thodg/libgit2

diff --git a/include/git2/apply.h b/include/git2/apply.h
index eb9cad4..7cc1c22 100644
--- a/include/git2/apply.h
+++ b/include/git2/apply.h
@@ -38,6 +38,22 @@ typedef int (*git_apply_delta_cb)(
 	void *payload);
 
 /**
+ * When applying a patch, callback that will be made per hunk.
+ *
+ * When the callback:
+ * - returns < 0, the apply process will be aborted.
+ * - returns > 0, the hunk will not be applied, but the apply process
+ *      continues
+ * - returns 0, the hunk is applied, and the apply process continues.
+ *
+ * @param hunk The hunk to be applied
+ * @param payload User-specified payload
+ */
+typedef int (*git_apply_hunk_cb)(
+	const git_diff_hunk *hunk,
+	void *payload);
+
+/**
  * Apply options structure
  *
  * Initialize with `GIT_APPLY_OPTIONS_INIT`. Alternatively, you can
@@ -49,6 +65,7 @@ typedef struct {
 	unsigned int version;
 
 	git_apply_delta_cb delta_cb;
+	git_apply_hunk_cb hunk_cb;
 	void *payload;
 } git_apply_options;
 
diff --git a/src/apply.c b/src/apply.c
index 9534267..8d94e78 100644
--- a/src/apply.c
+++ b/src/apply.c
@@ -167,15 +167,36 @@ static int update_hunk(
 	return 0;
 }
 
+typedef struct {
+	git_apply_options opts;
+	size_t skipped_new_lines;
+	size_t skipped_old_lines;
+} apply_hunks_ctx;
+
 static int apply_hunk(
 	patch_image *image,
 	git_patch *patch,
-	git_patch_hunk *hunk)
+	git_patch_hunk *hunk,
+	apply_hunks_ctx *ctx)
 {
 	patch_image preimage = PATCH_IMAGE_INIT, postimage = PATCH_IMAGE_INIT;
 	size_t line_num, i;
 	int error = 0;
 
+	if (ctx->opts.hunk_cb) {
+		error = ctx->opts.hunk_cb(&hunk->hunk, ctx->opts.payload);
+
+		if (error) {
+			if (error > 0) {
+				ctx->skipped_new_lines += hunk->hunk.new_lines;
+				ctx->skipped_old_lines += hunk->hunk.old_lines;
+				error = 0;
+			}
+
+			goto done;
+		}
+	}
+
 	for (i = 0; i < hunk->line_count; i++) {
 		size_t linenum = hunk->line_start + i;
 		git_diff_line *line = git_array_get(patch->lines, linenum);
@@ -198,7 +219,14 @@ static int apply_hunk(
 		}
 	}
 
-	line_num = hunk->hunk.new_start ? hunk->hunk.new_start - 1 : 0;
+	if (hunk->hunk.new_start) {
+		line_num = hunk->hunk.new_start -
+			ctx->skipped_new_lines +
+			ctx->skipped_old_lines -
+			1;
+	} else {
+		line_num = 0;
+	}
 
 	if (!find_hunk_linenum(&line_num, image, &preimage, line_num)) {
 		error = apply_err("hunk at line %d did not apply",
@@ -219,7 +247,8 @@ static int apply_hunks(
 	git_buf *out,
 	const char *source,
 	size_t source_len,
-	git_patch *patch)
+	git_patch *patch,
+	apply_hunks_ctx *ctx)
 {
 	git_patch_hunk *hunk;
 	git_diff_line *line;
@@ -231,7 +260,7 @@ static int apply_hunks(
 		goto done;
 
 	git_array_foreach(patch->hunks, i, hunk) {
-		if ((error = apply_hunk(&image, patch, hunk)) < 0)
+		if ((error = apply_hunk(&image, patch, hunk, ctx)) < 0)
 			goto done;
 	}
 
@@ -339,14 +368,19 @@ int git_apply__patch(
 	unsigned int *mode_out,
 	const char *source,
 	size_t source_len,
-	git_patch *patch)
+	git_patch *patch,
+	const git_apply_options *given_opts)
 {
+	apply_hunks_ctx ctx = { GIT_APPLY_OPTIONS_INIT };
 	char *filename = NULL;
 	unsigned int mode = 0;
 	int error = 0;
 
 	assert(contents_out && filename_out && mode_out && (source || !source_len) && patch);
 
+	if (given_opts)
+		memcpy(&ctx.opts, given_opts, sizeof(git_apply_options));
+
 	*filename_out = NULL;
 	*mode_out = 0;
 
@@ -361,7 +395,7 @@ int git_apply__patch(
 	if (patch->delta->flags & GIT_DIFF_FLAG_BINARY)
 		error = apply_binary(contents_out, source, source_len, patch);
 	else if (patch->hunks.size)
-		error = apply_hunks(contents_out, source, source_len, patch);
+		error = apply_hunks(contents_out, source, source_len, patch, &ctx);
 	else
 		error = git_buf_put(contents_out, source, source_len);
 
