file-store/pkg/erasureencode/params.go

package erasureencode

import (
  "fmt"
)

type Params struct {
  Size   int64  `json:"s,string"`
  Stride int32  `json:"t"`
  Shards uint16 `json:"h"`
  Parity uint16 `json:"p"`
}

type ChunkShardMeta struct {
  Shard        uint16
  ShardOffset  int64
  Chunk        uint16
  ChunkOffset  int64
  GlobalOffset int64
  Size         int32
}

func (params Params) Plan(offset, size int64) []ChunkShardMeta {
  outputs := []ChunkShardMeta{}
  end := offset + size
  if (end - 1) > params.Size {
    panic(fmt.Errorf("attempted read beyond end of file"))
  }
  // constant
  shards := int64(params.Shards)
  baseStride := int64(params.Stride)
  baseStripeWidth := baseStride * shards

  oddStripeOffset := (params.Size / baseStripeWidth) * baseStripeWidth
  oddChunkOffset := oddStripeOffset / shards
  oddSize := (params.Size - oddStripeOffset)
  oddStride := oddSize / shards
  if oddSize%shards > 0 {
    oddStride += 1
  }

  for offset < end {
    output := ChunkShardMeta{GlobalOffset: offset}
    // var chunk uint64       // which chunk the shard is in
    // var chunkOffset uint64 // the location within the chunk that the shard begins
    // var shardOffset uint64 // the location within the shard at which the data begins
    // var readSize uint64    // the number of bytes to read from the shard

    if offset >= oddStripeOffset {
      localOffset := offset - oddStripeOffset // the location relative to the odd data at which the desired data begins
      output.Chunk = uint16(localOffset / oddStride)
      output.Shard = uint16(oddStripeOffset/baseStride) + output.Chunk
      output.ShardOffset = localOffset % oddStride
      output.ChunkOffset = oddChunkOffset + output.ShardOffset
      output.Size = int32(min(end-offset, oddStride-output.ShardOffset))
    } else {
      shard := offset / baseStride
      output.Shard = uint16(offset / baseStride) // which shard the data is in
      output.Chunk = uint16(shard % shards)
      output.ShardOffset = offset % baseStride
      output.ChunkOffset = ((shard / shards) * baseStride) + output.ShardOffset
      output.Size = int32(min(end-offset, baseStride-output.ShardOffset))
    }
    if output.Size <= 0 {
      panic(fmt.Errorf("invalid read size"))
    }
    outputs = append(outputs, output)
    offset += int64(output.Size)
  }
  return outputs
}