10 files changed, 577 insertions, 205 deletions

diff --git a/examples/Makefile b/examples/Makefile
index 156a5ba6d..fe99c75cb 100644
--- a/examples/Makefile
+++ b/examples/Makefile
@@ -3,12 +3,13 @@
 CC = gcc
 CFLAGS = -g -I../include -I../src
 LFLAGS = -L../build -lgit2 -lz
+APPS = general showindex diff
 
-all: general showindex diff
+all: $(APPS)
 
 % : %.c
 	$(CC) -o $@ $(CFLAGS) $< $(LFLAGS)
 
 clean:
-	$(RM) general showindex diff
+	$(RM) $(APPS)
 	$(RM) -r *.dSYM
diff --git a/examples/diff.c b/examples/diff.c
index 9b696dad5..5eb0f3179 100644
--- a/examples/diff.c
+++ b/examples/diff.c
@@ -87,46 +87,123 @@ int printer(void *data, char usage, const char *line)
 	return 0;
 }
 
+int check_uint16_param(const char *arg, const char *pattern, uint16_t *val)
+{
+	size_t len = strlen(pattern);
+	uint16_t strval;
+	char *endptr = NULL;
+	if (strncmp(arg, pattern, len))
+		return 0;
+	strval = strtoul(arg + len, &endptr, 0);
+	if (endptr == arg)
+		return 0;
+	*val = strval;
+	return 1;
+}
+
+int check_str_param(const char *arg, const char *pattern, char **val)
+{
+	size_t len = strlen(pattern);
+	if (strncmp(arg, pattern, len))
+		return 0;
+	*val = (char *)(arg + len);
+	return 1;
+}
+
+void usage(const char *message, const char *arg)
+{
+	if (message && arg)
+		fprintf(stderr, "%s: %s\n", message, arg);
+	else if (message)
+		fprintf(stderr, "%s\n", message);
+	fprintf(stderr, "usage: diff <tree-oid> <tree-oid>\n");
+	exit(1);
+}
+
 int main(int argc, char *argv[])
 {
 	char path[GIT_PATH_MAX];
 	git_repository *repo = NULL;
-	git_tree *a, *b;
+	git_tree *t1 = NULL, *t2 = NULL;
 	git_diff_options opts = {0};
 	git_diff_list *diff;
-	char *dir = ".";
-	int color = -1;
+	int i, color = -1, compact = 0;
+	char *a, *dir = ".", *treeish1 = NULL, *treeish2 = NULL;
 
-	if (argc != 3) {
-		fprintf(stderr, "usage: diff <tree-oid> <tree-oid>\n");
-		exit(1);
+	/* parse arguments as copied from git-diff */
+
+	for (i = 1; i < argc; ++i) {
+		a = argv[i];
+
+		if (a[0] != '-') {
+			if (treeish1 == NULL)
+				treeish1 = a;
+			else if (treeish2 == NULL)
+				treeish2 = a;
+			else
+				usage("Only one or two tree identifiers can be provided", NULL);
+		}
+		else if (!strcmp(a, "-p") || !strcmp(a, "-u") ||
+			!strcmp(a, "--patch"))
+			compact = 0;
+		else if (!strcmp(a, "--name-status"))
+			compact = 1;
+		else if (!strcmp(a, "--color"))
+			color = 0;
+		else if (!strcmp(a, "--no-color"))
+			color = -1;
+		else if (!strcmp(a, "-R"))
+			opts.flags |= GIT_DIFF_REVERSE;
+		else if (!strcmp(a, "-a") || !strcmp(a, "--text"))
+			opts.flags |= GIT_DIFF_FORCE_TEXT;
+		else if (!strcmp(a, "--ignore-space-at-eol"))
+			opts.flags |= GIT_DIFF_IGNORE_WHITESPACE_EOL;
+		else if (!strcmp(a, "-b") || !strcmp(a, "--ignore-space-change"))
+			opts.flags |= GIT_DIFF_IGNORE_WHITESPACE_CHANGE;
+		else if (!strcmp(a, "-w") || !strcmp(a, "--ignore-all-space"))
+			opts.flags |= GIT_DIFF_IGNORE_WHITESPACE;
+		else if (!check_uint16_param(a, "-U", &opts.context_lines) &&
+			!check_uint16_param(a, "--unified=", &opts.context_lines) &&
+			!check_uint16_param(a, "--inter-hunk-context=",
+				&opts.interhunk_lines) &&
+			!check_str_param(a, "--src-prefix=", &opts.src_prefix) &&
+			!check_str_param(a, "--dst-prefix=", &opts.dst_prefix))
+			usage("Unknown arg", a);
 	}
 
+	if (!treeish1)
+		usage("Must provide at least one tree identifier (for now)", NULL);
+
+	/* open repo */
+
 	check(git_repository_discover(path, sizeof(path), dir, 0, "/"),
 		"Could not discover repository");
 	check(git_repository_open(&repo, path),
 		"Could not open repository");
 
-	check(resolve_to_tree(repo, argv[1], &a), "Looking up first tree");
-	check(resolve_to_tree(repo, argv[2], &b), "Looking up second tree");
-
-	check(git_diff_tree_to_tree(repo, &opts, a, b, &diff), "Generating diff");
-
-	fputs(colors[0], stdout);
-
-	check(git_diff_print_compact(diff, &color, printer), "Displaying diff summary");
+	check(resolve_to_tree(repo, treeish1, &t1), "Looking up first tree");
+	if (treeish2)
+		check(resolve_to_tree(repo, treeish2, &t2), "Looking up second tree");
 
-	fprintf(stdout, "--\n");
+	if (!treeish2)
+		check(git_diff_index_to_tree(repo, &opts, t1, &diff), "Generating diff");
+	else
+		check(git_diff_tree_to_tree(repo, &opts, t1, t2, &diff), "Generating diff");
 
