parse_line breaks on quoted fields

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Final diff

diff --git a/csvline.py b/csvline.py
index c00ae65..d639ee8 100644
--- a/csvline.py
+++ b/csvline.py
@@ -1,2 +1,33 @@
 def parse_line(line: str) -> list[str]:
-    return line.split(",")
+    fields: list[str] = []
+    buf: list[str] = []
+    in_quotes = False
+    at_field_start = True
+    i = 0
+    n = len(line)
+    while i < n:
+        ch = line[i]
+        if in_quotes:
+            if ch == '"':
+                if i + 1 < n and line[i + 1] == '"':
+                    # doubled quote inside a quoted field: literal quote
+                    buf.append('"')
+                    i += 2
+                    continue
+                in_quotes = False
+            else:
+                buf.append(ch)
+        elif ch == ",":
+            fields.append("".join(buf))
+            buf = []
+            at_field_start = True
+            i += 1
+            continue
+        elif ch == '"' and at_field_start:
+            in_quotes = True
+        else:
+            buf.append(ch)
+        at_field_start = False
+        i += 1
+    fields.append("".join(buf))
+    return fields
diff --git a/test_csvline.py b/test_csvline.py
index 3dff251..63aa895 100644
--- a/test_csvline.py
+++ b/test_csvline.py
@@ -3,3 +3,40 @@ from csvline import parse_line
 
 def test_simple():
     assert parse_line("a,b,c") == ["a", "b", "c"]
+
+
+def test_quoted_comma():
+    assert parse_line('a,"b,c",d') == ["a", "b,c", "d"]
+
+
+def test_doubled_quote_is_literal():
+    assert parse_line('"say ""hi""",x') == ['say "hi"', "x"]
+
+
+def test_empty_fields_kept():
+    assert parse_line("a,,b") == ["a", "", "b"]
+    assert parse_line("a,b,") == ["a", "b", ""]
+    assert parse_line(",") == ["", ""]
+    assert parse_line("") == [""]
+
+
+def test_empty_quoted_field():
+    assert parse_line('"",x') == ["", "x"]
+    assert parse_line('""""') == ['"']
+
+
+def test_quoted_field_at_end():
+    assert parse_line('x,"y"') == ["x", "y"]
+
+
+def test_unterminated_quote_takes_rest_of_line():
+    assert parse_line('"abc,d') == ["abc,d"]
+
+
+def test_doubled_quote_at_field_end():
+    assert parse_line('"a""",b') == ['a"', "b"]
+    assert parse_line('"""",x') == ['"', "x"]
+
+
+def test_mid_field_quote_is_literal():
+    assert parse_line('ab"c,d') == ['ab"c', "d"]