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authorKaz Kylheku <kaz@kylheku.com>2021-07-21 07:34:12 -0700
committerKaz Kylheku <kaz@kylheku.com>2021-07-21 07:34:12 -0700
commit17a0300e7d8858623feff27f5f43660ba90a0c32 (patch)
tree39f5ff69fa594681950fdcac4f4a1758d76d318a /HACKING
parent4c760c3b98fb4f0375b61b7b41e2c58bfca5fb59 (diff)
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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.
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