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diff --git a/subprojects/exess/src/decimal.h b/subprojects/exess/src/decimal.h
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+/*
+  Copyright 2019-2021 David Robillard <d@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
+  copyright notice and this permission notice appear in all copies.
+
+  THIS SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+
+#ifndef EXESS_DECIMAL_H
+#define EXESS_DECIMAL_H
+
+#include "exess/exess.h"
+
+#include <stddef.h>
+
+// Define C11 numeric constants if the compiler hasn't already
+#ifndef FLT_DECIMAL_DIG
+#  define FLT_DECIMAL_DIG 9
+#endif
+#ifndef DBL_DECIMAL_DIG
+#  define DBL_DECIMAL_DIG 17
+#endif
+
+typedef enum {
+  EXESS_NEGATIVE,
+  EXESS_NEGATIVE_INFINITY,
+  EXESS_NEGATIVE_ZERO,
+  EXESS_POSITIVE_ZERO,
+  EXESS_POSITIVE,
+  EXESS_POSITIVE_INFINITY,
+  EXESS_NAN,
+} ExessNumberKind;
+
+typedef struct {
+  ExessNumberKind kind;                        ///< Kind of number
+  int             expt;                        ///< Power of 10 exponent
+  unsigned        n_digits;                    ///< Number of significant digits
+  char            digits[DBL_DECIMAL_DIG + 2]; ///< Significant digits
+} ExessDecimalDouble;
+
+ExessDecimalDouble
+exess_measure_decimal(double d, unsigned max_precision);
+
+ExessDecimalDouble
+exess_measure_float(float f);
+
+ExessDecimalDouble
+exess_measure_double(double d);
+
+ExessResult
+exess_write_decimal_double(ExessDecimalDouble decimal,
+                           size_t             buf_size,
+                           char*              buf);
+
+#endif // EXESS_DECIMAL_H