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128 lines
3.1 KiB
Go
128 lines
3.1 KiB
Go
// Package dbhash implements the dropbox hash as described in
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//
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// https://www.dropbox.com/developers/reference/content-hash
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package dbhash
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import (
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"crypto/sha256"
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"hash"
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)
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const (
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// BlockSize of the checksum in bytes.
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BlockSize = sha256.BlockSize
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// Size of the checksum in bytes.
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Size = sha256.BlockSize
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bytesPerBlock = 4 * 1024 * 1024
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hashReturnedError = "hash function returned error"
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)
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type digest struct {
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n int // bytes written into blockHash so far
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blockHash hash.Hash
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totalHash hash.Hash
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sumCalled bool
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writtenMore bool
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}
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// New returns a new hash.Hash computing the Dropbox checksum.
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func New() hash.Hash {
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d := &digest{}
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d.Reset()
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return d
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}
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// writeBlockHash writes the current block hash into the total hash
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func (d *digest) writeBlockHash() {
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blockHash := d.blockHash.Sum(nil)
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_, err := d.totalHash.Write(blockHash)
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if err != nil {
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panic(hashReturnedError)
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}
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// reset counters for blockhash
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d.n = 0
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d.blockHash.Reset()
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}
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// Write writes len(p) bytes from p to the underlying data stream. It returns
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// the number of bytes written from p (0 <= n <= len(p)) and any error
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// encountered that caused the write to stop early. Write must return a non-nil
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// error if it returns n < len(p). Write must not modify the slice data, even
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// temporarily.
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//
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// Implementations must not retain p.
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func (d *digest) Write(p []byte) (n int, err error) {
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n = len(p)
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for len(p) > 0 {
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d.writtenMore = true
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toWrite := bytesPerBlock - d.n
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if toWrite > len(p) {
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toWrite = len(p)
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}
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_, err = d.blockHash.Write(p[:toWrite])
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if err != nil {
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panic(hashReturnedError)
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}
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d.n += toWrite
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p = p[toWrite:]
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// Accumulate the total hash
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if d.n == bytesPerBlock {
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d.writeBlockHash()
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}
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}
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return n, nil
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}
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// Sum appends the current hash to b and returns the resulting slice.
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// It does not change the underlying hash state.
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//
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// TODO(ncw) Sum() can only be called once for this type of hash.
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// If you call Sum(), then Write() then Sum() it will result in
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// a panic. Calling Write() then Sum(), then Sum() is OK.
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func (d *digest) Sum(b []byte) []byte {
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if d.sumCalled && d.writtenMore {
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panic("digest.Sum() called more than once")
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}
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d.sumCalled = true
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d.writtenMore = false
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if d.n != 0 {
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d.writeBlockHash()
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}
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return d.totalHash.Sum(b)
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}
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// Reset resets the Hash to its initial state.
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func (d *digest) Reset() {
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d.n = 0
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d.totalHash = sha256.New()
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d.blockHash = sha256.New()
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d.sumCalled = false
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d.writtenMore = false
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}
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// Size returns the number of bytes Sum will return.
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func (d *digest) Size() int {
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return d.totalHash.Size()
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}
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// BlockSize returns the hash's underlying block size.
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// The Write method must be able to accept any amount
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// of data, but it may operate more efficiently if all writes
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// are a multiple of the block size.
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func (d *digest) BlockSize() int {
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return d.totalHash.BlockSize()
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}
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// Sum returns the Dropbox checksum of the data.
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func Sum(data []byte) [Size]byte {
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var d digest
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d.Reset()
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_, _ = d.Write(data)
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var out [Size]byte
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d.Sum(out[:0])
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return out
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}
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// must implement this interface
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var _ hash.Hash = (*digest)(nil)
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