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diff --git a/subprojects/exess/src/digits.h b/subprojects/exess/src/digits.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_DIGITS_H
+#define EXESS_DIGITS_H
+
+typedef struct {
+  unsigned count; ///< Number of digits
+  int      expt;  ///< Power of 10 exponent
+} ExessDigitCount;
+
+/**
+   Write significant digits digits for `d` into `buf`.
+
+   Writes only significant digits, without any leading or trailing zeros.  The
+   actual number is given by the exponent in the return value.
+
+   @param d The number to convert to digits, must be finite and non-zero.
+   @param buf The output buffer at least `max_digits` long.
+   @param max_digits The maximum number of digits to write.
+*/
+ExessDigitCount
+exess_digits(double d, char* buf, unsigned max_digits);
+
+#endif // EXESS_DIGITS_H