-	color = 0;
+	if (color >= 0)
+		fputs(colors[0], stdout);
 
-	check(git_diff_print_patch(diff, &color, printer), "Displaying diff");
+	if (compact)
+		check(git_diff_print_compact(diff, &color, printer), "Displaying diff summary");
+	else
+		check(git_diff_print_patch(diff, &color, printer), "Displaying diff");
 
-	fputs(colors[0], stdout);
+	if (color >= 0)
+		fputs(colors[0], stdout);
 
 	git_diff_list_free(diff);
-	git_tree_free(a);
-	git_tree_free(b);
+	git_tree_free(t1);
+	git_tree_free(t2);
 	git_repository_free(repo);
 
 	return 0;
diff --git a/include/git2/diff.h b/include/git2/diff.h
index 56801ca01..e9ef5c356 100644
--- a/include/git2/diff.h
+++ b/include/git2/diff.h
@@ -29,17 +29,33 @@
  */
 GIT_BEGIN_DECL
 
+enum {
+	GIT_DIFF_NORMAL = 0,
+	GIT_DIFF_REVERSE = (1 << 0),
+	GIT_DIFF_FORCE_TEXT = (1 << 1),
+	GIT_DIFF_IGNORE_WHITESPACE = (1 << 2),
+	GIT_DIFF_IGNORE_WHITESPACE_CHANGE = (1 << 3),
+	GIT_DIFF_IGNORE_WHITESPACE_EOL = (1 << 4),
+	GIT_DIFF_IGNORE_SUBMODULES = (1 << 5),
+	GIT_DIFF_PATIENCE = (1 << 6)
+};
+
 /**
  * Structure describing options about how the diff should be executed.
  *
+ * Setting all values of the structure to zero will yield the default
+ * values.  Similarly, passing NULL for the options structure will
+ * give the defaults.  The default values are marked below.
+ *
  * @todo Most of the parameters here are not actually supported at this time.
  */
 typedef struct {
-	int context_lines;
-	int interhunk_lines;
-	int ignore_whitespace;
-	int force_text;			/**< generate text diffs even for binaries */
-	git_strarray pathspec;
+	uint32_t flags;				/**< defaults to GIT_DIFF_NORMAL */
+	uint16_t context_lines;		/**< defaults to 3 */
+	uint16_t interhunk_lines;	/**< defaults to 3 */
+	char *src_prefix;			/**< defaults to "a" */
+	char *dst_prefix;			/**< defaults to "b" */
+	git_strarray pathspec;		/**< defaults to show all paths */
 } git_diff_options;
 
 /**
@@ -158,7 +174,7 @@ typedef int (*git_diff_output_fn)(
  */
 GIT_EXTERN(int) git_diff_tree_to_tree(
 	git_repository *repo,
-	const git_diff_options *opts,
+	const git_diff_options *opts, /**< can be NULL for defaults */
 	git_tree *old,
 	git_tree *new,
 	git_diff_list **diff);
@@ -169,7 +185,7 @@ GIT_EXTERN(int) git_diff_tree_to_tree(
  */
 GIT_EXTERN(int) git_diff_index_to_tree(
 	git_repository *repo,
-	const git_diff_options *opts,
+	const git_diff_options *opts, /**< can be NULL for defaults */
 	git_tree *old,
 	git_diff_list **diff);
 
@@ -179,7 +195,7 @@ GIT_EXTERN(int) git_diff_index_to_tree(
  */
 GIT_EXTERN(int) git_diff_workdir_to_tree(
 	git_repository *repo,
-	const git_diff_options *opts,
+	const git_diff_options *opts, /**< can be NULL for defaults */
 	git_tree *old,
 	git_diff_list **diff);
 
@@ -189,7 +205,7 @@ GIT_EXTERN(int) git_diff_workdir_to_tree(
  */
 GIT_EXTERN(int) git_diff_workdir_to_index(
 	git_repository *repo,
-	const git_diff_options *opts,
+	const git_diff_options *opts, /**< can be NULL for defaults */
 	git_diff_list **diff);
 
 /**
diff --git a/src/diff.c b/src/diff.c
index a5b5e6198..9a12aa07c 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -12,33 +12,6 @@
 #include "blob.h"
 #include <ctype.h>
 
-static git_diff_delta *file_delta_new(
-	git_diff_list *diff,
-	const git_tree_diff_data *tdiff)
-{
-	git_diff_delta *delta = git__calloc(1, sizeof(git_diff_delta));
-
-	if (!delta) {
-		git__rethrow(GIT_ENOMEM, "Could not allocate diff record");
-		return NULL;
-	}
-
-	/* copy shared fields */
-	delta->status   = tdiff->status;
-	delta->old_attr = tdiff->old_attr;
-	delta->new_attr = tdiff->new_attr;
-	delta->old_oid  = tdiff->old_oid;
-	delta->new_oid  = tdiff->new_oid;
-	delta->path     = git__strdup(diff->pfx.ptr);
-	if (delta->path == NULL) {
-		git__free(delta);
-		git__rethrow(GIT_ENOMEM, "Could not allocate diff record path");
-		return NULL;
-	}
-
-	return delta;
-}
-
 static void file_delta_free(git_diff_delta *delta)
 {
 	if (!delta)
@@ -55,77 +28,119 @@ static void file_delta_free(git_diff_delta *delta)
 	git__free(delta);
 }
 
-static int tree_add_cb(const char *root, git_tree_entry *entry, void *data)
+static int file_delta_new__from_one(
+	git_diff_list *diff,
+	git_status_t status,
+	unsigned int attr,
+	const git_oid *oid,
+	const char *path)
 {
 	int error;
-	git_diff_list *diff = data;
-	ssize_t pfx_len = diff->pfx.size;
-	git_tree_diff_data tdiff;
-	git_diff_delta *delta;
+	git_diff_delta *delta = git__calloc(1, sizeof(git_diff_delta));
 
