Factor out decimal digit generation

1 files changed, 63 insertions, 1 deletions

diff --git a/src/decimal.c b/src/decimal.c
index 7f78c8a7..6c0bad59 100644
--- a/src/decimal.c
+++ b/src/decimal.c
@@ -1,5 +1,5 @@
 /*
-  Copyright 2019 David Robillard <http://drobilla.net>
+  Copyright 2019-2020 David Robillard <http://drobilla.net>
 
   Permission to use, copy, modify, and/or distribute this software for any
   purpose with or without fee is hereby granted, provided that the above
@@ -18,8 +18,70 @@
 
 #include "int_math.h"
 
+#include <assert.h>
+#include <float.h>
+#include <math.h>
+
 int
 serd_count_digits(const uint64_t i)
 {
 	return i == 0 ? 1 : (int)serd_ilog10(i) + 1;
 }
+
+unsigned
+serd_double_int_digits(const double abs)
+{
+	const double lg = ceil(log10(floor(abs) + 1.0));
+	return lg < 1.0 ? 1U : (unsigned)lg;
+}
+
+unsigned
+serd_decimals(const double d, char* const buf, const unsigned frac_digits)
+{
+	assert(isfinite(d));
+
+	// Point s to decimal point location
+	const double   abs_d      = fabs(d);
+	const double   int_part   = floor(abs_d);
+	const unsigned int_digits = serd_double_int_digits(abs_d);
+	unsigned       n_bytes    = 0;
+	char*          s          = buf + int_digits;
+	if (d < 0.0) {
+		*buf = '-';
+		++s;
+	}
+
+	// Write integer part (right to left)
+	char*    t   = s - 1;
+	uint64_t dec = (uint64_t)int_part;
+	do {
+		*t-- = (char)('0' + dec % 10);
+	} while ((dec /= 10) > 0);
+
+
+	*s++ = '.';
+
+	// Write fractional part (right to left)
+	double frac_part = fabs(d - int_part);
+	if (frac_part < DBL_EPSILON) {
+		*s++ = '0';
+		n_bytes = (unsigned)(s - buf);
+	} else {
+		uint64_t frac = (uint64_t)llround(frac_part * pow(10.0, (int)frac_digits));
+		s += frac_digits - 1;
+		unsigned i = 0;
+
+		// Skip trailing zeros
+		for (; i < frac_digits - 1 && !(frac % 10); ++i, --s, frac /= 10) {}
+
+		n_bytes = (unsigned)(s - buf) + 1u;
+
+		// Write digits from last trailing zero to decimal point
+		for (; i < frac_digits; ++i) {
+			*s-- = (char)('0' + (frac % 10));
+			frac /= 10;
+		}
+	}
+
+	return n_bytes;
+}