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package digest
import (
"crypto"
"fmt"
"hash"
"io"
)
// Algorithm identifies and implementation of a digester by an identifier.
// Note the that this defines both the hash algorithm used and the string
// encoding.
type Algorithm string
// supported digest types
const (
SHA256 Algorithm = "sha256" // sha256 with hex encoding
SHA384 Algorithm = "sha384" // sha384 with hex encoding
SHA512 Algorithm = "sha512" // sha512 with hex encoding
// Canonical is the primary digest algorithm used with the distribution
// project. Other digests may be used but this one is the primary storage
// digest.
Canonical = SHA256
)
var (
// TODO(stevvooe): Follow the pattern of the standard crypto package for
// registration of digests. Effectively, we are a registerable set and
// common symbol access.
// algorithms maps values to hash.Hash implementations. Other algorithms
// may be available but they cannot be calculated by the digest package.
algorithms = map[Algorithm]crypto.Hash{
SHA256: crypto.SHA256,
SHA384: crypto.SHA384,
SHA512: crypto.SHA512,
}
)
// Available returns true if the digest type is available for use. If this
// returns false, New and Hash will return nil.
func (a Algorithm) Available() bool {
h, ok := algorithms[a]
if !ok {
return false
}
// check availability of the hash, as well
return h.Available()
}
func (a Algorithm) String() string {
return string(a)
}
// Size returns number of bytes returned by the hash.
func (a Algorithm) Size() int {
h, ok := algorithms[a]
if !ok {
return 0
}
return h.Size()
}
// Set implemented to allow use of Algorithm as a command line flag.
func (a *Algorithm) Set(value string) error {
if value == "" {
*a = Canonical
} else {
// just do a type conversion, support is queried with Available.
*a = Algorithm(value)
}
return nil
}
// New returns a new digester for the specified algorithm. If the algorithm
// does not have a digester implementation, nil will be returned. This can be
// checked by calling Available before calling New.
func (a Algorithm) New() Digester {
return &digester{
alg: a,
hash: a.Hash(),
}
}
// Hash returns a new hash as used by the algorithm. If not available, the
// method will panic. Check Algorithm.Available() before calling.
func (a Algorithm) Hash() hash.Hash {
if !a.Available() {
// NOTE(stevvooe): A missing hash is usually a programming error that
// must be resolved at compile time. We don't import in the digest
// package to allow users to choose their hash implementation (such as
// when using stevvooe/resumable or a hardware accelerated package).
//
// Applications that may want to resolve the hash at runtime should
// call Algorithm.Available before call Algorithm.Hash().
panic(fmt.Sprintf("%v not available (make sure it is imported)", a))
}
return algorithms[a].New()
}
// FromReader returns the digest of the reader using the algorithm.
func (a Algorithm) FromReader(rd io.Reader) (Digest, error) {
digester := a.New()
if _, err := io.Copy(digester.Hash(), rd); err != nil {
return "", err
}
return digester.Digest(), nil
}
// FromBytes digests the input and returns a Digest.
func (a Algorithm) FromBytes(p []byte) Digest {
digester := a.New()
if _, err := digester.Hash().Write(p); err != nil {
// Writes to a Hash should never fail. None of the existing
// hash implementations in the stdlib or hashes vendored
// here can return errors from Write. Having a panic in this
// condition instead of having FromBytes return an error value
// avoids unnecessary error handling paths in all callers.
panic("write to hash function returned error: " + err.Error())
}
return digester.Digest()
}
// TODO(stevvooe): Allow resolution of verifiers using the digest type and
// this registration system.
// Digester calculates the digest of written data. Writes should go directly
// to the return value of Hash, while calling Digest will return the current
// value of the digest.
type Digester interface {
Hash() hash.Hash // provides direct access to underlying hash instance.
Digest() Digest
}
// digester provides a simple digester definition that embeds a hasher.
type digester struct {
alg Algorithm
hash hash.Hash
}
func (d *digester) Hash() hash.Hash {
return d.hash
}
func (d *digester) Digest() Digest {
return NewDigest(d.alg, d.hash)
}
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