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45 lines
1.5 KiB
45 lines
1.5 KiB
-- A generic fifo module. See docs/lua-modules/fifo.md for use examples.
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local tr, ti = table.remove, table.insert
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-- Remove an element and pass it to k, together with a boolean indicating that
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-- this is the last element in the queue; if that returns a value, leave that
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-- pending at the top of the fifo.
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--
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-- If k returns nil, the fifo will be advanced. Moreover, k may return a
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-- second result, a boolean, indicating "phantasmic" nature of this element.
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-- If this boolean is true, then the fifo will advance again, passing the next
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-- value, if there is one, to k, or priming itself for immediate execution at
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-- the next call to queue.
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--
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-- If the queue is empty, do not invoke k but flag it to enable immediate
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-- execution at the next call to queue.
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--
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-- Returns 'true' if the queue contained at least one non-phantom entry,
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-- 'false' otherwise.
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local function dequeue(q,k)
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if #q > 0
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then
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local new, again = k(q[1], #q == 1)
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if new == nil
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then tr(q,1)
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if again then return dequeue(q, k) end -- note tail call
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else q[1] = new
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end
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return true
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else q._go = true ; return false
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end
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end
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-- Queue a on queue q and dequeue with `k` if the fifo had previously emptied.
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local function queue(q,a,k)
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ti(q,a)
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if k ~= nil and q._go then q._go = false; dequeue(q, k) end
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end
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-- return a table containing just the FIFO constructor
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return {
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['new'] = function()
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return { ['_go'] = true ; ['queue'] = queue ; ['dequeue'] = dequeue }
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end
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}
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