Merge branch 'js/builtin-add-i'

The beginning of rewriting "git add -i" in C.

* js/builtin-add-i:
  built-in add -i: implement the `help` command
  built-in add -i: use color in the main loop
  built-in add -i: support `?` (prompt help)
  built-in add -i: show unique prefixes of the commands
  built-in add -i: implement the main loop
  built-in add -i: color the header in the `status` command
  built-in add -i: implement the `status` command
  diff: export diffstat interface
  Start to implement a built-in version of `git add --interactive`

9 files changed, 743 insertions, 23 deletions

diff --git a/Documentation/config/add.txt b/Documentation/config/add.txt
index 4d753f006e..c9f748f81c 100644
--- a/Documentation/config/add.txt
+++ b/Documentation/config/add.txt
@@ -5,3 +5,8 @@ add.ignore-errors (deprecated)::
 	option of linkgit:git-add[1].  `add.ignore-errors` is deprecated,
 	as it does not follow the usual naming convention for configuration
 	variables.
+
+add.interactive.useBuiltin::
+	[EXPERIMENTAL] Set to `true` to use the experimental built-in
+	implementation of the interactive version of linkgit:git-add[1]
+	instead of the Perl script version. Is `false` by default.
diff --git a/Makefile b/Makefile
index 44997f6f57..b7d7374dac 100644
--- a/Makefile
+++ b/Makefile
@@ -824,6 +824,7 @@ LIB_H := $(sort $(patsubst ./%,%,$(shell git ls-files '*.h' ':!t/' ':!Documentat
 	-name '*.h' -print)))
 
