py/formatfloat: Add ability to format doubles with exponents > 99.

For single prec, exponents never get larger than about 37.  For double
prec, exponents can be larger than 99 and need 3 bytes to format.  This
patch makes the number of bytes needed configurable.

Addresses issue #1772.

py/formatfloat.c

@@ -56,6 +56,7 @@
 #define FPCONST(x) x##F
 #define FPROUND_TO_ONE 0.9999995F
 #define FPDECEXP 32
+#define FPMIN_BUF_SIZE 6 // +9e+99
 
 #define FLT_SIGN_MASK   0x80000000
 #define FLT_EXP_MASK    0x7F800000
@@ -79,6 +80,7 @@ static inline int fp_isless1(float x) { union floatbits fb = {x}; return fb.u <
 #define FPCONST(x) x
 #define FPROUND_TO_ONE 0.999999999995
 #define FPDECEXP 256
+#define FPMIN_BUF_SIZE 7 // +9e+199
 #include <math.h>
 #define fp_signbit(x) signbit(x)
 #define fp_isspecial(x) 1
@@ -105,11 +107,11 @@ static const FPTYPE g_neg_pow[] = {
 int mp_format_float(FPTYPE f, char *buf, size_t buf_size, char fmt, int prec, char sign) {
 
     char *s = buf;
-    int buf_remaining = buf_size - 1;
 
-    if (buf_size < 7) {
-        // Smallest exp notion is -9e+99 which is 6 chars plus terminating
-        // null.
+    if (buf_size <= FPMIN_BUF_SIZE) {
+        // FPMIN_BUF_SIZE is the minimum size needed to store any FP number.
+        // If the buffer does not have enough room for this (plus null terminator)
+        // then don't try to format the float.
 
         if (buf_size >= 2) {
             *s++ = '?';
@@ -127,7 +129,10 @@ int mp_format_float(FPTYPE f, char *buf, size_t buf_size, char fmt, int prec, ch
             *s++ = sign;
         }
     }
-    buf_remaining -= (s - buf); // Adjust for sign
+
+    // buf_remaining contains bytes available for digits and exponent.
+    // It is buf_size minus room for the sign and null byte.
+    int buf_remaining = buf_size - 1 - (s - buf);
 
     if (fp_isspecial(f)) {
         char uc = fmt & 0x20;
@@ -226,8 +231,8 @@ int mp_format_float(FPTYPE f, char *buf, size_t buf_size, char fmt, int prec, ch
             e_sign = e_sign_char;
             dec = 0;
 
-            if (prec > (buf_remaining - 6)) {
-                prec = buf_remaining - 6;
+            if (prec > (buf_remaining - FPMIN_BUF_SIZE)) {
+                prec = buf_remaining - FPMIN_BUF_SIZE;
                 if (fmt == 'g') {
                     prec++;
                 }
@@ -258,8 +263,8 @@ int mp_format_float(FPTYPE f, char *buf, size_t buf_size, char fmt, int prec, ch
                 }
             }
         }
-        if (fmt == 'e' && prec > (buf_remaining - 6)) {
-            prec = buf_remaining - 6;
+        if (fmt == 'e' && prec > (buf_remaining - FPMIN_BUF_SIZE)) {
+            prec = buf_remaining - FPMIN_BUF_SIZE;
         }
         // If the user specified 'g' format, and e is < prec, then we'll switch
         // to the fixed format.
@@ -377,7 +382,10 @@ int mp_format_float(FPTYPE f, char *buf, size_t buf_size, char fmt, int prec, ch
     if (e_sign) {
         *s++ = e_char;
         *s++ = e_sign;
-        *s++ = '0' + (e / 10);
+        if (FPMIN_BUF_SIZE == 7 && e >= 100) {
+            *s++ = '0' + (e / 100);
+        }
+        *s++ = '0' + ((e / 10) % 10);
         *s++ = '0' + (e % 10);
     }
     *s = '\0';