package authn

import (
	"context"
	"fmt"
	"log"
	"net/http"
	"strings"

	"sourcecraft.dev/bigbes/sr-ht-spec/core"
)

// Resolver turns a request into a Principal. It holds the instance owner
// username — the one name a cookie has to match to carry authority — and, when
// the instance is configured for it, the tokens.sr.ht plane every agent
// credential is checked against.
type Resolver struct {
	owner string

	// bearer and users are the agent plane, installed by WithInstancePlane. Both
	// are nil when the instance config has no [tokens.sr.ht] section, and a
	// resolver in that state authenticates no agent at all: since spec stopped
	// minting its own credential there is nothing else for a bearer token to be
	// checked against. It is still a legal resolver — the CLI paths that
	// authenticate nobody build one — but a bearer credential presented to it is
	// a hard ErrNoAgentPlane, never a shrug.
	bearer BearerValidator
	users  UserLookup
}

// ResolverOption configures a Resolver at construction. Options rather than a
// second constructor because the plane is genuinely absent in some processes:
// `specsrht doc` builds a Service, resolves nobody, and has no use for an HTTP
// client to tokens.sr.ht.
type ResolverOption func(*Resolver) error

// WithInstancePlane wires the tokens.sr.ht bearer plane in: v validates a
// presented working token, users resolves its owner to a local row.
//
// Both are required together. A validator with no way to resolve an owner would
// authenticate a token and then have nothing to say about who presented it,
// which is the whole of what an agent credential is for here.
func WithInstancePlane(v BearerValidator, users UserLookup) ResolverOption {
	return func(rs *Resolver) error {
		if v == nil {
			return fmt.Errorf("authn: nil BearerValidator")
		}
		if users == nil {
			return fmt.Errorf("authn: nil UserLookup")
		}
		rs.bearer = v
		rs.users = users
		return nil
	}
}

// NewResolver builds a Resolver for the instance owner named in
// [sr.ht] owner-name.
//
// Pass WithInstancePlane to give it an agent plane. Without one it resolves
// cookies and refuses every bearer credential with ErrNoAgentPlane; the daemon
// therefore builds one with the plane and fails startup if it cannot, while the
// CLI paths that authenticate nobody build one without.
func NewResolver(owner string, opts ...ResolverOption) (*Resolver, error) {
	owner = strings.TrimPrefix(owner, "~")
	if err := core.ValidateOwner(owner); err != nil {
		return nil, fmt.Errorf("authn: instance owner: %w", err)
	}
	rs := &Resolver{owner: owner}
	for _, opt := range opts {
		if err := opt(rs); err != nil {
			return nil, err
		}
	}
	return rs, nil
}

// HasInstancePlane reports whether this resolver can authenticate an agent at
// all. Startup logging and tests only; never an authorization input.
func (rs *Resolver) HasInstancePlane() bool { return rs.bearer != nil }

// Owner returns the instance owner username this resolver recognises.
func (rs *Resolver) Owner() string { return rs.owner }

// Resolve determines who is making a request.
//
// A bearer token wins over a cookie when both are present: an agent that went
// to the trouble of presenting a credential is asking to be treated as an
// agent, and letting a stale browser cookie promote it to the owner would hand
// it the approved branch. The two credentials are checked in that order and
// never merged.
//
// A presented bearer token goes to the tokens.sr.ht plane and nowhere else.
// There is no second store behind it since spec stopped minting its own
// credential, so every refusal that plane returns is final — see
// resolveInstanceToken.
//
// The error contract is asymmetric on purpose:
//
//   - No bearer token: never an error. The cookie decides between KindOwner and
//     KindAnonymous, and any cookie problem is anonymity, not failure.
//   - A bearer token that fails: an error. StatusFor separates the 401 case
//     (malformed, foreign, revoked) from the 403 case (a good token this
//     instance has nothing to grant) and the 503 case (tokens.sr.ht
//     unreachable, or no plane wired at all).
//
// The agent identity and session headers are read here but not required: they
// are demanded at the write, by AgentWrite.Validate, which is the only place
// the design requires them and the only place a missing one can do harm.
func (rs *Resolver) Resolve(ctx context.Context, r *http.Request) (Principal, error) {
	if presented := BearerFromRequest(r); presented != "" {
		return rs.ResolveAgent(ctx, presented,
			r.Header.Get(HeaderAgent), r.Header.Get(HeaderAgentSession))
	}

