Lesson 08 · Walking sibling boxes - Build a Video Container Parser

Walking sibling boxes

A file, and every container inside it, is a run of boxes laid one after another. Today you walk a byte range and collect every box in it, advancing past each one by its own size.

The goal

Parse a byte range into a flat list of the boxes it contains.

Start here - the target

TO DO

Scenario: Adjacent boxes are collected into a list

Givena 24-byte buffer holding an ftyp box of size 16 followed by a free box of size 8

Whenthe buffer is walked into a list of boxes

Thenthere are 2 boxes: "ftyp" at offset 0 and "free" at offset 16

Andeach reported box carries its own Size (16 and 8)

Background

Boxes at one level sit adjacent: parse a header, then jump forward by that box’s full Size to land exactly on the next box’s first byte. Repeat until you run out of bytes. The key move is advancing by the whole box size, not just the header - the payload in between is skipped over wholesale at this level.

Here the ftyp box is 16 bytes, so after reading it you jump to offset 16 and find the free box (a padding box you can ignore) of size 8, ending at 24. This same loop parses the top level of a file and, once boxes nest, the children inside any container. Tomorrow you turn that observation into recursion.

Make it work

// step through the range, parsing a header then skipping its whole size
func parseBoxes(b []byte) []Box {
  var boxes []Box
  pos := 0
  for pos < len(b) {
    box := parseHeaderAt(b, pos, len(b))
    boxes = append(boxes, box)
    pos += int(box.Size) // advance past the entire box
  }
  return boxes
}

CheckpointDONE

You can walk a run of sibling boxes. Commit and stop here.