lib/number/subtraction.cc

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//
// canola - canon canola 1614p emulator
// Copyright (C) 2011, 2012 Peter Miller
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License, version 3, as
// published by the Free Software Foundation.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// General Public License for more details.
//
// You should have received a copy of the GNU General Public License along
// with this program. If not, see <http://www.gnu.org/licenses/>.
//

#include <lib/ac/assert.h>

#include <lib/number.h>


void
number::operator-=(const number &rhs)
{
    *this = operator-(rhs);
}


number
number::operator-(const number &rhs)
    const
{
    if (negative)
    {
        if (rhs.negative)
        {
            // negative - negative
            // lhs2 = -lhs
            number lhs2 = -*this;
            // rhs2 = -rhs
            number rhs2 = -rhs;
            // ans = -lhs2 - -rhs2
            // ans + lhs2 = rhs2
            // ans = rhs2 - lhs2
            return positive_minus_positive(rhs2, lhs2);
        }
        else
        {
            // negative - positive
            // lhs2 = -lhs
            number lhs2 = -*this;
            // ans = -lhs2 - rhs
            // ans = -(lhs2 + rhs)
            return -positive_plus_positive(lhs2, rhs);
        }
    }
    else
    {
        if (rhs.negative)
        {
            // positive - negative
            return positive_plus_positive(*this, -rhs);
        }
        else
        {
            // positive - positive
            return positive_minus_positive(*this, rhs);
        }
    }
}


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