 LIB_OBJS += abspath.o
+LIB_OBJS += add-interactive.o
 LIB_OBJS += advice.o
 LIB_OBJS += alias.o
 LIB_OBJS += alloc.o
diff --git a/add-interactive.c b/add-interactive.c
new file mode 100644
index 0000000000..d6cb98cd40
--- /dev/null
+++ b/add-interactive.c
@@ -0,0 +1,652 @@
+#include "cache.h"
+#include "add-interactive.h"
+#include "color.h"
+#include "config.h"
+#include "diffcore.h"
+#include "revision.h"
+#include "refs.h"
+#include "string-list.h"
+
+struct add_i_state {
+	struct repository *r;
+	int use_color;
+	char header_color[COLOR_MAXLEN];
+	char help_color[COLOR_MAXLEN];
+	char prompt_color[COLOR_MAXLEN];
+	char error_color[COLOR_MAXLEN];
+	char reset_color[COLOR_MAXLEN];
+};
+
+static void init_color(struct repository *r, struct add_i_state *s,
+		       const char *slot_name, char *dst,
+		       const char *default_color)
+{
+	char *key = xstrfmt("color.interactive.%s", slot_name);
+	const char *value;
+
+	if (!s->use_color)
+		dst[0] = '\0';
+	else if (repo_config_get_value(r, key, &value) ||
+		 color_parse(value, dst))
+		strlcpy(dst, default_color, COLOR_MAXLEN);
+
+	free(key);
+}
+
+static void init_add_i_state(struct add_i_state *s, struct repository *r)
+{
+	const char *value;
+
+	s->r = r;
+
+	if (repo_config_get_value(r, "color.interactive", &value))
+		s->use_color = -1;
+	else
+		s->use_color =
+			git_config_colorbool("color.interactive", value);
+	s->use_color = want_color(s->use_color);
+
+	init_color(r, s, "header", s->header_color, GIT_COLOR_BOLD);
+	init_color(r, s, "help", s->help_color, GIT_COLOR_BOLD_RED);
+	init_color(r, s, "prompt", s->prompt_color, GIT_COLOR_BOLD_BLUE);
+	init_color(r, s, "error", s->error_color, GIT_COLOR_BOLD_RED);
+	init_color(r, s, "reset", s->reset_color, GIT_COLOR_RESET);
+}
+
+/*
+ * A "prefix item list" is a list of items that are identified by a string, and
+ * a unique prefix (if any) is determined for each item.
+ *
+ * It is implemented in the form of a pair of `string_list`s, the first one
+ * duplicating the strings, with the `util` field pointing at a structure whose
+ * first field must be `size_t prefix_length`.
+ *
+ * That `prefix_length` field will be computed by `find_unique_prefixes()`; It
+ * will be set to zero if no valid, unique prefix could be found.
+ *
+ * The second `string_list` is called `sorted` and does _not_ duplicate the
+ * strings but simply reuses the first one's, with the `util` field pointing at
+ * the `string_item_list` of the first `string_list`. It  will be populated and
+ * sorted by `find_unique_prefixes()`.
+ */
+struct prefix_item_list {
+	struct string_list items;
+	struct string_list sorted;
+	size_t min_length, max_length;
+};
+#define PREFIX_ITEM_LIST_INIT \
+	{ STRING_LIST_INIT_DUP, STRING_LIST_INIT_NODUP, 1, 4 }
+
+static void prefix_item_list_clear(struct prefix_item_list *list)
+{
+	string_list_clear(&list->items, 1);
+	string_list_clear(&list->sorted, 0);
+}
+
+static void extend_prefix_length(struct string_list_item *p,
+				 const char *other_string, size_t max_length)
+{
+	size_t *len = p->util;
+
+	if (!*len || memcmp(p->string, other_string, *len))
+		return;
+
+	for (;;) {
+		char c = p->string[*len];
+
+		/*
+		 * Is `p` a strict prefix of `other`? Or have we exhausted the
+		 * maximal length of the prefix? Or is the current character a
+		 * multi-byte UTF-8 one? If so, there is no valid, unique
+		 * prefix.
+		 */
+		if (!c || ++*len > max_length || !isascii(c)) {
+			*len = 0;
+			break;
+		}
+
+		if (c != other_string[*len - 1])
+			break;
+	}
+}
+
+static void find_unique_prefixes(struct prefix_item_list *list)
+{
+	size_t i;
+
+	if (list->sorted.nr == list->items.nr)
+		return;
+
+	string_list_clear(&list->sorted, 0);
+	/* Avoid reallocating incrementally */
+	list->sorted.items = xmalloc(st_mult(sizeof(*list->sorted.items),
+					     list->items.nr));
+	list->sorted.nr = list->sorted.alloc = list->items.nr;
+
+	for (i = 0; i < list->items.nr; i++) {
+		list->sorted.items[i].string = list->items.items[i].string;
+		list->sorted.items[i].util = list->items.items + i;
+	}
+
+	string_list_sort(&list->sorted);
+
+	for (i = 0; i < list->sorted.nr; i++) {
+		struct string_list_item *sorted_item = list->sorted.items + i;
+		struct string_list_item *item = sorted_item->util;
+		size_t *len = item->util;
+
+		*len = 0;
+		while (*len < list->min_length) {
+			char c = item->string[(*len)++];
+
+			if (!c || !isascii(c)) {
+				*len = 0;
+				break;
+			}
+		}
+
+		if (i > 0)
+			extend_prefix_length(item, sorted_item[-1].string,
+					     list->max_length);
+		if (i + 1 < list->sorted.nr)
