A Kaitai Struct YAML specification for finding the log2 of a 32-bit integer

log2_int32.ksy

meta:
  id: log2_int32
  title: Find the log base 2 of a 32-bit integer
  license: CC0-1.0
doc-ref: https://graphics.stanford.edu/~seander/bithacks.html#IntegerLog
# params:
#   - id: val
#     type: u4
seq:
  - id: log2
    type: 'log2_step(_index, _index == 0 ? val : log2[_index - 1].v, _index == 0 ? 0 : log2[_index - 1].r)'
    repeat: expr
    repeat-expr: _root.b.size
instances:
  val:
    value: 0b00000000_00011011_01011001_10111011
    #        ^        ^  ^     ^        ^      ^
    #        31       23 20    15       7      0
  result:
    value: log2.last.r
  b:
    value: '[0xFFFF0000, 0xFF00, 0xF0, 0xC, 0x2]'
  s:
    value: '[16, 8, 4, 2, 1]'
types:
  log2_step:
    params:
      - id: i
        type: s4
      - id: prev_v
        type: u4
      - id: prev_r
        type: u1
    instances:
      v:
        value: '(prev_v & _root.b[i]) != 0 ? prev_v >> _root.s[i] : prev_v'
      r:
        value: '(prev_v & _root.b[i]) != 0 ? prev_r | _root.s[i] : prev_r'