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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.
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5 changed files with 178 additions and 0 deletions
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@ -39,6 +39,34 @@ String copy_string_from_user(const char* user_str, size_t user_str_size)
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return String(user_str, length);
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}
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namespace {
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const static void* bitap_bitwise(const void* haystack, size_t haystack_length, const void* needle, size_t needle_length)
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{
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ASSERT(needle_length < 32);
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u64 lookup = 0xfffffffe;
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constexpr size_t mask_length = (size_t)((u8)-1) + 1;
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u64 needle_mask[mask_length];
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for (size_t i = 0; i < mask_length; ++i)
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needle_mask[i] = 0xffffffff;
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for (size_t i = 0; i < needle_length; ++i)
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needle_mask[((const u8*)needle)[i]] &= ~(0x00000001 << i);
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for (size_t i = 0; i < haystack_length; ++i) {
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lookup |= needle_mask[((const u8*)haystack)[i]];
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lookup <<= 1;
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if (!(lookup & (0x00000001 << needle_length)))
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return ((const u8*)haystack) + i - needle_length + 1;
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}
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return nullptr;
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}
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}
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extern "C" {
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void copy_to_user(void* dest_ptr, const void* src_ptr, size_t n)
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@ -91,6 +119,31 @@ void* memmove(void* dest, const void* src, size_t n)
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return dest;
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}
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const void* memmem(const void* haystack, size_t haystack_length, const void* needle, size_t needle_length)
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{
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if (needle_length == 0)
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return haystack;
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if (haystack_length < needle_length)
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return nullptr;
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if (haystack_length == needle_length)
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return memcmp(haystack, needle, haystack_length) == 0 ? haystack : nullptr;
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if (needle_length < 32)
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return bitap_bitwise(haystack, haystack_length, needle, needle_length);
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// Fallback to a slower search.
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auto length_diff = haystack_length - needle_length;
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for (size_t i = 0; i < length_diff; ++i) {
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const auto* start = ((const u8*)haystack) + i;
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if (memcmp(start, needle, needle_length) == 0)
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return start;
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}
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return nullptr;
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}
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char* strcpy(char* dest, const char* src)
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{
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auto* dest_ptr = dest;
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