+			extend_prefix_length(item, sorted_item[1].string,
+					     list->max_length);
+	}
+}
+
+static ssize_t find_unique(const char *string, struct prefix_item_list *list)
+{
+	int index = string_list_find_insert_index(&list->sorted, string, 1);
+	struct string_list_item *item;
+
+	if (list->items.nr != list->sorted.nr)
+		BUG("prefix_item_list in inconsistent state (%"PRIuMAX
+		    " vs %"PRIuMAX")",
+		    (uintmax_t)list->items.nr, (uintmax_t)list->sorted.nr);
+
+	if (index < 0)
+		item = list->sorted.items[-1 - index].util;
+	else if (index > 0 &&
+		 starts_with(list->sorted.items[index - 1].string, string))
+		return -1;
+	else if (index + 1 < list->sorted.nr &&
+		 starts_with(list->sorted.items[index + 1].string, string))
+		return -1;
+	else if (index < list->sorted.nr)
+		item = list->sorted.items[index].util;
+	else
+		return -1;
+	return item - list->items.items;
+}
+
+struct list_options {
+	int columns;
+	const char *header;
+	void (*print_item)(int i, struct string_list_item *item, void *print_item_data);
+	void *print_item_data;
+};
+
+static void list(struct add_i_state *s, struct string_list *list,
+		 struct list_options *opts)
+{
+	int i, last_lf = 0;
+
+	if (!list->nr)
+		return;
+
+	if (opts->header)
+		color_fprintf_ln(stdout, s->header_color,
+				 "%s", opts->header);
+
+	for (i = 0; i < list->nr; i++) {
+		opts->print_item(i, list->items + i, opts->print_item_data);
+
+		if ((opts->columns) && ((i + 1) % (opts->columns))) {
+			putchar('\t');
+			last_lf = 0;
+		}
+		else {
+			putchar('\n');
+			last_lf = 1;
+		}
+	}
+
+	if (!last_lf)
+		putchar('\n');
+}
+struct list_and_choose_options {
+	struct list_options list_opts;
+
+	const char *prompt;
+	void (*print_help)(struct add_i_state *s);
+};
+
+#define LIST_AND_CHOOSE_ERROR (-1)
+#define LIST_AND_CHOOSE_QUIT  (-2)
+
+/*
+ * Returns the selected index.
+ *
+ * If an error occurred, returns `LIST_AND_CHOOSE_ERROR`. Upon EOF,
+ * `LIST_AND_CHOOSE_QUIT` is returned.
+ */
+static ssize_t list_and_choose(struct add_i_state *s,
+			       struct prefix_item_list *items,
+			       struct list_and_choose_options *opts)
+{
+	struct strbuf input = STRBUF_INIT;
+	ssize_t res = LIST_AND_CHOOSE_ERROR;
+
+	find_unique_prefixes(items);
+
+	for (;;) {
+		char *p;
+
+		strbuf_reset(&input);
+
+		list(s, &items->items, &opts->list_opts);
+
+		color_fprintf(stdout, s->prompt_color, "%s", opts->prompt);
+		fputs("> ", stdout);
+		fflush(stdout);
+
+		if (strbuf_getline(&input, stdin) == EOF) {
+			putchar('\n');
+			res = LIST_AND_CHOOSE_QUIT;
+			break;
+		}
+		strbuf_trim(&input);
+
+		if (!input.len)
+			break;
+
+		if (!strcmp(input.buf, "?")) {
+			opts->print_help(s);
+			continue;
+		}
+
+		p = input.buf;
+		for (;;) {
+			size_t sep = strcspn(p, " \t\r\n,");
+			ssize_t index = -1;
+
+			if (!sep) {
+				if (!*p)
+					break;
+				p++;
+				continue;
+			}
+
+			if (isdigit(*p)) {
+				char *endp;
+				index = strtoul(p, &endp, 10) - 1;
+				if (endp != p + sep)
+					index = -1;
+			}
+
+			if (p[sep])
+				p[sep++] = '\0';
+			if (index < 0)
+				index = find_unique(p, items);
+
+			if (index < 0 || index >= items->items.nr)
+				color_fprintf_ln(stdout, s->error_color,
+						 _("Huh (%s)?"), p);
+			else {
+				res = index;
+				break;
+			}
+
+			p += sep;
+		}
+
+		if (res != LIST_AND_CHOOSE_ERROR)
+			break;
+	}
+
+	strbuf_release(&input);
+	return res;
+}
+
+struct adddel {
+	uintmax_t add, del;
+	unsigned seen:1, binary:1;
+};
+
+struct file_item {
+	struct adddel index, worktree;
+};
+
+static void add_file_item(struct string_list *files, const char *name)
+{
+	struct file_item *item = xcalloc(sizeof(*item), 1);
+
+	string_list_append(files, name)->util = item;
+}
+
+struct pathname_entry {
+	struct hashmap_entry ent;
+	const char *name;
+	struct file_item *item;
+};
+
+static int pathname_entry_cmp(const void *unused_cmp_data,
+			      const struct hashmap_entry *he1,
+			      const struct hashmap_entry *he2,
+			      const void *name)
+{
+	const struct pathname_entry *e1 =
+		container_of(he1, const struct pathname_entry, ent);
+	const struct pathname_entry *e2 =
+		container_of(he2, const struct pathname_entry, ent);
+
+	return strcmp(e1->name, name ? (const char *)name : e2->name);
+}
+
+struct collection_status {
+	enum { FROM_WORKTREE = 0, FROM_INDEX = 1 } phase;
+
+	const char *reference;
+
+	struct string_list *files;
+	struct hashmap file_map;
+};
+
+static void collect_changes_cb(struct diff_queue_struct *q,
+			       struct diff_options *options,
+			       void *data)
+{
+	struct collection_status *s = data;
+	struct diffstat_t stat = { 0 };
+	int i;
+
