package link

import (
	"context"
	"encoding/json"
	"fmt"
	"time"
)

// Asking a role to build something, and being told what came of it.
//
// A build is work submitted to a role, not a message to a machine (novox/hq ADR 0121). The
// build-machine seat accepts a build and emits an outcome, so the same publish that answers whoever
// asked also reaches the controller that records it and the catalogue that places it in the module
// graph — and no build machine needs permission to publish into anybody's inbox.
//
// **This is the one flow whose shape differs from every other**, which is why it has its own seam
// rather than living in `Bus`. Everything else the controller sends is either an event nobody must
// act on or a declaration a node reconciles toward; a build is a request that takes minutes and has
// exactly one answer. Too long for request/reply, too particular to be an event.

// TheBuildMachine is the role a build is submitted to: node-scoped, held on every machine that
// builds, and the work shared among them (novox/hq ADR 0190). The name stays for every caller; what
// it names moved from the mesh's one build machine to whichever build agent is idle.
//
// **Switching a live mesh over, in order** — and why no step strands a build. The old seat's
// stream and worker (SEAT_MESH_BUILD_MACHINE, SEAT_MESH_BUILD_MACHINE_worker) stay on the bus until
// removed by hand, and the builder keeps draining them while it is assigned, because a machine
// serves the seat its credential claims (BuildSeatClaimed) and the controller asks the seat that
// has a holder (buildSeatAmong in the command) and hears both seats' outcomes:
//
//  1. Merge the controller and the host's first user list together; the new controller rolls and,
//     seeing only the builder assigned, still asks mesh-build-machine — which the builder holds.
//  2. Merge the catalogue's build-agent; the builder builds it and the controller registers it.
//  3. On each machine that builds: `module issue build-agent --node <n>`, then `assign`, then
//     `push`. The first holder appears, and from then on asks go to node-build-agent.
//  4. Unassign builder everywhere and `module forget` it.
//  5. By hand: delete SEAT_MESH_BUILD_MACHINE and its worker, drop the retired seat row and
//     TheBuildMachineBefore with it, and the second entries in seatsTheControllerAsks and
//     ControllerFollows.
const TheBuildMachine = "node-build-agent"

// TheBuildMachineBefore is the role a build was submitted to until ADR 0190: the mesh's one build
// machine, mesh-scoped. Kept named while the handover runs — a machine whose credential claims it
// still serves it, and the controller still hears its outcomes — and dropped with the retired seat
// row once nothing claims it.
const TheBuildMachineBefore = "mesh-build-machine"

// BuildSeatClaimed is the build role a machine serves: the first seat its credential claims, or the
// current role when the credential names none (a credential from before claims travelled in it, or
// one written by hand). **The credential decides, not the binary** (ADR 0190 handover): one build
// machine binary runs as the old `builder` on the old seat and as a `build-agent` on the new one,
// each taking the work the mesh issued it a credential for, so neither drains the other's queue
// and the switch needs no flag day.
func BuildSeatClaimed(claimed []string) string {
	for _, seat := range claimed {
		if seat != "" {
			return seat
		}
	}
	return TheBuildMachine
}

// BuildWork is where a build request lands, and BuildOutcome is where its result does. Derived from
// the seat, so both sides name the role and neither names the other. The no-argument forms name the
// current role; the `Of` forms take the seat, for the handover during which two roles exist.
func BuildWork() string              { return BuildWorkOf(TheBuildMachine) }
func BuildOutcome() string           { return BuildOutcomeOf(TheBuildMachine) }
func BuildWorkOf(seat string) string { return "mesh.seat." + seat + ".accept.build" }
func BuildOutcomeOf(seat string) string {
	return "mesh.seat." + seat + ".event.built"
}

// BuildStarted is where a build machine says it has taken a build, and BuildLog is where it says
// what it is doing, one line per message, under the build's own id (novox/hq ADR 0157).
//
// **The whole build is on the bus as it happens.** The outcome alone told a person that a build
// failed and its first line why; everything between — which command, how long, where it hung —
// lived in one container's stderr on one machine. Every line is now an event of the role, retained
// with the rest of the mesh's events, so a reader follows a build live by subscribing its subject,
// or reads it back afterwards from the stream, and a viewer is a subscriber and nothing more.
func BuildStarted() string              { return BuildStartedOf(TheBuildMachine) }
func BuildLog(id string) string         { return BuildLogOf(TheBuildMachine, id) }
func BuildStartedOf(seat string) string { return "mesh.seat." + seat + ".event.started" }
func BuildLogOf(seat, id string) string { return "mesh.seat." + seat + ".event.log." + id }

