package catalogue

import (
	"fmt"
	"regexp"
	"strings"
)

// What a module may call an event, and what a consumer may ask for.
//
// A module names an event **locally**: `order.placed`, not a subject and not a routing key
// (design 29 §1). A consumer names the emitter and the event: `billing.order.placed`. The mesh
// derives the subject from those, so reorganising the subject space leaves every manifest correct.
//
// **Nothing checked this until every manifest in the catalogue was wrong the same way**
// (novox/hq 04-ISSUES/127). All thirty-seven kept the old bus's routing key —
// `module.<module>.<verb>` — which the derivation read as "a module called `module`", so every
// cross-module subscription in the mesh pointed at a namespace nobody publishes to. Nothing failed:
// the services started and none of them reacted. The documentation on these fields taught the old
// form too, which is why the drift was uniform rather than scattered.

// eventName is one name in a local event: lower-case, and no wildcard.
var eventName = regexp.MustCompile(`^[a-z0-9][a-z0-9-]*$`)

// The wildcards a consumer may use, spelled the mesh's way and derived to whatever the transport
// spells them as.
//
// **A manifest holds no transport token**, which is the whole point of naming locally: the bus the
// mesh runs on today spells these `*` and `#`, and the one being built spells them `*` and `>`. A
// manifest that said either would be a manifest that stopped being true when the wire changed.
const (
	// OneName stands for exactly one name.
	OneName = "*"
	// TheRest stands for one or more names, and may only come last.
	TheRest = "**"
)

// EventProblems is what is wrong with a manifest's events.
//
// Refused at registration, because the alternative is a module that installs, starts, connects and
// reacts to nothing — and every log line says it is fine.
func EventProblems(m Manifest) []string {
	var problems []string

	for _, e := range m.Emits {
		if was, stale := staleEventForm(e, m.Module); stale {
			problems = append(problems, fmt.Sprintf(
				"%s emits %q, which is the old bus's routing key. An event is named locally now, so "+
					"write %q — the mesh derives the subject (novox/hq design 29 §1)",
				m.Module, was, strings.TrimPrefix(was, "module."+m.Module+".")))
			continue
		}
		if strings.HasPrefix(e, "module.") {
			problems = append(problems, fmt.Sprintf(
				"%s emits %q: `module.` is reserved, because it is how the old bus spelled a "+
					"routing key and an event named that way derives into a namespace nobody owns",
				m.Module, e))
			continue
		}
		if err := localName(e); err != nil {
			problems = append(problems, fmt.Sprintf("%s emits %q: %v", m.Module, e, err))
			continue
		}
		// **Its own name, never another's.** The bus enforces that a namespace belongs to the module
		// it is named for, so an event named for somebody else cannot be published at all. If the
		// event is about a role rather than about this module, it belongs on the seat: a name that
		// is stable across whoever fills it (04-ISSUES/127).
		if first, _, split := strings.Cut(e, "."); split && isAModuleNameOtherThan(first, m.Module) {
			problems = append(problems, fmt.Sprintf(
				"%s emits %q, which reads as another module's event. A module publishes under its "+
					"own name only. If this is about a role rather than about %s, declare it on that "+
					"seat, where the name survives the holder changing",
				m.Module, e, m.Module))
		}
	}

	for _, c := range m.Consumes {
		if strings.HasPrefix(c, "module.") {
			problems = append(problems, fmt.Sprintf(
				"%s consumes %q, which is the old bus's pattern. A consumed event names its emitter "+
					"and the event: write %q", m.Module, c, strings.TrimPrefix(c, "module.")))
			continue
		}
		if c == "#" {
			problems = append(problems, fmt.Sprintf(
				"%s consumes %q, which is the old bus's wildcard for everything. Write %q",
				m.Module, c, TheRest))
			continue
		}
		if err := consumePattern(c); err != nil {
			problems = append(problems, fmt.Sprintf("%s consumes %q: %v", m.Module, c, err))
		}
	}
	return problems
}

// staleEventForm says an emitted name is this module's own old routing key, and what it was.
func staleEventForm(event, module string) (string, bool) {
	return event, module != "" && strings.HasPrefix(event, "module."+module+".")
}

// isAModuleNameOtherThan says a first token names some module of this mesh that is not this one.
//
// Only the mesh's own seats and the catalogue could answer this properly, and neither is reachable
// from a parser given one manifest. So this catches the case that actually happened — a name that
// is a *provision* the mesh defines, which is where "another module's event" comes from in practice
// — and the whole-catalogue check catches the rest.
func isAModuleNameOtherThan(first, module string) bool {
	if first == module || first == "" {
		return false
	}
	if _, isASeat := SeatNamed(first); isASeat {
		return true
	}
	if _, isASeat := SeatDelivering(first); isASeat {
		return true
	}
	return false
}

// localName checks one event name: dot-separated names, no wildcards, nothing else.
func localName(event string) error {
	if event == "" {
		return fmt.Errorf("an event needs a name")
	}
	for _, part := range strings.Split(event, ".") {
		if part == OneName || part == TheRest {
			return fmt.Errorf("an emitted event names one event, so it carries no wildcard")
		}
		if !eventName.MatchString(part) {
			return fmt.Errorf("%q is not a usable name: lower-case letters, digits and dashes", part)
		}
	}
	return nil
}

// consumePattern checks a consumed pattern: the emitter, then the event, with wildcards.
func consumePattern(pattern string) error {
	if pattern == "" {
		return fmt.Errorf("a consumed event needs an emitter and an event")
	}
	parts := strings.Split(pattern, ".")
	for i, part := range parts {
		switch {
		case part == TheRest:
			if i != len(parts)-1 {
				return fmt.Errorf("%q stands for the rest of a name, so nothing may follow it", TheRest)
			}
		case part == OneName:
		case !eventName.MatchString(part):
			return fmt.Errorf("%q is not a usable name: lower-case letters, digits and dashes", part)
		}
	}
	// `**` alone is every event from every module, which the audit logger wants and says plainly.
	if len(parts) == 1 && parts[0] != TheRest {
		return fmt.Errorf(
			"%q names an emitter and no event. Write <emitter>.<event>, or %q for every event",
			pattern, TheRest)
	}
	return nil
}