feat(searching): add exponential_search2 implementation with self-tests and Doxygen header

Author: hobostay

searching/exponential_search2.c

@@ -0,0 +1,95 @@
+/**
+ * @file exponential_search2.c
+ * @brief Alternative implementation of [Exponential Search](https://en.wikipedia.org/wiki/Exponential_search).
+ * @details
+ * This version performs range expansion using safe bounds and a tight binary
+ * search. It returns the index of the target in a sorted array or -1 if not found.
+ * @author [hobostay](https://github.com/hobostay)
+ */
+
+#include <assert.h>   /// for assert
+#include <stdio.h>    /// for printf
+
+/**
+ * @brief Binary search within the closed interval [left, right]
+ */
+static int bsearch_closed(const int *arr, int left, int right, int x) {
+    while (left <= right) {
+        int mid = left + (right - left) / 2;
+        int v = arr[mid];
+        if (v == x) {
+            return mid;
+        }
+        if (v < x) {
+            left = mid + 1;
+        } else {
+            right = mid - 1;
+        }
+    }
+    return -1;
+}
+
+/**
+ * @brief Exponential search on a sorted array
+ * @param arr pointer to sorted array
+ * @param n length of array
+ * @param x target value to search for
+ * @returns index of x if found, otherwise -1
+ */
+int exponential_search2(const int *arr, int n, int x) {
+    if (arr == NULL || n <= 0) {
+        return -1;
+    }
+    if (arr[0] == x) {
+        return 0;
+    }
+    int bound = 1;
+    while (bound < n && arr[bound] < x) {
+        bound <<= 1;  // double
+    }
+    int left = bound >> 1;
+    int right = (bound < n) ? bound : (n - 1);
+    return bsearch_closed(arr, left, right, x);
+}
+
+/**
+ * @brief Self tests for exponential_search2
+ */
+static void test(void) {
+    printf("Test 1.... ");
+    int a1[] = {1, 3, 5, 7, 9, 11, 13, 15};
+    int n1 = (int)(sizeof(a1) / sizeof(a1[0]));
+    assert(exponential_search2(a1, n1, 1) == 0);
+    assert(exponential_search2(a1, n1, 15) == 7);
+    assert(exponential_search2(a1, n1, 7) == 3);
+    assert(exponential_search2(a1, n1, 4) == -1);
+    printf("passed\n");
+
+    printf("Test 2.... ");
+    int a2[] = {-10, -5, -1, 0, 2, 4, 8, 16, 32, 64};
+    int n2 = (int)(sizeof(a2) / sizeof(a2[0]));
+    assert(exponential_search2(a2, n2, -10) == 0);
+    assert(exponential_search2(a2, n2, 64) == 9);
+    assert(exponential_search2(a2, n2, 32) == 8);
+    assert(exponential_search2(a2, n2, 100) == -1);
+    printf("passed\n");
+
+    printf("Test 3.... ");
+    int a3[] = {2};
+    int n3 = (int)(sizeof(a3) / sizeof(a3[0]));
+    assert(exponential_search2(a3, n3, 2) == 0);
+    assert(exponential_search2(a3, n3, 3) == -1);
+    printf("passed\n");
+
+    printf("Test 4.... ");
+    assert(exponential_search2(NULL, 0, 1) == -1);
+    printf("passed\n");
+}
+
+/**
+ * @brief Main function
+ */
+int main(void) {
+    test();
+    return 0;
+}