skip an array access

Author: SamWilsn

rounds/1_histogram/solution.py

@@ -16,16 +16,20 @@ def i2b(x: int) -> bytes:
 def compute_histogram(path: str) -> dict[bytes, int]:
     """Frequency of every 2-byte bigram in the file at ``path``."""
     # Step 1: read the whole file into memory as a single bytes object.
-    counts = [0 for _ in range(0, 2**16)]
+    counts = [0 for _ in range(2**16)]
 
     source = open(path, "rb", buffering=0)
     data = mmap(source.fileno(), 0, access=ACCESS_READ)
 
     # Step 2: slide a 2-byte window across the buffer. For ``b"ABCD"`` the
     # iterations produce ``b"AB"``, ``b"BC"``, then ``b"CD"``. For each window,
     # bump the matching bucket in a ``dict`` keyed by the bigram itself.
+    previous = data[0]
     for i in range(len(data) - 1):
-        bigram = b2i(data[i], data[i + 1])
-        counts[bigram] += 1
+        current = data[i + 1]
+        counts[current + (previous << 8)] += 1
+        previous = current
 
-    return {i2b(idx): value for idx, value in enumerate(counts) if value != 0}
+    return {
+        i2b(idx): value for idx, value in enumerate(counts) if value != 0
+    }