Merge pull request #329 from herobank110/master

Add Boyer Moore horspool algorithm (compile time evaluated)

Author: amritansh22

Diff for: boyer_moore_horspool.cpp

@@ -0,0 +1,76 @@
+// Compile Time Evaluated Boyer Moore Horspool String Search Algorithm
+// Author: David Kanekanian
+//
+// Requires C++17.
+
+#include <string_view>  // std::string_view
+#include <optional>  // std::optional
+#include <array>  // std::array
+#include <iostream>  // std::cout
+
+
+struct SearchPattern {
+    constexpr static std::size_t num_chars{ 256 };
+    const std::string_view needle;
+    std::array<char, num_chars> bad_char_table{};
+
+    constexpr SearchPattern(const std::string_view needle) noexcept
+        : needle(needle) {
+        // compute bad char table
+
+        // Rule 1
+        // Characters not in the needle jump the length of the needle.
+        for (std::size_t i = 0; i < num_chars; i++) {
+            bad_char_table[i] = 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. */
+[[nodiscard]] constexpr std::optional<std::size_t> boyer_moore_horspool (
+    const SearchPattern& pattern,
+    const std::string_view haystack) noexcept {
+    for (std::size_t h = 0; h < haystack.length() - pattern.needle.length() + 1;) {
+        // Search this placement of the needle.
+        bool found = true;
+        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() {
+    constexpr SearchPattern needle{ "abc" };
+    constexpr auto haystack = "aa abc ddef";
+    constexpr auto index = boyer_moore_horspool(needle, haystack);
+    static_assert(index == 3, "index is supposed to be 3!");
+
+    if (index) {
+        std::cout << "found at index " << index.value() << '\n';
+    } else {
+        std::cout << "not found" << '\n';
+    }
+}