AK: Implement memmem() for iterator haystacks

This uses the KMP algorithm to implement the search.
Also replaces the slow route of the normal memmem() with KMP, which
should be fairly faster (O(n + m) as opposed to O(n * m)) :^)

AK/MemMem.h

@@ -26,8 +26,11 @@
 
 #pragma once
 
+#include <AK/Array.h>
 #include <AK/Assertions.h>
+#include <AK/Span.h>
 #include <AK/Types.h>
+#include <AK/Vector.h>
 
 namespace AK {
 
@@ -59,6 +62,64 @@ const static void* bitap_bitwise(const void* haystack, size_t haystack_length, c
 }
 }
 
+template<typename HaystackIterT>
+static inline Optional<size_t> memmem(const HaystackIterT& haystack_begin, const HaystackIterT& haystack_end, Span<const u8> needle) requires(requires { (*haystack_begin).data(); (*haystack_begin).size(); })
+{
+    auto prepare_kmp_partial_table = [&] {
+        Vector<int, 64> table;
+        table.resize(needle.size());
+
+        size_t position = 1;
+        int candidate = 0;
+
+        table[0] = -1;
+        while (position < needle.size()) {
+            if (needle[position] == needle[candidate]) {
+                table[position] = table[candidate];
+            } else {
+                table[position] = candidate;
+                do {
+                    candidate = table[candidate];
+                } while (candidate >= 0 && needle[candidate] != needle[position]);
+            }
+            ++position;
+            ++candidate;
+        }
+        return table;
+    };
+
+    auto table = prepare_kmp_partial_table();
+    size_t total_haystack_index = 0;
+    size_t current_haystack_index = 0;
+    int needle_index = 0;
+    auto haystack_it = haystack_begin;
+
+    while (haystack_it != haystack_end) {
+        auto&& chunk = *haystack_it;
+        if (current_haystack_index >= chunk.size()) {
+            current_haystack_index = 0;
+            ++haystack_it;
+            continue;
+        }
+        if (needle[needle_index] == chunk[current_haystack_index]) {
+            ++needle_index;
+            ++current_haystack_index;
+            ++total_haystack_index;
+            if ((size_t)needle_index == needle.size())
+                return total_haystack_index - needle_index;
+            continue;
+        }
+        needle_index = table[needle_index];
+        if (needle_index < 0) {
+            ++needle_index;
+            ++current_haystack_index;
+            ++total_haystack_index;
+        }
+    }
+
+    return {};
+}
+
 static inline const void* memmem(const void* haystack, size_t haystack_length, const void* needle, size_t needle_length)
 {
     if (needle_length == 0)
@@ -73,15 +134,14 @@ static inline const void* memmem(const void* haystack, size_t haystack_length, c
     if (needle_length < 32)
         return bitap_bitwise(haystack, haystack_length, needle, needle_length);
 
-    // Fallback to a slower search.
-    auto length_diff = haystack_length - needle_length;
-    for (size_t i = 0; i < length_diff; ++i) {
-        const auto* start = ((const u8*)haystack) + i;
-        if (__builtin_memcmp(start, needle, needle_length) == 0)
-            return start;
-    }
+    // Fallback to KMP.
+    Array<Span<const u8>, 1> spans { Span<const u8> { (const u8*)haystack, haystack_length } };
+    auto result = memmem(spans.begin(), spans.end(), { (const u8*)needle, needle_length });
 
-    return nullptr;
+    if (result.has_value())
+        return (const u8*)haystack + result.value();
+
+    return {};
 }
 
 }

AK/Tests/TestMemMem.cpp

@@ -0,0 +1,90 @@
+/*
+ * Copyright (c) 2020, the SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/TestSuite.h>
+
+#include <AK/MemMem.h>
+
+TEST_CASE(bitap)
+{
+    Array<u8, 8> haystack { 1, 0, 1, 2, 3, 4, 5, 0 };
+    Array<u8, 4> needle_0 { 2, 3, 4, 5 };
+    Array<u8, 4> needle_1 { 1, 2, 3, 4 };
+    Array<u8, 4> needle_2 { 3, 4, 5, 0 };
+    Array<u8, 4> needle_3 { 3, 4, 5, 6 };
+
+    auto result_0 = AK::memmem(haystack.data(), haystack.size(), needle_0.data(), needle_0.size());
+    auto result_1 = AK::memmem(haystack.data(), haystack.size(), needle_1.data(), needle_1.size());
+    auto result_2 = AK::memmem(haystack.data(), haystack.size(), needle_2.data(), needle_2.size());
+    auto result_3 = AK::memmem(haystack.data(), haystack.size(), needle_3.data(), needle_3.size());
+
+    EXPECT_EQ(result_0, &haystack[3]);
+    EXPECT_EQ(result_1, &haystack[2]);
+    EXPECT_EQ(result_2, &haystack[4]);
+    EXPECT_EQ(result_3, nullptr);
+}
+
+TEST_CASE(kmp_one_chunk)
+{
+    Array<u8, 8> haystack { 1, 0, 1, 2, 3, 4, 5, 0 };
+    Array<Array<u8, 8>, 1> haystack_arr { haystack };
+    Array<u8, 4> needle_0 { 2, 3, 4, 5 };
+    Array<u8, 4> needle_1 { 1, 2, 3, 4 };
+    Array<u8, 4> needle_2 { 3, 4, 5, 0 };
+    Array<u8, 4> needle_3 { 3, 4, 5, 6 };
+
+    auto result_0 = AK::memmem(haystack_arr.begin(), haystack_arr.end(), needle_0);
+    auto result_1 = AK::memmem(haystack_arr.begin(), haystack_arr.end(), needle_1);
+    auto result_2 = AK::memmem(haystack_arr.begin(), haystack_arr.end(), needle_2);
+    auto result_3 = AK::memmem(haystack_arr.begin(), haystack_arr.end(), needle_3);
+
+    EXPECT_EQ(result_0.value_or(9), 3u);
+    EXPECT_EQ(result_1.value_or(9), 2u);
+    EXPECT_EQ(result_2.value_or(9), 4u);
+    EXPECT(!result_3.has_value());
+}
+
+TEST_CASE(kmp_two_chunks)
+{
+    Array<u8, 4> haystack_first_half { 1, 0, 1, 2 }, haystack_second_half { 3, 4, 5, 0 };
+    Array<Array<u8, 4>, 2> haystack { haystack_first_half, haystack_second_half };
+    Array<u8, 4> needle_0 { 2, 3, 4, 5 };
+    Array<u8, 4> needle_1 { 1, 2, 3, 4 };
+    Array<u8, 4> needle_2 { 3, 4, 5, 0 };
+    Array<u8, 4> needle_3 { 3, 4, 5, 6 };
+
+    auto result_0 = AK::memmem(haystack.begin(), haystack.end(), needle_0);
+    auto result_1 = AK::memmem(haystack.begin(), haystack.end(), needle_1);
+    auto result_2 = AK::memmem(haystack.begin(), haystack.end(), needle_2);
+    auto result_3 = AK::memmem(haystack.begin(), haystack.end(), needle_3);
+
+    EXPECT_EQ(result_0.value_or(9), 3u);
+    EXPECT_EQ(result_1.value_or(9), 2u);
+    EXPECT_EQ(result_2.value_or(9), 4u);
+    EXPECT(!result_3.has_value());
+}
+
+TEST_MAIN(MemMem)