MT4/MT5 的指标计算都带缓存，不用每次都从头到尾的在每一根 bar 上计算。指标提供了 OnCalculate 回调函数。也就是在初次加载指标时，OnCalculate 回调函数会计算一下历史 bar，然后就等待 tick 更新最后一根 bar，tick 每更新一次就刷新一次 OnCalculate，这时可能还会出现断网再重连补数据的情况，那么就需要不光要绘制最后一根 bar, 要多绘制最后几根 bar。

那么在 OnCalculate 中就可以正向循环计算和反向循环计算两种方法。

从后往前计算

//+------------------------------------------------------------------+
//| Bill Williams' Alligator                                         |
//+------------------------------------------------------------------+
int OnCalculate(const int rates_total,
                const int prev_calculated,
                const datetime &time[],
                const double &open[],
                const double &high[],
                const double &low[],
                const double &close[],
                const long &tick_volume[],
                const long &volume[],
                const int &spread[]) {
    int limit = rates_total - prev_calculated;

//---- main loop
    for(int i = 0; i < limit; i++) {

        double Range = 0;
        double AvgRange = 0;

        for(int counter = 0; counter <= 0 + 9; counter++) {
            AvgRange = AvgRange + MathAbs(High[counter] - Low[counter]);
        }
        Range = AvgRange / 10;

        //---- ma_shift set to 0 because SetIndexShift called abowe
        ExtBlueBuffer[i] = iMA(NULL, 0, InpJawsPeriod, 0, MODE_SMMA, PRICE_MEDIAN, i);
        ExtRedBuffer[i] = iMA(NULL, 0, InpTeethPeriod, 0, MODE_SMMA, PRICE_MEDIAN, i);
        ExtLimeBuffer[i] = iMA(NULL, 0, InpLipsPeriod, 0, MODE_SMMA, PRICE_MEDIAN, i);

        double rosokuValueCloseP = iClose(NULL, 0, i);

        double JAW_ATAI_1 = ExtBlueBuffer[i];
        double JAW_ATAI_2 = iMA(NULL, 0, InpJawsPeriod, 0, MODE_SMMA, PRICE_MEDIAN, i + 1);

        double TEETH_ATAI_1 = ExtRedBuffer[i];
        double TEETH_ATAI_2 = iMA(NULL, 0, InpTeethPeriod, 0, MODE_SMMA, PRICE_MEDIAN, i + 1);

        double LIPS_ATAI_1 = ExtLimeBuffer[i];
        double LIPS_ATAI_2 = iMA(NULL, 0, InpLipsPeriod, 0, MODE_SMMA, PRICE_MEDIAN, i + 1);

        if(LIPS_ATAI_1 > TEETH_ATAI_1 && LIPS_ATAI_1 > JAW_ATAI_1 && JAW_ATAI_2 < JAW_ATAI_1 && rosokuValueCloseP > LIPS_ATAI_1) {

            CrossUp[i] = Low[i] - Range * 0.5;
        }

        else if(LIPS_ATAI_1 < TEETH_ATAI_1 && LIPS_ATAI_1 < JAW_ATAI_1 && JAW_ATAI_2 > JAW_ATAI_1 && rosokuValueCloseP < LIPS_ATAI_1) {

            CrossDown[i] = High[i] + Range * 0.5;
        } else {

        }

    }

    return(rates_total);
}

从前往后计算

//+------------------------------------------------------------------+
//| Bill Williams' Alligator                                         |
//+------------------------------------------------------------------+
int OnCalculate(const int rates_total,
                const int prev_calculated,
                const datetime &time[],
                const double &open[],
                const double &high[],
                const double &low[],
                const double &close[],
                const long &tick_volume[],
                const long &volume[],
                const int &spread[]) {
    int limit = rates_total - prev_calculated - 1;    

//---- main loop
    for(int i = limit; i >= 0; i--) {

        double Range = 0;
        double AvgRange = 0;

        for(int counter = 0; counter <= 0 + 9; counter++) {
            AvgRange = AvgRange + MathAbs(High[counter] - Low[counter]);
        }
        Range = AvgRange / 10;

        //---- ma_shift set to 0 because SetIndexShift called abowe
        ExtBlueBuffer[i] = iMA(NULL, 0, InpJawsPeriod, 0, MODE_SMMA, PRICE_MEDIAN, i);
        ExtRedBuffer[i] = iMA(NULL, 0, InpTeethPeriod, 0, MODE_SMMA, PRICE_MEDIAN, i);
        ExtLimeBuffer[i] = iMA(NULL, 0, InpLipsPeriod, 0, MODE_SMMA, PRICE_MEDIAN, i);

        double rosokuValueCloseP = iClose(NULL, 0, i);

        double JAW_ATAI_1 = ExtBlueBuffer[i];
        double JAW_ATAI_2 = iMA(NULL, 0, InpJawsPeriod, 0, MODE_SMMA, PRICE_MEDIAN, i + 1);

        double TEETH_ATAI_1 = ExtRedBuffer[i];
        double TEETH_ATAI_2 = iMA(NULL, 0, InpTeethPeriod, 0, MODE_SMMA, PRICE_MEDIAN, i + 1);

        double LIPS_ATAI_1 = ExtLimeBuffer[i];
        double LIPS_ATAI_2 = iMA(NULL, 0, InpLipsPeriod, 0, MODE_SMMA, PRICE_MEDIAN, i + 1);

        if(LIPS_ATAI_1 > TEETH_ATAI_1 && LIPS_ATAI_1 > JAW_ATAI_1 && JAW_ATAI_2 < JAW_ATAI_1 && rosokuValueCloseP > LIPS_ATAI_1) {

            CrossUp[i] = Low[i] - Range * 0.5;
        }

        else if(LIPS_ATAI_1 < TEETH_ATAI_1 && LIPS_ATAI_1 < JAW_ATAI_1 && JAW_ATAI_2 > JAW_ATAI_1 && rosokuValueCloseP < LIPS_ATAI_1) {

            CrossDown[i] = High[i] + Range * 0.5;
        } else {

        }

    }

    return(rates_total);
}