exp

static VALUE
BigMath_s_exp(VALUE klass, VALUE x, VALUE vprec)
{
    ssize_t prec, n, i;
    Real* vx = NULL;
    VALUE one, d, y;
    int negative = 0;
    int infinite = 0;
    int nan = 0;
    double flo;

    prec = NUM2SSIZET(vprec);
    if (prec <= 0) {
        rb_raise(rb_eArgError, "Zero or negative precision for exp");
    }

    /* TODO: the following switch statement is almostly the same as one in the
     *       BigDecimalCmp function. */
    switch (TYPE(x)) {
      case T_DATA:
        if (!is_kind_of_BigDecimal(x)) break;
        vx = DATA_PTR(x);
        negative = VpGetSign(vx) < 0;
        infinite = VpIsPosInf(vx) || VpIsNegInf(vx);
        nan = VpIsNaN(vx);
        break;

      case T_FIXNUM:
        /* fall through */
      case T_BIGNUM:
        vx = GetVpValue(x, 0);
        break;

      case T_FLOAT:
        flo = RFLOAT_VALUE(x);
        negative = flo < 0;
        infinite = isinf(flo);
        nan = isnan(flo);
        if (!infinite && !nan) {
            vx = GetVpValueWithPrec(x, DBL_DIG+1, 0);
        }
        break;

      case T_RATIONAL:
        vx = GetVpValueWithPrec(x, prec, 0);
        break;

      default:
        break;
    }
    if (infinite) {
        if (negative) {
            return ToValue(GetVpValueWithPrec(INT2FIX(0), prec, 1));
        }
        else {
            Real* vy;
            vy = VpCreateRbObject(prec, "#0");
            VpSetInf(vy, VP_SIGN_POSITIVE_INFINITE);
            RB_GC_GUARD(vy->obj);
            return ToValue(vy);
        }
    }
    else if (nan) {
        Real* vy;
        vy = VpCreateRbObject(prec, "#0");
        VpSetNaN(vy);
        RB_GC_GUARD(vy->obj);
        return ToValue(vy);
    }
    else if (vx == NULL) {
        cannot_be_coerced_into_BigDecimal(rb_eArgError, x);
    }
    x = vx->obj;

    n = prec + rmpd_double_figures();
    negative = VpGetSign(vx) < 0;
    if (negative) {
        VpSetSign(vx, 1);
    }

    one = ToValue(VpCreateRbObject(1, "1"));
    y   = one;
    d   = y;
    i   = 1;

    while (!VpIsZero((Real*)DATA_PTR(d))) {
        SIGNED_VALUE const ey = VpExponent10(DATA_PTR(y));
        SIGNED_VALUE const ed = VpExponent10(DATA_PTR(d));
        ssize_t m = n - vabs(ey - ed);

        rb_thread_check_ints();

        if (m <= 0) {
            break;
        }
        else if ((size_t)m < rmpd_double_figures()) {
            m = rmpd_double_figures();
        }

        d = BigDecimal_mult(d, x);                             /* d <- d * x */
        d = BigDecimal_div2(d, SSIZET2NUM(i), SSIZET2NUM(m));  /* d <- d / i */
        y = BigDecimal_add(y, d);                              /* y <- y + d  */
        ++i;                                                   /* i  <- i + 1 */
    }

    if (negative) {
        return BigDecimal_div2(one, y, vprec);
    }
    else {
        vprec = SSIZET2NUM(prec - VpExponent10(DATA_PTR(y)));
        return BigDecimal_round(1, &vprec, y);
    }

    RB_GC_GUARD(one);
    RB_GC_GUARD(x);
    RB_GC_GUARD(y);
    RB_GC_GUARD(d);
}