init

Telegram/lib_base/base/crc32hash.cpp

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+// This file is part of Desktop App Toolkit,
+// a set of libraries for developing nice desktop applications.
+//
+// For license and copyright information please follow this link:
+// https://github.com/desktop-app/legal/blob/master/LEGAL
+//
+#include "base/crc32hash.h"
+
+namespace base {
+namespace {
+
+class Crc32Table {
+public:
+	Crc32Table() {
+		auto poly = std::uint32_t(0x04c11db7);
+		for (auto i = 0; i != 256; ++i) {
+			_data[i] = reflect(i, 8) << 24;
+			for (auto j = 0; j != 8; ++j) {
+				_data[i] = (_data[i] << 1) ^ (_data[i] & (1 << 31) ? poly : 0);
+			}
+			_data[i] = reflect(_data[i], 32);
+		}
+	}
+
+	std::uint32_t operator[](int index) const {
+		return _data[index];
+	}
+
+private:
+	std::uint32_t reflect(std::uint32_t val, char ch) {
+		auto result = std::uint32_t(0);
+		for (int i = 1; i < (ch + 1); ++i) {
+			if (val & 1) {
+				result |= 1 << (ch - i);
+			}
+			val >>= 1;
+		}
+		return result;
+	}
+
+	std::uint32_t _data[256];
+
+};
+
+} // namespace
+
+std::int32_t crc32(const void *data, int len) {
+	static const auto kTable = Crc32Table();
+
+	const auto buffer = static_cast<const std::uint8_t*>(data);
+
+	auto crc = std::uint32_t(0xffffffff);
+	for (auto i = 0; i != len; ++i) {
+		crc = (crc >> 8) ^ kTable[(crc & 0xFF) ^ buffer[i]];
+	}
+
+	return static_cast<std::int32_t>(crc ^ 0xffffffff);
+}
+
+} // namespace base