@@ -457,7 +491,7 @@ static int apply_one(
 
 	if (delta->status != GIT_DELTA_DELETED) {
 		if ((error = git_apply__patch(&post_contents, &filename, &mode,
-				pre_contents.ptr, pre_contents.size, patch)) < 0 ||
+				pre_contents.ptr, pre_contents.size, patch, opts)) < 0 ||
 			(error = git_blob_create_frombuffer(&post_id, repo,
 				post_contents.ptr, post_contents.size)) < 0)
 			goto done;
diff --git a/src/apply.h b/src/apply.h
index b29460c..11ec756 100644
--- a/src/apply.h
+++ b/src/apply.h
@@ -10,6 +10,7 @@
 #include "common.h"
 
 #include "git2/patch.h"
+#include "git2/apply.h"
 #include "buffer.h"
 
 extern int git_apply__patch(
@@ -18,6 +19,7 @@ extern int git_apply__patch(
 	unsigned int *mode,
 	const char *source,
 	size_t source_len,
-	git_patch *patch);
+	git_patch *patch,
+	const git_apply_options *opts);
 
 #endif
diff --git a/tests/apply/callbacks.c b/tests/apply/callbacks.c
index 91bdfa3..1b759dc 100644
--- a/tests/apply/callbacks.c
+++ b/tests/apply/callbacks.c
@@ -88,3 +88,41 @@ void test_apply_callbacks__delta_can_skip(void)
 
 	git_diff_free(diff);
 }
+
+static int hunk_skip_odds_cb(const git_diff_hunk *hunk, void *payload)
+{
+	int *count = (int *)payload;
+	GIT_UNUSED(hunk);
+
+	return ((*count)++ % 2 == 1);
+}
+
+void test_apply_callbacks__hunk_can_skip(void)
+{
+	git_diff *diff;
+	git_apply_options opts = GIT_APPLY_OPTIONS_INIT;
+	int count = 0;
+
+	struct merge_index_entry workdir_expected[] = {
+		{ 0100644, "f51658077d85f2264fa179b4d0848268cb3475c3", 0, "asparagus.txt" },
+		{ 0100644, "68f6182f4c85d39e1309d97c7e456156dc9c0096", 0, "beef.txt" },
+		{ 0100644, "4b7c5650008b2e747fe1809eeb5a1dde0e80850a", 0, "bouilli.txt" },
+		{ 0100644, "c4e6cca3ec6ae0148ed231f97257df8c311e015f", 0, "gravy.txt" },
+		{ 0100644, "68af1fc7407fd9addf1701a87eb1c95c7494c598", 0, "oyster.txt" },
+		{ 0100644, "06f751b6ba4f017ddbf4248015768300268e092a", 0, "veal.txt" },
+	};
+	size_t workdir_expected_cnt = sizeof(workdir_expected) /
+	    sizeof(struct merge_index_entry);
+
+	opts.hunk_cb = hunk_skip_odds_cb;
+	opts.payload = &count;
+
+	cl_git_pass(git_diff_from_buffer(&diff,
+		DIFF_MANY_CHANGES_ONE, strlen(DIFF_MANY_CHANGES_ONE)));
+	cl_git_pass(git_apply(repo, diff, GIT_APPLY_LOCATION_WORKDIR, &opts));
+
+	validate_index_unchanged(repo);
+	validate_apply_workdir(repo, workdir_expected, workdir_expected_cnt);
+
+	git_diff_free(diff);
+}
diff --git a/tests/apply/fromdiff.c b/tests/apply/fromdiff.c
index 6110609..3b15602 100644
--- a/tests/apply/fromdiff.c
+++ b/tests/apply/fromdiff.c
@@ -49,7 +49,7 @@ static int apply_gitbuf(
 		cl_assert_equal_s(patch_expected, patchbuf.ptr);
 	}
 
-	error = git_apply__patch(&result, &filename, &mode, old ? old->ptr : NULL, old ? old->size : 0, patch);
+	error = git_apply__patch(&result, &filename, &mode, old ? old->ptr : NULL, old ? old->size : 0, patch, NULL);
 
 	if (error == 0 && new == NULL) {
 		cl_assert_equal_i(0, result.size);
diff --git a/tests/apply/fromfile.c b/tests/apply/fromfile.c
index 0ace639..6d4379b 100644
--- a/tests/apply/fromfile.c
+++ b/tests/apply/fromfile.c
@@ -39,7 +39,7 @@ static int apply_patchfile(
 
 	cl_git_pass(git_patch_from_buffer(&patch, patchfile, strlen(patchfile), NULL));
 
-	error = git_apply__patch(&result, &filename, &mode, old, old_len, patch);
+	error = git_apply__patch(&result, &filename, &mode, old, old_len, patch, NULL);
 
 	if (error == 0) {
 		cl_assert_equal_i(new_len, result.size);