package resolution

import (
	"fmt"
	"sort"
	"strings"

	mcat "git.novox.be/novox/mesh-sdk/go/catalogue"
)

// heldKey is what one holder holds: the seat, and its kind on a kinded bench.
func heldKey(claim, kind string) string {
	if kind == "" {
		return canonicalSeat(claim)
	}
	return canonicalSeat(claim) + "/" + kind
}

// Refusal is why a set of assignments cannot become a declaration.
//
// Every reason at once rather than the first, and each says what to do about it. A person
// resolving these fixes them in one pass or in four.
type Refusal struct{ Problems []string }

func (r *Refusal) Error() string {
	return "these assignments cannot be applied:\n  - " + strings.Join(r.Problems, "\n  - ")
}

// Resolve works out everything a node runs, from what was assigned to it.
//
// The catalogue is every module the mesh knows about; assigned is what a person put on this node.
// What comes back is the closure — assigned modules plus everything they require — or a refusal
// naming every reason it could not be closed.
func Resolve(catalogue map[string]mcat.Manifest, assigned []string, node mcat.Node, world mcat.World) (mcat.Resolution, error) {
	elsewhere := world.Held
	var problems []string

	// Which names are answered from elsewhere in the mesh rather than from this machine. A
	// property of the name, not of each provider: two modules disagreeing about whether a
	// database is local would make the same requirement mean different things depending on which
	// one happened to answer it.
	brokered := map[string]bool{}
	local := map[string]bool{}
	// And which of them keep their consumers' data (novox/hq ADR 0232) — also a property of the name.
	keeps := map[string]bool{}
	for _, m := range catalogue {
		for _, o := range m.Provides {
			if o.At() == mcat.ScopeMesh {
				brokered[o.Name] = true
				if m.KeepsConsumerData(o.Name) {
					keeps[o.Name] = true
				}
				continue
			}
			local[o.Name] = true
		}
	}
	// Which names are usable only on their provider's own machine (novox/hq ADR 0208 §3). Also a
	// property of the name: a display one provider says is the machine's and another says is not
	// would be pulled in for one consumer and refused for the next.
	machineReach := map[string]bool{}
	plainLocal := map[string]bool{}
	for _, m := range catalogue {
		for _, o := range m.Provides {
			if o.MachineReach() {
				machineReach[o.Name] = true
			} else if o.At() == mcat.ScopeNode {
				plainLocal[o.Name] = true
			}
		}
	}
	for want := range machineReach {
		if plainLocal[want] {
			problems = append(problems, fmt.Sprintf(
				"the catalogue disagrees about %q: some modules provide it with the machine's reach and "+
					"others without, so a requirement for it would be met differently by each", want))
		}
	}
	for want := range brokered {
		if local[want] {
			problems = append(problems, fmt.Sprintf(
				"the catalogue disagrees about %q: some modules provide it here and others from "+
					"anywhere in the mesh, so the same requirement would mean two things", want))
		}
	}

	// What each name can be satisfied by. Built once from the whole catalogue, because "how many
	// modules provide this" is the question the whole rule turns on.
	offers := map[string][]string{}
	for _, m := range catalogue {
		for _, o := range m.Offers() {
			offers[o] = append(offers[o], m.Module)
		}
	}
	for k := range offers {
		sort.Strings(offers[k])
	}

	chosen := map[string]bool{}
	because := map[string]string{}
	var order []string
	var needs []mcat.Needed

	// What the set already offers, which is the first thing a requirement is checked against.
	//
	// Without this, assigning zsh does not satisfy something that requires a shell: the
	// requirement is counted against the catalogue, three modules provide it, and the answer is
	// still "choose one" after somebody has chosen one. That makes the remedy useless, and it is
	// how this read when first used.
	satisfied := map[string]bool{}
	// Which modules a person assigned here, hostable. The walk marks a module chosen only when it
	// reaches it, and a consumer may be reached before the provider beside it — so the provider of
	// something already satisfied is looked for among these as well as among the chosen.
	assignedHere := map[string]bool{}

	// Everything a person assigned goes in first, except what this machine cannot run. Those are
	// choices already made, and a requirement one of them answers is not a choice to put back to
	// anybody.
	//
	// **A module the machine cannot host is left out here rather than refusing the whole set.**
	// Assigning fail2ban to a machine whose profile reports no firewall is one module on the wrong
	// machine (novox/hq ADR 0009's sibling case): the person meant something, the remedy is theirs,
	// and it is a fact about this one module — not a reason the healthy modules beside it should
	// fail to resolve and converge. It is reported as un-applied on the resolution, so it is neither
	// silently dropped nor fatal to the rest. The distinction is who wanted it: a module *required*
	// by something running here that cannot be hosted makes the set itself incoherent, and that is
	// still refused, by checkCapabilities, where it stays in the closure.
	var unhostable []mcat.Unhostable
	var queue []string
	for _, a := range assigned {
		if m, known := catalogue[a]; known {
			if missing := missingCapabilities(m, node); len(missing) > 0 {
				unhostable = append(unhostable, mcat.Unhostable{Module: a, Missing: missing})
				continue
			}
			for _, o := range m.Offers() {
				satisfied[o] = true
			}
			assignedHere[a] = true
		}
		because[a] = "assigned"
		queue = append(queue, a)
	}

	// What has already been complained about. A requirement can be wanted by several modules at
	// once, and saying the same thing twice makes a person hunt for the difference between two
	// identical lines before realising there is none.
	reported := map[string]bool{}

