// residue.mbt
//
// Vorbis residue setup 解析。
//
// residue 是 floor 与最终频域系数之间的残差，用 codebook 对系数分区编码。
// 三种类型：type 0（interleaved，≤8 通道）、type 1（interleaved，≤256 通道）、
// type 2（non-interleaved）。本阶段解析 setup 配置，实际解码在音频 packet 阶段。

///|
/// residue 的配置。
///
/// * `begin` / `end` : 作用的系数范围（end 是开区间上限）；
/// * `partition_size` : 每个 partition 的系数数量；
/// * `classifications` / `classbook` : 分区分类与对应 codebook；
/// * `cascade` : 每个 classification 的残差级联编码描述。
pub struct Residue {
  residue_type : Int
  begin : Int
  end : Int
  partition_size : Int
  classifications : Int
  classbook : Int
  cascade : Array[Int]
  residue_books : Array[Array[Int]]
}

///|
/// 从 `br` 的当前位置解析一个 residue 配置。
pub fn Residue::parse(br : BitReader) -> Result[Residue, String] {
  let residue_type = br.read_bits(16)
  if residue_type > 2 {
    return Err("invalid residue type")
  }
  let begin = br.read_bits(24)
  let end = br.read_bits(24)
  let partition_size = br.read_bits(24) + 1
  let classifications = br.read_bits(6) + 1
  let classbook = br.read_bits(8)

  // 每个 classification 的级联编码
  let cascade : Array[Int] = []
  for _ in 0.. Result[Unit, String] {
  let rtype = self.residue_type
  let classbook = codebooks[self.classbook]
  let classwords = classbook.dimensions
  let ch = targets.length()
  let actual_size = if rtype == 2 { n * 2 } else { n }
  let limit_begin = if self.begin < actual_size {
    self.begin
  } else {
    actual_size
  }
  let limit_end = if self.end < actual_size { self.end } else { actual_size }
  let n_read = limit_end - limit_begin
  let part_read = n_read / self.partition_size

  let clear_len = if ch > 1 { n } else { actual_size }
  for c in 0.. 1

  for pass in 0..<8 {
    let mut pcount = 0
    while pcount < part_read {
      if pass == 0 {
        if interleaved {
          let temp = match classbook.decode_scalar(br) {
            Ok(t) => t
            Err(e) => return Err(e)
          }
          spread_classification(
            class_of[0],
            pcount,
            classwords,
            temp,
            self.classifications,
          )
        } else {
          for j in 0.. t
              Err(e) => return Err(e)
            }
            spread_classification(
              class_of[j],
              pcount,
              classwords,
              temp,
              self.classifications,
            )
          }
        }
      }
      let mut i = 0
      while i < classwords && pcount < part_read {
        if interleaved {
          // type 2 多声道：VQ 系数按声道交错分配
          let c = class_of[0][pcount]
          let b = self.residue_books[c][pass]
          if b >= 0 {
            let book = codebooks[b]
            let z = limit_begin + pcount * self.partition_size
            let mut c_inter = z % ch
            let mut p_inter = z / ch
            let mut k = 0
            while k < self.partition_size {
              let entry = match book.decode_scalar(br) {
                Ok(e) => e
                Err(err) => return Err(err)
              }
              let dim = book.dimensions
              let base = entry * dim
              let nn = if dim < self.partition_size - k {
                dim
              } else {
                self.partition_size - k
              }
              for ii in 0..= 0 {
              match
                decode_partition(
                  codebooks[b],
                  br,
                  targets[j],
                  offset,
                  self.partition_size,
                  rtype,
                ) {
                Ok(_) => ()
                Err(e) => return Err(e)
              }
            }
          }
        }
        pcount += 1
        i += 1
      }
    }
  }
  Ok(())
}

///|
/// 把一个 classbook 码字按 classification 数展开成 classwords 个数字，
/// 写在 row 的 pcount 起、由高到低的位置上。
fn spread_classification(
  row : Array[Int],
  pcount : Int,
  classwords : Int,
  codeword : Int,
  count : Int,
) -> Unit {
  let mut q = codeword
  let mut k = 0
  while k < classwords {
    row[pcount + classwords - 1 - k] = q % count
    q = q / count
    k += 1
  }
}

///|
/// 解码一个 partition，把结果累加进 target。
///
/// type 0 是交织写法：每个码字的分量按下标步进而非相邻地摆放，step 个码字
/// 交错铺满整个 partition。type 1 及单声道 type 2 则是顺序摆放。
fn decode_partition(
  book : Codebook,
  br : BitReader,
  target : Array[Float],
  offset : Int,
  part_size : Int,
  rtype : Int,
) -> Result[Unit, String] {
  if book.dimensions <= 0 {
    return Err("codebook dimensions is zero")
  }
  if rtype == 0 {
    let step = part_size / book.dimensions
    for k in 0.. ()
        Err(e) => return Err(e)
      }
    }
  } else {
    let mut k = 0
    while k < part_size {
      match book.decode_vq(br, target, offset + k, part_size - k) {
        Ok(_) => ()
        Err(e) => return Err(e)
      }
      k += book.dimensions
    }
  }
  Ok(())
}