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/*
* Copyright (c) 2026 Dylan Araps
*
* 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.
*/
#ifndef DYLAN_DATE_H
#define DYLAN_DATE_H
//
// Fast date algorithm, C implementation.
// Source: https://www.benjoffe.com/fast-date-64
//
#include "util.h"
#define C1 505054698555331ull // floor(2^64 * 4 / 146097)
#define C2 50504432782230121ull // ceil(2^64 * 4 / 1461)
#define C3 8619973866219416ull // floor(2^64 / 2140)
#define SCALE 32u
#define SHIFT0 (30556u * SCALE)
#define SHIFT1 (5980u * SCALE)
#if defined(__SIZEOF_INT128__)
#define ERAS 4726498270ull
#define D_SHIFT ((u64)(146097ull * ERAS - 719469ull))
#define Y_SHIFT ((u64)(400ull * ERAS - 1ull))
static inline u64
mulhi(u64 a, u64 b)
{
__uint128_t p = (__uint128_t)a * (__uint128_t)b;
return (u64)(p >> 64);
}
static inline void
mul128(u64 a, u64 b, u64 *hi, u64 *lo)
{
__uint128_t p = (__uint128_t)a * (__uint128_t)b;
*hi = (u64)(p >> 64);
*lo = (u64)p;
}
#else
#define ERAS 14704u
#define D_SHIFT ((u64)(146097ull * (u64)ERAS - 719469ull))
#define Y_SHIFT ((u64)(400ull * (u64)ERAS - 1ull))
static inline u64
mulhi(u64 a, u64 b)
{
u64 a0 = (u32)a;
u64 a1 = a >> 32;
u64 b0 = (u32)b;
u64 b1 = b >> 32;
u64 p00 = a0 * b0;
u64 p01 = a0 * b1;
u64 p10 = a1 * b0;
u64 p11 = a1 * b1;
u64 mid = (p00 >> 32) + (u32)p01 + (u32)p10;
return p11 + (p01 >> 32) + (p10 >> 32) + (mid >> 32);
}
static inline void
mul128(u64 a, u64 b, u64 *hi, u64 *lo)
{
u64 a0 = (u32)a;
u64 a1 = a >> 32;
u64 b0 = (u32)b;
u64 b1 = b >> 32;
u64 p00 = a0 * b0;
u64 p01 = a0 * b1;
u64 p10 = a1 * b0;
u64 p11 = a1 * b1;
u64 mid = (p00 >> 32) + (u32)p01 + (u32)p10;
*hi = p11 + (p01 >> 32) + (p10 >> 32) + (mid >> 32);
*lo = (mid << 32) | (u32)p00;
}
#endif // defined(__SIZEOF_INT128__)
static inline void
ut_to_date(s32 day, s32 *Y, u32 *M, u32 *D)
{
u64 rev = (u64)D_SHIFT - (u64)(s64)day; // Reverse day count.
u64 cen = mulhi(C1, rev); // Divide 36524.25
u64 jul = rev - cen / 4u + cen; // Julian map.
u64 num_hi, num_lo;
mul128(C2, jul, &num_hi, &num_lo); // Divide 365.25
u64 yrs64 = (u64)Y_SHIFT - (u64)num_hi; // Forward year.
u64 yrs = (u32)yrs64;
u32 ypt = (u32)mulhi((u64)(24451u * SCALE), num_lo); // Year (backwards).
u32 bump = (ypt < (3952u * SCALE)); // Jan or Feb.
u32 shift = bump ? SHIFT1 : SHIFT0; // Month offset.
u32 N = (u32)((yrs % 4u) * (16u * SCALE) + shift - ypt); // Leap years.
u32 m = N / (2048u * SCALE);
u32 d = (u32)mulhi(C3, (u64)(N % (2048u * SCALE))); // Divide 2140
*Y = (s32)yrs + (s32)bump;
*M = m;
*D = d + 1u;
}
static inline void
ut_to_date_time(s64 tz, s32 day, s32 *Y, u32 *M, u32 *D, u32 *h, u32 *m, u32 *s)
{
s64 us = tz + day;
s32 days = (s32)(us / 86400);
s32 r = (s32)(us % 86400);
if (r < 0) { r += 86400; days -= 1; }
ut_to_date(days, Y, M, D);
u32 hr = r / 3600;
r -= hr * 3600;
*h = hr;
u32 mn = r / 60;
*m = mn;
*s = r - mn * 60;
}
#endif // DYLAN_DATE_H
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