+	if (!q->nr)
+		return;
+
+	compute_diffstat(options, &stat, q);
+
+	for (i = 0; i < stat.nr; i++) {
+		const char *name = stat.files[i]->name;
+		int hash = strhash(name);
+		struct pathname_entry *entry;
+		struct file_item *file_item;
+		struct adddel *adddel;
+
+		entry = hashmap_get_entry_from_hash(&s->file_map, hash, name,
+						    struct pathname_entry, ent);
+		if (!entry) {
+			add_file_item(s->files, name);
+
+			entry = xcalloc(sizeof(*entry), 1);
+			hashmap_entry_init(&entry->ent, hash);
+			entry->name = s->files->items[s->files->nr - 1].string;
+			entry->item = s->files->items[s->files->nr - 1].util;
+			hashmap_add(&s->file_map, &entry->ent);
+		}
+
+		file_item = entry->item;
+		adddel = s->phase == FROM_INDEX ?
+			&file_item->index : &file_item->worktree;
+		adddel->seen = 1;
+		adddel->add = stat.files[i]->added;
+		adddel->del = stat.files[i]->deleted;
+		if (stat.files[i]->is_binary)
+			adddel->binary = 1;
+	}
+	free_diffstat_info(&stat);
+}
+
+static int get_modified_files(struct repository *r, struct string_list *files,
+			      const struct pathspec *ps)
+{
+	struct object_id head_oid;
+	int is_initial = !resolve_ref_unsafe("HEAD", RESOLVE_REF_READING,
+					     &head_oid, NULL);
+	struct collection_status s = { FROM_WORKTREE };
+
+	if (discard_index(r->index) < 0 ||
+	    repo_read_index_preload(r, ps, 0) < 0)
+		return error(_("could not read index"));
+
+	string_list_clear(files, 1);
+	s.files = files;
+	hashmap_init(&s.file_map, pathname_entry_cmp, NULL, 0);
+
+	for (s.phase = FROM_WORKTREE; s.phase <= FROM_INDEX; s.phase++) {
+		struct rev_info rev;
+		struct setup_revision_opt opt = { 0 };
+
+		opt.def = is_initial ?
+			empty_tree_oid_hex() : oid_to_hex(&head_oid);
+
+		init_revisions(&rev, NULL);
+		setup_revisions(0, NULL, &rev, &opt);
+
+		rev.diffopt.output_format = DIFF_FORMAT_CALLBACK;
+		rev.diffopt.format_callback = collect_changes_cb;
+		rev.diffopt.format_callback_data = &s;
+
+		if (ps)
+			copy_pathspec(&rev.prune_data, ps);
+
+		if (s.phase == FROM_INDEX)
+			run_diff_index(&rev, 1);
+		else {
+			rev.diffopt.flags.ignore_dirty_submodules = 1;
+			run_diff_files(&rev, 0);
+		}
+	}
+	hashmap_free_entries(&s.file_map, struct pathname_entry, ent);
+
+	/* While the diffs are ordered already, we ran *two* diffs... */
+	string_list_sort(files);
+
+	return 0;
+}
+
+static void render_adddel(struct strbuf *buf,
+				struct adddel *ad, const char *no_changes)
+{
+	if (ad->binary)
+		strbuf_addstr(buf, _("binary"));
+	else if (ad->seen)
+		strbuf_addf(buf, "+%"PRIuMAX"/-%"PRIuMAX,
+			    (uintmax_t)ad->add, (uintmax_t)ad->del);
+	else
+		strbuf_addstr(buf, no_changes);
+}
+
+/* filters out prefixes which have special meaning to list_and_choose() */
+static int is_valid_prefix(const char *prefix, size_t prefix_len)
+{
+	return prefix_len && prefix &&
+		/*
+		 * We expect `prefix` to be NUL terminated, therefore this
+		 * `strcspn()` call is okay, even if it might do much more
+		 * work than strictly necessary.
+		 */
+		strcspn(prefix, " \t\r\n,") >= prefix_len &&	/* separators */
+		*prefix != '-' &&				/* deselection */
+		!isdigit(*prefix) &&				/* selection */
+		(prefix_len != 1 ||
+		 (*prefix != '*' &&				/* "all" wildcard */
+		  *prefix != '?'));				/* prompt help */
+}
+
+struct print_file_item_data {
+	const char *modified_fmt;
+	struct strbuf buf, index, worktree;
+};
+
+static void print_file_item(int i, struct string_list_item *item,
+			    void *print_file_item_data)
+{
+	struct file_item *c = item->util;
+	struct print_file_item_data *d = print_file_item_data;
+
+	strbuf_reset(&d->index);
+	strbuf_reset(&d->worktree);
+	strbuf_reset(&d->buf);
+
+	render_adddel(&d->worktree, &c->worktree, _("nothing"));
+	render_adddel(&d->index, &c->index, _("unchanged"));
+	strbuf_addf(&d->buf, d->modified_fmt,
+		    d->index.buf, d->worktree.buf, item->string);
+
+	printf(" %2d: %s", i + 1, d->buf.buf);
+}
+
+static int run_status(struct add_i_state *s, const struct pathspec *ps,
+		      struct string_list *files, struct list_options *opts)
+{
+	if (get_modified_files(s->r, files, ps) < 0)
+		return -1;
+
+	list(s, files, opts);
+	putchar('\n');
+
+	return 0;
+}
+
+static int run_help(struct add_i_state *s, const struct pathspec *unused_ps,
+		    struct string_list *unused_files,
+		    struct list_options *unused_opts)
+{
+	color_fprintf_ln(stdout, s->help_color, "status        - %s",
+			 _("show paths with changes"));
+	color_fprintf_ln(stdout, s->help_color, "update        - %s",
+			 _("add working tree state to the staged set of changes"));
+	color_fprintf_ln(stdout, s->help_color, "revert        - %s",
