Re: absence of a __hash metamethod
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- Subject: Re: absence of a __hash metamethod
- From: Mark Hamburg <mhamburg@...>
- Date: 2007年11月15日 09:50:41 -0800
Canonicalized, write-once tuples. Not that those are available...
But essentially, if you have a way to hash a table, then what you probably
want to do is:
hashToTable = setmetatable( {}, { __mode = "v" } )
tableWithComplexKeys = setmetatable( { },
__index = function( t, k )
local hash_k = hash( k )
local kk = hashToTable[ hash_k ]
return t[ kk ]
-- Uses the fact that t[ nil ] will be nil
end,
__index = function( t, k, v )
local hash_k = hash( k )
local kk = hashToTable[ hash_k ]
if kk == nil then
hashToTable[ hash_k ] = k
kk = k
end
t[ kk ] = v
end
}
You could also choose to index the resultant table with uniquely generated
values tied to the hashes in order to avoid problems with the values of the
keys being mutated:
hashToTable = setmetatable( {}, { __mode = "v" } )
tableWithComplexKeys = setmetatable( { },
__index = function( t, k )
local hash_k = hash( k )
local kk = hashToTable[ hash_k ]
return t[ kk ]
-- Uses the fact that t[ nil ] will be nil
end,
__index = function( t, k, v )
local hash_k = hash( k )
local kk = hashToTable[ hash_k ]
if kk == nil then
kk = { } -- private value
hashToTable[ hash_k ] = kk
end
t[ kk ] = v
end
}
Mark