fix color mayhem caused by Image.copyBytes()

src/image.zig

@@ -166,24 +166,35 @@ pub const Image = struct {
         end: usize,
     ) !void {
         const total_bytes = end - start;
-
         var i: usize = start;
-        while (i < end) : (i += buf.len) {
-            const read_bytes = c.sf_readf_float(self.sndfile, buf.ptr, @intCast(i64, buf.len));
 
+        // we do sf_seek() calls to make sure we are actually on the start
+        // and actually end at the end position for the file.
+        _ = c.sf_seek(self.sndfile, @intCast(i64, start), c.SEEK_SET);
+
+        while (i <= end) : (i += buf.len) {
+            std.debug.warn("i={}, buf.len={}, end={}\n", i, buf.len, end);
+            const excess = @intCast(i64, i + buf.len) - @intCast(i64, end);
+
+            var read_bytes: i64 = undefined;
             var view: []f32 = buf[0..buf.len];
-            if (i + buf.len > end) {
-                const excess = i + buf.len - end;
-                view = buf[0..excess];
-                _ = c.sf_seek(
-                    self.sndfile,
-                    -@intCast(i64, excess),
-                    c.SEEK_CUR,
+
+            if (excess > 0) {
+                std.debug.warn(
+                    "excess of {} bytes, reading {} instead\n",
+                    excess,
+                    @intCast(i64, buf.len) - excess,
                 );
+                read_bytes = c.sf_readf_float(self.sndfile, buf.ptr, @intCast(i64, buf.len) - excess);
+                view = buf[0..@intCast(usize, excess)];
+            } else {
+                read_bytes = c.sf_readf_float(self.sndfile, buf.ptr, @intCast(i64, buf.len));
             }
 
             try swrite(out_file, view.ptr, @intCast(i64, view.len));
         }
+
+        _ = c.sf_seek(self.sndfile, @intCast(i64, end), c.SEEK_SET);
     }
 
     /// Run a plugin over the image.
@@ -265,28 +276,21 @@ pub const Image = struct {
         // - plugin
         // - post-plugin
 
-        var header_buf = try self.allocator.alloc(f32, BMPHeaderSize);
-        defer self.allocator.free(header_buf);
-
         var file_copy_buf = try self.allocator.alloc(f32, BufferSize);
         defer self.allocator.free(file_copy_buf);
 
-        // bmp header copy
-        const header_len = @intCast(i64, header_buf.len);
-        _ = c.sf_readf_float(self.sndfile, header_buf.ptr, header_len);
-        try swrite(out_file, header_buf.ptr, header_len);
-
-        // pre-plugin copy
+        // pre-plugin copy, merged with bmp header copy
         try self.copyBytes(
             out_file,
             file_copy_buf,
-            BMPHeaderSize,
-            seek_pos.start,
+            usize(0),
+            seek_pos.start - 1,
         );
 
         var i: usize = seek_pos.start;
+        std.debug.warn("\tseek pos start: {} end: {}\n", seek_pos.start, seek_pos.end);
 
-        while (i < seek_pos.end) : (i += 1) {
+        while (i <= seek_pos.end) : (i += 1) {
             const read_bytes = c.sf_readf_float(self.sndfile, rctx.in_buf.ptr, 1);
             if (read_bytes == 0) {
                 std.debug.warn("WARN! reached EOF at idx={}\n", i);
@@ -301,7 +305,7 @@ pub const Image = struct {
         try self.copyBytes(
             out_file,
             file_copy_buf,
-            seek_pos.end,
+            seek_pos.end + 1,
             file_end,
         );