From bdc08b4f0507bc826c14737361f550ae0fc6b4c5 Mon Sep 17 00:00:00 2001
From: Martin Ashby <martin@ashbysoft.com>
Date: Mon, 14 Nov 2022 21:21:31 +0000
Subject: Exercise 4-1

---
 ex4-1.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)
 create mode 100644 ex4-1.c

diff --git a/ex4-1.c b/ex4-1.c
new file mode 100644
index 0000000..ba3660e
--- /dev/null
+++ b/ex4-1.c
@@ -0,0 +1,53 @@
+#include <stdio.h>
+#include <stdbool.h>
+
+static int _strlen(char s[]) {
+    int i;
+    for (i=0; s[i] != '\0'; i++) {}
+    return i;
+}
+
+/**
+ * returns the start index of the rightmost occurrence of t in s.
+ * or -1 if t does not appear in s
+ * 
+ * strings must be null-terminated
+ */ 
+int strrindex(char s[], char t[]) {
+    int s_len = _strlen(s);
+    int t_len = _strlen(t);
+    char last_t = t[t_len-1];
+    for (int i=(s_len-1); i>=0; i--) {
+        if (s[i] == last_t) {
+            int j = i;
+            int k = t_len-1;
+            bool full_match = true;
+            while (k >= 0) {
+                if (j < 0 || s[j] != t[k]) {
+                    full_match = false;
+                    break;
+                }
+                j--;
+                k--;
+            }
+            if (full_match) {
+                return j+1;
+            }
+        }
+    }
+    return -1;
+}
+
+static void test(char s[], char t[]) {
+    printf("strrindex s = [%s] t = [%s] result = %d\n", s, t, strrindex(s, t));
+}
+
+int main(void) {
+    test("", "");
+    test("x", "");
+    test("x", "x");
+    test("x", "xx");
+    test("xox", "x");
+    test("xox", "ox");
+    test("xox", "xox");
+}
\ No newline at end of file
-- 
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