+			 _("revert staged set of changes back to the HEAD version"));
+	color_fprintf_ln(stdout, s->help_color, "patch         - %s",
+			 _("pick hunks and update selectively"));
+	color_fprintf_ln(stdout, s->help_color, "diff          - %s",
+			 _("view diff between HEAD and index"));
+	color_fprintf_ln(stdout, s->help_color, "add untracked - %s",
+			 _("add contents of untracked files to the staged set of changes"));
+
+	return 0;
+}
+
+typedef int (*command_t)(struct add_i_state *s, const struct pathspec *ps,
+			 struct string_list *files,
+			 struct list_options *opts);
+
+struct command_item {
+	size_t prefix_length;
+	command_t command;
+};
+
+struct print_command_item_data {
+	const char *color, *reset;
+};
+
+static void print_command_item(int i, struct string_list_item *item,
+			       void *print_command_item_data)
+{
+	struct print_command_item_data *d = print_command_item_data;
+	struct command_item *util = item->util;
+
+	if (!util->prefix_length ||
+	    !is_valid_prefix(item->string, util->prefix_length))
+		printf(" %2d: %s", i + 1, item->string);
+	else
+		printf(" %2d: %s%.*s%s%s", i + 1,
+		       d->color, (int)util->prefix_length, item->string,
+		       d->reset, item->string + util->prefix_length);
+}
+
+static void command_prompt_help(struct add_i_state *s)
+{
+	const char *help_color = s->help_color;
+	color_fprintf_ln(stdout, help_color, "%s", _("Prompt help:"));
+	color_fprintf_ln(stdout, help_color, "1          - %s",
+			 _("select a numbered item"));
+	color_fprintf_ln(stdout, help_color, "foo        - %s",
+			 _("select item based on unique prefix"));
+	color_fprintf_ln(stdout, help_color, "           - %s",
+			 _("(empty) select nothing"));
+}
+
+int run_add_i(struct repository *r, const struct pathspec *ps)
+{
+	struct add_i_state s = { NULL };
+	struct print_command_item_data data = { "[", "]" };
+	struct list_and_choose_options main_loop_opts = {
+		{ 4, N_("*** Commands ***"), print_command_item, &data },
+		N_("What now"), command_prompt_help
+	};
+	struct {
+		const char *string;
+		command_t command;
+	} command_list[] = {
+		{ "status", run_status },
+		{ "help", run_help },
+	};
+	struct prefix_item_list commands = PREFIX_ITEM_LIST_INIT;
+
+	struct print_file_item_data print_file_item_data = {
+		"%12s %12s %s", STRBUF_INIT, STRBUF_INIT, STRBUF_INIT
+	};
+	struct list_options opts = {
+		0, NULL, print_file_item, &print_file_item_data
+	};
+	struct strbuf header = STRBUF_INIT;
+	struct string_list files = STRING_LIST_INIT_DUP;
+	ssize_t i;
+	int res = 0;
+
+	for (i = 0; i < ARRAY_SIZE(command_list); i++) {
+		struct command_item *util = xcalloc(sizeof(*util), 1);
+		util->command = command_list[i].command;
+		string_list_append(&commands.items, command_list[i].string)
+			->util = util;
+	}
+
+	init_add_i_state(&s, r);
+
+	/*
+	 * When color was asked for, use the prompt color for
+	 * highlighting, otherwise use square brackets.
+	 */
+	if (s.use_color) {
+		data.color = s.prompt_color;
+		data.reset = s.reset_color;
+	}
+
+	strbuf_addstr(&header, "      ");
+	strbuf_addf(&header, print_file_item_data.modified_fmt,
+		    _("staged"), _("unstaged"), _("path"));
+	opts.header = header.buf;
+
+	if (discard_index(r->index) < 0 ||
+	    repo_read_index(r) < 0 ||
+	    repo_refresh_and_write_index(r, REFRESH_QUIET, 0, 1,
+					 NULL, NULL, NULL) < 0)
+		warning(_("could not refresh index"));
+
+	res = run_status(&s, ps, &files, &opts);
+
+	for (;;) {
+		i = list_and_choose(&s, &commands, &main_loop_opts);
+		if (i == LIST_AND_CHOOSE_QUIT) {
+			printf(_("Bye.\n"));
+			res = 0;
+			break;
+		}
+		if (i != LIST_AND_CHOOSE_ERROR) {
+			struct command_item *util =
+				commands.items.items[i].util;
+			res = util->command(&s, ps, &files, &opts);
+		}
+	}
+
+	string_list_clear(&files, 1);
+	strbuf_release(&print_file_item_data.buf);
+	strbuf_release(&print_file_item_data.index);
+	strbuf_release(&print_file_item_data.worktree);
+	strbuf_release(&header);
+	prefix_item_list_clear(&commands);
+
+	return res;
+}
diff --git a/add-interactive.h b/add-interactive.h
new file mode 100644
index 0000000000..7043b8741d
--- /dev/null
+++ b/add-interactive.h
@@ -0,0 +1,8 @@
+#ifndef ADD_INTERACTIVE_H
+#define ADD_INTERACTIVE_H
+
+struct repository;
+struct pathspec;
+int run_add_i(struct repository *r, const struct pathspec *ps);
+
+#endif
diff --git a/builtin/add.c b/builtin/add.c
index dd18e5c9b6..d4686d5218 100644
--- a/builtin/add.c
+++ b/builtin/add.c
@@ -20,6 +20,7 @@
 #include "bulk-checkin.h"
 #include "argv-array.h"
 #include "submodule.h"
+#include "add-interactive.h"
 
