perf: optimize ASCII lowercase conversion

Author: Miladkhoshdel

strings/lower.py

@@ -1,6 +1,11 @@
+ASCII_UPPERCASE_START = ord("A")
+ASCII_UPPERCASE_END = ord("Z")
+ASCII_CASE_OFFSET = ord("a") - ord("A")
+
+
 def lower(word: str) -> str:
     """
-    Will convert the entire string to lowercase letters
+    Convert ASCII uppercase letters in a string to lowercase.
 
     >>> lower("wow")
     'wow'
@@ -13,11 +18,15 @@ def lower(word: str) -> str:
     >>> lower("whAT")
     'what'
     """
+    result = []
+
+    for char in word:
+        code = ord(char)
+        if ASCII_UPPERCASE_START <= code <= ASCII_UPPERCASE_END:
+            char = chr(code + ASCII_CASE_OFFSET)
+        result.append(char)
 
-    # Converting to ASCII value, obtaining the integer representation
-    # and checking to see if the character is a capital letter.
-    # If it is a capital letter, it is shifted by 32, making it a lowercase letter.
-    return "".join(chr(ord(char) + 32) if "A" <= char <= "Z" else char for char in word)
+    return "".join(result)
 
 
 if __name__ == "__main__":