apply: refactor code to drop line_allocated

The `struct image` has two members `line` and `line_allocated`. The
former member is the one that should be used throughout the code,
whereas the latter one is used to track whether the lines have been
allocated or not.

In practice, the array of lines is always allocated. The reason why we
have `line_allocated` is that `remove_first_line()` will advance the
array pointer to drop the first entry, and thus it points into the array
instead of to the array header.

Refactor the function to use memmove(3P) instead, which allows us to get
rid of this double bookkeeping. This is less efficient, but I doubt that
this matters much in practice. If this judgement call is found to be
wrong at a later point in time we can likely refactor the surrounding
loop such that we first calculate the number of leading context lines to
remove and then remove them in a single call to memmove(3P).

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

apply.c

@@ -281,7 +281,6 @@ struct image {
 	size_t len;
 	size_t nr;
 	size_t alloc;
-	struct line *line_allocated;
 	struct line *line;
 };
 #define IMAGE_INIT { 0 }
@@ -295,7 +294,7 @@ static void image_init(struct image *image)
 static void image_clear(struct image *image)
 {
 	free(image->buf);
-	free(image->line_allocated);
+	free(image->line);
 	image_init(image);
 }
 
@@ -313,10 +312,10 @@ static uint32_t hash_line(const char *cp, size_t len)
 
 static void image_add_line(struct image *img, const char *bol, size_t len, unsigned flag)
 {
-	ALLOC_GROW(img->line_allocated, img->nr + 1, img->alloc);
-	img->line_allocated[img->nr].len = len;
-	img->line_allocated[img->nr].hash = hash_line(bol, len);
-	img->line_allocated[img->nr].flag = flag;
+	ALLOC_GROW(img->line, img->nr + 1, img->alloc);
+	img->line[img->nr].len = len;
+	img->line[img->nr].hash = hash_line(bol, len);
+	img->line[img->nr].flag = flag;
 	img->nr++;
 }
 
@@ -348,15 +347,15 @@ static void image_prepare(struct image *image, char *buf, size_t len,
 		image_add_line(image, cp, next - cp, 0);
 		cp = next;
 	}
-	image->line = image->line_allocated;
 }
 
 static void image_remove_first_line(struct image *img)
 {
 	img->buf += img->line[0].len;
 	img->len -= img->line[0].len;
-	img->line++;
 	img->nr--;
+	if (img->nr)
+		MOVE_ARRAY(img->line, img->line + 1, img->nr);
 }
 
 static void image_remove_last_line(struct image *img)
@@ -2335,7 +2334,7 @@ static void update_pre_post_images(struct image *preimage,
 	       : fixed_preimage.nr <= preimage->nr);
 	for (i = 0; i < fixed_preimage.nr; i++)
 		fixed_preimage.line[i].flag = preimage->line[i].flag;
-	free(preimage->line_allocated);
+	free(preimage->line);
 	*preimage = fixed_preimage;
 
 	/*
@@ -2879,14 +2878,12 @@ static void update_image(struct apply_state *state,
 
 	/* Adjust the line table */
 	nr = img->nr + postimage->nr - preimage_limit;
-	if (preimage_limit < postimage->nr) {
+	if (preimage_limit < postimage->nr)
 		/*
 		 * NOTE: this knows that we never call image_remove_first_line()
 		 * on anything other than pre/post image.
 		 */
 		REALLOC_ARRAY(img->line, nr);
-		img->line_allocated = img->line;
-	}
 	if (preimage_limit != postimage->nr)
 		MOVE_ARRAY(img->line + applied_pos + postimage->nr,
 			   img->line + applied_pos + preimage_limit,
@@ -3027,8 +3024,8 @@ static int apply_one_fragment(struct apply_state *state,
 	    newlines.len > 0 && newlines.buf[newlines.len - 1] == '\n') {
 		old--;
 		strbuf_setlen(&newlines, newlines.len - 1);
-		preimage.line_allocated[preimage.nr - 1].len--;
-		postimage.line_allocated[postimage.nr - 1].len--;
+		preimage.line[preimage.nr - 1].len--;
+		postimage.line[postimage.nr - 1].len--;
 	}
 
 	leading = frag->leading;
@@ -3062,8 +3059,6 @@ static int apply_one_fragment(struct apply_state *state,
 	preimage.len = old - oldlines;
 	postimage.buf = newlines.buf;
 	postimage.len = newlines.len;
-	preimage.line = preimage.line_allocated;
-	postimage.line = postimage.line_allocated;
 
 	for (;;) {
 
@@ -3151,8 +3146,8 @@ static int apply_one_fragment(struct apply_state *state,
 out:
 	free(oldlines);
 	strbuf_release(&newlines);
-	free(preimage.line_allocated);
-	free(postimage.line_allocated);
+	free(preimage.line);
+	free(postimage.line);
 
 	return (applied_pos < 0);
 }
@@ -3752,7 +3747,7 @@ static int apply_data(struct apply_state *state, struct patch *patch,
 	patch->result = image.buf;
 	patch->resultsize = image.len;
 	add_to_fn_table(state, patch);
-	free(image.line_allocated);
+	free(image.line);
 
 	if (0 < patch->is_delete && patch->resultsize)
 		return error(_("removal patch leaves file contents"));