	// **An optional requirement is settled last** (novox/hq ADR 0330). Whether a name is optional is
	// a question about every module that wants it, and a module pulled in later may want it hard; so
	// a name only optional modules want so far is put aside until the walk has nothing else, and only
	// then, if still nobody wants it hard, settled by the optional rule: bound to a provider that is
	// already assigned or reachable, exactly as a hard one, and otherwise absent — never refused,
	// and never answered by installing anything.
	var deferred []string
	settling := map[string]bool{}
	absent := map[string]bool{}
	wantedHard := func(want string) bool {
		for _, name := range order {
			if manifestWantsHard(catalogue[name], want) {
				return true
			}
		}
		return false
	}
	wantedOptionally := func(want string) bool {
		for _, name := range order {
			if catalogue[name].IsOptional(want) {
				return true
			}
		}
		return false
	}
	leaveAbsent := func(want string) {
		absent[want] = true
		reported[want] = true
	}

	for {
		if len(queue) == 0 {
			if len(deferred) == 0 {
				break
			}
			// What a module chosen since wants hard goes back to the walk, which may install for
			// it; only when none does is the rest settled, since settling installs nothing and so
			// can change no other name's answer.
			var hard, optional []string
			for _, d := range deferred {
				if wantedHard(d) {
					hard = append(hard, d)
				} else {
					optional = append(optional, d)
				}
			}
			if len(hard) > 0 {
				queue, deferred = hard, optional
				continue
			}
			for _, d := range optional {
				settling[d] = true
			}
			queue, deferred = optional, nil
		}
		want := queue[0]
		queue = queue[1:]
		if chosen[want] || reported[want] {
			continue
		}
		if !settling[want] && because[want] != "assigned" && !wantedHard(want) && wantedOptionally(want) {
			deferred = append(deferred, want)
			continue
		}
		// Walked as a hard requirement: a refusal names a module that wants it hard, never one that
		// wanted it only optionally and met it first — whose remedy it is not.
		if first, known := catalogue[because[want]]; !settling[want] && known && !manifestWantsHard(first, want) {
			for _, name := range order {
				if manifestWantsHard(catalogue[name], want) {
					because[want] = name
					break
				}
			}
		}
		// Already answered by something in the set. This is the case that makes assigning zsh do
		// what a person meant by it.
		//
		// **Except when the thing answering it grants a credential** (novox/hq 04-ISSUES/021). A
		// shell answered here needs nothing more; a database answered here still needs a
		// password, because the consumer reaches it over TCP from its own container and the
		// database asks exactly as it would from another machine. **The machine stops being a
		// trust boundary the moment both ends are containers**, and treating it as one gave the
		// commonest arrangement of all — a service and its database on one node — the weakest
		// handling, silently.
		if satisfied[want] && !isModule(catalogue, want) && !answeredElsewhere(want, node, world, brokered, keeps) {
			here := func(name string) bool { return chosen[name] || assignedHere[name] }
			local := providersHere(catalogue, here, want)
			// Which of them it matters to choose between. A plain capability — a shell, a display
			// server — asks nothing of whoever answers it, and three shells beside an editor are
			// not a choice to put to anybody. One that grants a credential, or serves a fact the
			// consumer cannot guess, becomes a binding, and a binding is to one provider.
			matter := local
			if !brokered[want] {
				matter = nil
				for _, name := range local {
					if _, ok := catalogue[name].Serves[want]; ok {
						matter = append(matter, name)
					}
				}
			}
			var by mcat.Manifest
			switch len(matter) {
			case 0:
				// Nothing to bind to; satisfied by its presence, as it was.
			case 1:
				by = catalogue[matter[0]]
			default:
				// Two modules on this machine answer it. Taking whichever a map walk met first
				// was the rule until novox/hq #258 — random, per plan — and the same stance as
				// across machines applies: ambiguity is refused, never resolved by picking.
				//
				// **Not in the first pass.** That pass exists only to answer *what does this node
				// offer*, and refusing there makes the machine vanish rather than report a problem
				// (the sibling case below says why): every other node then loses what this one
				// provides — the vault, the identity provider — and refuses for a fault that is
				// this node's to settle. The second pass refuses it properly, where it is asked.
				if world.Unchecked {
					continue
				}
				c, pinned := world.Pinned[want]
				if !pinned || c.Node != node.Name || !oneOf(matter, c.Module) {
					reported[want] = true
					problems = append(problems, fmt.Sprintf(
						"%s provides %q %d times, wanted by %s — say which with `pin %s %s %s <module>`: %s",
						node.Name, want, len(matter), because[want], node.Name, want, node.Name,
						strings.Join(matter, ", ")))
					continue
				}
				by = catalogue[c.Module]
			}
			if brokered[want] {
				// Answered here, and still a need: the provider is this node.
				//
				// **The same loopback fallback as the branch below, and it was missing here.** A
				// machine with no private network has no `at`, and this branch passed that through
				// — so a consumer whose file says `${bound:<provision>:at}:${bound:...:port}` was
				// handed `:5000`, a name with no host, written into its environment without
				// complaint. The sibling case a few lines down had the fallback and the reasoning
				// for it; only this one did not. A machine off the network still reaches itself.
				at := node.At
				if at == "" {
					at = "127.0.0.1"
				}
				needs = append(needs, mcat.Needed{
					Name: want, From: node.Name, At: at, Module: by.Module,
					Serves: servedByOne(by, want), For: because[want],
					SharedOwn: sharedByOne(by, want), CredentialForm: by.CredentialFormOf(want), CredentialTool: by.CredentialToolOf(want),
					Identity: by.IdentityBoundOf(want)})
			} else if served := servedByOne(by, want); len(served) > 0 {
				// Answered here with no credential to mint, but the provider serves facts the
				// consumer cannot guess — a port, a model name — and so still needs a binding.
				// **The reachability rule does not apply**: both ends are on this same machine, so
				// there is no private network to share and nothing to refuse (novox/hq ADR 0024's
				// sibling case — a same-node keyless endpoint that the consumer still cannot invent
				// the port for, exactly what declaration.go's here() was left to patch after the
				// fact). Delivering it here as a need is what lets knownFor see it, so a file that
				// says ${bound:<provision>:port} is filled rather than refused.
				//
				// The address is this machine's own name on the private network when it has one, and
				// loopback when it does not — a machine off the network still reaches itself, and the
				// consumer's binding must carry a usable `at`. The same fallback declaration.go's
				// here() applies, done here because this need is now found before here() would run.
				at := node.At
				if at == "" {
					at = "127.0.0.1"
				}
				needs = append(needs, mcat.Needed{
					Name: want, From: node.Name, At: at, Serves: served, For: because[want], Module: by.Module,
					CredentialForm: by.CredentialFormOf(want), CredentialTool: by.CredentialToolOf(want)})
			}
			continue
		}

