package observation

import (
	"net/http"
	"sort"
	"strings"

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

// CohortRule assigns a request to a cohort when every stated condition holds.
//
// Rules are deterministic and declarative rather than learned. Blueprint 6.3
// wants cohorts stable enough to compare over time, and a classifier that
// drifts makes last month's measurement incomparable with this month's.
type CohortRule struct {
	// Cohort is the assignment this rule produces.
	Cohort contract.CohortID

	// Header matches a header value exactly, when both are set.
	Header      string
	HeaderValue string

	// HeaderPrefix matches a header by prefix, for SDK version families.
	HeaderPrefix string

	// PathPrefix matches the request path.
	PathPrefix string

	// Description explains the population, for the operator reading a report.
	Description string
}

func (r CohortRule) matches(req *http.Request) bool {
	if r.Header != "" {
		got := req.Header.Get(r.Header)
		switch {
		case r.HeaderValue != "":
			if !strings.EqualFold(got, r.HeaderValue) {
				return false
			}
		case r.HeaderPrefix != "":
			if !strings.HasPrefix(strings.ToLower(got), strings.ToLower(r.HeaderPrefix)) {
				return false
			}
		default:
			if got == "" {
				return false
			}
		}
	}
	if r.PathPrefix != "" && !strings.HasPrefix(req.URL.Path, r.PathPrefix) {
		return false
	}
	return true
}

// CohortEngine groups consumers into analytically useful populations.
type CohortEngine struct {
	rules    []CohortRule
	fallback contract.CohortID
	policy   RedactionPolicy
}

// NewCohortEngine returns an engine. Rules are evaluated in order, first match
// wins, so ordering is how an operator expresses precedence.
func NewCohortEngine(fallback contract.CohortID, policy RedactionPolicy, rules ...CohortRule) *CohortEngine {
	return &CohortEngine{rules: rules, fallback: fallback, policy: policy}
}

// Cohort implements the runtime's CohortResolver.
//
// It returns the pseudonymous consumer reference alongside the cohort, so the
// identity never reaches the data plane in raw form: redaction happens at
// assignment rather than later in the pipeline, where an intervening component
// could have logged it.
func (e *CohortEngine) Cohort(r *http.Request) (contract.CohortID, string) {
	consumer := e.policy.Pseudonymize(consumerIdentity(r))

	for _, rule := range e.rules {
		if rule.matches(r) {
			return rule.Cohort, consumer
		}
	}
	return e.fallback, consumer
}

// consumerIdentity extracts the raw identity a request claims.
func consumerIdentity(r *http.Request) string {
	for _, header := range []string{"X-FLUID-Consumer", "X-Consumer-ID"} {
		if v := r.Header.Get(header); v != "" {
			return v
		}
	}
	return ""
}

// Describe lists the configured cohorts, for operator display.
func (e *CohortEngine) Describe() []CohortRule {
	out := make([]CohortRule, len(e.rules))
	copy(out, e.rules)
	sort.Slice(out, func(i, j int) bool { return out[i].Cohort < out[j].Cohort })
	return out
}

// Population counts distinct consumers per cohort over a set of events.
//
// Counts below the policy's minimum are reported as suppressed rather than as
// a number, so a report cannot accidentally single out an individual.
type Population struct {
	Cohort     contract.CohortID `json:"cohort"`
	Consumers  int               `json:"consumers"`
	Events     int               `json:"events"`
	Suppressed bool              `json:"suppressed"`
}

// Populations summarizes cohort sizes across events.
func (e *CohortEngine) Populations(events []contract.FluidTelemetry) []Population {
	consumers := map[contract.CohortID]map[string]struct{}{}
	counts := map[contract.CohortID]int{}

	for _, ev := range events {
		if ev.Cohort == nil {
			continue
		}
		c := *ev.Cohort
		counts[c]++
		if consumers[c] == nil {
			consumers[c] = map[string]struct{}{}
		}
		if ev.ConsumerRef != "" {
			consumers[c][ev.ConsumerRef] = struct{}{}
		}
	}

	out := make([]Population, 0, len(counts))
	for c, n := range counts {
		distinct := len(consumers[c])
		out = append(out, Population{
			Cohort:     c,
			Consumers:  distinct,
			Events:     n,
			Suppressed: e.policy.SuppressSmallCohort(distinct),
		})
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Cohort < out[j].Cohort })
	return out
}