-	memset(&tdiff, 0, sizeof(tdiff));
-	tdiff.new_attr = git_tree_entry_attributes(entry);
-	if (S_ISDIR(tdiff.new_attr))
-		return GIT_SUCCESS;
+	/* This fn is just for single-sided diffs */
+	assert(status == GIT_STATUS_ADDED || status == GIT_STATUS_DELETED);
 
-	git_oid_cpy(&tdiff.new_oid, git_tree_entry_id(entry));
-	tdiff.status = GIT_STATUS_ADDED;
-	tdiff.path = git_tree_entry_name(entry);
+	if (!delta)
+		return git__rethrow(GIT_ENOMEM, "Could not allocate diff record");
 
-	if ((error = git_buf_joinpath(&diff->pfx, diff->pfx.ptr, root)) ||
-		(error = git_buf_joinpath(&diff->pfx, diff->pfx.ptr, tdiff.path)))
-		return error;
+	if ((delta->path = git__strdup(path)) == NULL) {
+		git__free(delta);
+		return git__rethrow(GIT_ENOMEM, "Could not allocate diff record path");
+	}
+
+	if (diff->opts.flags & GIT_DIFF_REVERSE)
+		status = (status == GIT_STATUS_ADDED) ?
+			GIT_STATUS_DELETED : GIT_STATUS_ADDED;
 
-	delta = file_delta_new(diff, &tdiff);
-	if (delta  == NULL)
-		error = GIT_ENOMEM;
-	else if ((error = git_vector_insert(&diff->files, delta)) < GIT_SUCCESS)
+	delta->status = status;
+
+	if (status == GIT_STATUS_ADDED) {
+		delta->new_attr = attr;
+		git_oid_cpy(&delta->new_oid, oid);
+	} else {
+		delta->old_attr = attr;
+		git_oid_cpy(&delta->old_oid, oid);
+	}
+
+	if ((error = git_vector_insert(&diff->files, delta)) < GIT_SUCCESS)
 		file_delta_free(delta);
 
-	git_buf_truncate(&diff->pfx, pfx_len);
+	return error;
+}
+
+static int file_delta_new__from_tree_diff(
+	git_diff_list *diff,
+	const git_tree_diff_data *tdiff)
+{
+	int error;
+	git_diff_delta *delta = git__calloc(1, sizeof(git_diff_delta));
+
+	if (!delta)
+		return git__rethrow(GIT_ENOMEM, "Could not allocate diff record");
+
+	if ((diff->opts.flags & GIT_DIFF_REVERSE) == 0) {
+		delta->status   = tdiff->status;
+		delta->old_attr = tdiff->old_attr;
+		delta->new_attr = tdiff->new_attr;
+		delta->old_oid  = tdiff->old_oid;
+		delta->new_oid  = tdiff->new_oid;
+	} else {
+		/* reverse the polarity of the neutron flow */
+		switch (tdiff->status) {
+		case GIT_STATUS_ADDED:   delta->status = GIT_STATUS_DELETED; break;
+		case GIT_STATUS_DELETED: delta->status = GIT_STATUS_ADDED; break;
+		default:                 delta->status = tdiff->status;
+		}
+		delta->old_attr = tdiff->new_attr;
+		delta->new_attr = tdiff->old_attr;
+		delta->old_oid  = tdiff->new_oid;
+		delta->new_oid  = tdiff->old_oid;
+	}
+
+	delta->path = git__strdup(diff->pfx.ptr);
+	if (delta->path == NULL) {
+		git__free(delta);
+		return git__rethrow(GIT_ENOMEM, "Could not allocate diff record path");
+	}
+
+	if ((error = git_vector_insert(&diff->files, delta)) < GIT_SUCCESS)
+		file_delta_free(delta);
 
 	return error;
 }
 
-static int tree_del_cb(const char *root, git_tree_entry *entry, void *data)
+static int tree_walk_cb(const char *root, git_tree_entry *entry, void *data)
 {
 	int error;
 	git_diff_list *diff = data;
 	ssize_t pfx_len = diff->pfx.size;
-	git_tree_diff_data tdiff;
-	git_diff_delta *delta;
 
-	memset(&tdiff, 0, sizeof(tdiff));
-	tdiff.old_attr = git_tree_entry_attributes(entry);
-	if (S_ISDIR(tdiff.old_attr))
+	if (S_ISDIR(git_tree_entry_attributes(entry)))
 		return GIT_SUCCESS;
 
-	git_oid_cpy(&tdiff.old_oid, git_tree_entry_id(entry));
-	tdiff.status = GIT_STATUS_DELETED;
-	tdiff.path = git_tree_entry_name(entry);
-
+	/* join pfx, root, and entry->filename into one */
 	if ((error = git_buf_joinpath(&diff->pfx, diff->pfx.ptr, root)) ||
-		(error = git_buf_joinpath(&diff->pfx, diff->pfx.ptr, tdiff.path)))
+		(error = git_buf_joinpath(
+			&diff->pfx, diff->pfx.ptr, git_tree_entry_name(entry))))
 		return error;
 
-	delta = file_delta_new(diff, &tdiff);
-	if (delta  == NULL)
-		error = GIT_ENOMEM;
-	else if ((error = git_vector_insert(&diff->files, delta)) < GIT_SUCCESS)
-		file_delta_free(delta);
+	error = file_delta_new__from_one(
+		diff, diff->mode, git_tree_entry_attributes(entry),
+		git_tree_entry_id(entry), diff->pfx.ptr);
 
 	git_buf_truncate(&diff->pfx, pfx_len);
 
 	return error;
 }
 
-static int tree_diff_cb(const git_tree_diff_data *ptr, void *data)
+static int tree_diff_cb(const git_tree_diff_data *tdiff, void *data)
 {
 	int error;
 	git_diff_list *diff = data;
 	ssize_t pfx_len = diff->pfx.size;
 