		// Answered from elsewhere in the mesh, if it is that kind of name. **Never installed
		// here.** Choosing a machine to put a database on is a decision with consequences
		// nobody would want made silently by something resolving a web application.
		if brokered[want] {
			reported[want] = true
			where := world.Offered[want]
			if settling[want] && len(where) == 0 {
				// Nothing assigned anywhere answers it: absent, and nothing is put on a machine
				// for it (ADR 0330 §3), however few modules in the catalogue could.
				leaveAbsent(want)
				continue
			}
			names := providerNames(where)
			take := func(p mcat.Provider) {
				if node.At != "" && p.At == "" || node.At == "" && p.At != "" || node.At == "" && p.At == "" {
					// One of them is not on the private network, so there is no path between
					// them. Said here rather than discovered as a connection timing out on a
					// machine that the mesh reported as configured.
					problems = append(problems, fmt.Sprintf(
						"%s needs %q from %s, and they are not both on the private network — "+
							"assign %s to whichever is missing it",
						node.Name, want, p.Node, meshNetwork))
					return
				}
				shared, form, tool := "", "", ""
				// A provider whose definition is not in hand is held to the tightest bound the
				// mesh knows, not to none: what it keeps is not known here (ADR 0225).
				bound := mcat.DefaultIdentityBound
				if pm, known := catalogue[p.Module]; known {
					shared, _ = pm.SharedCredentialOf(want)
					form, tool = pm.CredentialFormOf(want), pm.CredentialToolOf(want)
					bound = pm.IdentityBoundOf(want)
				}
				needs = append(needs, mcat.Needed{Name: want, From: p.Node, At: p.At, Module: p.Module,
					Serves: p.Serves, For: because[want], SharedOwn: shared, CredentialForm: form, CredentialTool: tool, Identity: bound,
					LeftOut: p.LeftOut})
			}
			switch {
			case world.Unchecked:
				// First pass. Whether anything answers this is exactly the question this pass
				// exists to make answerable, so it is not asked here.
			case len(where) == 0:
				remedy := "and nothing in the catalogue could"
				if answers := offers[want]; len(answers) == 1 {
					remedy = "— assign " + answers[0] + " to a node"
				} else if len(answers) > 1 {
					remedy = "— assign one of these to a node: " + strings.Join(answers, ", ")
				}
				problems = append(problems, fmt.Sprintf(
					"nothing in this mesh provides %q, wanted by %s %s",
					want, because[want], remedy))
			case len(where) == 1:
				if c, pinned := world.Pinned[want]; pinned && !chosenMatches(c, where[0]) {
					// One provider, and it is not the one this machine was told to use. Silently
					// using the other would be the mesh overruling a choice somebody made.
					problems = append(problems, fmt.Sprintf(
						"%s was told to get %q from %s, and only %s provides it",
						node.Name, want, c, nameOf(where[0])))
					break
				}
				take(where[0])
			default:
				c, pinned := world.Pinned[want]
				if !pinned {
					// **The seat's holder answers, when a seat delivers this** (novox/hq ADR 0110).
					// Not a guess, which ADR 0009 refuses: the choice was made once, mesh-wide, by
					// assigning the holder, where a pin makes it again on every consumer's node. A
					// pin still wins — it is a consumer coupled to one provider's contents, and has
					// said so.
					if holder, held := HolderAmong(want, where, world.Held); held {
						take(holder)
						break
					}
					problems = append(problems, fmt.Sprintf(
						"%d providers of %q, wanted by %s — say which with `pin %s %s <node> <module>`: %s",
						len(where), want, because[want], node.Name, want,
						strings.Join(names, ", ")))
					break
				}
				matching := chosenAmong(c, where)
				switch len(matching) {
				case 0:
					// Pointed at a provider that does not answer this. Refused rather than
					// falling back to another: a fallback would quietly move somebody's data to
					// a machine they did not choose, which is the whole reason this is asked.
					problems = append(problems, fmt.Sprintf(
						"%s was told to get %q from %s, and %s does not provide it — these do: %s",
						node.Name, want, c, c, strings.Join(names, ", ")))
				case 1:
					take(matching[0])
				default:
					// A record naming only the node, from before a pin named the module, and that
					// node answers twice. This once took the last one listed (novox/hq #258): a
					// coin flip, handed to whoever reads the certificate it chose.
					problems = append(problems, fmt.Sprintf(
						"%s was told to get %q from %s, and %s provides it %d times — say which with "+
							"`pin %s %s %s <module>`: %s",
						node.Name, want, c, c.Node, len(matching), node.Name, want, c.Node,
						strings.Join(providerNames(matching), ", ")))
				}
			}
			continue
		}

		// Usable only on its provider's own machine, and not here: refused, never answered by
		// installing a provider (novox/hq ADR 0208 §3). The display server is the machine's own role,
		// gated by its graphical session; one pulled in for a window manager is the misassignment
		// research 026 found. Another node's provider never counts — node scope is never brokered.
		if machineReach[want] && !isModule(catalogue, want) {
			reported[want] = true
			if settling[want] {
				leaveAbsent(want)
				continue
			}
			if world.Unchecked {
				// The first pass's refusals take a machine off the network; the second says it.
				continue
			}
			problems = append(problems, fmt.Sprintf(
				"%q is wanted by %s and is usable only on the machine that provides it, and nothing "+
					"assigned to %s does — %s", want, because[want], node.Name,
				machineReachRemedy(catalogue, want, node.Name)))
			continue
		}

