.\"	$Id$
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.\" Copyright (c) 2020 Kristaps Dzonsons <kristaps@bsd.lv>
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.Dd $Mdocdate$
.Dt LOWDOWN_DIFF 3
.Os
.Sh NAME
.Nm lowdown_diff
.Nd compute difference between parsed Markdown trees
.Sh LIBRARY
.Lb liblowdown
.Sh SYNOPSIS
.In sys/queue.h
.In stdio.h
.In lowdown.h
.Ft "struct lowdown_node *"
.Fo lowdown_diff
.Fa "const struct lowdown_node *nold"
.Fa "const struct lowdown_node *nnew"
.Fa "size_t *maxn"
.Fc
.Sh DESCRIPTION
Computes the difference between two Markdown trees, the source
.Fa nold
and destination
.Fa nnew ,
parsed by
.Xr lowdown_doc_parse 3 .
It uses the
.Vt enum lowdown_chng
type in the return tree's nodes to dictate insertions into and deletions
from
.Fa nold .
The
.Fa maxn
argument, if not
.Dv NULL ,
is set to one greater than the highest node identifier of the returned
tree.
.Sh RETURN VALUES
Returns a pointer to the difference tree or
.Dv NULL
on memory exhaution.
The pointer must be freed with
.Xr lowdown_node_free 3 .
.Sh EXAMPLES
The following parses and compares
.Va old
of length
.Va osz
and
.Va new
of length
.Va nsz .
It first allocates the parser, then the document, then the renderer
(HTML is used in this case).
Then it passes output to the renderer, prints it, and cleans up
resources.
On any memory errors, it exits with
.Xr err 3 .
.Bd -literal -offset indent
struct lowdown_doc *doc;
struct lowdown_node *no, *nn, *diff;
struct lowdown_buf *ob;
void *rndr;

if ((doc = lowdown_doc_new(NULL)) == NULL)
	err(1, NULL);
if ((no = lowdown_doc_parse(doc, NULL, old, osz, NULL)) == NULL)
	err(1, NULL);
if ((nn = lowdown_doc_parse(doc, NULL, new, nsz, NULL)) == NULL)
	err(1, NULL);
if ((diff = lowdown_diff(no, nn, NULL)) == NULL)
	err(1, NULL);
if ((rndr = lowdown_html_new(NULL)) == NULL)
	err(1, NULL);
if ((ob = lowdown_buf_new(1024)) == NULL)
	err(1, NULL);
if (!lowdown_html_rndr(ob, rndr, diff))
	err(1, NULL);

fwrite(stdout, 1, ob->size, ob->data);

lowdown_buf_free(ob);
lowdown_html_rndr_free(rndr);
lowdown_node_free(no);
lowdown_node_free(nn);
lowdown_node_free(diff);
lowdown_doc_free(doc);
.Ed
.Sh SEE ALSO
.Xr lowdown 3
.Rs
.%A Gregory Cobena
.%A Serge Abiteboul
.%A Amelie Marian
.%D 2002
.%T "Detecting Changes in XML Documents"
.%U https://www.cs.rutgers.edu/~amelie/papers/2002/diff.pdf
.Re
.Rs
.%A Wu Sun
.%A Manber Udi
.%A Myers Gene
.%T "An O(NP) sequence comparison algorithm"
.%J Information Processing Letters
.%V Volume 35
.%I Issue 6
.%D 1990
.Re