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-- result_view(key_length, fetch) returns a table which stores only a slice
-- of a result set plus some control values, yet presents read-only virtual
-- access to the entire result set.
-- key_length: {result set key, total result set length}
-- fetch: function(key, start) that returns (slice, adjusted start)
local function result_view(key_length, fetch)
return setmetatable(
{
key=key_length[1],
length=key_length[2],
-- C++ result sets use 0-based indexing, so internally we do too
start=0,
-- start with a dummy array with length 0
slice={}
},
{
__len = function(this)
return this.length
end,
__index = function(this, i)
-- right away, convert to 0-relative indexing
i -= 1
-- can we find this index within the current slice?
local reli = i - this.start
if 0 <= reli and reli < #this.slice then
-- Lua 1-relative indexing
return this.slice[reli + 1]
end
-- is this index outside the overall result set?
if not (0 <= i and i < this.length) then
return nil
end
-- fetch a new slice starting at i, using provided fetch()
local start
this.slice, start = fetch(this.key, i)
-- It's possible that caller-provided fetch() function forgot
-- to return the adjusted start index of the new slice. In
-- Lua, 0 tests as true, so if fetch() returned (slice, 0),
-- we'll duly reset this.start to 0. Otherwise, assume the
-- requested index was not adjusted: that the returned slice
-- really does start at i.
this.start = start or i
-- Hopefully this slice contains the desired i.
-- Back to 1-relative indexing.
return this.slice[i - this.start + 1]
end,
-- We purposely avoid putting any array entries (int keys) into
-- our table so that access to any int key will always call our
-- __index() metamethod. Moreover, we want any table iteration to
-- call __index(table, i) however many times; we do NOT want it to
-- retrieve key, length, start, slice.
-- So turn 'for k, v in result' into 'for k, v in ipairs(result)'.
__iter = ipairs,
-- This result set provides read-only access.
-- We do not support pushing updates to individual items back to
-- C++; for the intended use cases, that makes no sense.
__newindex = function(this, i, value)
error("result_view is a read-only data structure", 2)
end
}
)
end
return result_view
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