// Package oidc implements OpenID Connect client logic for the golang.org/x/oauth2 package.
package oidc

import (
	"context"
	"crypto/sha256"
	"crypto/sha512"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"hash"
	"io/ioutil"
	"mime"
	"net/http"
	"strings"
	"time"

	"golang.org/x/oauth2"
	jose "gopkg.in/square/go-jose.v2"
)

const (
	// ScopeOpenID is the mandatory scope for all OpenID Connect OAuth2 requests.
	ScopeOpenID = "openid"

	// ScopeOfflineAccess is an optional scope defined by OpenID Connect for requesting
	// OAuth2 refresh tokens.
	//
	// Support for this scope differs between OpenID Connect providers. For instance
	// Google rejects it, favoring appending "access_type=offline" as part of the
	// authorization request instead.
	//
	// See: https://openid.net/specs/openid-connect-core-1_0.html#OfflineAccess
	ScopeOfflineAccess = "offline_access"
)

var (
	errNoAtHash      = errors.New("id token did not have an access token hash")
	errInvalidAtHash = errors.New("access token hash does not match value in ID token")
)

// ClientContext returns a new Context that carries the provided HTTP client.
//
// This method sets the same context key used by the golang.org/x/oauth2 package,
// so the returned context works for that package too.
//
//    myClient := &http.Client{}
//    ctx := oidc.ClientContext(parentContext, myClient)
//
//    // This will use the custom client
//    provider, err := oidc.NewProvider(ctx, "https://accounts.example.com")
//
func ClientContext(ctx context.Context, client *http.Client) context.Context {
	return context.WithValue(ctx, oauth2.HTTPClient, client)
}

func doRequest(ctx context.Context, req *http.Request) (*http.Response, error) {
	client := http.DefaultClient
	if c, ok := ctx.Value(oauth2.HTTPClient).(*http.Client); ok {
		client = c
	}
	return client.Do(req.WithContext(ctx))
}

// Provider represents an OpenID Connect server's configuration.
type Provider struct {
	issuer      string
	authURL     string
	tokenURL    string
	userInfoURL string

	// Raw claims returned by the server.
	rawClaims []byte

	remoteKeySet KeySet
}

type cachedKeys struct {
	keys   []jose.JSONWebKey
	expiry time.Time
}

type providerJSON struct {
	Issuer      string `json:"issuer"`
	AuthURL     string `json:"authorization_endpoint"`
	TokenURL    string `json:"token_endpoint"`
	JWKSURL     string `json:"jwks_uri"`
	UserInfoURL string `json:"userinfo_endpoint"`
}

// NewProvider uses the OpenID Connect discovery mechanism to construct a Provider.
//
// The issuer is the URL identifier for the service. For example: "https://accounts.google.com"
// or "https://login.salesforce.com".
func NewProvider(ctx context.Context, issuer string) (*Provider, error) {
	wellKnown := strings.TrimSuffix(issuer, "/") + "/.well-known/openid-configuration"
	req, err := http.NewRequest("GET", wellKnown, nil)
	if err != nil {
		return nil, err
	}
	resp, err := doRequest(ctx, req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()

	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return nil, fmt.Errorf("unable to read response body: %v", err)
	}

	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("%s: %s", resp.Status, body)
	}

	var p providerJSON
	err = unmarshalResp(resp, body, &p)
	if err != nil {
		return nil, fmt.Errorf("oidc: failed to decode provider discovery object: %v", err)
	}

	if p.Issuer != issuer {
		return nil, fmt.Errorf("oidc: issuer did not match the issuer returned by provider, expected %q got %q", issuer, p.Issuer)
	}
	return &Provider{
		issuer:       p.Issuer,
		authURL:      p.AuthURL,
		tokenURL:     p.TokenURL,
		userInfoURL:  p.UserInfoURL,
		rawClaims:    body,
		remoteKeySet: NewRemoteKeySet(ctx, p.JWKSURL),
	}, nil
}

// Claims unmarshals raw fields returned by the server during discovery.
//
//    var claims struct {
//        ScopesSupported []string `json:"scopes_supported"`
//        ClaimsSupported []string `json:"claims_supported"`
//    }
//
//    if err := provider.Claims(&claims); err != nil {
//        // handle unmarshaling error
//    }
//
// For a list of fields defined by the OpenID Connect spec see:
// https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderMetadata
func (p *Provider) Claims(v interface{}) error {
	if p.rawClaims == nil {
		return errors.New("oidc: claims not set")
	}
	return json.Unmarshal(p.rawClaims, v)
}

// Endpoint returns the OAuth2 auth and token endpoints for the given provider.
func (p *Provider) Endpoint() oauth2.Endpoint {
	return oauth2.Endpoint{AuthURL: p.authURL, TokenURL: p.tokenURL}
}

// UserInfo represents the OpenID Connect userinfo claims.
type UserInfo struct {
	Subject       string `json:"sub"`
	Profile       string `json:"profile"`
	Email         string `json:"email"`
	EmailVerified bool   `json:"email_verified"`

	claims []byte
}

// Claims unmarshals the raw JSON object claims into the provided object.
func (u *UserInfo) Claims(v interface{}) error {
	if u.claims == nil {
		return errors.New("oidc: claims not set")
	}
	return json.Unmarshal(u.claims, v)
}

