restore dtostrf when floats are disabled in printf/scanf + round fix (#7093)

* restore dtostrf+fix when float printing is disabled at link time

* fix include file

* fix with proposal per review

* always use dtostrf

Author: d-a-v

cores/esp8266/core_esp8266_noniso.cpp

@@ -22,12 +22,12 @@
 
  */
 
-#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <math.h>
+#include <limits>
 #include "stdlib_noniso.h"
 
 extern "C" {
@@ -41,9 +41,79 @@ char* ultoa(unsigned long value, char* result, int base) {
 }
 
 char * dtostrf(double number, signed char width, unsigned char prec, char *s) {
-    char fmt[32];
-    sprintf(fmt, "%%%d.%df", width, prec);
-    sprintf(s, fmt, number);
+    bool negative = false;
+
+    if (isnan(number)) {
+        strcpy(s, "nan");
+        return s;
+    }
+    if (isinf(number)) {
+        strcpy(s, "inf");
+        return s;
+    }
+
+    char* out = s;
+
+    int fillme = width; // how many cells to fill for the integer part
+    if (prec > 0) {
+        fillme -= (prec+1);
+    }
+
+    // Handle negative numbers
+    if (number < 0.0) {
+        negative = true;
+        fillme--;
+        number = -number;
+    }
+
+    // Round correctly so that print(1.999, 2) prints as "2.00"
+    // I optimized out most of the divisions
+    double rounding = 2.0;
+    for (uint8_t i = 0; i < prec; ++i)
+        rounding *= 10.0;
+    rounding = 1.0 / rounding;
+
+    number += rounding;
+
+    // Figure out how big our number really is
+    double tenpow = 1.0;
+    int digitcount = 1;
+    double nextpow;
+    while (number >= (nextpow = (10.0 * tenpow))) {
+        tenpow = nextpow;
+        digitcount++;
+    }
+
+    // minimal compensation for possible lack of precision (#7087 addition)
+    number *= 1 + std::numeric_limits<decltype(number)>::epsilon();
+
+    number /= tenpow;
+    fillme -= digitcount;
+
+    // Pad unused cells with spaces
+    while (fillme-- > 0) {
+        *out++ = ' ';
+    }
+
+    // Handle negative sign
+    if (negative) *out++ = '-';
+
+    // Print the digits, and if necessary, the decimal point
+    digitcount += prec;
+    int8_t digit = 0;
+    while (digitcount-- > 0) {
+        digit = (int8_t)number;
+        if (digit > 9) digit = 9; // insurance
+        *out++ = (char)('0' | digit);
+        if ((digitcount == prec) && (prec > 0)) {
+            *out++ = '.';
+        }
+        number -= digit;
+        number *= 10.0;
+    }
+
+    // make sure the string is terminated
+    *out = 0;
     return s;
 }