Add code to build keydir

Author: abaksy

Diff for: README.md

@@ -1,5 +1,5 @@
 # pycask
-A log-structured disk-based key-value store implemented in Python
+A log-structured disk-based key-value store implemented in Python. 
 
 This is an implementation of the BitCask database described in [this paper](https://riak.com/assets/bitcask-intro.pdf). 
 The implementation is ideally independent of machine and environment considerations. 
@@ -22,3 +22,9 @@ datastore.close()
 [driver.py](https://github.com/abaksy/pycask/blob/main/driver.py) runs some benchmark tests for key access times for different database sizes (ranging from 10 entries to 1mn entries)
 
 ## How bitcask works
+The database consists of two major portions, the disk-based datastore and an in-memory data structure called the ```keydir```. 
+
+
+## Changelog
+1. Made sure to avoid using local timestamps, use UTC timestamps instead for internalization (avoid various kinds of timing bugs)
+2. Renamed class ```BitCaskDiskStore``` to ```BitCaskKVPair``` to more accurately reflect what the class actually does

Diff for: bitcask.py

@@ -9,6 +9,21 @@ def __init__(self, datafile_name="database.db") -> None:
         self.datafile = datafile_name
         self.file_handle =  open(self.datafile, "a+b")
         self.keydir: Dict[str, BitCaskKeyDirEntry] = dict()
+        self.build_keydir()
+    
+    def build_keydir(self):
+        filesize = os.path.getsize(self.datafile)
+        if filesize == 0:
+            return 
+        with open(self.datafile, 'rb') as f:
+            while f.tell() < filesize:
+                offset = f.tell()
+                byte_data = f.read(16)
+                timestamp, keysize, valuesize = struct.unpack("<QLL", byte_data)
+                key = f.read(keysize).decode()
+                value = f.read(valuesize).decode()
+                self.keydir[key] = BitCaskKeyDirEntry(valuesize, offset)
+            
 
     def get(self, key):
         """