survey: add ability to track prioritized lists

In future changes, we will make use of these methods. The intention is to
keep track of the top contributors according to some metric. We don't want
to store all of the entries and do a sort at the end, so track a
constant-size table and remove rows that get pushed out depending on the
chosen sorting algorithm.

Co-authored-by: Jeff Hostetler <git@jeffhostetler.com>
Signed-off-by; Jeff Hostetler <git@jeffhostetler.com>
Signed-off-by: Derrick Stolee <stolee@gmail.com>

builtin/survey.c

@@ -73,6 +73,119 @@ struct survey_report_object_size_summary {
 	size_t num_missing;
 };
 
+typedef int (*survey_top_cmp)(void *v1, void *v2);
+
+MAYBE_UNUSED
+static int cmp_by_nr(void *v1, void *v2)
+{
+	struct survey_report_object_size_summary *s1 = v1;
+	struct survey_report_object_size_summary *s2 = v2;
+
+	if (s1->nr < s2->nr)
+		return -1;
+	if (s1->nr > s2->nr)
+		return 1;
+	return 0;
+}
+
+MAYBE_UNUSED
+static int cmp_by_disk_size(void *v1, void *v2)
+{
+	struct survey_report_object_size_summary *s1 = v1;
+	struct survey_report_object_size_summary *s2 = v2;
+
+	if (s1->disk_size < s2->disk_size)
+		return -1;
+	if (s1->disk_size > s2->disk_size)
+		return 1;
+	return 0;
+}
+
+MAYBE_UNUSED
+static int cmp_by_inflated_size(void *v1, void *v2)
+{
+	struct survey_report_object_size_summary *s1 = v1;
+	struct survey_report_object_size_summary *s2 = v2;
+
+	if (s1->inflated_size < s2->inflated_size)
+		return -1;
+	if (s1->inflated_size > s2->inflated_size)
+		return 1;
+	return 0;
+}
+
+/**
+ * Store a list of "top" categories by some sorting function. When
+ * inserting a new category, reorder the list and free the one that
+ * got ejected (if any).
+ */
+struct survey_report_top_table {
+	const char *name;
+	survey_top_cmp cmp_fn;
+	size_t nr;
+	size_t alloc;
+
+	/**
+	 * 'data' stores an array of structs and must be cast into
+	 * the proper array type before evaluating an index.
+	 */
+	void *data;
+};
+
+MAYBE_UNUSED
+static void init_top_sizes(struct survey_report_top_table *top,
+			   size_t limit, const char *name,
+			   survey_top_cmp cmp)
+{
+	struct survey_report_object_size_summary *sz_array;
+
+	top->name = name;
+	top->cmp_fn = cmp;
+	top->alloc = limit;
+	top->nr = 0;
+
+	CALLOC_ARRAY(sz_array, limit);
+	top->data = sz_array;
+}
+
+MAYBE_UNUSED
+static void clear_top_sizes(struct survey_report_top_table *top)
+{
+	struct survey_report_object_size_summary *sz_array = top->data;
+
+	for (size_t i = 0; i < top->nr; i++)
+		free(sz_array[i].label);
+	free(sz_array);
+}
+
+MAYBE_UNUSED
+static void maybe_insert_into_top_size(struct survey_report_top_table *top,
+				       struct survey_report_object_size_summary *summary)
+{
+	struct survey_report_object_size_summary *sz_array = top->data;
+	size_t pos = top->nr;
+
+	/* Compare against list from the bottom. */
+	while (pos > 0 && top->cmp_fn(&sz_array[pos - 1], summary) < 0)
+		pos--;
+
+	/* Not big enough! */
+	if (pos >= top->alloc)
+		return;
+
+	/* We need to shift the data. */
+	if (top->nr == top->alloc)
+		free(sz_array[top->nr - 1].label);
+	else
+		top->nr++;
+
+	for (size_t i = top->nr - 1; i > pos; i--)
+		memcpy(&sz_array[i], &sz_array[i - 1], sizeof(*sz_array));
+
+	memcpy(&sz_array[pos], summary, sizeof(*summary));
+	sz_array[pos].label = xstrdup(summary->label);
+}
+
 /**
  * This struct contains all of the information that needs to be printed
  * at the end of the exploration of the repository and its references.