		candidates := offers[want]
		if settling[want] && !(len(candidates) == 0 && len(world.Licences[want]) > 0) {
			// Not answered by anything in this node's set, and only installing could answer it
			// here — which is never done for an optional requirement (ADR 0330 §3), not even
			// for the only candidate. A licence record still answers it, in the post-pass.
			leaveAbsent(want)
			continue
		}
		switch len(candidates) {
		case 0:
			if len(world.Licences[want]) > 0 {
				// Answered by a record rather than by a module, and the post-pass below settles
				// which one. Left alone here: the two questions a refusal must answer — *is
				// there anything* and *which one* — have different remedies, and answering the
				// first wrongly would send somebody looking for a module to install.
				reported[want] = true
				continue
			}
			if world.Unchecked {
				// The first pass, which exists only to answer *what does this node offer*.
				//
				// **Refusing here makes the machine vanish** rather than reporting a problem: the
				// caller takes a failed resolution to mean it learned nothing about this node, so
				// one unanswerable requirement on one machine removes that machine from the
				// private network as far as every OTHER machine is concerned — and they are then
				// told, wrongly, that the two of them share no network.
				//
				// That is a wrong answer about a machine nobody asked about, caused by a fault on
				// a third. The second pass refuses it properly, where the question is being asked.
				reported[want] = true
				continue
			}
			reported[want] = true
			problems = append(problems, fmt.Sprintf(
				"nothing provides %q, wanted by %s", want, because[want]))
			continue
		case 1:
			// No choice to make, so none is made. This is the case that lets `install i3` bring
			// in xorg without anybody being asked anything.
		default:
			reported[want] = true
			problems = append(problems, fmt.Sprintf(
				"%q is wanted by %s and %d modules provide it — choose one and assign it: %s",
				want, because[want], len(candidates), strings.Join(candidates, ", ")))
			continue
		}

		m := catalogue[candidates[0]]
		if chosen[m.Module] {
			continue
		}
		chosen[m.Module] = true
		order = append(order, m.Module)
		for _, o := range m.Offers() {
			satisfied[o] = true
		}
		if _, ok := because[m.Module]; !ok {
			because[m.Module] = requiredByWords + because[want]
		}

		for _, r := range m.Wants() {
			if _, ok := because[r]; !ok {
				because[r] = m.Module
			}
			queue = append(queue, r)
		}
	}

	// One need per module that wants it, rather than one per name (novox/hq 04-ISSUES/022).
	//
	// **A consumer is a module on a machine, not a machine.** The walk above is a work-list over
	// names, so a requirement three modules share is visited once and produced one need, carrying
	// whichever module happened to mention it first. Everything downstream inherited that: one
	// credential, named after a node, and the other two consumers given nothing at all — a
	// service that resolves cleanly and then cannot authenticate, which is the exact shape of
	// 021.
	//
	// The record pass below already gets this right and says so. It is the same rule.
	needs = perConsumer(needs, order, catalogue)

	// **A consumer bound to its data stays bound to it** (novox/hq ADR 0232). Per consumer, after
	// the fan-out, because a binding is a consumer's: the walk above chose one provider per name for
	// the whole machine, and two consumers of it may have been bound at different times.
	var kept []mcat.KeptBinding
	if !world.Unchecked {
		here := func(name string) bool { return chosen[name] || assignedHere[name] }
		var stuck []string
		needs, kept, stuck = keepBound(needs, catalogue, node, world, keeps, here)
		problems = append(problems, stuck...)
	}

	// What is answered by a record rather than by a machine.
	//
	// A post-pass, deliberately: nothing about it depends on the order requirements were walked
	// in, and putting it in the queue would mean the reachability rule — which must not apply
	// here — sitting one branch away from a case it would be wrong for.
	//
	// **Refused when the consumer has not said which.** ADR 0024 warns this will be felt: a mesh
	// holding three ways to reach a model refuses every consumer that has not chosen, which is
	// correct and is a great deal of saying-which the first time. So the refusal names the
	// candidates and the exact command, because being right is not the same as being usable.
	for _, name := range order {
		m := catalogue[name]
		for _, want := range m.Wants() {
			offered, byRecord := world.Licences[want]
			if !byRecord || len(offered) == 0 {
				continue
			}
			if satisfied[want] {
				// Something in this node's own set answers it -- a model the mesh runs itself,
				// most obviously. A record is not consulted when there is a local answer.
				continue
			}
			using, said := world.Using[m.Module][want]
			if !said {
				if world.Unchecked {
					continue
				}
				names := make([]string, 0, len(offered))
				for _, r := range offered {
					names = append(names, r.Name)
				}
				sort.Strings(names)
				problems = append(problems, fmt.Sprintf(
					"%s on %s needs %q and has not been told which one to use — say which with "+
						"`licence use <name> %s %s`: %s",
					m.Module, node.Name, want, node.Name, m.Module, strings.Join(names, ", ")))
				continue
			}
			needs = append(needs, mcat.Needed{
				Name: want, From: using.Name, Serves: using.Serves, ByRecord: true,
				// The module that required it, not whatever first mentioned the name: the key is
				// sealed per consumer, and a consumer here is a module on a machine.
				For: m.Module,
			})
		}
	}

	resolution := mcat.Resolution{Node: node.Name, At: node.At, PublicDomain: node.PublicDomain,
		Account: node.Account, AccountHome: node.AccountHome, AgentAccount: node.AgentAccount,
		AgentAccountHome: node.AgentAccountHome, Capabilities: node.Capabilities,
		Because: because, Needs: needs, Unhostable: unhostable, Kept: kept,
		Absent: absentOf(absent, order, catalogue)}
	for _, n := range providersFirst(order, catalogue) {
		resolution.Modules = append(resolution.Modules, catalogue[n])
	}

	problems = append(problems, checkCapabilities(resolution.Modules, node)...)
	claims, claimProblems := checkClaims(resolution.Modules, node, elsewhere, world.Holdings)
	problems = append(problems, claimProblems...)
	problems = append(problems, checkResources(resolution.Modules)...)
	resolution.Claims = claims