 static const char * const builtin_add_usage[] = {
 	N_("git add [<options>] [--] <pathspec>..."),
@@ -185,6 +186,16 @@ int run_add_interactive(const char *revision, const char *patch_mode,
 {
 	int status, i;
 	struct argv_array argv = ARGV_ARRAY_INIT;
+	int use_builtin_add_i =
+		git_env_bool("GIT_TEST_ADD_I_USE_BUILTIN", -1);
+
+	if (!patch_mode) {
+		if (use_builtin_add_i < 0)
+			git_config_get_bool("add.interactive.usebuiltin",
+					    &use_builtin_add_i);
+		if (use_builtin_add_i == 1)
+			return !!run_add_i(the_repository, pathspec);
+	}
 
 	argv_array_push(&argv, "add--interactive");
 	if (patch_mode)
@@ -319,6 +330,7 @@ static int add_config(const char *var, const char *value, void *cb)
 		ignore_add_errors = git_config_bool(var, value);
 		return 0;
 	}
+
 	return git_default_config(var, value, cb);
 }
 
diff --git a/diff.c b/diff.c
index 087213466e..8e2914c031 100644
--- a/diff.c
+++ b/diff.c
@@ -2495,22 +2495,6 @@ static void pprint_rename(struct strbuf *name, const char *a, const char *b)
 	}
 }
 
