[unix-history] / .ref-BSD-3 / usr / doc / trofftut / tt14

.NH
Diversions
.PP
There are numerous occasions in page layout when it is necessary to store some text
for a period of time without actually printing it.
Footnotes are the most obvious example:
the text of the footnote usually appears in the input well before the place
on the page where it is to be printed is reached.
In fact,
the place where it is output normally depends on how big it is,
which implies that there must be a way
to process the footnote at least
enough to decide its size
without printing it.
.PP
.UL troff
provides a mechanism called a diversion
for doing this processing.
Any part of the output may be diverted into a macro instead
of being printed,
and then at some convenient time the macro may be put back into
the input.
.PP
The command
.BD .di\ xy
begins a diversion _ all subsequent output is collected into the macro
.BD xy
until the command
.BD .di
with no arguments is encountered.
This terminates the diversion.
The processed text is available at any time thereafter, simply
by giving the command
.P1
^xy
.P2
The vertical size of the last finished diversion is contained in
the built-in number register
.BD dn .
.PP
As a simple example,
suppose we want to implement a `keep-release'
operation,
so that text between the commands
.BD .KS 
and
.BD .KE
will not be split across a page boundary
(as for a figure or table).
Clearly, when a
.BD .KS
is encountered, we have to begin diverting
the output so we can find out how big it is.
Then when a
.BD .KE
is seen, we decide
whether the diverted text will fit on the current page,
and print it either there if it fits, or at the top of the next page if it doesn't.
So:
.P1 2
.ta .6i
^de KS	\e" start keep
^br	\e" start fresh line
^ev 1	\e" collect in new environment
^fi	\e" make it filled text
^di XX	\e" collect in XX
^^
.P2
.P1 2
.ta .6i
^de KE	\e" end keep
^br	\e" get last partial line
^di	\e" end diversion
^if \e\en(dn>=\e\en(.t .bp   \e" bp if doesn't fit
^nf	\e" bring it back in no-fill
^XX	\e" text
^ev	\e" return to normal environment
^^
.P2
Recall that number register
.BD nl
is the current position
on the output page.
Since output was being diverted, this remains
at its value when the diversion started.
.BD dn
is the amount of text in the diversion;
.BD .t
(another built-in register)
is the distance to the next trap,
which we assume is at the bottom margin of the page.
If the diversion is large enough to go past the trap,
the
.BD .if
is satisfied, and
a
.BD .bp
is issued.
In either case, the diverted output is then brought back with
.BD .XX .
It is essential to bring it back in no-fill mode so
.UL troff
will do no further processing on it.
.PP
This is not the most general keep-release,
nor is it robust in the face of all conceivable inputs,
but it would require more space than we have here to write it
in full generality.
This section is not intended
to teach everything about diversions,
but to sketch out enough that you can read
existing macro packages with some comprehension.
Commit	Line	Data
8340f87c BJ	1	.NH
	2	Diversions
	3	.PP
	4	There are numerous occasions in page layout when it is necessary to store some text
	5	for a period of time without actually printing it.
	6	Footnotes are the most obvious example:
	7	the text of the footnote usually appears in the input well before the place
	8	on the page where it is to be printed is reached.
	9	In fact,
	10	the place where it is output normally depends on how big it is,
	11	which implies that there must be a way
	12	to process the footnote at least
	13	enough to decide its size
	14	without printing it.
	15	.PP
	16	.UL troff
	17	provides a mechanism called a diversion
	18	for doing this processing.
	19	Any part of the output may be diverted into a macro instead
	20	of being printed,
	21	and then at some convenient time the macro may be put back into
	22	the input.
	23	.PP
	24	The command
	25	.BD .di\ xy
	26	begins a diversion _ all subsequent output is collected into the macro
	27	.BD xy
	28	until the command
	29	.BD .di
	30	with no arguments is encountered.
	31	This terminates the diversion.
	32	The processed text is available at any time thereafter, simply
	33	by giving the command
	34	.P1
	35	^xy
	36	.P2
	37	The vertical size of the last finished diversion is contained in
	38	the built-in number register
	39	.BD dn .
	40	.PP
	41	As a simple example,
	42	suppose we want to implement a `keep-release'
	43	operation,
	44	so that text between the commands
	45	.BD .KS
	46	and
	47	.BD .KE
	48	will not be split across a page boundary
	49	(as for a figure or table).
	50	Clearly, when a
	51	.BD .KS
	52	is encountered, we have to begin diverting
	53	the output so we can find out how big it is.
	54	Then when a
	55	.BD .KE
	56	is seen, we decide
	57	whether the diverted text will fit on the current page,
	58	and print it either there if it fits, or at the top of the next page if it doesn't.
	59	So:
	60	.P1 2
	61	.ta .6i
	62	^de KS \e" start keep
	63	^br \e" start fresh line
	64	^ev 1 \e" collect in new environment
65	^fi \e" make it filled text
66	^di XX \e" collect in XX
67	^^
68	.P2
69	.P1 2
70	.ta .6i
71	^de KE \e" end keep
72	^br \e" get last partial line
73	^di \e" end diversion
74	^if \e\en(dn>=\e\en(.t .bp \e" bp if doesn't fit
75	^nf \e" bring it back in no-fill
76	^XX \e" text
77	^ev \e" return to normal environment
78	^^
79	.P2
80	Recall that number register
81	.BD nl
82	is the current position
83	on the output page.
84	Since output was being diverted, this remains
85	at its value when the diversion started.
86	.BD dn
87	is the amount of text in the diversion;
88	.BD .t
89	(another built-in register)
90	is the distance to the next trap,
91	which we assume is at the bottom margin of the page.
92	If the diversion is large enough to go past the trap,
93	the
94	.BD .if
95	is satisfied, and
96	a
97	.BD .bp
98	is issued.
99	In either case, the diverted output is then brought back with
100	.BD .XX .
101	It is essential to bring it back in no-fill mode so
102	.UL troff
103	will do no further processing on it.
104	.PP
105	This is not the most general keep-release,
106	nor is it robust in the face of all conceivable inputs,
107	but it would require more space than we have here to write it
108	in full generality.
109	This section is not intended
110	to teach everything about diversions,
111	but to sketch out enough that you can read
112	existing macro packages with some comprehension.