// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package tka (WIP) implements the Tailnet Key Authority.
package tka

import (
	"bytes"
	"crypto/ed25519"
	"errors"
	"fmt"
	"os"
	"sort"
)

// Authority is a Tailnet Key Authority. This type is the main coupling
// point to the rest of the tailscale client.
//
// Authority objects can either be created from an existing, non-empty
// tailchonk (via tka.Open()), or created from scratch using tka.Bootstrap()
// or tka.Create().
type Authority struct {
	head           AUM
	oldestAncestor AUM
	state          State

	storage Chonk
}

// A chain describes a linear sequence of updates from Oldest to Head,
// resulting in some State at Head.
type chain struct {
	Oldest AUM
	Head   AUM

	state State

	// Set to true if the AUM chain intersects with the active
	// chain from a previous run.
	chainsThroughActive bool
}

// computeChainCandidates returns all possible chains based on AUMs stored
// in the given tailchonk. A chain is defined as a unique (oldest, newest)
// AUM tuple. chain.state is not yet populated in returned chains.
//
// If lastKnownOldest is provided, any chain that includes the given AUM
// has the chainsThroughActive field set to true. This bit is leveraged
// in computeActiveAncestor() to filter out irrelevant chains when determining
// the active ancestor from a list of distinct chains.
func computeChainCandidates(storage Chonk, lastKnownOldest *AUMHash, maxIter int) ([]chain, error) {
	heads, err := storage.Heads()
	if err != nil {
		return nil, fmt.Errorf("reading heads: %v", err)
	}
	candidates := make([]chain, len(heads))
	for i := range heads {
		// Oldest is iteratively computed below.
		candidates[i] = chain{Oldest: heads[i], Head: heads[i]}
	}
	// Not strictly necessary, but simplifies checks in tests.
	sort.Slice(candidates, func(i, j int) bool {
		ih, jh := candidates[i].Oldest.Hash(), candidates[j].Oldest.Hash()
		return bytes.Compare(ih[:], jh[:]) < 0
	})

	// candidates.Oldest needs to be computed by working backwards from
	// head as far as we can.
	iterAgain := true // if theres still work to be done.
	for i := 0; iterAgain; i++ {
		if i >= maxIter {
			return nil, fmt.Errorf("iteration limit exceeded (%d)", maxIter)
		}

		iterAgain = false
		for j := range candidates {
			parent, hasParent := candidates[j].Oldest.Parent()
			if hasParent {
				parent, err := storage.AUM(parent)
				if err != nil {
					if err == os.ErrNotExist {
						continue
					}
					return nil, fmt.Errorf("reading parent: %v", err)
				}
				candidates[j].Oldest = parent
				if lastKnownOldest != nil && *lastKnownOldest == parent.Hash() {
					candidates[j].chainsThroughActive = true
				}
				iterAgain = true
			}
		}
	}
	return candidates, nil
}

// pickNextAUM returns the AUM which should be used as the next
// AUM in the chain, possibly applying fork resolution logic.
//
// In other words: given an AUM with 3 children like this:
//    / - 1
//  P   - 2
//    \ - 3
//
// pickNextAUM will determine and return the correct branch.
//
// This method takes ownership of the provided slice.
func pickNextAUM(state State, candidates []AUM) AUM {
	switch len(candidates) {
	case 0:
		panic("pickNextAUM called with empty candidate set")
	case 1:
		return candidates[0]
	}

	// Oooof, we have some forks in the chain. We need to pick which
	// one to use by applying the Fork Resolution Algorithm ✨
	//
	// The rules are this:
	// 1. The child with the highest signature weight is chosen.
	// 2. If equal, the child which is a RemoveKey AUM is chosen.
	// 3. If equal, the child with the lowest AUM hash is chosen.
	sort.Slice(candidates, func(j, i int) bool {
		// Rule 1.
		iSigWeight, jSigWeight := candidates[i].Weight(state), candidates[j].Weight(state)
		if iSigWeight != jSigWeight {
			return iSigWeight < jSigWeight
		}

		// Rule 2.
		if iKind, jKind := candidates[i].MessageKind, candidates[j].MessageKind; iKind != jKind &&
			(iKind == AUMRemoveKey || jKind == AUMRemoveKey) {
			return jKind == AUMRemoveKey
		}

