gh-137627: Make csv.Sniffer.sniff() delimiter detection 1.6x faster (#137628)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Lib/csv.py

@@ -362,31 +362,33 @@ def _guess_delimiter(self, data, delimiters):
         try and evaluate the smallest portion of the data possible, evaluating
         additional chunks as necessary.
         """
+        from collections import Counter, defaultdict
 
         data = list(filter(None, data.split('\n')))
 
-        ascii = [chr(c) for c in range(127)] # 7-bit ASCII
-
         # build frequency tables
         chunkLength = min(10, len(data))
         iteration = 0
-        charFrequency = {}
+        num_lines = 0
+        # {char -> {count_per_line -> num_lines_with_that_count}}
+        char_frequency = defaultdict(Counter)
         modes = {}
         delims = {}
         start, end = 0, chunkLength
         while start < len(data):
             iteration += 1
             for line in data[start:end]:
-                for char in ascii:
-                    metaFrequency = charFrequency.get(char, {})
-                    # must count even if frequency is 0
-                    freq = line.count(char)
-                    # value is the mode
-                    metaFrequency[freq] = metaFrequency.get(freq, 0) + 1
-                    charFrequency[char] = metaFrequency
+                num_lines += 1
+                for char, count in Counter(line).items():
+                    if char.isascii():
+                        char_frequency[char][count] += 1
 
-            for char in charFrequency.keys():
-                items = list(charFrequency[char].items())
+            for char, counts in char_frequency.items():
+                items = list(counts.items())
+                missed_lines = num_lines - sum(counts.values())
+                if missed_lines:
+                    # Store the number of lines 'char' was missing from.
+                    items.append((0, missed_lines))
                 if len(items) == 1 and items[0][0] == 0:
                     continue
                 # get the mode of the frequencies