	// Judged over the closure — what this node will actually run — so a holder pulled in by a
	// requirement counts, and the holders' mutual dependence resolves (novox/hq ADR 0207 §3).
	// Reported until the switch; refused after it, though never in the first pass, whose refusals
	// make a machine vanish from the network rather than report anything.
	resolution.Unheld = UnheldDependencies(catalogue, node.Name, resolution.Modules, nil)
	// Only a dependency some module in the catalogue could meet is refused: one with no possible
	// holder has no remedy to name, and stays a report in `status` (novox/hq ADR 0207 §4, read with
	// the assign rule above it).
	if enforceSeatDependencies && !world.Unchecked {
		for _, u := range resolution.Unheld {
			if len(u.Holders) > 0 {
				problems = append(problems, u.String())
			}
		}
	}

	if len(problems) > 0 {
		sort.Strings(problems)
		return mcat.Resolution{}, &Refusal{Problems: problems}
	}
	return resolution, nil
}

// isModule reports whether a name is a module in its own right rather than only something
// modules provide.
//
// A requirement naming a module is not satisfied by something else providing that name: `i3`
// requires `xorg` and means xorg, not "anything calling itself a display server".
func isModule(catalogue map[string]mcat.Manifest, want string) bool {
	_, ok := catalogue[want]
	return ok
}

// missingCapabilities is the capabilities a module declares that this machine does not have.
//
// Only the absent ones: a capability the machine reports having is nothing to say. Empty means the
// machine can host the module as far as its capabilities go.
func missingCapabilities(m mcat.Manifest, node mcat.Node) []string {
	var missing []string
	for _, c := range append(append([]string(nil), m.Capabilities...), impliedCapabilities(m)...) {
		if !node.Capabilities[c] {
			missing = append(missing, c)
		}
	}
	return missing
}

// WrongMachine is why a module cannot run here, said as a fact about the machine.
//
// Said the same whether a module was refused because something running here requires it or reported
// as un-applied because a person assigned it directly: the reason is identical, and nothing can be
// installed to change it.
func WrongMachine(module, capability, node string) string {
	return fmt.Sprintf(
		"%s needs the capability %q and %s does not have it — this is the wrong machine, "+
			"not a missing module", module, capability, node)
}

// checkCapabilities refuses a module the machine cannot run that something running here requires.
//
// A directly-assigned module the machine cannot host is left out of the closure before this runs
// and reported as un-applied instead (see Resolve); what reaches here is a module still in the
// closure because something requires it, and that set is genuinely incoherent — the requiring
// module cannot have its requirement met on this machine.
func checkCapabilities(modules []mcat.Manifest, node mcat.Node) []string {
	var problems []string
	for _, m := range modules {
		for _, c := range missingCapabilities(m, node) {
			problems = append(problems, WrongMachine(m.Module, c, node.Name))
		}
	}
	return problems
}

// checkClaims refuses two modules holding one singular thing.
//
// Within this node's own set, and against what is already held elsewhere for the wider scopes. A
// claim at mesh scope is the same idea as the mesh's one hub, said once instead of hard-coded.
func checkClaims(modules []mcat.Manifest, node mcat.Node, elsewhere []mcat.Held, holdings []mcat.Held) ([]mcat.Held, []string) {
	var problems []string
	var held []mcat.Held

	// onRecord is every recorded holder of a seat, if a handover ever named one: one for most seats,
	// and as many as were added for a replicated one (novox/hq ADR 0223).
	onRecord := func(claim, scope string) []mcat.Held {
		var out []mcat.Held
		for _, h := range holdings {
			hs, ok := mcat.SeatNamed(h.Claim)
			cs, cok := mcat.SeatNamed(claim)
			if ok && cok && hs.Name == cs.Name && h.Scope == scope {
				out = append(out, h)
			}
		}
		return out
	}
	// recordedHere says this node's module is one of a seat's holders on record.
	recordedHere := func(claim, scope, module string) bool {
		for _, rec := range onRecord(claim, scope) {
			if rec.Node == node.Name && rec.Module == module {
				return true
			}
		}
		return false
	}

	byScope := map[string]map[string]string{} // scope → claim → module
	for _, m := range modules {
		for _, c := range m.Claims {
			scope := c.At()
			// **A recorded holder settles it before any counting.** An assignment that could hold
			// the seat but is not the one on record is eligible, and that is all: it is not a second
			// holder, so it is not refused, and it does not hold (novox/hq ADR 0131). This is what
			// lets the next holder stand beside the current one until the seat is handed over.
			if recs := onRecord(c.Name, scope); len(recs) > 0 {
				// **A seat held once is on record once** (novox/hq ADR 0223). Only a replicated seat
				// may have several holders on record; several for any other seat is a store that
				// disagrees with the mesh's definition of the role, and it is refused, named, rather
				// than letting two machines answer for what the mesh has one of.
				if s, known := mcat.SeatNamed(c.Name); known && !s.Replicated && len(recs) > 1 {
					problems = append(problems, fmt.Sprintf(
						"%q is on record as held by %d assignments, and it is held once per %s — "+
							"`seat %s --to <node>/<module>` records one", c.Name, len(recs), scope, c.Name))
					continue
				}
				if !recordedHere(c.Name, scope, m.Module) {
					continue
				}
			}
			if byScope[scope] == nil {
				byScope[scope] = map[string]string{}
			}
			// **One seat under either of its names** (novox/hq ADR 0122): a manifest registered before
			// a rename claims the former name, and one written after it the current — two claimants of
			// one seat, compared by the seat they resolve to and not by how each spelled it.
			seat := heldKey(c.Name, c.Kind)
			if other, taken := byScope[scope][seat]; taken {
				problems = append(problems, fmt.Sprintf(
					"%s and %s both claim %q, and only one thing may hold it per %s",
					other, m.Module, seat, scope))
				continue
			}
			byScope[scope][seat] = m.Module
			held = append(held, mcat.Held{Claim: c.Name, Scope: scope, Node: node.Name,
				Module: m.Module, Site: node.Site, Kind: c.Kind})
		}
	}