		// Rule 3.
		iHash, jHash := candidates[i].Hash(), candidates[j].Hash()
		return bytes.Compare(iHash[:], jHash[:]) > 0
	})

	return candidates[0]
}

// advanceChain computes the next AUM to advance with based on all child
// AUMs, returning the chosen AUM & the state obtained by applying that
// AUM.
//
// The return value for next is nil if there are no children AUMs, hence
// the provided state is at head (up to date).
func advanceChain(state State, candidates []AUM) (next *AUM, out State, err error) {
	if len(candidates) == 0 {
		return nil, state, nil
	}

	aum := pickNextAUM(state, candidates)
	if state, err = state.applyVerifiedAUM(aum); err != nil {
		return nil, State{}, fmt.Errorf("advancing state: %v", err)
	}
	return &aum, state, nil
}

// fastForward iteratively advances the current state based on known AUMs until
// the given termination function returns true or there is no more progress possible.
//
// The last-processed AUM, and the state computed after applying the last AUM,
// are returned.
func fastForward(storage Chonk, maxIter int, startState State, done func(curAUM AUM, curState State) bool) (AUM, State, error) {
	if startState.LastAUMHash == nil {
		return AUM{}, State{}, errors.New("invalid initial state")
	}
	nextAUM, err := storage.AUM(*startState.LastAUMHash)
	if err != nil {
		return AUM{}, State{}, fmt.Errorf("reading next: %v", err)
	}

	curs := nextAUM
	state := startState
	for i := 0; i < maxIter; i++ {
		if done != nil && done(curs, state) {
			return curs, state, nil
		}

		children, err := storage.ChildAUMs(curs.Hash())
		if err != nil {
			return AUM{}, State{}, fmt.Errorf("getting children of %X: %v", curs.Hash(), err)
		}
		next, nextState, err := advanceChain(state, children)
		if err != nil {
			return AUM{}, State{}, fmt.Errorf("advance %X: %v", curs.Hash(), err)
		}
		if next == nil {
			// There were no more children, we are at 'head'.
			return curs, state, nil
		}
		curs = *next
		state = nextState
	}

	return AUM{}, State{}, fmt.Errorf("iteration limit exceeded (%d)", maxIter)
}

// computeStateAt returns the State at wantHash.
func computeStateAt(storage Chonk, maxIter int, wantHash AUMHash) (State, error) {
	// TODO(tom): This is going to get expensive for really long
	//            chains. We should make nodes emit a checkpoint every
	//            X updates or something.

	topAUM, err := storage.AUM(wantHash)
	if err != nil {
		return State{}, err
	}

	// Iterate backwards till we find a starting point to compute
	// the state from.
	//
	// Valid starting points are either a checkpoint AUM, or a
	// genesis AUM.
	curs := topAUM
	var state State
	for i := 0; true; i++ {
		if i > maxIter {
			return State{}, fmt.Errorf("iteration limit exceeded (%d)", maxIter)
		}

		// Checkpoints encapsulate the state at that point, dope.
		if curs.MessageKind == AUMCheckpoint {
			state = curs.State.cloneForUpdate(&curs)
			break
		}
		parent, hasParent := curs.Parent()
		if !hasParent {
			// This is a 'genesis' update: there are none before it, so
			// this AUM can be applied to the empty state to determine
			// the state at this AUM.
			//
			// It is only valid for NoOp, AddKey, and Checkpoint AUMs
			// to be a genesis update. Checkpoint was handled earlier.
			if mk := curs.MessageKind; mk == AUMNoOp || mk == AUMAddKey {
				var err error
				if state, err = (State{}).applyVerifiedAUM(curs); err != nil {
					return State{}, fmt.Errorf("applying genesis (%+v): %v", curs, err)
				}
				break
			}
			return State{}, fmt.Errorf("invalid genesis update: %+v", curs)
		}

		// If we got here, the current state is dependent on the previous.
		// Keep iterating backwards till thats not the case.
		if curs, err = storage.AUM(parent); err != nil {
			return State{}, fmt.Errorf("reading parent: %v", err)
		}
	}

	// We now know some starting point state. Iterate forward till we
	// are at the AUM we want state for.
	_, state, err = fastForward(storage, maxIter, state, func(curs AUM, _ State) bool {
		return curs.Hash() == wantHash
	})
	// fastForward only terminates before the done condition if it
	// doesnt have any later AUMs to process. This cant be the case
	// as we've already iterated through them above so they must exist,
	// but we check anyway to be super duper sure.
	if err == nil && *state.LastAUMHash != wantHash {
		panic("unexpected fastForward outcome")
	}
	return state, err
}

