Add boyer moore search

Author: herobank110

boyer_moore_horspool.cpp

@@ -0,0 +1,64 @@
+#include <string_view>  // std::string_view
+#include <optional>  // std::optional
+#include <array>  // std::array
+#include <iostream>  // std::cout
+
+
+struct SearchPattern {
+    std::string_view needle;
+    std::array<char, 256> bad_char_table;
+    SearchPattern(const std::string_view needle) : needle(needle) {
+        // compute bad char table
+        bad_char_table = {};
+
+        // Rule 1
+        // Characters not in the needle jump the length of the needle.
+        bad_char_table.fill(needle.length());
+
+
+        // Rule 2
+        // Characters in the needle except last character jump:
+        // length of needle - last position of character - 1
+        for (std::size_t i = 0; i < needle.length(); i++) {
+            bad_char_table[needle[i]] = needle.length() - i - 1;
+        }
+    }
+};
+
+/** Returns index of first occurrance of pattern in haystack. */
+std::optional<std::size_t> boyer_moore_horspool(SearchPattern pattern, std::string_view haystack) {
+    for (std::size_t h = 0; h < haystack.length() - pattern.needle.length() + 1;) {
+        // Search this placement of the needle.
+        bool found = false;
+        for (std::size_t n = pattern.needle.length() - 1; ; n--) {
+            // Check the needle in reverse.
+            if (haystack[h + n] != pattern.needle[n]) {
+                // Bad character found.
+                found = false;
+                // Jump based on the bad character table.
+                h += pattern.bad_char_table[haystack[h + n]];
+                break;
+            }
+            if (n == 0) {
+                break;
+            }
+        }
+        if (found) {
+            return h;
+        }
+    }
+
+    // Not found in the haystack.
+    return std::nullopt;
+}
+
+int main() {
+    auto needle = SearchPattern{ "abc" };
+    auto haystack = "aa abc ddef";
+    auto index = boyer_moore_horspool(needle, haystack);
+    if (index) {
+        std::cout << "found at index " << index.value() << '\n' << std::flush;
+    } else {
+        std::cout << "not found" << std::endl;
+    }
+}