hash: support both semantics of weak keys + values.

Hash tables with weak keys and values now support a choice of both
possible semantics: under and-semantics, an entry lapses when both the
key and value are unreachable. Under or-semantics, an entry lapses if
either the key or value is unreachable.

The and-semantics is new. Until TXR 266, only or-semantics was
supported. This will be the default: when a hash table is specified
as :weak-keys and :weak-vals, it will have or-semantics.

The keywords :weak-or and :weak-and specify weak keys and values,
with the specific semantics. They are utually exclusive, but tolerate
the presence of :weak-keys and :weak-vals.

The make-hash function is being extended such that if its leftmost
argument, <weak-keys>, is specified as one of the keywords :weak-and
or :weak-or, then the hash table will have weak keys and values with the
specified semantics, and the <weak-vals> argument is ignored
(values are weak even if that argument is false).

* eval.c (eval_init): Initially register the top_vb,
top_mb, top_smb, special and builtin hashes as ordinary hashes: no weak
keys or values. Then use tweak_hash to switch to weak keys+vals
with and-semantics. We do it this way because the keywords are not yet
initialized; we cannot use them.

* hash.h (enum hash_flags, hash_flags_t): Moved to header. Member
hash_weak_both renamed to hash_weak_or. New member hash_weak_and.
(weak_and_k, weak_or_k): New keyword variables.
(hash_print_op): Handle hash_weak_and by printing :weak-and.
(hash_mark): Handle hash_weak_and by marking nothing, like hash_weak_or.
(do_make_hash): Check first argument against the two new keywords and
set flags accordingly. This function is called from eval_init before
the keywords have been initialized, in which case weak_keys ==
weak_and_k is true when both are nil; we watch for that.
(tweak_hash): Now returns void and takes a hash_flags_t argument which
is simply planted.
(do_wak_tables): Implement hash_weak_and case. Remove the compat 266
stuff from hash_weak_or. Compatibility is no longer required since we
are not changing the default semantics of hash tables. Phew; that's a
load of worry off the plate.
(hashv): Parse the two new keywords, validate and provide semantics.
(hash_init): Initialize weak_and_k and weak_or_k kewyords.

* hash.h (enum hash_flags, hash_flags_t): Moved here now.
(weak_and_k, weak_or_k): Declared.

* lib.c (compat_fixup): Remove call to parse_compat_fixup.

* parser.c (parse_init): Create stream_parser_hash with and-semantics.
(parse_compat_fixup): Function removed.

* parser.h (parse_compat_fixup): Declaration removed.

* txr.1: Hash documentation updated.

1 files changed, 1 insertions, 7 deletions

diff --git a/parser.c b/parser.c
index 169b58b0..ff79ec3f 100644
--- a/parser.c
+++ b/parser.c
@@ -1878,7 +1878,7 @@ void parse_init(void)
   parser_cls = cobj_register(parser_s);
 
   protect(&stream_parser_hash, &unique_s, &catch_all, convert(val *, 0));
-  stream_parser_hash = make_hash(t, t, nil);
+  stream_parser_hash = make_hash(weak_and_k, nil, nil);
   catch_all = cons(t, nil);
 
   parser_l_init();
@@ -1897,9 +1897,3 @@ void parse_init(void)
   reg_fun(intern(lit("repl"), system_package), func_n4(repl));
   reg_mac(json_s, func_n2(me_json));
 }
-
-void parse_compat_fixup(int compat_ver)
-{
-  if (compat_ver <= 266)
-    tweak_hash(stream_parser_hash, t, nil);
-}