package runtime

import (
	"fmt"
	"hash/fnv"
	"sort"

	"github.com/tegwick/fluid-core/internal/contract"
)

// Request is the subset of an inbound request that revision resolution may
// consider. Nothing else is allowed to influence the decision: resolution must
// be a pure function of these fields and the loaded policy, or it stops being
// auditable (ArchitectureBlueprint.md section 5.2).
type Request struct {
	// ExplicitRevision is a revision the consumer asked for by name.
	ExplicitRevision contract.RevisionID
	// BoundRevision comes from a client contract binding.
	BoundRevision contract.RevisionID
	// Cohort is the consumer's cohort assignment.
	Cohort contract.CohortID
	// Tenant identifies the calling tenant, where the interface is multi-tenant.
	Tenant string
	// ConsumerRef is the pseudonymous, stable consumer identity used to keep a
	// long-lived consumer on one side of an experiment.
	ConsumerRef string
	// CorrelationID ties this request to its telemetry.
	CorrelationID string
}

// Resolution is the outcome of revision resolution, including why.
//
// The reason is not decoration. Blueprint section 5.2 requires resolution to be
// auditable, and "which revision served this request" is unanswerable later
// without recording how it was chosen.
type Resolution struct {
	Revision   contract.RevisionID
	Reason     contract.FluidTelemetryResolutionReason
	Experiment *contract.ExperimentID
	// PolicyGeneration records which policy produced this decision.
	PolicyGeneration int64
}

// Resolver implements the deterministic precedence chain.
type Resolver struct {
	registry *Registry
	// allowExplicit controls whether consumers may pin a revision by name. Some
	// interfaces want this for migration testing; others must not expose it.
	allowExplicit bool
}

// NewResolver returns a resolver over reg.
func NewResolver(reg *Registry, allowExplicit bool) *Resolver {
	return &Resolver{registry: reg, allowExplicit: allowExplicit}
}

// Resolve selects the revision that will serve req.
//
// The order is fixed by ArchitectureBlueprint.md section 5.2:
//
//	explicit revision -> bound client contract -> experiment assignment -> stable default
//
// Each step is skipped rather than failed when the candidate is not routable,
// so a retired pin or an ineligible cohort degrades to the default instead of
// erroring the request.
func (r *Resolver) Resolve(req Request) (Resolution, error) {
	policy, err := r.registry.Policy()
	if err != nil {
		return Resolution{}, err
	}

	if r.allowExplicit && req.ExplicitRevision != "" {
		if err := r.registry.CheckRoutable(req.ExplicitRevision, req.Cohort); err != nil {
			// An explicit request for something unroutable is a consumer error
			// worth surfacing, not something to silently reinterpret.
			return Resolution{}, fmt.Errorf("explicit revision %s: %w", req.ExplicitRevision, err)
		}
		return Resolution{
			Revision:         req.ExplicitRevision,
			Reason:           contract.FluidTelemetryResolutionReasonExplicitRevision,
			PolicyGeneration: policy.Generation,
		}, nil
	}

	if req.BoundRevision != "" {
		if err := r.registry.CheckRoutable(req.BoundRevision, req.Cohort); err == nil {
			return Resolution{
				Revision:         req.BoundRevision,
				Reason:           contract.FluidTelemetryResolutionReasonBoundContract,
				PolicyGeneration: policy.Generation,
			}, nil
		}
	}

	if rule, ok := matchRule(policy.Rules, req); ok {
		chosen, ok := allocate(rule, req, policy.DefaultRevision)
		if ok {
			if err := r.registry.CheckRoutable(chosen, req.Cohort); err == nil {
				reason := contract.FluidTelemetryResolutionReasonCohortRule
				if rule.Experiment != nil {
					reason = contract.FluidTelemetryResolutionReasonExperimentAssignment
				}
				return Resolution{
					Revision:         chosen,
					Reason:           reason,
					Experiment:       rule.Experiment,
					PolicyGeneration: policy.Generation,
				}, nil
			}
		}
	}

	if err := r.registry.CheckRoutable(policy.DefaultRevision, req.Cohort); err != nil {
		return Resolution{}, fmt.Errorf("default revision %s: %w", policy.DefaultRevision, err)
	}
	return Resolution{
		Revision:         policy.DefaultRevision,
		Reason:           contract.FluidTelemetryResolutionReasonStableDefault,
		PolicyGeneration: policy.Generation,
	}, nil
}

// matchRule returns the first rule matching the request. Rules are evaluated in
// document order and the first match wins, so policy authors control precedence
// by ordering rather than by scoring.
func matchRule(rules []contract.RoutingPolicyRulesItem, req Request) (contract.RoutingPolicyRulesItem, bool) {
	for _, rule := range rules {
		if rule.Cohort != nil && *rule.Cohort != req.Cohort {
			continue
		}
		if rule.Tenant != "" && rule.Tenant != req.Tenant {
			continue
		}
		return rule, true
	}
	return contract.RoutingPolicyRulesItem{}, false
}

// allocate picks a revision from a rule's traffic shares.
//
// Assignment is a deterministic function of the sticky key, so a given consumer
// lands on the same side of an experiment for its whole duration. Random
// per-request assignment would make within-consumer comparisons meaningless and
// would let a client observe both revisions at once.
func allocate(rule contract.RoutingPolicyRulesItem, req Request, fallback contract.RevisionID) (contract.RevisionID, bool) {
	if len(rule.Allocation) == 0 {
		return "", false
	}

	// Sorting makes the traversal order independent of Go's map iteration, which
	// is what turns a hash bucket into a stable assignment.
	ids := make([]string, 0, len(rule.Allocation))
	var total float64
	for id, share := range rule.Allocation {
		ids = append(ids, id)
		total += float64(share)
	}
	sort.Strings(ids)

	if total <= 0 {
		return "", false
	}

	key := stickyKey(rule, req)
	position := bucket(key) * total

	var cumulative float64
	for _, id := range ids {
		cumulative += float64(rule.Allocation[id])
		if position < cumulative {
			return contract.RevisionID(id), true
		}
	}
	// Floating-point drift at the top of the range.
	return contract.RevisionID(ids[len(ids)-1]), true
}

// stickyKey chooses what keeps a consumer on one side of an experiment.
func stickyKey(rule contract.RoutingPolicyRulesItem, req Request) string {
	mode := contract.RoutingPolicyRulesItemStickyByConsumerID
	if rule.StickyBy != nil {
		mode = *rule.StickyBy
	}

	var subject string
	switch mode {
	case contract.RoutingPolicyRulesItemStickyByTenant:
		subject = req.Tenant
	case contract.RoutingPolicyRulesItemStickyByCorrelationID:
		subject = req.CorrelationID
	case contract.RoutingPolicyRulesItemStickyByNone:
		subject = req.CorrelationID
	default:
		subject = req.ConsumerRef
	}

	// Namespacing by experiment stops one consumer from landing in the same
	// arm of every concurrent experiment, which would confound their results.
	if rule.Experiment != nil {
		return string(*rule.Experiment) + "\x00" + subject
	}
	return subject
}

// bucket maps a key into [0, 1).
func bucket(key string) float64 {
	h := fnv.New64a()
	_, _ = h.Write([]byte(key))
	// 53 bits keeps the result exactly representable as a float64.
	const mask = 1<<53 - 1
	return float64(h.Sum64()&mask) / float64(mask+1)
}