🚀 go-pugleaf

I'm writing a program that produces a Makefile to build fonts using
metafont.

The input is a striped version of the \font declerations in lfonts.tex,
i.e.:

cmssi10 scaled 900

One option to this program is the DPI for metafont. E.g., at 85dpi,
the above yields:
#
# NAME=cmssi10
# SCALE = 900
# EXTENSION = 77
#
cmssi10.77pk:
	-mf '\mode:=sun;mag=900/1000;input cmssi10'
	-gftopk cmssi10.77gf && rm -f cmssi10.77gf

The question is ``how does metafont generate the number used in the
pk/gf suffix.''

I'm using ROUND( DPI * scale * mag ) / ( 1000 * 1000 ) ),
which gives me ``77gf'' above. Metafont generates a .76gf.
Aha, you say, it must TRUNCATE, not ROUND, i.e.
	ROUND( 85 * 900 * 1000 ) / (1000 * 1000 ) -> 77
	TRUNC( 85 * 900 * 1000 ) / (1000 * 1000 ) -> 76

But, then, consider:
	mf '\mode:=sun;mag=700/1000;input cmssi10'

this gives me cmssi10.60gf, but I compute
	ROUND( 85 * 700 * 1000 ) / (1000 * 1000 ) -> 60
	TRUNC( 85 * 700 * 1000 ) / (1000 * 1000 ) -> 59

and in this case, metafont rounds. So what gives? Anyone have a simple
way to compute what the PK/GF extension is going to be? The reason it's
important is that the next entry in the generated makefile depends on
that PK/GF file, to wit:

cmssi10.77.snf:	cmssi10.77pk
	-mftobdf -dpi 85 cmssi10.77pk
	bdftosnf < cmssi10.77.bdf > cmssi10.77.snf


Dirk Grunwald -- Univ. of Colorado at Boulder	(grunwald@foobar.colorado.edu)