graph: move doc to graph.h and graph.c

Move the documentation from Documentation/technical/api-history-graph.txt to
graph.h and graph.c as it's easier for the developers to find the usage
information beside the code instead of looking for it in another doc file.

The graph library was already well documented, so few comments were added to
both graph.h and graph.c

Also documentation/technical/api-history-graph.txt is removed because
the information it has is now redundant and it'll be hard to keep it up to
date and synchronized with the documentation in the header file.

Signed-off-by: Heba Waly <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: HebaWaly

Documentation/technical/api-history-graph.txt

@@ -34,6 +34,7 @@ static void graph_padding_line(struct git_graph *graph, struct strbuf *sb);
  * handle directly. It is assumed that this is the same file handle as the
  * file specified by the graph diff options. This is necessary so that
  * graph_show_strbuf can be called even with a NULL graph.
+ * If a NULL graph is supplied, the strbuf is printed as-is.
  */
 static void graph_show_strbuf(struct git_graph *graph,
 			      FILE *file,

graph.c

@@ -2,6 +2,103 @@
 #define GRAPH_H
 #include "diff.h"
 
+/**
+ * The graph API is used to draw a text-based representation of the commit
+ * history. The API generates the graph in a line-by-line fashion.
+ *
+ * Calling sequence
+ * ----------------
+ *
+ * - Create a `struct git_graph` by calling `graph_init()`.  When using the
+ *   revision walking API, this is done automatically by `setup_revisions()` if
+ *   the '--graph' option is supplied.
+ *
+ * - Use the revision walking API to walk through a group of contiguous commits.
+ *   The `get_revision()` function automatically calls `graph_update()` each time
+ *   it is invoked.
+ *
+ * - For each commit, call `graph_next_line()` repeatedly, until
+ *   `graph_is_commit_finished()` returns non-zero.  Each call to
+ *   `graph_next_line()` will output a single line of the graph.  The resulting
+ *   lines will not contain any newlines.  `graph_next_line()` returns 1 if the
+ *   resulting line contains the current commit, or 0 if this is merely a line
+ *   needed to adjust the graph before or after the current commit.  This return
+ *   value can be used to determine where to print the commit summary information
+ *   alongside the graph output.
+ *
+ * Limitations
+ * -----------
+ * - Check the graph_update() function for its limitations.
+ *
+ * - The graph API does not currently support reverse commit ordering.  In
+ *   order to implement reverse ordering, the graphing API needs an
+ *   (efficient) mechanism to find the children of a commit.
+ *
+ * Sample usage
+ * ------------
+ *
+ * ------------
+ * struct commit *commit;
+ * struct git_graph *graph = graph_init(opts);
+ *
+ * while ((commit = get_revision(opts)) != NULL) {
+ * 	while (!graph_is_commit_finished(graph))
+ * 	{
+ * 		struct strbuf sb;
+ * 		int is_commit_line;
+ *
+ * 		strbuf_init(&sb, 0);
+ * 		is_commit_line = graph_next_line(graph, &sb);
+ * 		fputs(sb.buf, stdout);
+ *
+ * 		if (is_commit_line)
+ * 			log_tree_commit(opts, commit);
+ * 		else
+ * 			putchar(opts->diffopt.line_termination);
+ * 	}
+ * }
+ * ------------
+ * Sample output
+ * -------------
+ *
+ * The following is an example of the output from the graph API.  This output does
+ * not include any commit summary information--callers are responsible for
+ * outputting that information, if desired.
+ * ------------
+ * *
+ * *
+ * *
+ * |\
+ * * |
+ * | | *
+ * | \ \
+ * |  \ \
+ * *-. \ \
+ * |\ \ \ \
+ * | | * | |
+ * | | | | | *
+ * | | | | | *
+ * | | | | | *
+ * | | | | | |\
+ * | | | | | | *
+ * | * | | | | |
+ * | | | | | *  \
+ * | | | | | |\  |
+ * | | | | * | | |
+ * | | | | * | | |
+ * * | | | | | | |
+ * | |/ / / / / /
+ * |/| / / / / /
+ * * | | | | | |
+ * |/ / / / / /
+ * * | | | | |
+ * | | | | | *
+ * | | | | |/
+ * | | | | *
+ * ------------
+ *
+ */
+
 /* A graph is a pointer to this opaque structure */
 struct git_graph;
 
@@ -50,6 +147,21 @@ struct git_graph *graph_init(struct rev_info *opt);
  * If graph_update() is called before graph_is_commit_finished() returns 1,
  * the next call to graph_next_line() will output an ellipsis ("...")
  * to indicate that a portion of the graph is missing.
+ *
+ * Limitations:
+ * -----------
+ *
+ * - `graph_update()` must be called with commits in topological order.  It should
+ *   not be called on a commit if it has already been invoked with an ancestor of
+ *   that commit, or the graph output will be incorrect.
+ *
+ * - `graph_update()` must be called on a contiguous group of commits.  If
+ *   `graph_update()` is called on a particular commit, it should later be called
+ *   on all parents of that commit.  Parents must not be skipped, or the graph
+ *   output will appear incorrect.
+ *
+ * - `graph_update()` may be used on a pruned set of commits only if the parent list
+ *   has been rewritten so as to include only ancestors from the pruned set.
  */
 void graph_update(struct git_graph *graph, struct commit *commit);
 
@@ -62,6 +174,10 @@ void graph_update(struct git_graph *graph, struct commit *commit);
  * for this commit.  If 0 is returned, graph_next_line() may still be
  * called without calling graph_update(), and it will merely output
  * appropriate "vertical padding" in the graph.
+ *
+ * If `graph_update()` is called before all lines for the current commit have
+ * been printed, the next call to `graph_next_line()` will output an ellipsis,
+ * to indicate that a portion of the graph was omitted.
  */
 int graph_is_commit_finished(struct git_graph const *graph);
 
@@ -112,6 +228,7 @@ void graph_show_padding(struct git_graph *graph);
 /*
  * If the graph is non-NULL, print the rest of the history graph for this
  * commit to stdout.  Does not print a terminating newline on the last line.
+ * Returns 1 if output was printed, and 0 if no output was necessary.
  */
 int graph_show_remainder(struct git_graph *graph);
 
@@ -121,6 +238,10 @@ int graph_show_remainder(struct git_graph *graph);
  * This is similar to graph_show_strbuf(), but it always prints the
  * remainder of the graph.
  *
+ * It is better than directly calling `graph_show_strbuf()` followed by
+ * `graph_show_remainder()` since it properly handles buffers that do not end in
+ * a terminating newline.
+ *
  * If the strbuf ends with a newline, the output printed by
  * graph_show_commit_msg() will end with a newline.  If the strbuf is
  * missing a terminating newline (including if it is empty), the output