// UserInfo uses the token source to query the provider's user info endpoint.
func (p *Provider) UserInfo(ctx context.Context, tokenSource oauth2.TokenSource) (*UserInfo, error) {
	if p.userInfoURL == "" {
		return nil, errors.New("oidc: user info endpoint is not supported by this provider")
	}

	req, err := http.NewRequest("GET", p.userInfoURL, nil)
	if err != nil {
		return nil, fmt.Errorf("oidc: create GET request: %v", err)
	}

	token, err := tokenSource.Token()
	if err != nil {
		return nil, fmt.Errorf("oidc: get access token: %v", err)
	}
	token.SetAuthHeader(req)

	resp, err := doRequest(ctx, req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return nil, err
	}
	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("%s: %s", resp.Status, body)
	}

	var userInfo UserInfo
	if err := json.Unmarshal(body, &userInfo); err != nil {
		return nil, fmt.Errorf("oidc: failed to decode userinfo: %v", err)
	}
	userInfo.claims = body
	return &userInfo, nil
}

// IDToken is an OpenID Connect extension that provides a predictable representation
// of an authorization event.
//
// The ID Token only holds fields OpenID Connect requires. To access additional
// claims returned by the server, use the Claims method.
type IDToken struct {
	// The URL of the server which issued this token. OpenID Connect
	// requires this value always be identical to the URL used for
	// initial discovery.
	//
	// Note: Because of a known issue with Google Accounts' implementation
	// this value may differ when using Google.
	//
	// See: https://developers.google.com/identity/protocols/OpenIDConnect#obtainuserinfo
	Issuer string

	// The client ID, or set of client IDs, that this token is issued for. For
	// common uses, this is the client that initialized the auth flow.
	//
	// This package ensures the audience contains an expected value.
	Audience []string

	// A unique string which identifies the end user.
	Subject string

	// Expiry of the token. Ths package will not process tokens that have
	// expired unless that validation is explicitly turned off.
	Expiry time.Time
	// When the token was issued by the provider.
	IssuedAt time.Time

	// Initial nonce provided during the authentication redirect.
	//
	// This package does NOT provided verification on the value of this field
	// and it's the user's responsibility to ensure it contains a valid value.
	Nonce string

	// at_hash claim, if set in the ID token. Callers can verify an access token
	// that corresponds to the ID token using the VerifyAccessToken method.
	AccessTokenHash string

	// signature algorithm used for ID token, needed to compute a verification hash of an
	// access token
	sigAlgorithm string

	// Raw payload of the id_token.
	claims []byte
}

// Claims unmarshals the raw JSON payload of the ID Token into a provided struct.
//
//		idToken, err := idTokenVerifier.Verify(rawIDToken)
//		if err != nil {
//			// handle error
//		}
//		var claims struct {
//			Email         string `json:"email"`
//			EmailVerified bool   `json:"email_verified"`
//		}
//		if err := idToken.Claims(&claims); err != nil {
//			// handle error
//		}
//
func (i *IDToken) Claims(v interface{}) error {
	if i.claims == nil {
		return errors.New("oidc: claims not set")
	}
	return json.Unmarshal(i.claims, v)
}

// VerifyAccessToken verifies that the hash of the access token that corresponds to the iD token
// matches the hash in the id token. It returns an error if the hashes  don't match.
// It is the caller's responsibility to ensure that the optional access token hash is present for the ID token
// before calling this method. See https://openid.net/specs/openid-connect-core-1_0.html#CodeIDToken
func (i *IDToken) VerifyAccessToken(accessToken string) error {
	if i.AccessTokenHash == "" {
		return errNoAtHash
	}
	var h hash.Hash
	switch i.sigAlgorithm {
	case RS256, ES256, PS256:
		h = sha256.New()
	case RS384, ES384, PS384:
		h = sha512.New384()
	case RS512, ES512, PS512:
		h = sha512.New()
	default:
		return fmt.Errorf("oidc: unsupported signing algorithm %q", i.sigAlgorithm)
	}
	h.Write([]byte(accessToken)) // hash documents that Write will never return an error
	sum := h.Sum(nil)[:h.Size()/2]
	actual := base64.RawURLEncoding.EncodeToString(sum)
	if actual != i.AccessTokenHash {
		return errInvalidAtHash
	}
	return nil
}

type idToken struct {
	Issuer   string   `json:"iss"`
	Subject  string   `json:"sub"`
	Audience audience `json:"aud"`
	Expiry   jsonTime `json:"exp"`
	IssuedAt jsonTime `json:"iat"`
	Nonce    string   `json:"nonce"`
	AtHash   string   `json:"at_hash"`
}

type audience []string

func (a *audience) UnmarshalJSON(b []byte) error {
	var s string
	if json.Unmarshal(b, &s) == nil {
		*a = audience{s}
		return nil
	}
	var auds []string
	if err := json.Unmarshal(b, &auds); err != nil {
		return err
	}
	*a = audience(auds)
	return nil
}

type jsonTime time.Time

func (j *jsonTime) UnmarshalJSON(b []byte) error {
	var n json.Number
	if err := json.Unmarshal(b, &n); err != nil {
		return err
	}
	var unix int64

	if t, err := n.Int64(); err == nil {
		unix = t
	} else {
		f, err := n.Float64()
		if err != nil {
			return err
		}
		unix = int64(f)
	}
	*j = jsonTime(time.Unix(unix, 0))
	return nil
}

func unmarshalResp(r *http.Response, body []byte, v interface{}) error {
	err := json.Unmarshal(body, &v)
	if err == nil {
		return nil
	}
	ct := r.Header.Get("Content-Type")
	mediaType, _, parseErr := mime.ParseMediaType(ct)
	if parseErr == nil && mediaType == "application/json" {
		return fmt.Errorf("got Content-Type = application/json, but could not unmarshal as JSON: %v", err)
	}
	return fmt.Errorf("expected Content-Type = application/json, got %q: %v", ct, err)
}
