44 lines
1.3 KiB
Go
44 lines
1.3 KiB
Go
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package filler
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import (
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"math/bits"
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"github.com/filecoin-project/go-state-types/abi"
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)
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func FillersFromRem(in abi.UnpaddedPieceSize) ([]abi.UnpaddedPieceSize, error) {
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// Convert to in-sector bytes for easier math:
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//
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// Sector size to user bytes ratio is constant, e.g. for 1024B we have 1016B
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// of user-usable data.
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//
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// (1024/1016 = 128/127)
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//
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// Given that we can get sector size by simply adding 1/127 of the user
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// bytes
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//
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// (we convert to sector bytes as they are nice round binary numbers)
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toFill := uint64(in + (in / 127))
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// We need to fill the sector with pieces that are powers of 2. Conveniently
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// computers store numbers in binary, which means we can look at 1s to get
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// all the piece sizes we need to fill the sector. It also means that number
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// of pieces is the number of 1s in the number of remaining bytes to fill
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out := make([]abi.UnpaddedPieceSize, bits.OnesCount64(toFill))
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for i := range out {
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// Extract the next lowest non-zero bit
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next := bits.TrailingZeros64(toFill)
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psize := uint64(1) << next
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// e.g: if the number is 0b010100, psize will be 0b000100
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// set that bit to 0 by XORing it, so the next iteration looks at the
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// next bit
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toFill ^= psize
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// Add the piece size to the list of pieces we need to create
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out[i] = abi.PaddedPieceSize(psize).Unpadded()
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}
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return out, nil
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}
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