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vendor/github.com/google/go-containerregistry/pkg/v1/partial/README.md

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+# `partial`
+
+[![GoDoc](https://godoc.org/github.com/google/go-containerregistry/pkg/v1/partial?status.svg)](https://godoc.org/github.com/google/go-containerregistry/pkg/v1/partial)
+
+## Partial Implementations
+
+There are roughly two kinds of image representations: compressed and uncompressed.
+
+The implementations for these kinds of images are almost identical, with the only
+major difference being how blobs (config and layers) are fetched. This common
+code lives in this package, where you provide a _partial_ implementation of a
+compressed or uncompressed image, and you get back a full `v1.Image` implementation.
+
+### Examples
+
+In a registry, blobs are compressed, so it's easiest to implement a `v1.Image` in terms
+of compressed layers. `remote.remoteImage` does this by implementing `CompressedImageCore`:
+
+```go
+type CompressedImageCore interface {
+	RawConfigFile() ([]byte, error)
+	MediaType() (types.MediaType, error)
+	RawManifest() ([]byte, error)
+	LayerByDigest(v1.Hash) (CompressedLayer, error)
+}
+```
+
+In a tarball, blobs are (often) uncompressed, so it's easiest to implement a `v1.Image` in terms
+of uncompressed layers. `tarball.uncompressedImage` does this by implementing `UncompressedImageCore`:
+
+```go
+type CompressedImageCore interface {
+	RawConfigFile() ([]byte, error)
+	MediaType() (types.MediaType, error)
+	LayerByDiffID(v1.Hash) (UncompressedLayer, error)
+}
+```
+
+## Optional Methods
+
+Where possible, we access some information via optional methods as an optimization.
+
+### [`partial.Descriptor`](https://godoc.org/github.com/google/go-containerregistry/pkg/v1/partial#Descriptor)
+
+There are some properties of a [`Descriptor`](https://github.com/opencontainers/image-spec/blob/master/descriptor.md#properties) that aren't derivable from just image data:
+
+* `MediaType`
+* `Platform`
+* `URLs`
+* `Annotations`
+
+For example, in a `tarball.Image`, there is a `LayerSources` field that contains
+an entire layer descriptor with `URLs` information for foreign layers. This
+information can be passed through to callers by implementing this optional
+`Descriptor` method.
+
+See [`#654`](https://github.com/google/go-containerregistry/pull/654).
+
+### [`partial.UncompressedSize`](https://godoc.org/github.com/google/go-containerregistry/pkg/v1/partial#UncompressedSize)
+
+Usually, you don't need to know the uncompressed size of a layer, since that
+information isn't stored in a config file (just he sha256 is needed); however,
+there are cases where it is very helpful to know the layer size, e.g. when
+writing the uncompressed layer into a tarball.
+
+See [`#655`](https://github.com/google/go-containerregistry/pull/655).