-- Calculate the CRC-16-CCITT checksum for an array of bytes

-- Copyright (C)2025, Philip Munts dba Munts Technologies.
--
-- Redistribution and use in source and binary forms, with or without
-- modification, are permitted provided that the following conditions are met:
--
-- * Redistributions of source code must retain the above copyright notice,
--   this list of conditions and the following disclaimer.
--
-- THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
-- AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
-- IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
-- ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
-- LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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-- POSSIBILITY OF SUCH DAMAGE.

-- The following CRC16-CCITT subroutine came from:
-- http://stackoverflow.com/questions/10564491/function-to-calculate-a-crc16-checksum

FUNCTION CRC16_CCITT(buf : BufferType; len : Natural) RETURN Unsigned_16 IS

  crc : Unsigned_16 := Seed;
  x   : Unsigned_8;

BEGIN
  IF len = 0 THEN
    RETURN crc;
  END IF;

  FOR i IN Buf'First .. Buf'First + IndexType(len) - 1 LOOP
    x   := Unsigned_8(Shift_Right(crc, 8)) XOR Unsigned_8(buf(i));
    x   := x XOR Shift_Right(x, 4);
    crc := Shift_Left(crc, 8) XOR Shift_Left(Unsigned_16(x), 12) XOR
      Shift_Left(Unsigned_16(x), 5) XOR Unsigned_16(x);
  END LOOP;

  RETURN crc;
END CRC16_CCITT;