package amp
import (
"encoding/base64"
"io"
)
// https://amp.dev/boilerplate/
// https://amp.dev/documentation/guides-and-tutorials/learn/spec/amp-boilerplate/?format=websites
// https://amp.dev/documentation/guides-and-tutorials/learn/spec/amphtml/?format=websites#the-amp-html-format
const (
boilerplateStart = `
`
boilerplateEnd = `
`
)
const (
// We restrict the amount of text may go inside an HTML element, in
// order to limit the amount a decoder may have to buffer.
elementSizeLimit = 32 * 1024
// The payload is conceptually a long base64-encoded string, but we
// break the string into short chunks separated by whitespace. This is
// to protect against modification by AMP caches, which reportedly may
// truncate long words in text:
// https://bugs.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/25985#note_2592348
bytesPerChunk = 32
// We set the number of chunks per element so as to stay under
// elementSizeLimit. Here, we assume that there is 1 byte of whitespace
// after each chunk (with an additional whitespace byte at the beginning
// of the element).
chunksPerElement = (elementSizeLimit - 1) / (bytesPerChunk + 1)
)
// The AMP armor encoder is a chain of a base64 encoder (base64.NewEncoder) and
// an HTML element encoder (elementEncoder). A top-level encoder (armorEncoder)
// coordinates these two, and handles prepending and appending the AMP
// boilerplate. armorEncoder's Write method writes data into the base64 encoder,
// where it makes its way through the chain.
// NewArmorEncoder returns a new AMP armor encoder. Anything written to the
// returned io.WriteCloser will be encoded and written to w. The caller must
// call Close to flush any partially written data and output the AMP boilerplate
// trailer.
func NewArmorEncoder(w io.Writer) (io.WriteCloser, error) {
// Immediately write the AMP boilerplate header.
_, err := w.Write([]byte(boilerplateStart))
if err != nil {
return nil, err
}
element := &elementEncoder{w: w}
// Write a server–client protocol version indicator, outside the base64
// layer.
_, err = element.Write([]byte{'0'})
if err != nil {
return nil, err
}
base64 := base64.NewEncoder(base64.StdEncoding, element)
return &armorEncoder{
w: w,
element: element,
base64: base64,
}, nil
}
type armorEncoder struct {
base64 io.WriteCloser
element *elementEncoder
w io.Writer
}
func (enc *armorEncoder) Write(p []byte) (int, error) {
// Write into the chain base64 | element | w.
return enc.base64.Write(p)
}
func (enc *armorEncoder) Close() error {
// Close the base64 encoder first, to flush out any buffered data and
// the final padding.
err := enc.base64.Close()
if err != nil {
return err
}
// Next, close the element encoder, to close any open elements.
err = enc.element.Close()
if err != nil {
return err
}
// Finally, output the AMP boilerplate trailer.
_, err = enc.w.Write([]byte(boilerplateEnd))
if err != nil {
return err
}
return nil
}
// elementEncoder arranges written data into pre elements, with the text within
// separated into chunks. It does no HTML encoding, so data written must not
// contain any bytes that are meaningful in HTML.
type elementEncoder struct {
w io.Writer
chunkCounter int
elementCounter int
}
func (enc *elementEncoder) Write(p []byte) (n int, err error) {
total := 0
for len(p) > 0 {
if enc.elementCounter == 0 && enc.chunkCounter == 0 {
_, err := enc.w.Write([]byte("
\n"))
if err != nil {
return total, err
}
}
n := bytesPerChunk - enc.chunkCounter
if n > len(p) {
n = len(p)
}
nn, err := enc.w.Write(p[:n])
if err != nil {
return total, err
}
total += nn
p = p[n:]
enc.chunkCounter += n
if enc.chunkCounter >= bytesPerChunk {
enc.chunkCounter = 0
enc.elementCounter += 1
nn, err := enc.w.Write([]byte("\n"))
if err != nil {
return total, err
}
total += nn
}
if enc.elementCounter >= chunksPerElement {
enc.elementCounter = 0
nn, err := enc.w.Write([]byte("
\n"))
if err != nil {
return total, err
}
total += nn
}
}
return total, nil
}
func (enc *elementEncoder) Close() error {
var err error
if !(enc.elementCounter == 0 && enc.chunkCounter == 0) {
if enc.chunkCounter == 0 {
_, err = enc.w.Write([]byte("\n"))
} else {
_, err = enc.w.Write([]byte("\n\n"))
}
}
return err
}