package provider

import (
	"bytes"
	"encoding/hex"
	"encoding/json"
	"fmt"

	datekeys "g.activething.com/go/DateKeys"
	"g.activething.com/go/DateKeys/codec"
	"g.activething.com/go/DateKeys/profile"
)

// Schema constants of the release object (spec v0.15, §47.1): the answer of
// the Release API, an entry of a Release Cache, and a release the caller
// gives from a file or from an archive.
const (
	ReleaseTypeTag       = "datekeys-release"
	ReleaseSchemaVersion = 1
)

// Limits of the release object (spec v0.15, §47.1). The object has no frame:
// a file or an input of more than MaxReleaseObjectSize bytes is rejected
// before it is decoded, with ErrNonCanonicalCBOR, and no valid encoding comes
// close to it. MaxSignatureLen is the longest compressed point of
// BLS12-381, one of G2; Quicknet signs with 48 bytes, a point of G1.
const (
	MaxReleaseObjectSize = 1024
	MaxSignatureLen      = 96
)

// MaxReleaseJSONSize bounds drand's JSON, which a reader accepts too as the
// input of the caller (spec v0.15, §47.1). It is the bound of a relay
// response in provider/drand.
const MaxReleaseJSONSize = 8 << 10

// releaseKeys is the number of keys of the release object, all required.
const releaseKeys = 5

// releaseWire is the CBOR map of the release object, keys 2 to 4; keys 0 and
// 1 are the constants ReleaseTypeTag and ReleaseSchemaVersion.
type releaseWire struct {
	ChainHash []byte // key 2, 32 bytes
	Round     uint64 // key 3, 1..2^53-1
	Signature []byte // key 4, 1..MaxSignatureLen bytes
}

func (w *releaseWire) encode(e *codec.Encoder) {
	e.Map(releaseKeys)
	e.Uint(0)
	e.Text(ReleaseTypeTag)
	e.Uint(1)
	e.Uint(ReleaseSchemaVersion)
	e.Uint(2)
	e.Bstr(w.ChainHash)
	e.Uint(3)
	e.Uint(w.Round)
	e.Uint(4)
	e.Bstr(w.Signature)
}

// decode reads the map with every CDDL rule of the release object, all of
// them ErrNonCanonicalCBOR: what each field means against the pinned profile
// and the DateKey is checked by Verify, at step 10.
func (w *releaseWire) decode(d *codec.Decoder) error {
	pairs, err := d.Map(releaseKeys)
	if err != nil {
		return err
	}
	if pairs != releaseKeys {
		return fmt.Errorf("%d keys, want all %d: %w", pairs, releaseKeys, datekeys.ErrNonCanonicalCBOR)
	}
	for want := range uint64(releaseKeys) {
		k, err := d.Key()
		if err != nil {
			return err
		}
		if k != want {
			return fmt.Errorf("key %d where key %d was expected: %w", k, want, datekeys.ErrNonCanonicalCBOR)
		}
		switch k {
		case 0:
			_, err = d.Text(len(ReleaseTypeTag))
		case 1:
			_, err = d.Uint(ReleaseSchemaVersion)
		case 2:
			w.ChainHash, err = d.Bstr(32, 32)
		case 3:
			if w.Round, err = d.Uint(codec.MaxSafeUint); err == nil && w.Round == 0 {
				err = fmt.Errorf("round 0: %w", datekeys.ErrNonCanonicalCBOR)
			}
		case 4:
			w.Signature, err = d.Bstr(1, MaxSignatureLen)
		}
		if err != nil {
			return fmt.Errorf("key %d: %w", k, err)
		}
	}
	return d.EndMap()
}

// EncodeRelease returns the release object of r (spec v0.15, §47.1): its
// chain hash, its round and its signature. It does
// not verify the release: Verify does, against the pinned profile.
func EncodeRelease(r Release) ([]byte, error) {
	switch {
	case len(r.ChainHash) != 32:
		return nil, fmt.Errorf("provider: release object: chain hash of %d bytes, want 32: %w", len(r.ChainHash), datekeys.ErrNonCanonicalCBOR)
	case r.Round == 0 || r.Round > codec.MaxSafeUint:
		return nil, fmt.Errorf("provider: release object: round %d outside 1..%d: %w", r.Round, uint64(codec.MaxSafeUint), datekeys.ErrNonCanonicalCBOR)
	case len(r.Signature) == 0 || len(r.Signature) > MaxSignatureLen:
		return nil, fmt.Errorf("provider: release object: signature of %d bytes outside 1..%d: %w", len(r.Signature), MaxSignatureLen, datekeys.ErrNonCanonicalCBOR)
	}
	w := releaseWire{ChainHash: r.ChainHash, Round: r.Round, Signature: r.Signature}
	var e codec.Encoder
	w.encode(&e)
	return e.Out()
}

