dash encoding is now like percent encoding but with dashes

Refs #1439 (comment)

datasette/utils/__init__.py

@@ -1141,13 +1141,34 @@ def add_cors_headers(headers):
     headers["Access-Control-Expose-Headers"] = "Link"
 
 
+_DASH_ENCODING_SAFE = frozenset(
+    b"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+    b"abcdefghijklmnopqrstuvwxyz"
+    b"0123456789_"
+    # This is the same as Python percent-encoding but I removed
+    # '.' and '-' and '~'
+)
+
+
+class DashEncoder(dict):
+    # Keeps a cache internally, via __missing__
+    def __missing__(self, b):
+        # Handle a cache miss, store encoded string in cache and return.
+        res = chr(b) if b in _DASH_ENCODING_SAFE else "-{:02X}".format(b)
+        self[b] = res
+        return res
+
+
+_dash_encoder = DashEncoder().__getitem__
+
+
 @documented
 def dash_encode(s: str) -> str:
-    "Returns dash-encoded string - for example ``/foo/bar`` -> ``-/foo-/bar``"
-    return s.replace("-", "--").replace(".", "-.").replace("/", "-/")
+    "Returns dash-encoded string - for example ``/foo/bar`` -> ``-2Ffoo-2Fbar``"
+    return "".join(_dash_encoder(char) for char in s.encode("utf-8"))
 
 
 @documented
 def dash_decode(s: str) -> str:
-    "Decodes a dash-encoded string, so ``-/foo-/bar`` -> ``/foo/bar``"
-    return s.replace("-/", "/").replace("-.", ".").replace("--", "-")
+    "Decodes a dash-encoded string, so ``-2Ffoo-2Fbar`` -> ``/foo/bar``"
+    return urllib.parse.unquote(s.replace("-", "%"))

docs/internals.rst

@@ -883,13 +883,16 @@ Dash encoding
 
 Datasette uses a custom encoding scheme in some places, called **dash encoding**. This is primarily used for table names and row primary keys, to avoid any confusion between ``/`` characters in those values and the Datasette URLs that reference them.
 
-Dash encoding applies the following rules, in order:
+Dash encoding uses the same algorithm as `URL percent-encoding <https://developer.mozilla.org/en-US/docs/Glossary/percent-encoding>`__, but with the ``-`` hyphen character used in place of ``%``.
 
-- All single ``-`` characters are replaced by ``--``
-- ``.`` characters are replaced by ``-.``
-- ``/`` characters are replaced by ``./``
+Any character other than ``ABCDEFGHIJKLMNOPQRSTUVWXYZ abcdefghijklmnopqrstuvwxyz 0123456789_`` will be replaced by the numeric equivalent preceded by a hyphen. For example:
 
-These rules are applied in reverse order to decode a dash encoded string.
+- ``/`` becomes ``-2F``
+- ``.`` becomes ``-2E``
+- ``%`` becomes ``-25``
+- ``-`` becomes ``-2D``
+- Space character becomes ``-20``
+- ``polls/2022.primary`` becomes ``polls-2F2022-2Eprimary``
 
 .. _internals_utils_dash_encode:
 

tests/test_utils.py

@@ -652,9 +652,11 @@ async def test_derive_named_parameters(sql, expected):
     "original,expected",
     (
         ("abc", "abc"),
-        ("/foo/bar", "-/foo-/bar"),
-        ("/-/bar", "-/---/bar"),
-        ("-/db-/table---.csv-.csv", "---/db---/table-------.csv---.csv"),
+        ("/foo/bar", "-2Ffoo-2Fbar"),
+        ("/-/bar", "-2F-2D-2Fbar"),
+        ("-/db-/table.csv", "-2D-2Fdb-2D-2Ftable-2Ecsv"),
+        (r"%~-/", "-25-7E-2D-2F"),
+        ("-25-7E-2D-2F", "-2D25-2D7E-2D2D-2D2F"),
     ),
 )
 def test_dash_encoding(original, expected):