parse /etc/localtime implementation

Author: Зишан Мирза

libc/src/time/timezone.cpp

@@ -6,6 +6,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <fcntl.h>
+#include <unistd.h>
+#include <stdio.h> // TODO: Remove all printf functions
+#include <sys/types.h>
+
 #include "src/time/time_utils.h"
 #include "src/__support/common.h"
 #include "src/time/timezone.h"
@@ -15,8 +20,106 @@ namespace timezone {
 
 using LIBC_NAMESPACE::time_utils::TimeConstants;
 
+void rev_str(char *str) {
+    int start = 0;
+    int end = 0;
+
+    while (str[end] != '\0') {
+        end++;
+    }
+    end--;
+
+    while (start < end) {
+        str[start] = str[start] ^ str[end];
+        str[end] = str[start] ^ str[end];
+        str[start] = str[start] ^ str[end];
+
+        start++;
+        end--;
+    }
+}
+
 int get_timezone_offset(char *timezone) {
   (void)timezone;
+
+  unsigned char hdr[TIMEZONE_HDR_SIZE];
+
+  int32_t magic;
+  unsigned char version;
+  __int128_t reserved;
+  int32_t tzh_ttisutcnt;
+  int32_t tzh_ttisstdcnt;
+  int32_t tzh_leapcnt;
+  int32_t tzh_timecnt;
+  int32_t tzh_typecnt;
+  int32_t tzh_charcnt;
+
+  int fd;
+  size_t bytes;
+
+  fd = open("/etc/localtime", O_RDONLY);
+  if (fd < 0) {
+    return 0;
+  }
+
+  bytes = read(fd, hdr, sizeof(hdr));
+  if (bytes != sizeof(hdr)) {
+    return 0;
+  }
+
+  size_t i;
+  __uint128_t tmp;
+
+  // these locations in timezone files are defined in `tzfile`
+  magic = (hdr[0] << 24) | (hdr[1] << 16) | (hdr[2] << 8) | hdr[3];
+  version = hdr[4];
+  for (i = 5; i <= 20; i++) {
+    tmp = (tmp << 8) | hdr[i];
+  }
+  reserved = tmp;
+  tzh_ttisutcnt = (hdr[20] << 24) | (hdr[21] << 16) | (hdr[22] << 8) | hdr[23];
+  tzh_ttisstdcnt = (hdr[24] << 24) | (hdr[25] << 16) | (hdr[26] << 8) | hdr[27];
+  tzh_leapcnt = (hdr[28] << 24) | (hdr[29] << 16) | (hdr[30] << 8) | hdr[31];
+  tzh_timecnt = (hdr[32] << 24) | (hdr[33] << 16) | (hdr[34] << 8) | hdr[35];
+  tzh_typecnt = (hdr[36] << 24) | (hdr[37] << 16) | (hdr[38] << 8) | hdr[39];
+  tzh_charcnt = (hdr[40] << 24) | (hdr[41] << 16) | (hdr[42] << 8) | hdr[43];
+  (void)tzh_ttisutcnt;
+  (void)tzh_ttisstdcnt;
+  (void)tzh_leapcnt;
+  (void)tzh_typecnt;
+  (void)tzh_charcnt;
+
+  if (magic != 0x545A6966) {
+    return 0;
+  }
+
+  // currently only supporting tzfile v2
+  if (version != 0x32) {
+    return 0;
+  }
+
+  // according to `tzfile`, 15 bytes should be 0
+  if ((reserved ^ 0x00) != 0) {
+    return 0;
+  }
+
+  for (i = 0; i < (size_t)tzh_timecnt; i++) {
+    uint8_t buf[4];
+    bytes = read(fd, buf, 4);
+    if (bytes != 4) {
+      continue;
+    }
+
+    int32_t transition = (buf[0] << 24) | (buf[1] << 16) | (buf[2] << 8) | buf[3];
+    transition = ((transition & 0xFF000000) >> 24) |
+        ((transition & 0x00FF0000) >> 8) |
+        ((transition & 0x0000FF00) << 8) |
+        ((transition & 0x000000FF) << 24);
+    printf("transition %d:   %d\n", i, transition);
+  }
+
+  close(fd);
+
   return 0;
 }
 

libc/src/time/timezone.h

@@ -15,6 +15,9 @@
 namespace LIBC_NAMESPACE_DECL {
 namespace timezone {
 
+// accoring to `tzfile`, timezone header if always 44 bytes
+#define TIMEZONE_HDR_SIZE 44
+
 int get_timezone_offset(char *timezone);
 
 } // namespace timezone

libc/test/src/time/localtime_test.cpp

@@ -26,7 +26,7 @@ void set_env_var(const char *env) {
   }
 }
 
-/*TEST(LlvmLibcLocaltime, ValidUnixTimestamp0) {
+TEST(LlvmLibcLocaltime, ValidUnixTimestamp0) {
   set_env_var("TZ=Europe/Berlin");
 
   const time_t t_ptr = 0;
@@ -42,7 +42,7 @@ void set_env_var(const char *env) {
   ASSERT_EQ(0, result->tm_isdst);
 }
 
-TEST(LlvmLibcLocaltime, ValidUnixTimestamp32Int) {
+/*TEST(LlvmLibcLocaltime, ValidUnixTimestamp32Int) {
   set_env_var("TZ=Europe/Berlin");
 
   time_t t_ptr = 2147483647;