68 lines
2.1 KiB
Go
68 lines
2.1 KiB
Go
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package erasureencode
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import (
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"fmt"
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)
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type Params struct {
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Size uint64 `json:"s,string"`
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Stride int32 `json:"t"`
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Shards uint16 `json:"h"`
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Parity uint16 `json:"p"`
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}
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type ChunkShardMeta struct {
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Chunk uint16
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ChunkOffset uint64
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GlobalOffset uint64
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Size int32
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}
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func (params Params) Plan(offset, size uint64) []ChunkShardMeta {
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outputs := []ChunkShardMeta{}
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end := offset + size
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if (end - 1) > params.Size {
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panic(fmt.Errorf("attempted read beyond end of file"))
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}
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// constant
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shards := uint64(params.Shards)
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baseStride := uint64(params.Stride)
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baseStripeWidth := baseStride * shards
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oddStripeOffset := (params.Size / baseStripeWidth) * baseStripeWidth
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oddChunkOffset := oddStripeOffset / shards
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oddSize := (params.Size - oddStripeOffset)
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oddStride := oddSize / shards
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if oddSize%shards > 0 {
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oddStride += 1
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}
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for offset < end {
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output := ChunkShardMeta{GlobalOffset: offset}
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// var chunk uint64 // which chunk the shard is in
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// var chunkOffset uint64 // the location within the chunk that the shard begins
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// var shardOffset uint64 // the location within the shard at which the data begins
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// var readSize uint64 // the number of bytes to read from the shard
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if offset >= oddStripeOffset {
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localOffset := offset - oddStripeOffset // the location relative to the odd data at which the desired data begins
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output.Chunk = uint16(localOffset / oddStride)
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shardOffset := localOffset % oddStride
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output.ChunkOffset = oddChunkOffset + shardOffset
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output.Size = int32(min(end-offset, oddStride-shardOffset))
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} else {
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shardNum := offset / baseStride // which shard the data is in
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output.Chunk = uint16(shardNum % shards)
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shardOffset := offset % baseStride
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output.ChunkOffset = ((shardNum / shards) * baseStride) + shardOffset
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output.Size = int32(min(end-offset, baseStride-shardOffset))
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}
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if output.Size <= 0 {
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panic(fmt.Errorf("invalid read size"))
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}
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outputs = append(outputs, output)
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offset += uint64(output.Size)
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}
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return outputs
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}
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