Make checksum codes easier to read.

UPDATES.md

@@ -1,2 +1,39 @@
+# UPDATES
+* 2.8.1:
+    * Improvement: Checksum calculation is faster.
+* 2.8.0:
+    * Feature: Now support endless ping, using `ping3 -c 0 example.com` with a count of 0 or `ping3.verbose_ping('example.com', count=0)` to start, using `ctrl + c` to stop.
 * 2.7.0:
-    * Using `SOCK_DGRAM` instead of `SOCK_RAW` on macOS. According to [this](https://apple.stackexchange.com/questions/312857/how-does-macos-allow-standard-users-to-ping), `SOCK_DGRAM` can be sent by standard user on macOS.
+    * Feature: Using `SOCK_DGRAM` instead of `SOCK_RAW` on macOS. According to [this](https://apple.stackexchange.com/questions/312857/how-does-macos-allow-standard-users-to-ping), `SOCK_DGRAM` can be sent by standard user on macOS.
+* 2.6.6:
+    * Bug Fix: `setsockopt` error for `SOL_IP.IP_TTL` on windows. ( #28 )
+* 2.6.5:
+    * Bug Fix: When multi-processing or multi-threading, icmp_id will no longer collision. ( #23 )
+* 2.6.1:
+    * Feature: Add network interface binding support for Linux. ( #22 )
+* 2.5.1:
+    * Features:
+        * Add interval support to `ping3.verbose_ping()`. ( #17 )
+        * Add `-i/--interval` argument for interval support in command-line.
+* 2.4.7:
+    * Bug Fix: Input parameter `size` in `ping()` should not include ICMP_Header size. ( #21 )
+* 2.4.4:
+    * Bug Fix: When there are a lot of incoming packets, and the destination address has no response, `ping()` never return. ( #14 )
+* 2.4.0:
+    * Feature: Return False if HostUnknown error raised (instead of print info to screen).
+    * Improvement: Increase usability of errors and provide a more precisely text.
+* 2.3.1:
+    * Features:
+        * Add `--debug` argument for DEBUG mode in command-line.
+        * Add `--exceptions` argument for EXCEPTIONS mode in command-line.
+* 2.2.3:
+    * Feature: Add command-line mode.
+* 2.0.1:
+    * Features:
+        * Add support of ICMP Payload size.
+        * Add support of EXCEPTIONS mode.
+        * Add support of DEBUG mode.
+* 1.2.1:
+    * Feature: Add support for multiple interfaces. Use `ping(..., src_addr="INTERFACE IP")`
+* 1.1.0:
+    * Improvement: Update tests and PyPI info.

ping3.py

@@ -14,7 +14,7 @@
 import errors
 from enums import ICMP_DEFAULT_CODE, IcmpType, IcmpTimeExceededCode, IcmpDestinationUnreachableCode
 
-__version__ = "2.8.0"
+__version__ = "2.8.1"
 DEBUG = False  # DEBUG: Show debug info for developers. (default False)
 EXCEPTIONS = False  # EXCEPTIONS: Raise exception when delay is not available.
 LOGGER = None  # LOGGER: Record logs into console or file.
@@ -93,15 +93,17 @@ def checksum(source: bytes) -> int:
     RFC792: https://tools.ietf.org/html/rfc792
 
     Args:
-        source: The input to be calculated.
+        source: Bytes. The input to be calculated.
 
     Returns:
-        Calculated checksum.
+        int: Calculated checksum.
     """
-    res = sum(source[::2]) + (sum(source[1::2]) << 8)
-    while res > 0xffff:
-        res = sum(divmod(res, 0x10000))
-    return ~res & 0xffff
+    BITS = 16  # 16-bit long
+    carry = 1 << BITS  # 0x10000
+    result = sum(source[::2]) + (sum(source[1::2]) << (BITS // 2))  # Even bytes (odd indexes) shift 1 byte to the left.
+    while result >= carry:  # Ones' complement sum.
+        result = sum(divmod(result, carry))  # Each carry add to right most bit.
+    return ~result & ((1 << BITS) - 1)  # Ensure 16-bit
 
 
 def read_icmp_header(raw: bytes) -> dict: