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diff --git a/src/bigint.h b/src/bigint.h
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+/*
+  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
+  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 SERD_BIGINT_H
+#define SERD_BIGINT_H
+
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+
+typedef uint32_t Bigit;
+
+/* We need enough precision for any double, the "largest" of which (using
+   absolute exponents) is the smallest subnormal ~= 5e-324.  This is 1076 bits
+   long, but we need a bit more space for arithmetic.  This is absurd, but such
+   is decimal.  These are only used on the stack so it doesn't hurt too much.
+*/
+
+#define BIGINT_MAX_SIGNIFICANT_BITS 1280
+#define BIGINT_BIGIT_BITS 32
+#define BIGINT_MAX_BIGITS (BIGINT_MAX_SIGNIFICANT_BITS / BIGINT_BIGIT_BITS)
+
+typedef struct
+{
+	Bigit    bigits[BIGINT_MAX_BIGITS];
+	unsigned n_bigits;
+} SerdBigint;
+
+void
+serd_bigint_zero(SerdBigint* num);
+
+size_t
+serd_bigint_print_hex(FILE* stream, const SerdBigint* num);
+
+size_t
+serd_bigint_to_hex_string(const SerdBigint* num, char* buf, size_t len);
+
+void
+serd_bigint_clamp(SerdBigint* num);
+
+void
+serd_bigint_shift_left(SerdBigint* num, unsigned amount);
+
+void
+serd_bigint_set(SerdBigint* num, const SerdBigint* value);
+
+void
+serd_bigint_set_u32(SerdBigint* num, uint32_t value);
+
+void
+serd_bigint_set_u64(SerdBigint* num, uint64_t value);
+
+void
+serd_bigint_set_pow10(SerdBigint* num, unsigned exponent);
+
+void
+serd_bigint_set_decimal_string(SerdBigint* num, const char* str);
+
+void
+serd_bigint_set_hex_string(SerdBigint* num, const char* str);
+
+void
+serd_bigint_multiply_u32(SerdBigint* num, uint32_t factor);
+
+void
+serd_bigint_multiply_u64(SerdBigint* num, uint64_t factor);
+
+void
+serd_bigint_multiply_pow10(SerdBigint* num, unsigned exponent);
+
+int
+serd_bigint_compare(const SerdBigint* lhs, const SerdBigint* rhs);
+
+void
+serd_bigint_add_u32(SerdBigint* lhs, uint32_t rhs);
+
+void
+serd_bigint_add(SerdBigint* lhs, const SerdBigint* rhs);
+
+void
+serd_bigint_subtract(SerdBigint* lhs, const SerdBigint* rhs);
+
+Bigit
+serd_bigint_left_shifted_bigit(const SerdBigint* num,
+                               unsigned          amount,
+                               unsigned          index);
+
+/// Faster implementation of serd_bigint_subtract(lhs, rhs << amount)
+void
+serd_bigint_subtract_left_shifted(SerdBigint*       lhs,
+                                  const SerdBigint* rhs,
+                                  unsigned          amount);
+
+/// Faster implementation of serd_bigint_compare(l + p, c)
+int
+serd_bigint_plus_compare(const SerdBigint* l,
+                         const SerdBigint* p,
+                         const SerdBigint* c);
+
+/// Divide and set `lhs` to modulo
+uint32_t
+serd_bigint_divmod(SerdBigint* lhs, const SerdBigint* rhs);
+
+#endif // SERD_BIGINT_H