[erlang-questions] update_counter on steroids

[Anthony Ramine]

Hello Ulf,

Not what you asked for, but I implemented ets:update_counter/4 which takes a default value yesterday:

	ets:new(counters, [set,named_table]),
	1 = ets:update_counter(counters, c1, 1, {c1,1}),
	3 = ets:update_counter(counters, c1, 2, {c1,1}),
	[{c1,3}] = ets:tab2list(counters).

Regards,

	https://github.com/erlang/otp/pull/362

-- 
Anthony Ramine

Le 7 août 2013 à 13:39, Ulf Wiger <ulf@REDACTED> a écrit :

> 
> There was some discussion recently on teaching update_counter some new tricks.
> 
> Here is a suggestion:
> 
> Right now, the function takes (Tab, Key, UpOp | [UpOp]), where
>  UpOp ::== {Pos, Incr} | {Pos, Incr, Threshold, WrapTo}
> 
> I'd like to save the result of an operation in a temporary variable.
> 
> An atomic reset, for example. (let's say counter c1 has the value 17):
> 
> ets:update_counter(counters, c1, [{2, 0, '$1'}, {2, {'-', '$1'}}]) ->
>  [17, 0]
> 
> (Increment by 0 means we read the existing value, just like today).
> 
> Why use this instead of simply ets:insert/2? Well, for one thing, we get the old value back, so it's an atomic read-reset.
> 
> Sum counter:
> 
> ets:update_counter(counters, c2, [{2,0,'$1'}, {3,0,'$2'}, {4, {'+', '$1', '$2'}}])
> 
> Wrap with parameterized threshold and reset value:
> 
> ets:update_counter(counters, c3, [{3,0,'$1'}, {4,0,'$2'}, {2, Incr, '$1', '$2'}])
> 
> This assumes the following changes:
> 
> - A number of temp variables, like in match specs.
> - A 3-tuple  {Pos, Incr, SaveTo}, where SaveTo is a variable name (e.g. '$1')
> - A 5-tuple  {Pos, Incr, Threshold, WrapTo, SaveTo}
> - The possibility to use, wherever an integer() is expected
>  VariableName | {UnaryOp, VariableName} | {BinaryOp, Var1, Var2}
> 
> We could also use this to e.g. store a value together with an increment operation. A reset with timestamp:
> 
> TS = timestamp(),  % in milliseconds
> ets:update_counter(counters, c4, [{2, 0, '$1'}, {2, {'-', '$1'}, {3, 0, '$2'}, {3, {'-', TS, '$2'}}])
> 
> …a bit bizarre perhaps, but still.
> 
> Comments?
> 
> BR,
> Ulf W
> 
> 
> Ulf Wiger, Co-founder & Developer Advocate, Feuerlabs Inc.
> http://feuerlabs.com
> 
> 
> 
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