From 43e26e583ca92524fd5993cc20b6d84fb9da5d8d Mon Sep 17 00:00:00 2001
From: apio <blobs.trading@gmail.com>
Date: Sun, 20 Nov 2022 18:30:40 +0100
Subject: [PATCH] Calculate the boot timestamp in UNIX time for further usage

---
 kernel/src/arch/Timer.cpp | 51 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/kernel/src/arch/Timer.cpp b/kernel/src/arch/Timer.cpp
index c06b003c..e9694a2c 100644
--- a/kernel/src/arch/Timer.cpp
+++ b/kernel/src/arch/Timer.cpp
@@ -1,8 +1,58 @@
 #include "arch/Timer.h"
+#include "arch/Serial.h"
+#include "boot/bootboot.h"
+#include <Result.h>
 
 static u64 timer_ticks = 0;
 static u64 boot_timestamp;
 
+static int isleap(int year)
+{
+    return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
+}
+
+static int make_yday(int year, int month)
+{
+    static const short int upto[12] = {0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334};
+    int yd;
+
+    yd = upto[month - 1];
+    if (month > 2 && isleap(year)) yd++;
+    return yd;
+}
+
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_16
+static u64 broken_down_to_unix(u64 year, u64 yday, u64 hour, u64 min, u64 sec)
+{
+    return sec + min * 60 + hour * 3600 + yday * 86400 + (year - 70) * 31536000 + ((year - 69) / 4) * 86400 -
+           ((year - 1) / 100) * 86400 + ((year + 299) / 400) * 86400;
+}
+
+// The bootloader encodes the date and time in Binary-Coded Decimal (BCD), which represents decimal digits using
+// hexadecimal digits. For example, BCD 0x22 is 22 in decimal.
+// https://gitlab.com/bztsrc/bootboot/-/blob/master/bootboot_spec_1st_ed.pdf, page 15.
+static int bcd_number_to_decimal(int num)
+{
+    return ((num >> 4) * 10) + (num & 0xf);
+}
+
+static u64 bootloader_time_to_unix(u8 boottime[8])
+{
+    int year = bcd_number_to_decimal(boottime[0]) * 100 + bcd_number_to_decimal(boottime[1]);
+    int month = bcd_number_to_decimal(boottime[2]);
+    int day = bcd_number_to_decimal(boottime[3]);
+    int hour = bcd_number_to_decimal(boottime[4]);
+    int minute = bcd_number_to_decimal(boottime[5]);
+    int second = bcd_number_to_decimal(boottime[6]);
+    // "The last byte can store 1/100th second precision, but in lack of support on most platforms, it is 0x00".
+    // Therefore, let's not rely on it.
+    Serial::printf("Current time: %.2d/%.2d/%d %.2d:%.2d:%.2d UTC\n", day, month, year, hour, minute, second)
+        .release_value();
+    return broken_down_to_unix(year - 1900, make_yday(year, month) + (day - 1), hour, minute, second);
+}
+
+extern BOOTBOOT bootboot;
+
 namespace Timer
 {
     void tick()
@@ -75,6 +125,7 @@ namespace Timer
 
     void init()
     {
+        boot_timestamp = bootloader_time_to_unix(bootboot.datetime);
         arch_init();
     }
 }
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