-	error = git_buf_joinpath(&diff->pfx, diff->pfx.ptr, ptr->path);
+	error = git_buf_joinpath(&diff->pfx, diff->pfx.ptr, tdiff->path);
 	if (error < GIT_SUCCESS)
 		return error;
 
@@ -138,56 +153,83 @@ static int tree_diff_cb(const git_tree_diff_data *ptr, void *data)
 	 *   and a deletion (thank you, git_tree_diff!)
 	 * otherwise, this is a blob-to-blob diff
 	 */
-	if (S_ISDIR(ptr->old_attr) && S_ISDIR(ptr->new_attr)) {
+	if (S_ISDIR(tdiff->old_attr) && S_ISDIR(tdiff->new_attr)) {
 		git_tree *old = NULL, *new = NULL;
 
-		if (!(error = git_tree_lookup(&old, diff->repo, &ptr->old_oid)) &&
-			!(error = git_tree_lookup(&new, diff->repo, &ptr->new_oid)))
-		{
+		if (!(error = git_tree_lookup(&old, diff->repo, &tdiff->old_oid)) &&
+			!(error = git_tree_lookup(&new, diff->repo, &tdiff->new_oid)))
 			error = git_tree_diff(old, new, tree_diff_cb, diff);
-		}
 
 		git_tree_free(old);
 		git_tree_free(new);
-	} else if (S_ISDIR(ptr->old_attr) && ptr->new_attr == 0) {
-		/* deleted a whole tree */
-		git_tree *old = NULL;
-		if (!(error = git_tree_lookup(&old, diff->repo, &ptr->old_oid))) {
-			error = git_tree_walk(old, tree_del_cb, GIT_TREEWALK_POST, diff);
-			git_tree_free(old);
-		}
-	} else if (S_ISDIR(ptr->new_attr) && ptr->old_attr == 0) {
-		/* added a whole tree */
-		git_tree *new = NULL;
-		if (!(error = git_tree_lookup(&new, diff->repo, &ptr->new_oid))) {
-			error = git_tree_walk(new, tree_add_cb, GIT_TREEWALK_POST, diff);
-			git_tree_free(new);
-		}
-	} else {
-		git_diff_delta *delta = file_delta_new(diff, ptr);
-		if (delta == NULL)
-			error = GIT_ENOMEM;
-		else if ((error = git_vector_insert(&diff->files, delta)) < GIT_SUCCESS)
-			file_delta_free(delta);
-	}
+	} else if (S_ISDIR(tdiff->old_attr) || S_ISDIR(tdiff->new_attr)) {
+		git_tree *tree     = NULL;
+		int added_dir      = S_ISDIR(tdiff->new_attr);
+		const git_oid *oid = added_dir ? &tdiff->new_oid : &tdiff->old_oid;
+		diff->mode         = added_dir ? GIT_STATUS_ADDED : GIT_STATUS_DELETED;
+
+		if (!(error = git_tree_lookup(&tree, diff->repo, oid)))
+			error = git_tree_walk(tree, tree_walk_cb, GIT_TREEWALK_POST, diff);
+		git_tree_free(tree);
+	} else
+		error = file_delta_new__from_tree_diff(diff, tdiff);
 
 	git_buf_truncate(&diff->pfx, pfx_len);
 
 	return error;
 }
 
+static char *git_diff_src_prefix_default = "a/";
+static char *git_diff_dst_prefix_default = "b/";
+#define PREFIX_IS_DEFAULT(A) \
+	((A) == git_diff_src_prefix_default || (A) == git_diff_dst_prefix_default)
+
+static char *copy_prefix(const char *prefix)
+{
+	size_t len = strlen(prefix);
+	char *str = git__malloc(len + 2);
+	if (str != NULL) {
+		memcpy(str, prefix, len + 1);
+		/* append '/' at end if needed */
+		if (len > 0 && str[len - 1] != '/') {
+			str[len] = '/';
+			str[len + 1] = '\0';
+		}
+	}
+	return str;
+}
+
 static git_diff_list *git_diff_list_alloc(
 	git_repository *repo, const git_diff_options *opts)
 {
 	git_diff_list *diff = git__calloc(1, sizeof(git_diff_list));
-	if (diff != NULL) {
-		if (opts != NULL) {
-			memcpy(&diff->opts, opts, sizeof(git_diff_options));
-			/* do something safer with the pathspec strarray */
-		}
-		diff->repo = repo;
-		git_buf_init(&diff->pfx, 0);
+	if (diff == NULL)
+		return NULL;
+
+	diff->repo = repo;
+	git_buf_init(&diff->pfx, 0);
+
+	if (opts == NULL)
+		return diff;
+
+	memcpy(&diff->opts, opts, sizeof(git_diff_options));
+
+	diff->opts.src_prefix = (opts->src_prefix == NULL) ?
+		git_diff_src_prefix_default : copy_prefix(opts->src_prefix);
+	diff->opts.dst_prefix = (opts->dst_prefix == NULL) ?
+		git_diff_dst_prefix_default : copy_prefix(opts->dst_prefix);
+	if (!diff->opts.src_prefix || !diff->opts.dst_prefix) {
+		git__free(diff);
+		return NULL;
+	}
+	if (diff->opts.flags & GIT_DIFF_REVERSE) {
+		char *swap = diff->opts.src_prefix;
+		diff->opts.src_prefix = diff->opts.dst_prefix;
+		diff->opts.dst_prefix = swap;
 	}
+
+	/* do something safe with the pathspec strarray */
+
 	return diff;
 }
 
@@ -205,6 +247,14 @@ void git_diff_list_free(git_diff_list *diff)
 		diff->files.contents[i] = NULL;
 	}
 	git_vector_free(&diff->files);
+	if (!PREFIX_IS_DEFAULT(diff->opts.src_prefix)) {
+		git__free(diff->opts.src_prefix);
+		diff->opts.src_prefix = NULL;
+	}
+	if (!PREFIX_IS_DEFAULT(diff->opts.dst_prefix)) {
+		git__free(diff->opts.dst_prefix);
+		diff->opts.dst_prefix = NULL;
+	}
 	git__free(diff);
 }
 