	// And against the rest of the mesh, for the scopes that reach past this machine.
	for _, h := range held {
		for _, e := range elsewhere {
			if e.Node == node.Name || heldKey(e.Claim, e.Kind) != heldKey(h.Claim, h.Kind) || e.Scope != h.Scope {
				continue
			}
			switch h.Scope {
			case mcat.ScopeMesh:
				// **A holder on record is never a second claimant** (novox/hq ADR 0223). Records are
				// the mesh's settled answer: one for most seats, several only for a replicated seat,
				// each added by an act. Two holders here are two records, and both hold.
				if recordedHere(h.Claim, h.Scope, h.Module) {
					continue
				}
				// Both claim and nobody is on record, or this refusal could not have happened.
				// The remedy is the handover that records the holder (novox/hq ADR 0131,
				// 04-ISSUES/170), so it is named here rather than left to be found.
				problems = append(problems, fmt.Sprintf(
					"%s on %s claims %q, which %s on %s already holds — one per mesh. Nothing is "+
						"on record for it; `seat %s --to %s/%s` records the holder, and the other "+
						"assignment then stands beside it, eligible and silent",
					h.Module, node.Name, h.Claim, e.Module, e.Node, h.Claim, e.Node, e.Module))
			case mcat.ScopeSite:
				if node.Site != "" && node.Site == e.Site {
					problems = append(problems, fmt.Sprintf(
						"%s on %s claims %q, which %s on %s already holds at %s — one per site",
						h.Module, node.Name, h.Claim, e.Module, e.Node, node.Site))
				}
			}
		}
	}
	return held, problems
}

// canonicalSeat is the seat a claimed name refers to, by its current name: itself for a name the mesh
// does not know (a module's own seat), its seat's name for a former one.
func canonicalSeat(name string) string {
	if s, known := mcat.SeatNamed(name); known {
		return s.Name
	}
	return name
}

// checkResources refuses two modules writing the same thing.
//
// This costs no manifest field: the mesh already holds every resource of every module, so two
// declaring one path or one unit are visible without either having to know about the other. A
// declared claim is only for the abstract conflicts nothing in the resources reveals.
//
// **The refusal is about ownership, not use** (novox/hq ADR 0051, 04-ISSUES/036). Two modules
// owning one path is the fault this catches — the class of collision this repository keeps
// recording. Two modules *accessing* one operator-owned path is not a collision: it is the whole
// point of a media stack, where the library server, the managers and the download client must see
// the same directories. An access is not a resource and never enters the owner map, so co-access
// resolves with no refusal. What is refused is the contradiction — a path one module owns and
// another merely accesses — because shared data is the operator's and nobody's to own.
func checkResources(modules []mcat.Manifest) []string {
	// Judged as the machine composes them: where the node-resolver holder writes the resolver file, the
	// uplink holder's rendering of it is not composed, so it is not a second owner (novox/hq ADR 0247).
	modules = steppedBack(modules)
	// And only a claimant of a seat naming a path declares it — the controller's generated resources
	// included, which are judged here at composition (novox/hq ADR 0287).
	problems := seatPathsOnMachine(modules)
	owner := map[string]string{}
	ownedPath := map[string]string{} // path → owning module, for the access check below
	packagePresent, packageAbsent := map[string][]string{}, map[string][]string{}

	for _, m := range modules {
		// Compared placed, not as written (novox/hq ADR 0112): ${dir:state} is the same six
		// characters in every module and a different directory in each — two modules' templates
		// being spelled alike is not two modules owning one path. The default root serves the
		// comparison: a collision is within one node, and any one root keeps distinct modules'
		// places distinct. A reference that cannot be placed is left as written — naming what
		// does not exist is the manifest's own problem, refused where it was made.
		dirs := dirsFor(m, mcat.Rendering{})
		for _, r := range m.Resources {
			if fmt.Sprint(r["type"]) == "user" {
				// **An account is shared; what it is set to is not.** Several modules may need one
				// login: the shell's module sets its shell, the container runtime's puts it in the
				// `docker` group. The host only ever adds groups, and takes back only a group it put the
				// account in, when no declared resource still asks for it (novox/hq ADR 0252) — so groups
				// from several modules cannot contradict each other and are not owned. A shell or a home
				// is one value, and two modules setting it would each be undone by the other's
				// apply: each stays one module's per node, and two are refused naming both.
				name, _ := r["name"].(string)
				for _, field := range []string{"shell", "home"} {
					if v, ok := r[field].(string); !ok || v == "" || name == "" {
						continue
					}
					key := "user " + field + " " + name
					if other, taken := owner[key]; taken && other != m.Module {
						problems = append(problems, fmt.Sprintf(
							"%s and %s both set the %s of the user %q", other, m.Module, field, name))
					}
					owner[key] = m.Module
				}
				continue
			}
			if fmt.Sprint(r["type"]) == "package" {
				// **A package may be declared by several modules** (novox/hq ADR 0303): declaring it says
				// it is needed, and the host installs a missing one, never removes one a declaration
				// stopped naming, so a second declaration changes nothing on the machine. What two
				// modules can contradict is presence: one declaring it present and another absent
				// (ADR 0180) would have the host install and remove it in turn, so that pair is refused.
				if name, _ := r["package"].(string); name != "" {
					if r["absent"] == true {
						packageAbsent[name] = append(packageAbsent[name], m.Module)
					} else {
						packagePresent[name] = append(packagePresent[name], m.Module)
					}
				}
				continue
			}
			for _, field := range []string{"path", "unit", "name"} {
				value, ok := r[field].(string)
				if !ok || value == "" {
					if field == "path" && fmt.Sprint(r["type"]) == "directory" {
						// A pathless directory owns its placed path — a module stating that
						// very path is exactly the collision this exists to catch.
						value = dirs[fmt.Sprint(r["id"])]
					}
					if value == "" {
						continue
					}
				}
				if field == "path" {
					if placed, err := dirFill(value, dirs, m.Module); err == nil {
						value = placed
					}
				}
				key := field + " " + value
				if other, taken := owner[key]; taken && other != m.Module {
					problems = append(problems, fmt.Sprintf(
						"%s and %s both declare the %s %q", other, m.Module, field, value))
				}
				owner[key] = m.Module
				if field == "path" {
					ownedPath[value] = m.Module
				}
			}
		}
		// **A file the mesh renders for a module is that module's path too** (novox/hq ADR 0223). The
		// machine's resolver file is a fact the uplink's holder asks for, and a second module asking
		// for it, or declaring it, would have the host write one path twice in every apply, the last
		// one winning. A fact under the operator's home is placed per account and compared nowhere.
		for _, name := range sortedFacts(m.Facts) {
			fact := m.Facts[name]
			if fact.Home || !strings.HasPrefix(fact.Path, "/") {
				continue
			}
			key := "path " + fact.Path
			if other, taken := owner[key]; taken && other != m.Module {
				problems = append(problems, fmt.Sprintf(
					"%s and %s both declare the path %q", other, m.Module, fact.Path))
			}
			owner[key] = m.Module
			ownedPath[fact.Path] = m.Module
		}
	}
	for _, name := range packageNames(packageAbsent) {
		if wanted := packagePresent[name]; len(wanted) > 0 {
			problems = append(problems, fmt.Sprintf("%s declares the package %q absent and %s declare it present (novox/hq ADR 0303)",
				strings.Join(packageAbsent[name], ", "), name, strings.Join(wanted, ", ")))
		}
	}

