string: Fix `to_cmangle` to always produce valid C identifiers.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>

diff --git a/lib/standard/string.nit b/lib/standard/string.nit
index e443433..3656e15 100644 (file)
--- a/lib/standard/string.nit
+++ b/lib/standard/string.nit
@@ -448,13 +448,36 @@ abstract class Text
                return " " * before + self + " " * (diff-before)
        end
 
-       # Mangle a string to be a unique string only made of alphanumeric characters
+       # Mangle a string to be a unique string only made of alphanumeric characters and underscores.
+       #
+       # This method is injective (two different inputs never produce the same
+       # output) and the returned string always respect the following rules:
+       #
+       # * Contains only US-ASCII letters, digits and underscores.
+       # * Never starts with a digit.
+       # * Never contains two contiguous underscores.
+       #
+       #     assert "42_is/The answer!".to_cmangle == "_52d2_is_47dThe_32danswer_33d"
+       #     assert "__d".to_cmangle == "_95d_d"
+       #     assert "_42".to_cmangle == "_95d42"
+       #     assert "foo".to_cmangle == "foo"
+       #     assert "".to_cmangle == ""
        fun to_cmangle: String
        do
+               if is_empty then return ""
                var res = new FlatBuffer
                var underscore = false
-               for i in [0..length[ do
-                       var c = chars[i]
+               var start = 0
+               var c = chars[0]
+
+               if c >= '0' and c <= '9' then
+                       res.add('_')
+                       res.append(c.ascii.to_s)
+                       res.add('d')
+                       start = 1
+               end
+               for i in [start..length[ do
+                       c = chars[i]
                        if (c >= 'a' and c <= 'z') or (c >='A' and c <= 'Z') then
                                res.add(c)
                                underscore = false