// computeActiveAncestor determines which ancestor AUM to use as the
// ancestor of the valid chain.
//
// If all the chains end up having the same ancestor, then thats the
// only possible ancestor, ezpz. However if there are multiple distinct
// ancestors, that means there are distinct chains, and we need some
// hint to choose what to use. For that, we rely on the chainsThroughActive
// bit, which signals to us that that ancestor was part of the
// chain in a previous run.
func computeActiveAncestor(storage Chonk, chains []chain) (AUMHash, error) {
	// Dedupe possible ancestors, tracking if they were part of
	// the active chain on a previous run.
	ancestors := make(map[AUMHash]bool, len(chains))
	for _, c := range chains {
		ancestors[c.Oldest.Hash()] = c.chainsThroughActive
	}

	if len(ancestors) == 1 {
		// There's only one. DOPE.
		for k, _ := range ancestors {
			return k, nil
		}
	}

	// Theres more than one, so we need to use the ancestor that was
	// part of the active chain in a previous iteration.
	// Note that there can only be one distinct ancestor that was
	// formerly part of the active chain, because AUMs can only have
	// one parent and would have converged to a common ancestor.
	for k, chainsThroughActive := range ancestors {
		if chainsThroughActive {
			return k, nil
		}
	}

	return AUMHash{}, errors.New("multiple distinct chains")
}

// computeActiveChain bootstraps the runtime state of the Authority when
// starting entirely off stored state.
//
// TODO(tom): Don't look at head states, just iterate forward from
// the ancestor.
//
// The algorithm is as follows:
// 1. Determine all possible 'head' (like in git) states.
// 2. Filter these possible chains based on whether the ancestor was
//    formerly (in a previous run) part of the chain.
// 3. Compute the state of the state machine at this ancestor. This is
//    needed for fast-forward, as each update operates on the state of
//    the update preceeding it.
// 4. Iteratively apply updates till we reach head ('fast forward').
func computeActiveChain(storage Chonk, lastKnownOldest *AUMHash, maxIter int) (chain, error) {
	chains, err := computeChainCandidates(storage, lastKnownOldest, maxIter)
	if err != nil {
		return chain{}, fmt.Errorf("computing candidates: %v", err)
	}

	// Find the right ancestor.
	oldestHash, err := computeActiveAncestor(storage, chains)
	if err != nil {
		return chain{}, fmt.Errorf("computing ancestor: %v", err)
	}
	ancestor, err := storage.AUM(oldestHash)
	if err != nil {
		return chain{}, err
	}

	// At this stage we know the ancestor AUM, so we have excluded distinct
	// chains but we might still have forks (so we don't know the head AUM).
	//
	// We iterate forward from the ancestor AUM, handling any forks as we go
	// till we arrive at a head.
	out := chain{Oldest: ancestor, Head: ancestor}
	if out.state, err = computeStateAt(storage, maxIter, oldestHash); err != nil {
		return chain{}, fmt.Errorf("bootstrapping state: %v", err)
	}
	out.Head, out.state, err = fastForward(storage, maxIter, out.state, nil)
	if err != nil {
		return chain{}, fmt.Errorf("fast forward: %v", err)
	}
	return out, nil
}

// aumVerify verifies if an AUM is well-formed, correctly signed, and
// can be accepted for storage.
func aumVerify(aum AUM, state State, isGenesisAUM bool) error {
	if err := aum.StaticValidate(); err != nil {
		return fmt.Errorf("invalid: %v", err)
	}
	if !isGenesisAUM {
		if err := checkParent(aum, state); err != nil {
			return err
		}
	}

	if len(aum.Signatures) == 0 {
		return errors.New("unsigned AUM")
	}
	sigHash := aum.SigHash()
	for i, sig := range aum.Signatures {
		key, err := state.GetKey(sig.KeyID)
		if err != nil {
			return fmt.Errorf("bad keyID on signature %d: %v", i, err)
		}
		if err := sig.Verify(sigHash, key); err != nil {
			return fmt.Errorf("signature %d: %v", i, err)
		}
	}
	return nil
}

func checkParent(aum AUM, state State) error {
	parent, hasParent := aum.Parent()
	if !hasParent {
		return errors.New("aum has no parent")
	}
	if state.LastAUMHash == nil {
		return errors.New("cannot check update parent hash against a state with no previous AUM")
	}
	if *state.LastAUMHash != parent {
		return fmt.Errorf("aum with parent %x cannot be applied to a state with parent %x", state.LastAUMHash, parent)
	}
	return nil
}

// Head returns the AUM digest of the latest update applied to the state
// machine.
func (a *Authority) Head() AUMHash {
	return *a.state.LastAUMHash
}

