package reference

import (
	"regexp"
	"strings"
)

const (
	// alphanumeric defines the alphanumeric atom, typically a
	// component of names. This only allows lower case characters and digits.
	alphanumeric = `[a-z0-9]+`

	// separator defines the separators allowed to be embedded in name
	// components. This allows one period, one or two underscore and multiple
	// dashes. Repeated dashes and underscores are intentionally treated
	// differently. In order to support valid hostnames as name components,
	// supporting repeated dash was added. Additionally double underscore is
	// now allowed as a separator to loosen the restriction for previously
	// supported names.
	separator = `(?:[._]|__|[-]*)`

	// domainNameComponent restricts the registry domain component of a
	// repository name to start with a component as defined by DomainRegexp.
	domainNameComponent = `(?:[a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9-]*[a-zA-Z0-9])`

	// tag matches valid tag names. From docker/docker:graph/tags.go.
	tag = `[\w][\w.-]{0,127}`

	// digestPat matches well-formed digests, including algorithm (e.g. "sha256:<encoded>").
	//
	// TODO(thaJeztah): this should follow the same rules as https://pkg.go.dev/github.com/opencontainers/go-digest@v1.0.0#DigestRegexp
	// so that go-digest defines the canonical format. Note that the go-digest is
	// more relaxed:
	//   - it allows multiple algorithms (e.g. "sha256+b64:<encoded>") to allow
	//     future expansion of supported algorithms.
	//   - it allows the "<encoded>" value to use urlsafe base64 encoding as defined
	//     in [rfc4648, section 5].
	//
	// [rfc4648, section 5]: https://www.rfc-editor.org/rfc/rfc4648#section-5.
	digestPat = `[A-Za-z][A-Za-z0-9]*(?:[-_+.][A-Za-z][A-Za-z0-9]*)*[:][[:xdigit:]]{32,}`

	// identifier is the format for a content addressable identifier using sha256.
	// These identifiers are like digests without the algorithm, since sha256 is used.
	identifier = `([a-f0-9]{64})`

	// ipv6address are enclosed between square brackets and may be represented
	// in many ways, see rfc5952. Only IPv6 in compressed or uncompressed format
	// are allowed, IPv6 zone identifiers (rfc6874) or Special addresses such as
	// IPv4-Mapped are deliberately excluded.
	ipv6address = `\[(?:[a-fA-F0-9:]+)\]`
)

var (
	// domainName defines the structure of potential domain components
	// that may be part of image names. This is purposely a subset of what is
	// allowed by DNS to ensure backwards compatibility with Docker image
	// names. This includes IPv4 addresses on decimal format.
	domainName = expression(domainNameComponent, optional(repeated(literal(`.`), domainNameComponent)))

	// host defines the structure of potential domains based on the URI
	// Host subcomponent on rfc3986. It may be a subset of DNS domain name,
	// or an IPv4 address in decimal format, or an IPv6 address between square
	// brackets (excluding zone identifiers as defined by rfc6874 or special
	// addresses such as IPv4-Mapped).
	host = `(?:` + domainName + `|` + ipv6address + `)`

	// allowed by the URI Host subcomponent on rfc3986 to ensure backwards
	// compatibility with Docker image names.
	domain = expression(host, optional(literal(`:`), `[0-9]+`))

	// DomainRegexp matches hostname or IP-addresses, optionally including a port
	// number. It defines the structure of potential domain components that may be
	// part of image names. This is purposely a subset of what is allowed by DNS to
	// ensure backwards compatibility with Docker image names. It may be a subset of
	// DNS domain name, an IPv4 address in decimal format, or an IPv6 address between
	// square brackets (excluding zone identifiers as defined by [rfc6874] or special
	// addresses such as IPv4-Mapped).
	//
	// [rfc6874]: https://www.rfc-editor.org/rfc/rfc6874.
	DomainRegexp = regexp.MustCompile(domain)

	// TagRegexp matches valid tag names. From docker/docker:graph/tags.go.
	TagRegexp = regexp.MustCompile(tag)

	// anchoredTagRegexp matches valid tag names, anchored at the start and
	// end of the matched string.
	anchoredTagRegexp = regexp.MustCompile(anchored(tag))

	// DigestRegexp matches well-formed digests, including algorithm (e.g. "sha256:<encoded>").
	DigestRegexp = regexp.MustCompile(digestPat)

	// anchoredDigestRegexp matches valid digests, anchored at the start and
	// end of the matched string.
	anchoredDigestRegexp = regexp.MustCompile(anchored(digestPat))

	// pathComponent restricts registry path-components to start with at least
	// one letter or number, with following parts able to be separated by one
	// period, one or two underscore and multiple dashes.
	pathComponent = expression(alphanumeric, optional(repeated(separator, alphanumeric)))
	namePat       = expression(optional(domain, literal(`/`)), pathComponent, optional(repeated(literal(`/`), pathComponent)))

	// NameRegexp is the format for the name component of references, including
	// an optional domain and port, but without tag or digest suffix.
	NameRegexp = regexp.MustCompile(namePat)

	// anchoredNameRegexp is used to parse a name value, capturing the
	// domain and trailing components.
	anchoredNameRegexp = regexp.MustCompile(anchored(optional(capture(domain), literal(`/`)), capture(pathComponent, optional(repeated(literal(`/`), pathComponent)))))

	referencePat = anchored(capture(namePat), optional(literal(":"), capture(tag)), optional(literal("@"), capture(digestPat)))

	// ReferenceRegexp is the full supported format of a reference. The regexp
	// is anchored and has capturing groups for name, tag, and digest
	// components.
	ReferenceRegexp = regexp.MustCompile(referencePat)

	// IdentifierRegexp is the format for string identifier used as a
	// content addressable identifier using sha256. These identifiers
	// are like digests without the algorithm, since sha256 is used.
	IdentifierRegexp = regexp.MustCompile(identifier)

	// anchoredIdentifierRegexp is used to check or match an
	// identifier value, anchored at start and end of string.
	anchoredIdentifierRegexp = regexp.MustCompile(anchored(identifier))
)

// literal compiles s into a literal regular expression, escaping any regexp
// reserved characters.
func literal(s string) string {
	re := regexp.MustCompile(regexp.QuoteMeta(s))

	if _, complete := re.LiteralPrefix(); !complete {
		panic("must be a literal")
	}

	return re.String()
}

// expression defines a full expression, where each regular expression must
// follow the previous.
func expression(res ...string) string {
	return strings.Join(res, "")
}

// optional wraps the expression in a non-capturing group and makes the
// production optional.
func optional(res ...string) string {
	return `(?:` + strings.Join(res, "") + `)?`
}

// repeated wraps the regexp in a non-capturing group to get one or more
// matches.
func repeated(res ...string) string {
	return `(?:` + strings.Join(res, "") + `)+`
}

// capture wraps the expression in a capturing group.
func capture(res ...string) string {
	return `(` + strings.Join(res, "") + `)`
}

// anchored anchors the regular expression by adding start and end delimiters.
func anchored(res ...string) string {
	return `^` + strings.Join(res, "") + `$`
}