floatApproxToString.cpp

// Clairvoire@gmail.com  \(v ` >`)_v
// feel free to use as you see fit, attribution appreciated but not required, not fit for any purpose

// if `lexicalCheck is false`
//		returns TRUE if the printed version of a float is being approximated when we limit the print
//		to a certain number of decimal places, as defined by `decimalPlaceLimit`, 
//		returns FALSE if the printed version is 100% accurate to the float itself.
// if `lexicalCheck is true` (expensive)
//		adds the following check after the prior check, if the function is about to return TRUE: 
//		  the value is printed using the decimal rounding, then converted back to a float; 
//		  if this parsed value is the same as the input value, the function returns FALSE instead of TRUE
// NOTE: This WILL return true for values like 0.1 without `lexicalCheck`, as they are approximated 
//		 at extremely small fractional parts in floats:  0.1 is actually 0.100000000000000005551115...
//       but the lexical check will avoid this situation by checking if the displayed value is mapped
//       to the floating representation during the string-to-float process
     

// truth table:
// f( val,  dec ) -> ret   
// -----------------------
// f( 0.5,    3 ) ->  0   
// f( 0.25,   3 ) ->  0   
// f( 0.125,  3 ) ->  0   
// f( 0.0625, 3 ) ->  1    

bool isFloatRoundingApprox(float value, uint32_t decimalPlaceLimit, bool lexicalCheck = false) {
	// 0.0 is special, it's the only value for which our assumption about the exponent kind of backfires
	// 1.0 and 0.5 are... not special, these are just kind of common, may as well make quick outs for them
	if(value == 0.f || value == 1.f || (value == 0.5f && decimalPlaceLimit > 0))
		return false; 
	uint32_t bits = *(uint32_t*)(&value);
	uint32_t exponent = (bits>>23)&0xFF;
	uint32_t mantissa = bits&0x7FFFFF;
	uint32_t lsb;
	#ifdef _MSC_VER
		{
			unsigned long result; // has to be this type
			_BitScanForward(&result, mantissa);
			lsb = result;
		}
	#elif __GNUC__
		#pragma message ("branch untested, if it works, feel free to delete this!")
		lsb = __builtin_ffs(mantissa);
	#else
		#pragma message ("branch untested, if it works, feel free to delete this!")
		#pragma message ("using an unoptimized fallback")
		lsb = 0; // treat as lsb until the end
		for (uint32_t i = 0; i < 23; ++i) {
			if((mantissa>>i)&1u)
				break;
			++lsb;
		}
		lsb = (lsb + 1) % 23; // 0 to 22 lsb
	#endif
	// invert so we get the distance from the left instead of the right
	lsb = (23 - lsb);
	if(lsb == 23)
		lsb = 0;

	// takes advantage of a property I noticed (that hopefully is true?) regarding float representation,
	// where:
	//		 [index of the least set bit in the mantissa] - ([exponent] - 127)
	// defines how many digits "deep" into the fractional part our value's entropy lies (or uh something like that?)
	// we can use this to DIRECTLY gather if our decimal point is being rounded due to the fact that...
	// once we touch a digit, it can NEVER become insignificant again (this however does NOT hold for 0.0)

	int32_t depth = int32_t(lsb) - (int32_t(exponent) - 127);
	bool result = depth > int32_t(decimalPlaceLimit);
	if (result && lexicalCheck) {
		char format[8];
		format[0] = '%';
		format[1] = '.';
		if(decimalPlaceLimit >= 10){
			format[2] = '0' + decimalPlaceLimit/10;
			format[3] = '0' + decimalPlaceLimit%10;
			format[4] = 'f';
			format[5] = 0;
		} else {
			format[2] = '0' + decimalPlaceLimit;
			format[3] = 'f';
			format[4] = 0;
		}
		char buffer[32];
		snprintf(buffer, 32, format, value);
		result = !(value == strtof(buffer, NULL));
	}
	return result;
}



// you can disable thread safety by setting this to 0 instead of 1, it may improve performance if you don't need it
#define THREAD_SAFE 1

// this prints a float to a string, appending '~' if rounding occurs beyond `decimalPlaces`, appends ' ' otherwise
// NOTE:   returned string is valid until next time this function is called within this thread (or among all threads if THREAD_SAFE is 0)
//         if you need it longer than that, you'll need to copy it somewhere
char * floatApproxToString(float value, bool plusSign, bool leadWithZeroes, bool alignLeft, uint32 minIntDigitCount, uint32 decimalPlaces, bool lexicalCheck = false) {
	static_assert(std::numeric_limits<float>::is_iec559, "Hello from 20XX");
	assert(minIntDigitCount <= 99); // limits impact how format string are constructed, if these change the impl must be updated
	assert(decimalPlaces <= 99);

	const uint32 formatSize = 16;

	#if THREAD_SAFE
	thread_local int size = 0;
	thread_local char * buffer = NULL;
	#else
	static int size = 0;
	static char * buffer = NULL;
	#endif

	char format[formatSize];
	// building the format string byte by byte
	uint32 n = 1;
	format[0] =  '%';
	if(alignLeft)
		format[n++] = '-';
	if(leadWithZeroes)
		format[n++] = '0';
	if((2+minIntDigitCount+decimalPlaces) / 10)
		format[n++] = '0' + (2+minIntDigitCount+decimalPlaces)/10;
	format[n++] = '0' + (2+minIntDigitCount+decimalPlaces)%10;
	format[n++] = '.';
	if(decimalPlaces > 10)
		format[n++] = '0' + decimalPlaces/10;
	format[n++] = '0' + decimalPlaces%10;
	format[n++] = 'f';
	bool approx = isFloatRoundingApprox(value, decimalPlaces, lexicalCheck);
	if(approx)
		format[n++] = '~';
	else
		format[n++] = ' ';
	format[n] = '\0';
	assert(n < formatSize);

	// resize our buffer if we're lacking enough room.  
	int newSize = snprintf(buffer, size, format, value);
	if (newSize + 1 > size) {
		// didn't have enough room; resize to fit then use snprintf() again
		size = newSize + 1;
		if(buffer != NULL) 
			delete [] buffer;
		buffer = new (std::nothrow) char[size];
		assert(buffer != NULL);
		newSize = snprintf(buffer, size, format, value); // second time should succeed without fail
		assert(newSize > 0);
	}
	// if aligning left, we need to shift the ~ left a bit
	if (alignLeft && approx) {
		n = decimalPlaces;
		while (buffer[n] != ' ' && buffer[n] != 0)
			++n;
		if(buffer[n] == ' ')
			buffer[n] = '~';
		++n;
		while (buffer[n] != '~' && buffer[n] != 0)
			++n;
		if(buffer[n] == '~')
			buffer[n] = ' ';
	}
	return buffer;
}