switch to array, use mmap

Author: SamWilsn

rounds/1_histogram/solution.py

@@ -4,11 +4,28 @@
 passes out of the box. Replace the body of ``compute_histogram`` with your
 own faster implementation.
 """
+from collections import defaultdict
+from mmap import mmap, ACCESS_READ
 
+def b2i(low: int, high: int) -> int:
+    return high + (low << 8)
+
+def i2b(x: int) -> bytes:
+    return bytes([(x & 0xFF00) >> 8, x & 0xFF])
 
 def compute_histogram(path: str) -> dict[bytes, int]:
     """Frequency of every 2-byte bigram in the file at ``path``."""
-    # TODO: remove this delegation and write your own implementation here.
-    from .baseline import compute_histogram as _baseline
+    # Step 1: read the whole file into memory as a single bytes object.
+    counts = [0 for _ in range(0, 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.
+    for i in range(len(data) - 1):
+        bigram = b2i(data[i], data[i + 1])
+        counts[bigram] += 1
 
-    return _baseline(path)
+    return {i2b(idx): value for idx, value in enumerate(counts) if value != 0}