From da28548905dab7c60c3fcec4975ccfa23e315909 Mon Sep 17 00:00:00 2001
From: Dylan Araps <dylan.araps@gmail.com>
Date: Fri, 27 Feb 2026 13:41:56 +0200
Subject: 0.99.0

---
 lib/date.h | 139 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 139 insertions(+)
 create mode 100644 lib/date.h

(limited to 'lib/date.h')

diff --git a/lib/date.h b/lib/date.h
new file mode 100644
index 0000000..6d8b060
--- /dev/null
+++ b/lib/date.h
@@ -0,0 +1,139 @@
+/*
+ * 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 <stddef.h>
+#include <stdint.h>
+
+#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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