lzw: Direct output into frame data, avoiding stack.

If the frame is non-interlaced, and has the same rowstride as the
full image, then we can decode lzw directly into the output image.

1 files changed, 81 insertions, 2 deletions

diff --git a/src/libnsgif.c b/src/libnsgif.c
index 752f8c2..3c49218 100644
--- a/src/libnsgif.c
+++ b/src/libnsgif.c
@@ -626,8 +626,8 @@ static gif_result gif__recover_previous_frame(const gif_animation *gif)
         return GIF_OK;
 }
 
-static inline gif_result
-gif__decode(gif_animation *gif,
+static gif_result
+gif__decode_complex(gif_animation *gif,
                 unsigned int frame,
                 unsigned int width,
                 unsigned int height,
@@ -701,6 +701,85 @@ gif__decode(gif_animation *gif,
         return ret;
 }
 
+static gif_result
+gif__decode_simple(gif_animation *gif,
+                unsigned int frame,
+                unsigned int height,
+                unsigned int offset_y,
+                uint8_t minimum_code_size,
+                unsigned int *restrict frame_data,
+                unsigned int *restrict colour_table)
+{
+        unsigned int transparency_index;
+        uint32_t pixels = gif->width * height;
+        uint32_t written = 0;
+        gif_result ret = GIF_OK;
+        lzw_result res;
+
+        /* Initialise the LZW decoding */
+        res = lzw_decode_init(gif->lzw_ctx, gif->gif_data,
+                        gif->buffer_size, gif->buffer_position,
+                        minimum_code_size);
+        if (res != LZW_OK) {
+                return gif_error_from_lzw(res);
+        }
+
+        transparency_index = gif->frames[frame].transparency ?
+                        gif->frames[frame].transparency_index :
+                        GIF_NO_TRANSPARENCY;
+
+        frame_data += (offset_y * gif->width);
+
+        while (pixels > 0) {
+                res = lzw_decode_map_continuous(gif->lzw_ctx,
+                                transparency_index, colour_table,
+                                frame_data, pixels, &written);
+                pixels -= written;
+                frame_data += written;
+                if (res != LZW_OK) {
+                        /* Unexpected end of frame, try to recover */
+                        if (res == LZW_OK_EOD) {
+                                ret = GIF_OK;
+                        } else {
+                                ret = gif_error_from_lzw(res);
+                        }
+                        break;
+                }
+        }
+
+        if (pixels == 0) {
+                ret = GIF_OK;
+        }
+
+        return ret;
+}
+
+static inline gif_result
+gif__decode(gif_animation *gif,
+                unsigned int frame,
+                unsigned int width,
+                unsigned int height,
+                unsigned int offset_x,
+                unsigned int offset_y,
+                unsigned int interlace,
+                uint8_t minimum_code_size,
+                unsigned int *restrict frame_data,
+                unsigned int *restrict colour_table)
+{
+        gif_result ret;
+
+        if (interlace == false && width == gif->width && offset_x == 0) {
+                ret = gif__decode_simple(gif, frame, height, offset_y,
+                                minimum_code_size, frame_data, colour_table);
+        } else {
+                ret = gif__decode_complex(gif, frame, width, height,
+                                offset_x, offset_y, interlace,
+                                minimum_code_size, frame_data, colour_table);
+        }
+
+        return ret;
+}
+
 /**
  * decode a gif frame
  *