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.
const TheBuildMachine = "mesh-build-machine"

// 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.
func BuildWork() string    { return "mesh.seat." + TheBuildMachine + ".accept.build" }
func BuildOutcome() string { return "mesh.seat." + TheBuildMachine + ".event.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)

	// 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 {
	// 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
}