51 lines
1.8 KiB
C
51 lines
1.8 KiB
C
/*
|
|
* 23-bit modular arithmetic. This file is part of Shairport Sync
|
|
* Copyright (c) Mike Brady 2025
|
|
|
|
* Modifications, including those associated with audio synchronization, multithreading and
|
|
* metadata handling copyright (c) Mike Brady 2014--2025
|
|
* All rights reserved.
|
|
*
|
|
* Permission is hereby granted, free of charge, to any person
|
|
* obtaining a copy of this software and associated documentation
|
|
* files (the "Software"), to deal in the Software without
|
|
* restriction, including without limitation the rights to use,
|
|
* copy, modify, merge, publish, distribute, sublicense, and/or
|
|
* sell copies of the Software, and to permit persons to whom the
|
|
* Software is furnished to do so, subject to the following conditions:
|
|
*
|
|
* The above copyright notice and this permission notice shall be
|
|
* included in all copies or substantial portions of the Software.
|
|
*
|
|
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
|
|
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
|
|
* OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
|
|
* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
|
|
* HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
|
|
* WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
|
|
* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
|
|
* OTHER DEALINGS IN THE SOFTWARE.
|
|
*/
|
|
|
|
#include <stdint.h>
|
|
#include "mod23.h"
|
|
|
|
#define MOD_23BIT 0x7FFFFF // 2^23 - 1
|
|
|
|
// Assumes 'a' and 'b' are within 2^22 of each other
|
|
int32_t a_minus_b_mod23(uint32_t a, uint32_t b) {
|
|
|
|
// Mask to 23 bits
|
|
a &= MOD_23BIT;
|
|
b &= MOD_23BIT;
|
|
|
|
// Compute difference modulo 2^23
|
|
uint32_t diff = (a - b) & MOD_23BIT;
|
|
|
|
// Interpret as signed 23-bit value
|
|
// If the top bit (bit 22) is set, it's negative
|
|
int32_t signed_diff = (diff & 0x400000) ? (diff | 0xFF800000) : diff;
|
|
|
|
return signed_diff;
|
|
}
|