	username := UsernameFromRequest(r)
	if username == "" {
		return Anonymous(), nil
	}
	if username != rs.owner {
		// A real user of the instance who is not bigbes. Single-user means
		// there is nothing to grant them, so they read exactly as an anonymous
		// viewer does; the name is kept for the log line and the "you are
		// signed in as" affordance only.
		return Principal{Kind: KindAnonymous, CookieUser: username}, nil
	}
	return Principal{Kind: KindOwner, Owner: username, CookieUser: username}, nil
}

// ResolveAgent authenticates a presented agent credential, with the provenance
// the caller collected alongside it, and is what Resolve calls once it has
// pulled all three out of an HTTP request.
//
// It is exported because the push path is not an HTTP request: a `git push`
// arrives over SSH and the credential reaches the daemon in a hook's
// environment, not in an Authorization header. That path used to check the
// agent_token table directly, which is precisely how it ended up unable to
// accept an instance token while the HTTP surfaces could. One credential plane
// deserves one implementation of "is this credential good?", so hooks calls this
// and the two surfaces cannot drift.
//
// It never returns an anonymous principal on failure: a presented credential
// that does not verify is an error, so the caller refuses at the door instead of
// silently downgrading an agent to a reader.
func (rs *Resolver) ResolveAgent(ctx context.Context, presented, agent, session string) (Principal, error) {
	if presented == "" {
		return Anonymous(), ErrNoToken
	}
	if rs.bearer == nil {
		// Not a bad credential: this process cannot check any credential. 503
		// via StatusFor, and the operator's clue is in the message rather than
		// in an agent's incident report about a token that "stopped working".
		return Anonymous(), fmt.Errorf(
			"%w: spec.sr.ht authenticates agents through tokens.sr.ht, and this instance's "+
				"config.ini has no [tokens.sr.ht] origin", ErrNoAgentPlane)
	}
	return rs.resolveInstanceToken(ctx, presented,
		strings.TrimSpace(agent), strings.TrimSpace(session))
}

// Middleware attaches the resolved Principal to the request context, where
// PrincipalFromContext reads it.
//
// It rejects only a failed bearer token — 401 for a bad credential, 403 for a
// good one that this instance has nothing to grant, 503 for a backend that could
// not answer, per StatusFor. Everything else, including every cookie problem,
// flows through as anonymous: the read plane is anonymous-capable and must never
// answer an error page on identity grounds.
func (rs *Resolver) Middleware() func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			p, err := rs.Resolve(r.Context(), r)
			if err != nil {
				status := StatusFor(err)
				if status >= 500 {
					// Fail closed and loudly. The alternative — degrading to
					// anonymous — would turn a Postgres blip or an unreachable
					// tokens.sr.ht into agents silently losing their write
					// access.
					log.Printf("authn: resolving bearer credential: %v", err)
				}
				http.Error(w, resolveFailureMessage(status), status)
				return
			}
			next.ServeHTTP(w, r.WithContext(WithPrincipal(r.Context(), p)))
		})
	}
}

// resolveFailureMessage is what a refused caller is told. It is keyed on the
// status and not on the error, so that nothing about which plane refused, whose
// token it was, or whether a row exists leaks to a caller holding a credential
// this service did not accept.
func resolveFailureMessage(status int) string {
	switch status {
	case http.StatusUnauthorized:
		return "invalid agent token"
	case http.StatusForbidden:
		return "this token does not authorize requests to spec.sr.ht"
	default:
		return "authentication backend unavailable"
	}
}