time-based-key-value-store.py

Author: Chris Wu

problems/time-based-key-value-store.py

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+"""
+We need a hash-table to store the key-value information.
+For every key, there are multiple values and we need it to sort by`timestamp`.
+
+First, we use two array, `t` and `v` to store timestamp and value separately.
+And becuase the problem says **The timestamps for all TimeMap.set operations are strictly increasing.**
+So for the `set()` method, we only need to simply append `value` and `timestamp` to the end of `v` and `t`.
+The array `t` and its corresponding `v` is sorted already.
+
+[bisect](https://docs.python.org/2/library/bisect.html) can help us implement our `get()` method.
+It returns an insertion point which comes after (to the right of) any existing entries, if we insert the **`timestamp` we are going to get()** into a the array, `t`.
+So `bisect(t, timestamp)` returns `i`:
+    * If `i==0`: it means that there are no value samller than the `timestamp`, `return ""`.
+    * If `i>0`: it means `i` is the index where its value just exceed the `timestamp`, return the value at `i-1`.
+
+We can also implement our own `bisect` by binary search.
+The implementation assume there are no duplicate value in the array.
+
+Time Complexity, O(1) for `set()`. O(LogN) for `get()`.
+Space complexity is O(N)
+N is the number of value stored.
+"""
+import collections
+import bisect
+
+class TimeMap(object):
+
+    def __init__(self):
+        self.t = collections.defaultdict(list)
+        self.v = collections.defaultdict(list)
+
+    def set(self, key, value, timestamp):
+        self.t[key].append(timestamp)
+        self.v[key].append(value)
+
+    def get(self, key, timestamp):
+        if len(self.t[key])==0: return ""
+        i = bisect.bisect(self.t[key], timestamp)
+        return self.v[key][i-1] if i>0 else ""
+
+
+import collections
+
+class TimeMap(object):
+    def __init__(self):
+        self.t = collections.defaultdict(list)
+        self.v = collections.defaultdict(list)
+
+    def set(self, key, value, timestamp):
+        self.t[key].append(timestamp)
+        self.v[key].append(value)
+
+    def get(self, key, timestamp):
+        if len(self.t[key])==0: return ""
+        i = self.bisect(self.t[key], timestamp)
+        return self.v[key][i-1] if i>0 else ""
+
+    def bisect(self, A, x):
+        if A is None or len(A)==0: return 0
+
+        l = 0
+        r = len(A)-1
+        while l<=r:
+            if x<A[l]: return l
+            elif A[l]==x: return l+1
+            elif A[r]<=x: return r+1
+
+            p = (l+r)/2
+            if x<A[p]:
+                r = p-1
+            elif A[p]==x:
+                return p+1
+            else:
+                l = p+1
+        return l
+
+t = TimeMap()
+t.set('love', 'low', 10)
+t.set('love', 'high', 20)
+print t.get('love', 5)
+print t.get('love', 10)
+print t.get('love', 15)
+print t.get('love', 20)
+print t.get('love', 25)
+
+
+