@@ -231,6 +281,111 @@ int git_diff_tree_to_tree(
 
 typedef struct {
 	git_diff_list *diff;
+	git_index *index;
+	unsigned int index_pos;
+} index_to_tree_info;
+
+static int add_new_index_deltas(
+	index_to_tree_info *info,
+	const char *stop_path)
+{
+	int error;
+	git_index_entry *idx_entry = git_index_get(info->index, info->index_pos);
+
+	while (idx_entry != NULL &&
+		(stop_path == NULL || strcmp(idx_entry->path, stop_path) < 0))
+	{
+		error = file_delta_new__from_one(
+			info->diff, GIT_STATUS_ADDED, idx_entry->mode,
+			&idx_entry->oid, idx_entry->path);
+		if (error < GIT_SUCCESS)
+			return error;
+
+		idx_entry = git_index_get(info->index, ++info->index_pos);
+	}
+
+	return GIT_SUCCESS;
+}
+
+static int diff_index_to_tree_cb(const char *root, git_tree_entry *tree_entry, void *data)
+{
+	int error;
+	index_to_tree_info *info = data;
+	git_index_entry *idx_entry;
+
+	/* TODO: submodule support for GIT_OBJ_COMMITs in tree */
+	if (git_tree_entry_type(tree_entry) != GIT_OBJ_BLOB)
+		return GIT_SUCCESS;
+
+	error = git_buf_joinpath(&info->diff->pfx, root, git_tree_entry_name(tree_entry));
+	if (error < GIT_SUCCESS)
+		return error;
+
+	/* create add deltas for index entries that are not in the tree */
+	error = add_new_index_deltas(info, info->diff->pfx.ptr);
+	if (error < GIT_SUCCESS)
+		return error;
+
+	/* create delete delta for tree entries that are not in the index */
+	idx_entry = git_index_get(info->index, info->index_pos);
+	if (idx_entry == NULL || strcmp(idx_entry->path, info->diff->pfx.ptr) > 0) {
+		return file_delta_new__from_one(
+			info->diff, GIT_STATUS_DELETED, git_tree_entry_attributes(tree_entry),
+			git_tree_entry_id(tree_entry), info->diff->pfx.ptr);
+	}
+
+	/* create modified delta for non-matching tree & index entries */
+	info->index_pos++;
+
+	if (git_oid_cmp(&idx_entry->oid, git_tree_entry_id(tree_entry)) ||
+		idx_entry->mode != git_tree_entry_attributes(tree_entry))
+	{
+		git_tree_diff_data tdiff;
+		tdiff.old_attr = git_tree_entry_attributes(tree_entry);
+		tdiff.new_attr = idx_entry->mode;
+		tdiff.status   = GIT_STATUS_MODIFIED;
+		tdiff.path     = idx_entry->path;
+		git_oid_cpy(&tdiff.old_oid, git_tree_entry_id(tree_entry));
+		git_oid_cpy(&tdiff.new_oid, &idx_entry->oid);
+
+		error = file_delta_new__from_tree_diff(info->diff, &tdiff);
+	}
+
+	return error;
+
+}
+
+int git_diff_index_to_tree(
+	git_repository *repo,
+	const git_diff_options *opts,
+	git_tree *old,
+	git_diff_list **diff_ptr)
+{
+	int error;
+	index_to_tree_info info = {0};
+
+	if ((info.diff = git_diff_list_alloc(repo, opts)) == NULL)
+		return GIT_ENOMEM;
+
+	if ((error = git_repository_index(&info.index, repo)) == GIT_SUCCESS) {
+		error = git_tree_walk(
+			old, diff_index_to_tree_cb, GIT_TREEWALK_POST, &info);
+		if (error == GIT_SUCCESS)
+			error = add_new_index_deltas(&info, NULL);
+		git_index_free(info.index);
+	}
+	git_buf_free(&info.diff->pfx);
+
+	if (error != GIT_SUCCESS)
+		git_diff_list_free(info.diff);
+	else
+		*diff_ptr = info.diff;
+
+	return error;
+}
+
+typedef struct {
+	git_diff_list *diff;
 	void *cb_data;
 	git_diff_hunk_fn hunk_cb;
 	git_diff_line_fn line_cb;
@@ -331,6 +486,26 @@ static int set_file_is_binary(
 	return GIT_SUCCESS;
 }
 
+static void setup_xdiff_config(git_diff_options *opts, xdemitconf_t *cfg)
+{
+	memset(cfg, 0, sizeof(xdemitconf_t));
+
+	cfg->ctxlen =
+		(!opts || !opts->context_lines) ? 3 : opts->context_lines;
+	cfg->interhunkctxlen =
+		(!opts || !opts->interhunk_lines) ? 3 : opts->interhunk_lines;
+
+	if (!opts)
+		return;
+
+	if (opts->flags & GIT_DIFF_IGNORE_WHITESPACE)
+		cfg->flags |= XDF_WHITESPACE_FLAGS;
+	if (opts->flags & GIT_DIFF_IGNORE_WHITESPACE_CHANGE)
+		cfg->flags |= XDF_IGNORE_WHITESPACE_CHANGE;
+	if (opts->flags & GIT_DIFF_IGNORE_WHITESPACE_EOL)
+		cfg->flags |= XDF_IGNORE_WHITESPACE_AT_EOL;
+}
+
 int git_diff_foreach(
 	git_diff_list *diff,
 	void *data,
@@ -341,17 +516,23 @@ int git_diff_foreach(
 	int error = GIT_SUCCESS;
 	diff_info di;
 	git_diff_delta *delta;
+	xpparam_t    xdiff_params;
+	xdemitconf_t xdiff_config;
+	xdemitcb_t   xdiff_callback;
 
 	di.diff    = diff;
 	di.cb_data = data;
 	di.hunk_cb = hunk_cb;
 	di.line_cb = line_cb;
 
