Add Printf extension %H

- This conversion has behavior similar to %g: It automatically behaves like
  %f or %e based on the number of output characters. However, unlike %g,
  this decision is also based on what will produce the smallest string
  without truncating the output. The precision field (the * in %.*f) is
  ignored. Converting a double to text with %H and then back to a double
  should be lossless.

src/zstrformat.cpp

@@ -638,6 +638,12 @@ namespace StringFormat
 			}
 			goto fp_begin;
 		}
+		else if (type == 'H')
+		{ // %H is an extension that behaves similarly to %g, except it automatically
+		  // selects precision based on whatever will produce the smallest string.
+			expchar = 'e';
+			goto fp_begin;
+		}
 #if 0
 		// The hdtoa function provided with FreeBSD uses a hexadecimal FP constant.
 		// Microsoft's compiler does not support these, so I would need to hack it
@@ -690,7 +696,7 @@ fp_begin:
 				precision = DEFPREC;
 			}
 			dblarg = va_arg(arglist, double);
-			obuff = dtoaresult = dtoa(dblarg, expchar ? 2 : 3, precision, &expt, &signflag, &dtoaend);
+			obuff = dtoaresult = dtoa(dblarg, type != 'H' ? (expchar ? 2 : 3) : 0, precision, &expt, &signflag, &dtoaend);
 //fp_common:
 			decimal_point = localeconv()->decimal_point;
 			flags |= F_SIGNED;
@@ -742,6 +748,22 @@ fp_begin:
 						}
 					}
 				}
+				else if (type == 'H')
+				{
+					if (expt > -(ndig + 2) && expt <= (ndig + 4))
+					{ // Make %H smell like %f
+						expchar = '\0';
+						precision = ndig - expt;
+						if (precision < 0)
+						{
+							precision = 0;
+						}
+					}
+					else
+					{// Make %H smell like %e
+						precision = ndig;
+					}
+				}
 				if (expchar)
 				{
 					expsize = exponent(expstr, expt - 1, expchar);