DO NOT MERGE sqrt: add review comments. #35

rjnn · 2017-03-17T19:03:45Z

This is a dummy PR to simulate a code review on the square root code
in the existing APD library.

Apologies that this is so late.

This change is

This is a dummy PR to simulate a code review on the square root code in the existing APD library.

maddyblue

I've addressed these in #36

maddyblue · 2017-03-17T19:47:18Z

context.go


 	switch x.Sign() {
 	case -1:
-		res := InvalidOperation
-		return c.goError(res)
+		return c.goError(InvalidOperation)


Done on the nan branch already.

maddyblue · 2017-03-17T19:48:09Z

context.go

+	// variables in scope low. This refactoring doesn't quite work, but perhaps
+	// you can refactor the calls below to use nc.Precision instead of keeping
+	// workp in scope throughout.
+	// TODO(mjibson): Also, what's nc?


nc is the "new context", which is used for the internal operations that require higher precision than the desired output. It is named consistently throughout many functions in apd.

maddyblue · 2017-03-17T19:52:14Z

context.go

@@ -967,6 +997,9 @@ func (c *Context) Pow(d, x, y *Decimal) (Condition, error) {

 // Quantize adjusts and rounds v as necessary so it is represented with
 // exponent exp and stores the result in d.
+// TODO(mjibson): "adjusts"?
+// TODO(mjibson): Could use a comment (not just here), or a wrapper Function
+// since you call this sometimes with the same argument for both d and v and that scares me.


There are tests in the nan branch that verify you can safely call all functions where the result is also any operand and they still work correctly. Is that good enough?

I'm not at all concerned about correctness because the testing throughout is ridiculously good, but it is weird to see, but that's fine. I'm OK with it.

maddyblue · 2017-03-17T19:53:04Z

context.go

@@ -399,7 +426,8 @@ func (c *Context) Sqrt(d, x *Decimal) (Condition, error) {
 		// approx = 0.5 * (approx + f / approx)
 		ed.Mul(approx, tmp, decimalHalf)
 	}
-	nc.Precision = workp + 1
+	// TODO(mjibson): why the plus 1?


Hey the tests pass without this. Removed.

maddyblue · 2017-03-17T20:00:27Z

context.go

 	p := uint32(3)
 	tmp := new(Decimal)

 	// The algorithm in the paper says to use c.Precision + 2. decNumber uses
 	// workp + 2. But we use workp + 5 to make the tests pass. This means it is
 	// possible there are inputs we don't compute correctly and could be 1ulp off.
-	for maxp := workp + 5; p != maxp; {
+	for maxp := nc.Precision + 5; p != maxp; {
+		// TODO(mjibson): why the minus 2?


Because that's what's written in the paper. Did I miss something?

No, I missed it.

maddyblue · 2017-03-17T20:46:58Z

context.go

+	var f *Decimal
+	// e is the number of digits in x.
+	var e int64
+	// ed is ...


ed is used consistently throughout as the error accumulator for internal operations. I think a comment describing that would be more annoying than useful since it's the same everywhere.

maddyblue · 2017-03-17T20:47:21Z

context.go

-		workp = 7
+
+	// TODO(mjibson): Too many variables without comments/definitions. And nd is a temporary variable that goes out of scope, perhaps it can be closed off? I've given it a try but I'm not too happy with this either... Also, these are my *guesses* for what these variables represent.
+	// f is ...


Both f and e are the same as in the paper. I don't think it makes sense to describe them here.

OK. Nevertheless, I think the paper is too terse! Not blaming you, but I couldn't make sense of what those variables were from both the code and the paper. Still, you can leave it alone. We can't be rewriting papers as well.

maddyblue · 2017-03-17T20:50:50Z

context.go

-	if workp < 7 {
-		workp = 7
+
+	// TODO(mjibson): Too many variables without comments/definitions. And nd is a temporary variable that goes out of scope, perhaps it can be closed off? I've given it a try but I'm not too happy with this either... Also, these are my *guesses* for what these variables represent.


nd is the only variable I can see as benefiting from this, but it doesn't even gain that much because it requires a var e blah above it, which I don't think is better. All the others are used throughout this function because that's how they work in the paper or decNumber.

maddyblue · 2017-03-17T20:51:36Z

context.go

+	// TODO(mjibson): Also it would be good if we could keep the number of
+	// variables in scope low. This refactoring doesn't quite work, but perhaps
+	// you can refactor the calls below to use nc.Precision instead of keeping
+	// workp in scope throughout.


I'm not sure how to do this without an extra variable because nc.Precision needs to be set to workp later down in the function, so we have to store it somewhere.

maddyblue · 2017-03-17T20:55:05Z

context.go

@@ -336,38 +336,62 @@ func (c *Context) Rem(d, x, y *Decimal) (Condition, error) {
 }

 // Sqrt sets d to the square root of x.
+// Sqrt uses the Babylonian method for computing the square root, which uses


I really like this basic description of the algorithm and the big O for the number of steps. However I'm going to leave the reference as is to be consistent with the other references, none of which show up in the godoc. If you think it should be in the godoc, we can have that discussion and move them all at the same time.

I'm okay with that. I didn't realize the change in position would have godoc effects.

rjnn · 2017-03-17T21:15:40Z

Looks like everything is addressed, and we can continue the conversation at #36.

DO NOT MERGE sqrt: add review comments.

ce5a471

This is a dummy PR to simulate a code review on the square root code in the existing APD library.

rjnn assigned maddyblue Mar 17, 2017

rjnn mentioned this pull request Mar 17, 2017

Sqrt needs a review #15

Closed

maddyblue reviewed Mar 17, 2017

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rjnn closed this Mar 17, 2017

rjnn deleted the review_sqrt branch March 17, 2017 21:15

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DO NOT MERGE sqrt: add review comments. #35

DO NOT MERGE sqrt: add review comments. #35

rjnn commented Mar 17, 2017 •

edited by RaduBerinde

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rjnn commented Mar 17, 2017

DO NOT MERGE sqrt: add review comments. #35

DO NOT MERGE sqrt: add review comments. #35

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rjnn commented Mar 17, 2017 • edited by RaduBerinde Loading

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rjnn commented Mar 17, 2017

rjnn commented Mar 17, 2017 •

edited by RaduBerinde

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