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diff --git a/Userland/Applications/Calculator/Keypad.h b/Userland/Applications/Calculator/Keypad.h
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+++ b/Userland/Applications/Calculator/Keypad.h
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+/*
+ * Copyright (c) 2019-2020, Sergey Bugaev <bugaevc@serenityos.org>
+ * 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.
+ */
+
+#pragma once
+
+#include <AK/String.h>
+
+// This type implements number typing and
+// displaying mechanics. It does not perform
+// any arithmetic operations or anything on
+// the values it deals with.
+
+class Keypad final {
+public:
+    Keypad();
+    ~Keypad();
+
+    void type_digit(int digit);
+    void type_decimal_point();
+    void type_backspace();
+
+    double value() const;
+    void set_value(double);
+
+    String to_string() const;
+
+private:
+    // Internal representation of the current decimal value.
+    bool m_negative { false };
+    long m_int_value { 0 };
+    long m_frac_value { 0 };
+    int m_frac_length { 0 };
+    // E.g. for -35.004200,
+    // m_negative = true
+    // m_int_value = 35
+    // m_frac_value = 4200
+    // m_frac_length = 6
+
+    enum class State {
+        External,
+        TypingInteger,
+        TypingDecimal
+    };
+
+    State m_state { State::External };
+};