// DecodeRelease decodes a release object (spec v0.15, §47.1) with the layers
// of spec §69.1 that it has: its size, at most MaxReleaseObjectSize bytes;
// its type and schema version (ErrNonCanonicalCBOR, then
// ErrUnsupportedVersion); its encoding and schema (ErrNonCanonicalCBOR). The
// release it returns names its chain, and Verify checks it against the pinned
// profile at step 10 of spec §63: the chain hash, the round, the signature.
func DecodeRelease(b []byte) (Release, error) {
	if len(b) == 0 || len(b) > MaxReleaseObjectSize {
		return Release{}, fmt.Errorf("provider: release object of %d bytes, outside 1..%d: %w", len(b), MaxReleaseObjectSize, datekeys.ErrNonCanonicalCBOR)
	}
	if err := codec.CheckSchema(b, ReleaseTypeTag, ReleaseSchemaVersion); err != nil {
		return Release{}, fmt.Errorf("provider: release object: %w", err)
	}
	var w releaseWire
	if err := codec.Unmarshal(b, w.decode, w.encode); err != nil {
		return Release{}, fmt.Errorf("provider: release object: %w", err)
	}
	return Release{Round: w.Round, Signature: w.Signature, ChainHash: w.ChainHash}, nil
}

// ParseRelease reads a release that the caller supplies: drand's JSON, when
// its first byte other than a JSON space is "{", or else a release object
// (spec v0.15, §47.1), with DecodeRelease. drand's JSON is the answer of a
// relay, {"round": …, "signature": "…"}, with an optional "randomness" that
// must be SHA-256 of the signature; it does not name its chain, so the
// release has no chain hash, and any failure to read it is
// ErrReleaseInvalid. It is accepted as input, never written.
func ParseRelease(b []byte) (Release, error) {
	if t := bytes.TrimLeft(b, " \t\r\n"); len(t) > 0 && t[0] == '{' {
		return parseDrandJSON(b)
	}
	return DecodeRelease(b)
}

// parseDrandJSON reads the JSON of a drand relay.
func parseDrandJSON(b []byte) (Release, error) {
	if len(b) > MaxReleaseJSONSize {
		return Release{}, fmt.Errorf("provider: drand JSON of %d bytes, larger than %d: %w", len(b), MaxReleaseJSONSize, datekeys.ErrReleaseInvalid)
	}
	var wire struct {
		Round      *uint64 `json:"round"`
		Signature  *string `json:"signature"`
		Randomness string  `json:"randomness"`
	}
	if err := json.Unmarshal(b, &wire); err != nil || wire.Round == nil || wire.Signature == nil {
		return Release{}, fmt.Errorf("provider: drand JSON: malformed, or without round or signature: %w", datekeys.ErrReleaseInvalid)
	}
	sig, err := hex.DecodeString(*wire.Signature)
	if err != nil {
		return Release{}, fmt.Errorf("provider: drand JSON: signature is not hex: %w", datekeys.ErrReleaseInvalid)
	}
	if wire.Randomness != "" && !randomnessMatches(wire.Randomness, sig) {
		return Release{}, fmt.Errorf("provider: drand JSON: randomness does not match the signature: %w", datekeys.ErrReleaseInvalid)
	}
	return Release{Round: *wire.Round, Signature: sig}, nil
}

// Supplier hands over a release that the caller has in hand (spec v0.15,
// §49, §63 step 9.c): a release object read from a file, drand's JSON that
// the person saved, or an entry of a local archive. It makes no network
// request, so capsule.Open asks it for the release without comparing its
// clock with the round time: a valid signature proves that the round was
// published.
//
// Supply returns the encoding of the release of c, as it is: a release
// object or drand's JSON, which capsule.Open decodes and verifies at step 10
// with the codes of that step. Without a release for c it returns an error
// that wraps datekeys.ErrReleaseUnavailable, the code of step 9.
type Supplier interface {
	Supply(p *profile.Profile, c Condition) ([]byte, error)
}

// Encoded is a release in hand, already read: the bytes of a release object
// or of drand's JSON. It supplies itself whatever the condition; step 10 compares
// its round with the DateKey.
type Encoded []byte

// Supply implements Supplier.
func (e Encoded) Supply(*profile.Profile, Condition) ([]byte, error) { return e, nil }

// NewReleaseObject returns the release object of a release of the profile p,
// with the chain hash of p: what a Release Cache or an archive stores, or
// the Release API serves, after verifying the release (spec v0.15, §45,
// §47, §47.1).
func NewReleaseObject(p *profile.Profile, r Release) ([]byte, error) {
	r.ChainHash = p.ChainHash[:]
	return EncodeRelease(r)
}

// chainHashHex is the chain hash of a release in the text of an error.
func chainHashHex(b []byte) string { return hex.EncodeToString(b) }