// BuildStart is what a build machine says the moment it takes a build.
type BuildStart struct {
	ID         string `json:"id"`
	Repository string `json:"repository"`
	Path       string `json:"path,omitempty"`
	Ref        string `json:"ref,omitempty"`
	On         string `json:"on"`
	At         string `json:"at"`
}

// BuildLine is one thing a build said while building.
type BuildLine struct {
	ID string `json:"id"`
	// Seq counts the lines of one build from 1, so a reader that joined late or read two copies
	// can order them and see a gap.
	Seq int    `json:"seq"`
	At  string `json:"at"`
	// Step is which part of the build spoke — clone, context, image, run, failed — and Message is
	// what it said, as the builder's own log prints it.
	Step    string `json:"step"`
	Message string `json:"message"`
}

// KeyRoleBuilt is the build outcome under the role's name, on the bus the mesh runs on today.
//
// The same event as KeyModuleBuilt and published beside it, because a catalogue installed before this
// change listens for the module's name and one installed after listens for the role's. Both, until
// this bus retires: a rename needs publisher and subscriber to change together, and a deployment
// cannot promise which arrives first.
const KeyRoleBuilt = "built"

// Builders is how work reaches a build machine and how the outcome comes back.
type Builders interface {
	// Submit asks for one build and waits for its outcome.
	//
	// The wait is long by nature. A build clones, pulls a base image and runs a container build, so
	// a timeout here says "nothing is doing builds" rather than "this build is slow" — and the two
	// need different remedies, which is why the message distinguishes them.
	Submit(ctx context.Context, request BuildRequest, wait time.Duration) (BuildResult, error)

	// Ask submits one build and does not wait: the outcome is the role's event, heard and taken in
	// by the controller whether or not anybody waited (novox/hq issue 176). For a caller that
	// cannot hold a connection for the minutes a build takes — a tool call — and follows the build
	// by its id instead.
	Ask(ctx context.Context, request BuildRequest) error

	// Close lets go of whatever was dialled.
	Close()
}

// BuildMachine is a machine taking work from the role it holds.
type BuildMachine interface {
	// Take hands each request to do until the context ends, and says why it stopped.
	Take(ctx context.Context, do func(context.Context, Build)) error
	Close()
}

// Build is one request a machine has been handed.
type Build interface {
	// Began says the build has been taken and is under way, before anything runs.
	Began(ctx context.Context) error

	// Say publishes one line of what the build is doing. Never fails the build: a line the bus
	// did not take is a line lost, and the outcome still comes.
	Say(step, message string)

	// Request is what to build.
	Request() BuildRequest

	// Announce publishes the outcome as the role's own event.
	//
	// One publish, three audiences: whoever asked matches it by the id their request carried, the
	// controller records it, and the catalogue places it. On the bus the mesh runs on today that
	// fan-out came from a shared exchange; here the mesh derived the subject.
	Announce(ctx context.Context, result BuildResult) error

	// Done settles the request. Called only after the outcome is away, so a machine that dies
	// before announcing leaves the work for another rather than losing it.
	Done() error

	// Hold hands the work back for another attempt after the delay.
	Hold(after time.Duration) error
}

// waitingFor is the message a caller gets when nothing answered. Its own function because both
// transports say it, and saying it differently in two places is how one of them ends up vague.
func waitingFor(wait time.Duration) error {
	return fmt.Errorf(
		"no build machine answered within %s. Either nothing holds %s — in which case the work is "+
			"queued and will be done when something does — or a build is taking longer than this",
		wait, TheBuildMachine)
}

// theOutcomeOf reads a result and says whether it is the answer to this request.
func theOutcomeOf(body []byte, id string) (BuildResult, bool, error) {
	var result BuildResult
	if err := json.Unmarshal(body, &result); err != nil {
		return BuildResult{}, false, fmt.Errorf("a build machine answered with something unreadable: %w", err)
	}
	// Somebody else's build. Skipped rather than returned, because returning it would attribute one
	// build's outcome to another's.
	return result, result.ID == id, nil
}

// ModuleOf reads the module's name out of a manifest a build produced, which is the only place it is
// authoritative — a request named a repository and a path, not a module.
func ModuleOf(manifest json.RawMessage) string {
	var named struct {
		Module string `json:"module"`
	}
	if err := json.Unmarshal(manifest, &named); err != nil {
		return ""
	}
	return named.Module
}