package contract

import (
	"fmt"
	"strings"
)

// Identifier prefixes from FluidHypothesisRevisionSchema.md section 16.
//
// Implementations may use UUIDs internally; these human-readable prefixes exist
// so that operational tooling and audit trails stay legible to people.
const (
	PrefixHypothesis         = "H-"
	PrefixRevision           = "R-"
	PrefixExperiment         = "E-"
	PrefixPressure           = "P-"
	PrefixBackendRequirement = "BR-"
	PrefixDecision           = "D-"
	PrefixEvent              = "EV-"
	PrefixFeedback           = "F-"
	PrefixCohort             = "C-"
)

// EntityKind names the FLUID artifact an identifier refers to.
type EntityKind string

const (
	KindHypothesis         EntityKind = "hypothesis"
	KindRevision           EntityKind = "revision"
	KindExperiment         EntityKind = "experiment"
	KindPressure           EntityKind = "pressure"
	KindBackendRequirement EntityKind = "backend_requirement"
	KindDecision           EntityKind = "decision"
	KindEvent              EntityKind = "event"
	KindFeedback           EntityKind = "feedback"
	KindCohort             EntityKind = "cohort"
	KindIntent             EntityKind = "intent"
	KindRoutingPolicy      EntityKind = "routing_policy"
)

// prefixOrder matters: "BR-" must be tested before "B"-less single letters
// would otherwise mis-claim it, and "EV-" before "E-".
var prefixOrder = []struct {
	prefix string
	kind   EntityKind
}{
	{PrefixBackendRequirement, KindBackendRequirement},
	{PrefixEvent, KindEvent},
	{PrefixHypothesis, KindHypothesis},
	{PrefixRevision, KindRevision},
	{PrefixExperiment, KindExperiment},
	{PrefixPressure, KindPressure},
	{PrefixDecision, KindDecision},
	{PrefixFeedback, KindFeedback},
	{PrefixCohort, KindCohort},
}

// KindOf reports which FLUID artifact an identifier names.
//
// Audit trails carry bare identifiers across record boundaries, so being able to
// classify one without knowing where it came from keeps trace reconstruction
// from needing a lookup table at every hop.
func KindOf(id string) (EntityKind, bool) {
	for _, p := range prefixOrder {
		if strings.HasPrefix(id, p.prefix) {
			return p.kind, true
		}
	}
	return "", false
}

// ErrWrongKind reports an identifier used in the wrong position.
type ErrWrongKind struct {
	ID   string
	Want EntityKind
	Got  EntityKind
}

func (e *ErrWrongKind) Error() string {
	if e.Got == "" {
		return fmt.Sprintf("identifier %q has no recognized FLUID prefix, wanted a %s id", e.ID, e.Want)
	}
	return fmt.Sprintf("identifier %q is a %s id, wanted a %s id", e.ID, e.Got, e.Want)
}

// RequireKind checks that id names the expected artifact.
func RequireKind(id string, want EntityKind) error {
	got, ok := KindOf(id)
	if !ok || got != want {
		return &ErrWrongKind{ID: id, Want: want, Got: got}
	}
	return nil
}