+	memset(&xdiff_params, 0, sizeof(xdiff_params));
+	setup_xdiff_config(&diff->opts, &xdiff_config);
+	memset(&xdiff_callback, 0, sizeof(xdiff_callback));
+	xdiff_callback.outf = diff_output_cb;
+	xdiff_callback.priv = &di;
+
 	git_vector_foreach(&diff->files, di.index, delta) {
-		mmfile_t old, new;
-		xpparam_t params;
-		xdemitconf_t cfg;
-		xdemitcb_t callback;
+		mmfile_t old_data, new_data;
 
 		/* map files */
 		if (hunk_cb || line_cb) {
@@ -365,12 +546,12 @@ int git_diff_foreach(
 			{
 				error = git_blob_lookup(
 					&delta->old_blob, diff->repo, &delta->old_oid);
-				old.ptr = (char *)git_blob_rawcontent(delta->old_blob);
-				old.size = git_blob_rawsize(delta->old_blob);
+				old_data.ptr = (char *)git_blob_rawcontent(delta->old_blob);
+				old_data.size = git_blob_rawsize(delta->old_blob);
 			} else {
 				delta->old_blob = NULL;
-				old.ptr = "";
-				old.size = 0;
+				old_data.ptr = "";
+				old_data.size = 0;
 			}
 
 			if (delta->status == GIT_STATUS_ADDED ||
@@ -378,21 +559,25 @@ int git_diff_foreach(
 			{
 				error = git_blob_lookup(
 					&delta->new_blob, diff->repo, &delta->new_oid);
-				new.ptr = (char *)git_blob_rawcontent(delta->new_blob);
-				new.size = git_blob_rawsize(delta->new_blob);
+				new_data.ptr = (char *)git_blob_rawcontent(delta->new_blob);
+				new_data.size = git_blob_rawsize(delta->new_blob);
 			} else {
 				delta->new_blob = NULL;
-				new.ptr = "";
-				new.size = 0;
+				new_data.ptr = "";
+				new_data.size = 0;
 			}
 		}
 
-		if (diff->opts.force_text)
+		if (diff->opts.flags & GIT_DIFF_FORCE_TEXT)
 			delta->binary = 0;
 		else if ((error = set_file_is_binary(
-			diff->repo, delta, &old, &new)) < GIT_SUCCESS)
+			diff->repo, delta, &old_data, &new_data)) < GIT_SUCCESS)
 			break;
 
+		/* TODO: if ignore_whitespace is set, then we *must* do text
+		 * diffs to tell if a file has really been changed.
+		 */
+
 		if (file_cb != NULL) {
 			error = file_cb(data, delta, (float)di.index / diff->files.length);
 			if (error != GIT_SUCCESS)
@@ -411,19 +596,8 @@ int git_diff_foreach(
 
 		di.delta = delta;
 
-		memset(&params, 0, sizeof(params));
-
-		memset(&cfg, 0, sizeof(cfg));
-		cfg.ctxlen = diff->opts.context_lines || 3;
-		cfg.interhunkctxlen = diff->opts.interhunk_lines || 3;
-		if (diff->opts.ignore_whitespace)
-			cfg.flags |= XDF_WHITESPACE_FLAGS;
-
-		memset(&callback, 0, sizeof(callback));
-		callback.outf = diff_output_cb;
-		callback.priv = &di;
-
-		xdl_diff(&old, &new, &params, &cfg, &callback);
+		xdl_diff(&old_data, &new_data,
+			&xdiff_params, &xdiff_config, &xdiff_callback);
 
 		git_blob_free(delta->old_blob);
 		delta->old_blob = NULL;
@@ -436,6 +610,7 @@ int git_diff_foreach(
 }
 
 typedef struct {
+	git_diff_list *diff;
 	git_diff_output_fn print_cb;
 	void *cb_data;
 	git_buf *buf;
@@ -483,7 +658,8 @@ static int print_compact(void *data, git_diff_delta *delta, float progress)
 	if (delta->new_path != NULL)
 		git_buf_printf(pi->buf, "%c\t%s%c -> %s%c\n", code,
 			delta->path, old_suffix, delta->new_path, new_suffix);
-	else if (delta->old_attr != delta->new_attr)
+	else if (delta->old_attr != delta->new_attr &&
+		delta->old_attr != 0 && delta->new_attr != 0)
 		git_buf_printf(pi->buf, "%c\t%s%c (%o -> %o)\n", code,
 			delta->path, new_suffix, delta->old_attr, delta->new_attr);
 	else if (old_suffix != ' ')
@@ -506,6 +682,7 @@ int git_diff_print_compact(
 	git_buf buf = GIT_BUF_INIT;
 	print_info pi;
 
+	pi.diff     = diff;
 	pi.print_cb = print_cb;
 	pi.cb_data  = cb_data;
 	pi.buf      = &buf;
@@ -549,15 +726,15 @@ static int print_patch_file(void *data, git_diff_delta *delta, float progress)
 {
 	int error;
 	print_info *pi = data;
-	const char *oldpfx = "a/";
+	const char *oldpfx = pi->diff->opts.src_prefix;
 	const char *oldpath = delta->path;
-	const char *newpfx = "b/";
+	const char *newpfx = pi->diff->opts.dst_prefix;
 	const char *newpath = delta->new_path ? delta->new_path : delta->path;
 
 	GIT_UNUSED_ARG(progress);
 
 	git_buf_clear(pi->buf);
-	git_buf_printf(pi->buf, "diff --git a/%s b/%s\n", delta->path, newpath);
+	git_buf_printf(pi->buf, "diff --git %s%s %s%s\n", oldpfx, delta->path, newpfx, newpath);
 	if ((error = print_oid_range(pi, delta)) < GIT_SUCCESS)
 		return error;
 
