %d\n", x2, arr[x2]);
+
+      // if x1 == x2, the change makes no difference
+
+      t = arr[x1];
+      arr[x1] = arr[x2];
+      arr[x2] = t;
+
+    }
+
+    if (x1 > idx) {
+      printf("Changing j %d to %d\n", j, x1);
+      j = x1;
+    } else {
+      // x1 <= idx
+      printf("Changing i %d to %d\n", i, x1);
+      i = x1;
+    }
+  }
+
+  // we should have i == j == idx
+  printf("i: %d, j: %d, idx: %d, v: %d\n", i, j, idx, arr[idx]);
+  return arr[idx]; // in order to make it work, we need to make sure that in this place is fully sorted, one way is to split into three pieces
+}
+
+/**
+ * n: 4
+ * 4
+ * 3
+ * 2
+ * 1
+ * idx: 4, 3, 2, 1 all work
+ */
+
+int main() {
+
+  int n;
+  printf("Provide n: ");
+  scanf("%d", &n);
+
+  int numbers[n];
+
+  for (int i = 0; i < n; i++) {
+    scanf("%d", &numbers[i]);
+  }
+
+  int idx;
+  printf("Idx: ");
+  scanf("%d", &idx);
+
+  int res = hoare(numbers, 0, n-1, idx);
+  printf("Res: %x\n", res);
+  return 0;
+}