[apps/graph/values] add a temporary methods to print float in a buffer

Change-Id: Ib81339b4b7e41a59f20432c90310a8aee68c9299

apps/graph/values/float_to_string.cpp

@@ -0,0 +1,77 @@
+#include "float_to_string.h"
+
+void FloatToString::printBase10IntegerWithDecimalMarker(char * buffer, int bufferLength,  int i, int decimalMarkerPosition) {
+  /* The decimal marker position is always preceded by a char, thus, it is never
+   * in first position. When called by convertFloatToText, the buffer length is
+   * always > 0 as we asserted a minimal number of available chars. */
+  assert(bufferLength > 0 && decimalMarkerPosition != 0);
+  int endChar = bufferLength - 1, startChar = 0;
+  int dividend = i, digit = 0, quotien = 0;
+  if (i < 0) {
+    buffer[startChar++] = '-';
+    dividend = -i;
+    decimalMarkerPosition += 1;
+  }
+  /* This loop acts correctly as we asserted the endChar >= 0 and
+   * decimalMarkerPosition != 0 */
+  do {
+    if (endChar == decimalMarkerPosition) {
+      buffer[endChar--] = '.';
+    }
+    quotien = dividend/10;
+    digit = dividend - quotien*10;
+    buffer[endChar--] = '0'+digit;
+    dividend = quotien;
+  }  while (endChar >= startChar);
+}
+
+void FloatToString::convertFloatToText(float f, char * buffer, int maxNumberOfChar, Mode mode) {
+  /* We here assert that the buffer is long enough to display any float in a
+   * understandable way: the worst case has the form -1e-38 (6+1 char). */
+  assert(maxNumberOfChar > 6);
+
+  //if (isinf(f)) {
+  float maximalFloat = 3.4f*powf(10, 38);
+  if (f > maximalFloat || f < -maximalFloat) {
+    buffer[0] = 'N';
+    buffer[1] = 'a';
+    buffer[2] = 'N';
+    buffer[3] = 0;
+    return;
+  }
+
+  float logBase10 = f != 0.0f ? log10f(fabsf(f)) : 0;
+  int exponentInBase10 = logBase10;
+  if ((int)f == 0 and logBase10 != exponentInBase10) {
+    /* For floats < 0, the exponent in base 10 is the inferior integer part of
+     * log10(float). We thus decrement the exponent for float < 0 whose exponent
+     * is not an integer. */
+    exponentInBase10--;
+  }
+
+  int numberOfCharExponent = exponentInBase10 != 0 ? log10f(fabsf((float)exponentInBase10)) + 1 : 1;
+  if (exponentInBase10 < 0){
+    // If the exponent is < 0, we need a additional char for the sign
+    numberOfCharExponent++;
+  }
+
+  /* Future optimisation, if mode > 0, find the position of decimalMarker and
+   * decide whether to display the exponent. */
+
+  // Number of char available for the mantissa
+  int availableCharsForMantissa = maxNumberOfChar - numberOfCharExponent - 2;
+
+  // Compute mantissa
+  /* The number of digits in an integer is capped because the maximal integer is
+   * 2^31 - 1. As our mantissa is an integer, we assert that we stay beyond this
+   * threshold during computation. */
+  int numberMaximalOfCharsInInteger = log10f(powf(2, 31));
+  assert(availableCharsForMantissa < numberMaximalOfCharsInInteger);
+  int mantissa = f >= 0 ? f * powf(10, availableCharsForMantissa - exponentInBase10 - 2) : f * powf(10, availableCharsForMantissa - exponentInBase10 - 3);
+
+  // Print sequentially mantissa and exponent
+  printBase10IntegerWithDecimalMarker(buffer, availableCharsForMantissa, mantissa, 1);
+  buffer[availableCharsForMantissa] = 'e';
+  printBase10IntegerWithDecimalMarker(buffer+availableCharsForMantissa+1, numberOfCharExponent, exponentInBase10, -1);
+  buffer[availableCharsForMantissa+1+numberOfCharExponent] = 0;
+}