// Open initializes an existing TKA from the given tailchonk.
//
// Only use this if the current node has initialized an Authority before.
// If a TKA exists on other nodes but theres nothing locally, use Bootstrap().
// If no TKA exists anywhere and you are creating it for the first
// time, use New().
func Open(storage Chonk) (*Authority, error) {
	a, err := storage.LastActiveAncestor()
	if err != nil {
		return nil, fmt.Errorf("reading last ancestor: %v", err)
	}

	c, err := computeActiveChain(storage, a, 2000)
	if err != nil {
		return nil, fmt.Errorf("active chain: %v", err)
	}

	return &Authority{
		head:           c.Head,
		oldestAncestor: c.Oldest,
		storage:        storage,
		state:          c.state,
	}, nil
}

// Create initializes a brand-new TKA, generating a genesis update
// and committing it to the given storage.
//
// The given signer must also be present in state as a trusted key.
//
// Do not use this to initialize a TKA that already exists, use Open()
// or Bootstrap() instead.
func Create(storage Chonk, state State, signer ed25519.PrivateKey) (*Authority, AUM, error) {
	// Generate & sign a checkpoint, our genesis update.
	genesis := AUM{
		MessageKind: AUMCheckpoint,
		State:       &state,
	}
	if err := genesis.StaticValidate(); err != nil {
		// This serves as an easy way to validate the given state.
		return nil, AUM{}, fmt.Errorf("invalid state: %v", err)
	}
	genesis.sign25519(signer)

	a, err := Bootstrap(storage, genesis)
	return a, genesis, err
}

// Bootstrap initializes a TKA based on the given checkpoint.
//
// Call this when setting up a new nodes' TKA, but other nodes
// with initialized TKA's exist.
//
// Pass the returned genesis AUM from Create(), or a later checkpoint AUM.
//
// TODO(tom): We should test an authority bootstrapped from a later checkpoint
// works fine with sync and everything.
func Bootstrap(storage Chonk, bootstrap AUM) (*Authority, error) {
	heads, err := storage.Heads()
	if err != nil {
		return nil, fmt.Errorf("reading heads: %v", err)
	}
	if len(heads) != 0 {
		return nil, errors.New("tailchonk is not empty")
	}

	// Check the AUM is well-formed.
	if bootstrap.MessageKind != AUMCheckpoint {
		return nil, fmt.Errorf("bootstrap AUMs must be checkpoint messages, got %v", bootstrap.MessageKind)
	}
	if bootstrap.State == nil {
		return nil, errors.New("bootstrap AUM is missing state")
	}
	if err := aumVerify(bootstrap, *bootstrap.State, true); err != nil {
		return nil, fmt.Errorf("invalid bootstrap: %v", err)
	}

	// Everything looks good, write it to storage.
	if err := storage.CommitVerifiedAUMs([]AUM{bootstrap}); err != nil {
		return nil, fmt.Errorf("commit: %v", err)
	}
	if err := storage.SetLastActiveAncestor(bootstrap.Hash()); err != nil {
		return nil, fmt.Errorf("set ancestor: %v", err)
	}

	return Open(storage)
}

// Inform is called to tell the authority about new updates. Updates
// should be ordered oldest to newest. An error is returned if any
// of the updates could not be processed.
func (a *Authority) Inform(updates []AUM) error {
	stateAt := make(map[AUMHash]State, len(updates)+1)
	toCommit := make([]AUM, 0, len(updates))

	for i, update := range updates {
		hash := update.Hash()
		if _, err := a.storage.AUM(hash); err == nil {
			// Already have this AUM.
			continue
		}

		parent, hasParent := update.Parent()
		if !hasParent {
			return fmt.Errorf("update %d: missing parent", i)
		}

		state, hasState := stateAt[parent]
		var err error
		if !hasState {
			if state, err = computeStateAt(a.storage, 2000, parent); err != nil {
				return fmt.Errorf("update %d computing state: %v", i, err)
			}
			stateAt[parent] = state
		}

		if err := aumVerify(update, state, false); err != nil {
			return fmt.Errorf("update %d invalid: %v", i, err)
		}
		if stateAt[hash], err = state.applyVerifiedAUM(update); err != nil {
			return fmt.Errorf("update %d cannot be applied: %v", i, err)
		}
		toCommit = append(toCommit, update)
	}

	if err := a.storage.CommitVerifiedAUMs(toCommit); err != nil {
		return fmt.Errorf("commit: %v", err)
	}

	// TODO(tom): Theres no need to recompute the state from scratch
	//            in every case. We should detect when updates were
	//            a linear, non-forking series applied to head, and
	//            just use the last State we computed.
	oldestAncestor := a.oldestAncestor.Hash()
	c, err := computeActiveChain(a.storage, &oldestAncestor, 2000)
	if err != nil {
		return fmt.Errorf("recomputing active chain: %v", err)
	}
	a.head = c.Head
	a.oldestAncestor = c.Oldest
	a.state = c.state
	return nil
}