Kernel+LibC: Implement 'memmem'

This commit adds an implementation of memmem, using the Bitap text
search algorithm for needles smaller than 32 bytes, and a naive loop
search for longer needles.

Kernel/StdLib.cpp

@@ -39,6 +39,34 @@ String copy_string_from_user(const char* user_str, size_t user_str_size)
     return String(user_str, length);
 }
 
+namespace {
+const static void* bitap_bitwise(const void* haystack, size_t haystack_length, const void* needle, size_t needle_length)
+{
+    ASSERT(needle_length < 32);
+
+    u64 lookup = 0xfffffffe;
+
+    constexpr size_t mask_length = (size_t)((u8)-1) + 1;
+    u64 needle_mask[mask_length];
+
+    for (size_t i = 0; i < mask_length; ++i)
+        needle_mask[i] = 0xffffffff;
+
+    for (size_t i = 0; i < needle_length; ++i)
+        needle_mask[((const u8*)needle)[i]] &= ~(0x00000001 << i);
+
+    for (size_t i = 0; i < haystack_length; ++i) {
+        lookup |= needle_mask[((const u8*)haystack)[i]];
+        lookup <<= 1;
+
+        if (!(lookup & (0x00000001 << needle_length)))
+            return ((const u8*)haystack) + i - needle_length + 1;
+    }
+
+    return nullptr;
+}
+}
+
 extern "C" {
 
 void copy_to_user(void* dest_ptr, const void* src_ptr, size_t n)
@@ -91,6 +119,31 @@ void* memmove(void* dest, const void* src, size_t n)
     return dest;
 }
 
+const void* memmem(const void* haystack, size_t haystack_length, const void* needle, size_t needle_length)
+{
+    if (needle_length == 0)
+        return haystack;
+
+    if (haystack_length < needle_length)
+        return nullptr;
+
+    if (haystack_length == needle_length)
+        return memcmp(haystack, needle, haystack_length) == 0 ? haystack : nullptr;
+
+    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 (memcmp(start, needle, needle_length) == 0)
+            return start;
+    }
+
+    return nullptr;
+}
+
 char* strcpy(char* dest, const char* src)
 {
     auto* dest_ptr = dest;