	// An accessed path is the operator's, so no module may declare it as one of its own
	// (novox/hq ADR 0051). Refused here rather than silently tolerated: an owner would create and
	// chown the very directory another module was told to expect the operator to provide, and the
	// two intentions cannot both hold. Two modules *accessing* it, by contrast, is never checked,
	// which is what lets the stack in 04-ISSUES/036 co-resolve.
	for _, m := range modules {
		for _, a := range m.Accesses {
			if a.Path == "" {
				continue // placed by the assignment; nothing to compare at registration
			}
			switch other := ownedPath[a.Path]; other {
			case "":
				// Nobody owns it — the ordinary, correct case for shared data.
			case m.Module:
				problems = append(problems, fmt.Sprintf(
					"%s both owns and accesses %q — it is one or the other", m.Module, a.Path))
			default:
				problems = append(problems, fmt.Sprintf(
					"%s accesses %q, which %s declares it owns — shared data is the operator's, "+
						"owned by no module (novox/hq ADR 0051)", m.Module, a.Path, other))
			}
		}
	}
	return problems
}

// meshNetwork is what to assign to a machine that needs to reach another one.
//
// A string here rather than an import, because the private network is a module the control plane
// ships and this package must not depend on the thing it resolves. The name being wrong would
// show up as a refusal naming a module nobody can assign, which a test checks
// (provided_test.go). The private network's own module since novox/hq ADR 0226 retired the
// `networking` bundle that required it.
const meshNetwork = "mesh-wireguard"

// providersFirst orders a node's modules so that what answers a requirement comes before what
// asked for it.
//
// **The host does not sort** (novox/hq ADR 0005), so the order written here is the order a machine
// applies. Selection walks outward from what was assigned, which puts a consumer before the thing
// it pulled in — and a module whose service reads a file another module writes then starts before
// the file exists.
//
// It fails, and the next reconcile fixes it. That is the worst shape a fault can take: what gets
// remembered is that it works, and nobody looks again
// ([04-ISSUES/013](novox/hq)). This is the same fault one level up from where that was found.
//
// **Stable where nothing requires anything.** Modules with no relation keep the order selection
// gave them, because that order is meaningful — assigned first, then what they pulled in — and a
// sort that reshuffled unrelated modules would make every declaration's diff unreadable.
func providersFirst(order []string, shelf map[string]mcat.Manifest) []string {
	// Who offers what, among the modules actually chosen.
	offeredBy := map[string]string{}
	for _, name := range order {
		for _, offered := range shelf[name].Offers() {
			if _, taken := offeredBy[offered]; !taken {
				offeredBy[offered] = name
			}
		}
	}

	needs := map[string][]string{}
	for _, name := range order {
		for _, want := range shelf[name].Wants() {
			provider, known := offeredBy[want]
			if !known || provider == name {
				continue
			}
			needs[name] = append(needs[name], provider)
		}
	}

	var out []string
	placed := map[string]bool{}
	var place func(string, map[string]bool)
	place = func(name string, visiting map[string]bool) {
		if placed[name] {
			return
		}
		if visiting[name] {
			// Two modules requiring each other. Left in the order selection gave them rather
			// than refused: a cycle here is not a machine that cannot work — both are applied,
			// and one of them starts before the other is ready and settles on the next pass.
			// Refusing would make a pair of modules that cooperate impossible to assign.
			return
		}
		visiting[name] = true
		for _, provider := range needs[name] {
			place(provider, visiting)
		}
		delete(visiting, name)
		placed[name] = true
		out = append(out, name)
	}
	for _, name := range order {
		place(name, map[string]bool{})
	}
	return out
}

