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Comment by g15jv2dp | Hacker News Reader

Time for nitpicks, sorry.

The formulas for the floats have typos. They should read (-1)^S, not -1^S (which always equals -1).

Interpreting the raw bit patterns isn't a piecewise linear approximation of the logarithm. The lines between the data points on the blue graph don't actually exist, it's not possible for a bit to be half set to 1. It's rather a discrete version of the logarithm: the only data points that exist - where the red and blue lines meet - are literally equal to the (scaled, shifted) logarithm.

Other than that, nice post!

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I'm not sure I understand. Imagine a very small 6-bit float, with 1 bit sign, 2 bits exponent, and 3 bits mantissa. The interval [010000, 010111] contains the numbers 2, 2.25, 2.5, 2.75, 3, 3.25, 3.5, 3.75.

But! These numbers' base two logarithms imply mantissae of

- .0000000 (equal to the float 000)

- .0010101 (not equal to the float 001)

- .0101001 (not equal to the float 010)

- .0111010 (not equal to the float 011)

- .1001010 (not equal to the float 100)

- .1011001 (not equal to the float 101)

- .1100111 (not equal to the float 110)

- .1110100 (not equal to the float 111)

because the floats in the [2,4) interval are linearly spaced, whereas the corresponding logarithms are not. In other words, the floats are a piecewise linear approximation of the logarithm – just as the article says.

You're right about the first part, of course. But don't you just need to truncate?

In any case, my point was more that it's not "piecewise linear". Piecewise linear means that the map is defined on some interval; it's not, it's defined on a discrete set of values. To take your example, the map isn't defined on e.g., 2.3. You can decide to interpolate linearly between the value at 2.25 and the value at 2.5, but that's a decision you make which isn't reflected in the code.

Or said differently: do you consider that any map defined on a discrete subset of R is really a piecewise linear map?

If your point is that it's more of an arithmetic sequence than a line segment, thanks to discretisation, then I agree but I don't have a good word for it other than "discrete piecewise linear".

I mean it's not continuous, but it's still on an interval scale.

It's not a continuous piecewise linear approximation of the logarithm, it's a discrete piecewise linear approximation of the logarithm. You are correct that the blue lines aren't continuous, but incorrect about their interpretation - there are 256 individual points evenly spaced along the that x axis that make up the blue graph, not just a handful at the intersections. (A full graph would have 2^32 options in a piecewise linear pattern, but that isn't what the OP has graphed.)

Given that the article is talking about operations on 32 bit integers and IEEE-754 32-bit floats, I personally think it's fine to leave out "discrete" in the description.

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