@@ -647,6 +824,7 @@ int git_diff_print_patch(
 	git_buf buf = GIT_BUF_INIT;
 	print_info pi;
 
+	pi.diff     = diff;
 	pi.print_cb = print_cb;
 	pi.cb_data  = cb_data;
 	pi.buf      = &buf;
@@ -671,11 +849,17 @@ int git_diff_blobs(
 	diff_info di;
 	git_diff_delta delta;
 	mmfile_t old, new;
-	xpparam_t params;
-	xdemitconf_t cfg;
-	xdemitcb_t callback;
+	xpparam_t xdiff_params;
+	xdemitconf_t xdiff_config;
+	xdemitcb_t xdiff_callback;
+
+	assert(repo);
 
-	assert(repo && options);
+	if (options && (options->flags & GIT_DIFF_REVERSE)) {
+		git_blob *swap = old_blob;
+		old_blob = new_blob;
+		new_blob = swap;
+	}
 
 	if (old_blob) {
 		old.ptr  = (char *)git_blob_rawcontent(old_blob);
@@ -712,21 +896,15 @@ int git_diff_blobs(
 	di.delta   = &delta;
 	di.cb_data = cb_data;
 	di.hunk_cb = hunk_cb;
-	di.line_cb = line_cb;	
-
-	memset(&params, 0, sizeof(params));
-
-	memset(&cfg, 0, sizeof(cfg));
-	cfg.ctxlen = options->context_lines || 3;
-	cfg.interhunkctxlen = options->interhunk_lines || 3;
-	if (options->ignore_whitespace)
-		cfg.flags |= XDF_WHITESPACE_FLAGS;
+	di.line_cb = line_cb;
 
-	memset(&callback, 0, sizeof(callback));
-	callback.outf = diff_output_cb;
-	callback.priv = &di;
+	memset(&xdiff_params, 0, sizeof(xdiff_params));
+	setup_xdiff_config(options, &xdiff_config);
+	memset(&xdiff_callback, 0, sizeof(xdiff_callback));
+	xdiff_callback.outf = diff_output_cb;
+	xdiff_callback.priv = &di;
 
-	xdl_diff(&old, &new, &params, &cfg, &callback);
+	xdl_diff(&old, &new, &xdiff_params, &xdiff_config, &xdiff_callback);
 
 	return GIT_SUCCESS;
 }
diff --git a/src/diff.h b/src/diff.h
index 1bb5c36f0..e7764a8eb 100644
--- a/src/diff.h
+++ b/src/diff.h
@@ -14,8 +14,11 @@
 struct git_diff_list {
 	git_repository   *repo;
 	git_diff_options opts;
-	git_buf          pfx;
 	git_vector       files;    /* vector of git_diff_file_delta */
+
+	/* the following are just used while processing the diff list */
+	git_buf          pfx;
+	git_status_t     mode;
 };
 
 #endif
diff --git a/tests-clar/diff/blob.c b/tests-clar/diff/blob.c
index 048b05c79..7aa8ceb22 100644
--- a/tests-clar/diff/blob.c
+++ b/tests-clar/diff/blob.c
@@ -22,7 +22,7 @@ void test_diff_blob__0(void)
 {
 	git_blob *a, *b, *c, *d;
 	git_oid a_oid, b_oid, c_oid, d_oid;
-	git_diff_options opts;
+	git_diff_options opts = {0};
 	diff_expects exp;
 
 	/* tests/resources/attr/root_test1 */
@@ -44,8 +44,7 @@ void test_diff_blob__0(void)
 	/* Doing the equivalent of a `git diff -U1` on these files */
 
 	opts.context_lines = 1;
-	opts.interhunk_lines = 0;
-	opts.ignore_whitespace = 0;
+	opts.interhunk_lines = 1;
 
