int arr_val[3];
  int arr_diff1[3];
  int arr_if1[3];
  int arr_diff2[3];
  int arr_if2[3];
  int arr_out[3];

void setup() {
  Serial.begin(115200);
}

void loop() {

  int val = analogRead(35);
  int out = my_envelope(val);

  //Serial.print(val);
  //Serial.print(";");
  //Serial.println(out);
  
  delay(100); // this speeds up the simulation

}

int my_envelope(int value) {

  //сдвигаем все массивы на такт
  arr_val[2] = arr_val[1];
  arr_val[1] = arr_val[0];
  arr_val[0] = value;

  arr_diff1[2] = arr_diff1[1];
  arr_diff1[1] = arr_diff1[0];
    
  arr_if1[2] = arr_if1[1];
  arr_if1[1] = arr_if1[0];

  arr_diff2[2] = arr_diff2[1];
  arr_diff2[1] = arr_diff2[0];
    
  arr_if2[2] = arr_if2[1];
  arr_if2[1] = arr_if2[0];
    
  arr_out[2] = arr_out[1];
  arr_out[1] = arr_out[0];

  //начинаем вычислять
  if (arr_val[1] - arr_val[2] > 0) arr_diff1[0] = 1;
  if (arr_val[1] - arr_val[2] == 0) arr_diff1[0] = 0;
  if (arr_val[1] - arr_val[2] < 0) arr_diff1[0] = -1;
    
  if (arr_diff1[0] != 0) {
    arr_if1[0] = arr_diff1[0];
  } else {
    arr_if1[0] = arr_if1[1];
  }
    
  arr_diff2[0] = arr_if1[0] - arr_if1[1];
  
  if (arr_diff2[0] == -2) {
    arr_if2[0] = 1;
  } else {
    arr_if2[0] = 0;
  }

  if (arr_if2[0] == 1) {
    arr_out[0] = arr_val[2];
  } else {
    arr_out[0] = arr_out[1];
  }
    
  Serial.print(value);
  Serial.print(";");
  Serial.println(arr_out[1]);

  return arr_out[1];
}