-struct diffstat_t {
-	int nr;
-	int alloc;
-	struct diffstat_file {
-		char *from_name;
-		char *name;
-		char *print_name;
-		const char *comments;
-		unsigned is_unmerged:1;
-		unsigned is_binary:1;
-		unsigned is_renamed:1;
-		unsigned is_interesting:1;
-		uintmax_t added, deleted;
-	} **files;
-};
-
 static struct diffstat_file *diffstat_add(struct diffstat_t *diffstat,
 					  const char *name_a,
 					  const char *name_b)
@@ -3157,7 +3141,7 @@ static void show_dirstat_by_line(struct diffstat_t *data, struct diff_options *o
 	gather_dirstat(options, &dir, changed, "", 0);
 }
 
-static void free_diffstat_info(struct diffstat_t *diffstat)
+void free_diffstat_info(struct diffstat_t *diffstat)
 {
 	int i;
 	for (i = 0; i < diffstat->nr; i++) {
@@ -6283,12 +6267,7 @@ void diff_flush(struct diff_options *options)
 	    dirstat_by_line) {
 		struct diffstat_t diffstat;
 
-		memset(&diffstat, 0, sizeof(struct diffstat_t));
-		for (i = 0; i < q->nr; i++) {
-			struct diff_filepair *p = q->queue[i];
-			if (check_pair_status(p))
-				diff_flush_stat(p, options, &diffstat);
-		}
+		compute_diffstat(options, &diffstat, q);
 		if (output_format & DIFF_FORMAT_NUMSTAT)
 			show_numstat(&diffstat, options);
 		if (output_format & DIFF_FORMAT_DIFFSTAT)
@@ -6621,6 +6600,20 @@ static int is_submodule_ignored(const char *path, struct diff_options *options)
 	return ignored;
 }
 
+void compute_diffstat(struct diff_options *options,
+		      struct diffstat_t *diffstat,
+		      struct diff_queue_struct *q)
+{
+	int i;
+
+	memset(diffstat, 0, sizeof(struct diffstat_t));
+	for (i = 0; i < q->nr; i++) {
+		struct diff_filepair *p = q->queue[i];
+		if (check_pair_status(p))
+			diff_flush_stat(p, options, diffstat);
+	}
+}
+
 void diff_addremove(struct diff_options *options,
 		    int addremove, unsigned mode,
 		    const struct object_id *oid,
diff --git a/diff.h b/diff.h
index 7f8f024feb..d986ddc3b5 100644
--- a/diff.h
+++ b/diff.h
@@ -245,6 +245,22 @@ void diff_emit_submodule_error(struct diff_options *o, const char *err);
 void diff_emit_submodule_pipethrough(struct diff_options *o,
 				     const char *line, int len);
 
