contract BNum is BConst {

    function btoi(uint a)
        internal pure 
        returns (uint)
    {
        return a / BONE;
    }

    function bfloor(uint a)
        internal pure
        returns (uint)
    {
        return btoi(a) * BONE;
    }

    function badd(uint a, uint b)
        internal pure
        returns (uint)
    {
        uint c = a + b;
        require(c >= a, "ERR_ADD_OVERFLOW");
        return c;
    }

    function bsub(uint a, uint b)
        internal pure
        returns (uint)
    {
        (uint c, bool flag) = bsubSign(a, b);
        require(!flag, "ERR_SUB_UNDERFLOW");
        return c;
    }

    function bsubSign(uint a, uint b)
        internal pure
        returns (uint, bool)
    {
        if (a >= b) {
            return (a - b, false);
        } else {
            return (b - a, true);
        }
    }

    function bmul(uint a, uint b)
        internal pure
        returns (uint)
    {
        uint c0 = a * b;
        require(a == 0 || c0 / a == b, "ERR_MUL_OVERFLOW");
        uint c1 = c0 + (BONE / 2);
        require(c1 >= c0, "ERR_MUL_OVERFLOW");
        uint c2 = c1 / BONE;
        return c2;
    }

    function bdiv(uint a, uint b)
        internal pure
        returns (uint)
    {
        require(b != 0, "ERR_DIV_ZERO");
        uint c0 = a * BONE;
        require(a == 0 || c0 / a == BONE, "ERR_DIV_INTERNAL"); // bmul overflow
        uint c1 = c0 + (b / 2);
        require(c1 >= c0, "ERR_DIV_INTERNAL"); //  badd require
        uint c2 = c1 / b;
        return c2;
    }

    // DSMath.wpow
    function bpowi(uint a, uint n)
        internal pure
        returns (uint)
    {
        uint z = n % 2 != 0 ? a : BONE;

        for (n /= 2; n != 0; n /= 2) {
            a = bmul(a, a);

            if (n % 2 != 0) {
                z = bmul(z, a);
            }
        }
        return z;
    }

    // Compute b^(e.w) by splitting it into (b^e)*(b^0.w).
    // Use `bpowi` for `b^e` and `bpowK` for k iterations
    // of approximation of b^0.w
    function bpow(uint base, uint exp)
        internal pure
        returns (uint)
    {
        require(base >= MIN_BPOW_BASE, "ERR_BPOW_BASE_TOO_LOW");
        require(base <= MAX_BPOW_BASE, "ERR_BPOW_BASE_TOO_HIGH");

        uint whole  = bfloor(exp);   
        uint remain = bsub(exp, whole);

        uint wholePow = bpowi(base, btoi(whole));

        if (remain == 0) {
            return wholePow;
        }

        uint partialResult = bpowApprox(base, remain, BPOW_PRECISION);
        return bmul(wholePow, partialResult);
    }

    function bpowApprox(uint base, uint exp, uint precision)
        internal pure
        returns (uint)
    {
        // term 0:
        uint a     = exp;
        (uint x, bool xneg)  = bsubSign(base, BONE);
        uint term = BONE;
        uint sum   = term;
        bool negative = false;


        // term(k) = numer / denom 
        //         = (product(a - i - 1, i=1-->k) * x^k) / (k!)
        // each iteration, multiply previous term by (a-(k-1)) * x / k
        // continue until term is less than precision
        for (uint i = 1; term >= precision; i++) {
            uint bigK = i * BONE;
            (uint c, bool cneg) = bsubSign(a, bsub(bigK, BONE));
            term = bmul(term, bmul(c, x));
            term = bdiv(term, bigK);
            if (term == 0) break;

            if (xneg) negative = !negative;
            if (cneg) negative = !negative;
            if (negative) {
                sum = bsub(sum, term);
            } else {
                sum = badd(sum, term);
            }
        }

        return sum;
    }

}
function bpow(uint base, uint exp)
    internal pure
    returns (uint)
{
    require(base >= MIN_BPOW_BASE, "ERR_BPOW_BASE_TOO_LOW");
    require(base <= MAX_BPOW_BASE, "ERR_BPOW_BASE_TOO_HIGH");

    uint whole  = bfloor(exp);   
    uint remain = bsub(exp, whole);

    uint wholePow = bpowi(base, btoi(whole));

    if (remain == 0) {
        return wholePow;
    }

    uint partialResult = bpowApprox(base, remain, BPOW_PRECISION);
    return bmul(wholePow, partialResult);
}

// Compute b^(e.w) by splitting it into (b^e)*(b^0.w).
// Use `bpowi` for `b^e` and `bpowK` for k iterations
// of approximation of b^0.w