count-lines and off-by-one error

count-lines and off-by-one error

[Thibaut Verron]

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Hello,

The documentation for `count-lines' is the following:

> Return number of lines between START and END.
> This is usually the number of newlines between them,
> but can be one more if START is not equal to END
> and the greater of them is not at the start of a line.

I can vaguely understand this behaviour: it counts the number of non-empty
lines between START and END... except that it also counts empty lines if
they are in the middle. So, the number of lines, including the last one if
non-empty. Is that so?

Is there no built-in function computing the number of newlines between two
positions? There are multiple ways to do it, for example a subtraction
between `line-number-at-pos' (which does call `count-lines' but returns the
correct result), a wrapper around `count-lines' working around the edge
cases, or a call to `count-matches'. But it seems to me that it's a common
enough task to warrant a built-in method.

At the very least, `count-screen-lines' takes an optional argument
specifying whether to _always_ count this extra line (so, even at the
beginning of a line), maybe `count-lines' could get this behaviour too?

Thanks, best regards,
Thibaut

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