+struct diffstat_t {
+	int nr;
+	int alloc;
+	struct diffstat_file {
+		char *from_name;
+		char *name;
+		char *print_name;
+		const char *comments;
+		unsigned is_unmerged:1;
+		unsigned is_binary:1;
+		unsigned is_renamed:1;
+		unsigned is_interesting:1;
+		uintmax_t added, deleted;
+	} **files;
+};
+
 enum color_diff {
 	DIFF_RESET = 0,
 	DIFF_CONTEXT = 1,
@@ -334,6 +350,10 @@ void diff_change(struct diff_options *,
 
 struct diff_filepair *diff_unmerge(struct diff_options *, const char *path);
 
+void compute_diffstat(struct diff_options *options, struct diffstat_t *diffstat,
+		      struct diff_queue_struct *q);
+void free_diffstat_info(struct diffstat_t *diffstat);
+
 #define DIFF_SETUP_REVERSE      	1
 #define DIFF_SETUP_USE_SIZE_CACHE	4
 
diff --git a/t/README b/t/README
index 60d5b77bcc..5132ec83f8 100644
--- a/t/README
+++ b/t/README
@@ -397,6 +397,10 @@ GIT_TEST_STASH_USE_BUILTIN=<boolean>, when false, disables the
 built-in version of git-stash. See 'stash.useBuiltin' in
 git-config(1).
 
+GIT_TEST_ADD_I_USE_BUILTIN=<boolean>, when true, enables the
+built-in version of git add -i. See 'add.interactive.useBuiltin' in
+git-config(1).
+
 GIT_TEST_INDEX_THREADS=<n> enables exercising the multi-threaded loading
 of the index for the whole test suite by bypassing the default number of
 cache entries and thread minimums. Setting this to 1 will make the
diff --git a/t/t3701-add-interactive.sh b/t/t3701-add-interactive.sh
index d50e165ca8..d4f9386621 100755
--- a/t/t3701-add-interactive.sh
+++ b/t/t3701-add-interactive.sh
@@ -647,4 +647,29 @@ test_expect_success 'checkout -p works with pathological context lines' '
 	test_write_lines a b a b a a b a b a >expect &&
 	test_cmp expect a
 '
+
+test_expect_success 'show help from add--helper' '
+	git reset --hard &&
+	cat >expect <<-EOF &&
+
+	<BOLD>*** Commands ***<RESET>
+	  1: <BOLD;BLUE>s<RESET>tatus	  2: <BOLD;BLUE>u<RESET>pdate	  3: <BOLD;BLUE>r<RESET>evert	  4: <BOLD;BLUE>a<RESET>dd untracked
+	  5: <BOLD;BLUE>p<RESET>atch	  6: <BOLD;BLUE>d<RESET>iff	  7: <BOLD;BLUE>q<RESET>uit	  8: <BOLD;BLUE>h<RESET>elp
+	<BOLD;BLUE>What now<RESET>> <BOLD;RED>status        - show paths with changes<RESET>
+	<BOLD;RED>update        - add working tree state to the staged set of changes<RESET>
+	<BOLD;RED>revert        - revert staged set of changes back to the HEAD version<RESET>
+	<BOLD;RED>patch         - pick hunks and update selectively<RESET>
+	<BOLD;RED>diff          - view diff between HEAD and index<RESET>
+	<BOLD;RED>add untracked - add contents of untracked files to the staged set of changes<RESET>
+	<BOLD>*** Commands ***<RESET>
+	  1: <BOLD;BLUE>s<RESET>tatus	  2: <BOLD;BLUE>u<RESET>pdate	  3: <BOLD;BLUE>r<RESET>evert	  4: <BOLD;BLUE>a<RESET>dd untracked
+	  5: <BOLD;BLUE>p<RESET>atch	  6: <BOLD;BLUE>d<RESET>iff	  7: <BOLD;BLUE>q<RESET>uit	  8: <BOLD;BLUE>h<RESET>elp
+	<BOLD;BLUE>What now<RESET>>$SP
+	Bye.
+	EOF
+	test_write_lines h | GIT_PAGER_IN_USE=true TERM=vt100 git add -i >actual.colored &&
+	test_decode_color <actual.colored >actual &&
+	test_i18ncmp expect actual
+'
+
 test_done