// perConsumer turns one need per provision into one need per module that wants it.
//
// Order follows the resolved modules rather than a map, so the same set always produces the same
// needs — a declaration whose contents move for no reason makes every push look like a change.
//
// A need nothing in the set wants is kept as it is rather than dropped. That should not happen;
// if it does, the honest outcome is an extra credential nobody reads, not a consumer silently
// losing the one it depends on.
func perConsumer(needs []mcat.Needed, order []string, catalogue map[string]mcat.Manifest) []mcat.Needed {
	out := make([]mcat.Needed, 0, len(needs))
	// Once per (provision, consumer, local name, provider). The walk over names visits a
	// same-node provision once per module that mentions it, so two consumers of one produced two
	// raw needs and, fanned out below, four — the same credential twice for each. Harmless
	// downstream, since a pair is one row however often it is asked for, and wrong all the same.
	seen := map[[4]string]bool{}
	keep := func(n mcat.Needed) {
		key := [4]string{n.Name, n.For, n.Local, n.From}
		if seen[key] {
			return
		}
		seen[key] = true
		out = append(out, n)
	}
	for _, n := range needs {
		var wanted bool
		for _, name := range order {
			m, known := catalogue[name]
			if !known {
				continue
			}
			for _, want := range m.Wants() {
				if want != n.Name {
					continue
				}
				copied := n
				copied.For = m.Module
				// And once per file THIS module keeps the credential in, where it keeps several
				// (ADR 0094) — here, where the consumer is finally known, not on the walk above.
				for _, one := range eachLocal(nil, catalogue, copied) {
					keep(one)
				}
				wanted = true
				break
			}
		}
		if !wanted {
			keep(n)
		}
	}
	return out
}

// eachLocal appends the need once per file the wanting module keeps the credential in: once, with
// no local name, in the ordinary case; once per local name where the module wants several values
// from one provider (ADR 0094). Each is its own pair credential downstream.
func eachLocal(needs []mcat.Needed, catalogue map[string]mcat.Manifest, n mcat.Needed) []mcat.Needed {
	files := catalogue[n.For].SecretFiles(n.Name)
	// One file under a local name is still a local name: the review found a module keeping ONE
	// named secret given a need with no local, and so no file, while everything reported success.
	if len(files) == 0 || (len(files) == 1 && files[0].Local == "") {
		return append(needs, n)
	}
	for _, f := range files {
		one := n
		one.Local = f.Local
		needs = append(needs, one)
	}
	return needs
}

// machineReachRemedy names what would meet a requirement with the machine's reach: every module in
// the catalogue that provides it, each with the node seats it holds — for a display, the holders of
// node-display-server (novox/hq ADR 0208 §3), named by the seat because that is the role being
// asked for, without this code knowing which seat any provision belongs to.
func machineReachRemedy(catalogue map[string]mcat.Manifest, want, node string) string {
	var named []string
	for _, name := range sortedKeys(catalogue) {
		m := catalogue[name]
		provides := false
		for _, o := range m.Provides {
			if o.Name == want {
				provides = true
			}
		}
		if !provides {
			continue
		}
		var held []string
		for _, c := range m.Claims {
			if c.At() == mcat.ScopeNode {
				held = append(held, c.Name)
			}
		}
		if len(held) > 0 {
			named = append(named, fmt.Sprintf("%s (holds %s)", name, strings.Join(held, ", ")))
		} else {
			named = append(named, name)
		}
	}
	if len(named) == 0 {
		return "and nothing in the catalogue provides it"
	}
	return fmt.Sprintf("assign one to %s: %s", node, strings.Join(named, "; "))
}

// answeredElsewhere says that a mesh-wide provision this machine could answer itself is answered by
// another machine all the same: one this machine was pinned to, or the holder of the mesh seat that
// delivers it (novox/hq ADR 0110, ADR 0194).
//
// **A provider on the machine was taken before either was asked.** The branch for a provision
// answered here bound the consumer to the local provider and consulted a pin only between two local
// ones, and the seat's holder never; both were read only for a provider on another machine. So when
// every machine ran a provider of `wildcard-resolution` — each machine's own resolver, still assigned
// while the mesh moved to one — every machine bound its resolver configuration to itself, with the
// mesh's one resolver recorded, held and pinned (novox/hq issue 258). The seat is the mesh's choice of
// who answers, made once; a provider that merely runs here does not overrule it, and neither does it
// overrule a pin somebody set on this machine.
//
// Not in the first pass, which asks only what this machine offers and has no providers to read.
//
// **And never by the seat alone for a provision that keeps its consumers' data** (novox/hq ADR
// 0232). The rule above was written for the mesh's resolver, which any provider answers alike. Read
// for the store's seat, it re-bound every database consumer on a machine running its own store to
// the seat's holder on another machine, which made each of them a fresh, empty database there and
// left their data behind (novox/hq issue 273). A provider beside a consumer of its data is where that
// data is; only a pin — a person — answers it from elsewhere.
func answeredElsewhere(want string, node mcat.Node, world mcat.World, brokered, keeps map[string]bool) bool {
	if world.Unchecked || !brokered[want] {
		return false
	}
	if c, pinned := world.Pinned[want]; pinned {
		return c.Node != node.Name
	}
	if keeps[want] {
		return false
	}
	// **A machine holding the seat answers itself** (novox/hq ADR 0223). With a replicated seat
	// another machine holds it too, and the first holder in the providers' order may be that one; a
	// holder is still where this machine's own requirement is answered, so its resolver file lists
	// itself first.
	if seat, delivered := mcat.SeatDelivering(want); delivered {
		for _, h := range append(append([]mcat.Held(nil), world.Holdings...), world.Held...) {
			if hs, ok := mcat.SeatNamed(h.Claim); ok && hs.Name == seat.Name && h.Scope == seat.Scope &&
				h.Node == node.Name {
				return false
			}
		}
	}
	holder, held := HolderAmong(want, world.Offered[want], world.Held)
	return held && holder.Node != node.Name
}

// packageNames are the packages a map names, in order.
func packageNames(m map[string][]string) []string {
	names := make([]string, 0, len(m))
	for n := range m {
		names = append(names, n)
	}
	sort.Strings(names)
	return names
}

// impliedCapabilities are the capabilities a module needs for what its resources say, beyond those it declares: a
// container that names groups needs an engine that reads them, so the module is left out of a machine whose report
// lacks the capability, naming it, and the field is sent to no engine that would refuse it. None for a module whose
// containers name no groups.
func impliedCapabilities(m mcat.Manifest) []string {
	for _, r := range m.Resources {
		if fmt.Sprint(r["type"]) == "container" && len(groupsOf(r)) > 0 {
			return []string{mcat.ContainerGroups}
		}
	}
	return nil
}