 	memset(&exp, 0, sizeof(exp));
 	cl_git_pass(git_diff_blobs(
diff --git a/tests-clar/diff/diff_helpers.c b/tests-clar/diff/diff_helpers.c
index 3fcf45c10..b32c4bc2d 100644
--- a/tests-clar/diff/diff_helpers.c
+++ b/tests-clar/diff/diff_helpers.c
@@ -82,3 +82,23 @@ int diff_line_fn(
 	}
 	return 0;
 }
+
+git_tree *resolve_commit_oid_to_tree(
+	git_repository *repo,
+	const char *partial_oid)
+{
+	size_t len = strlen(partial_oid);
+	git_oid oid;
+	git_object *obj;
+	git_tree *tree;
+
+	if (git_oid_fromstrn(&oid, partial_oid, len) == 0)
+		git_object_lookup_prefix(&obj, repo, &oid, len, GIT_OBJ_ANY);
+	cl_assert(obj);
+	if (git_object_type(obj) == GIT_OBJ_TREE)
+		return (git_tree *)obj;
+	cl_assert(git_object_type(obj) == GIT_OBJ_COMMIT);
+	cl_git_pass(git_commit_tree(&tree, (git_commit *)obj));
+	git_object_free(obj);
+	return tree;
+}
diff --git a/tests-clar/diff/diff_helpers.h b/tests-clar/diff/diff_helpers.h
index 4c3e7580e..035c000f5 100644
--- a/tests-clar/diff/diff_helpers.h
+++ b/tests-clar/diff/diff_helpers.h
@@ -38,3 +38,4 @@ extern int diff_line_fn(
 	char line_origin,
 	const char *content,
 	size_t content_len);
+
diff --git a/tests-clar/diff/index.c b/tests-clar/diff/index.c
new file mode 100644
index 000000000..0941c7c21
--- /dev/null
+++ b/tests-clar/diff/index.c
@@ -0,0 +1,96 @@
+#include "clar_libgit2.h"
+#include "diff_helpers.h"
+
+static git_repository *g_repo = NULL;
+
+void test_diff_index__initialize(void)
+{
+	cl_fixture_sandbox("status");
+	cl_git_pass(p_rename("status/.gitted", "status/.git"));
+	cl_git_pass(git_repository_open(&g_repo, "status/.git"));
+}
+
+void test_diff_index__cleanup(void)
+{
+	git_repository_free(g_repo);
+	g_repo = NULL;
+	cl_fixture_cleanup("status");
+}
+
+void test_diff_index__0(void)
+{
+	/* grabbed a couple of commit oids from the history of the attr repo */
+	const char *a_commit = "26a125ee1bf"; /* the current HEAD */
+	const char *b_commit = "0017bd4ab1ec3"; /* the start */
+	git_tree *a = resolve_commit_oid_to_tree(g_repo, a_commit);
+	git_tree *b = resolve_commit_oid_to_tree(g_repo, b_commit);
+	git_diff_options opts = {0};
+	git_diff_list *diff = NULL;
+	diff_expects exp;
+
+	cl_assert(a);
+	cl_assert(b);
+
+	opts.context_lines = 1;
+	opts.interhunk_lines = 1;
+
+	memset(&exp, 0, sizeof(exp));
+
+	cl_git_pass(git_diff_index_to_tree(g_repo, &opts, a, &diff));
+
+	cl_git_pass(git_diff_foreach(
+		diff, &exp, diff_file_fn, diff_hunk_fn, diff_line_fn));
+
+	/* to generate these values:
+	 * - cd to tests/resources/status,
+	 * - mv .gitted .git
+	 * - git diff --name-status --cached 26a125ee1bf
+	 * - git diff -U1 --cached 26a125ee1bf
+	 * - mv .git .gitted
+	 */
+	cl_assert(exp.files == 8);
+	cl_assert(exp.file_adds == 3);
+	cl_assert(exp.file_dels == 2);
+	cl_assert(exp.file_mods == 3);
+
+	cl_assert(exp.hunks == 8);
+
+	cl_assert(exp.lines == 11);
+	cl_assert(exp.line_ctxt == 3);
+	cl_assert(exp.line_adds == 6);
+	cl_assert(exp.line_dels == 2);
+
+	git_diff_list_free(diff);
+	diff = NULL;
+	memset(&exp, 0, sizeof(exp));
+
+	cl_git_pass(git_diff_index_to_tree(g_repo, &opts, b, &diff));
+
+	cl_git_pass(git_diff_foreach(
+		diff, &exp, diff_file_fn, diff_hunk_fn, diff_line_fn));
+
+	/* to generate these values:
+	 * - cd to tests/resources/status,
+	 * - mv .gitted .git
+	 * - git diff --name-status --cached 0017bd4ab1ec3
+	 * - git diff -U1 --cached 0017bd4ab1ec3
+	 * - mv .git .gitted
+	 */
+	cl_assert(exp.files == 12);
+	cl_assert(exp.file_adds == 7);
+	cl_assert(exp.file_dels == 2);
+	cl_assert(exp.file_mods == 3);
+
+	cl_assert(exp.hunks == 12);
+
+	cl_assert(exp.lines == 16);
+	cl_assert(exp.line_ctxt == 3);
+	cl_assert(exp.line_adds == 11);
+	cl_assert(exp.line_dels == 2);
+
+	git_diff_list_free(diff);
+	diff = NULL;
+
+	git_tree_free(a);
+	git_tree_free(b);
+}
diff --git a/tests-clar/diff/tree.c b/tests-clar/diff/tree.c
index 3c5d2a9f2..836db5765 100644
--- a/tests-clar/diff/tree.c
+++ b/tests-clar/diff/tree.c
@@ -18,34 +18,16 @@ void test_diff_tree__cleanup(void)
 	cl_fixture_cleanup("attr");
 }
 
-static git_tree *resolve_commit_oid_to_tree(const char *partial_oid)
-{
-	size_t len = strlen(partial_oid);
-	git_oid oid;
-	git_object *obj;
-	git_tree *tree;
-
-	if (git_oid_fromstrn(&oid, partial_oid, len) == 0)
-		git_object_lookup_prefix(&obj, g_repo, &oid, len, GIT_OBJ_ANY);
-	cl_assert(obj);
-	if (git_object_type(obj) == GIT_OBJ_TREE)
-		return (git_tree *)obj;
-	cl_assert(git_object_type(obj) == GIT_OBJ_COMMIT);
-	cl_git_pass(git_commit_tree(&tree, (git_commit *)obj));
-	git_object_free(obj);
-	return tree;
-}
-
 void test_diff_tree__0(void)
 {
 	/* grabbed a couple of commit oids from the history of the attr repo */
 	const char *a_commit = "605812a";
 	const char *b_commit = "370fe9ec22";
 	const char *c_commit = "f5b0af1fb4f5c";
-	git_tree *a = resolve_commit_oid_to_tree(a_commit);
-	git_tree *b = resolve_commit_oid_to_tree(b_commit);
-	git_tree *c = resolve_commit_oid_to_tree(c_commit);
-	git_diff_options opts;
+	git_tree *a = resolve_commit_oid_to_tree(g_repo, a_commit);
+	git_tree *b = resolve_commit_oid_to_tree(g_repo, b_commit);
+	git_tree *c = resolve_commit_oid_to_tree(g_repo, c_commit);
+	git_diff_options opts = {0};
 	git_diff_list *diff = NULL;
 	diff_expects exp;
 
@@ -53,8 +35,7 @@ void test_diff_tree__0(void)
 	cl_assert(b);
 
 	opts.context_lines = 1;
-	opts.interhunk_lines = 0;
-	opts.ignore_whitespace = 0;
+	opts.interhunk_lines = 1;
 
 	memset(&exp, 0, sizeof(exp));