package apiutil

import (
	"mime"
	"sort"
	"strconv"
	"strings"
)

type mediaRange struct {
	mt   string  // canonicalised media‑type, e.g. "application/json"
	q    float64 // quality factor (0‑1)
	raw  string  // original string – useful for logging/debugging
	spec int     // 2=exact, 1=type/*, 0=*/*
}

func parseMediaRange(field string) (mediaRange, bool) {
	field = strings.TrimSpace(field)

	mt, params, err := mime.ParseMediaType(field)
	if err != nil {
		log.WithError(err).Debug("Failed to parse header field")
		return mediaRange{}, false
	}

	r := mediaRange{mt: mt, q: 1, spec: 2, raw: field}

	if qs, ok := params["q"]; ok {
		v, err := strconv.ParseFloat(qs, 64)
		if err != nil || v < 0 || v > 1 {
			log.WithField("q", qs).Debug("Invalid quality factor (0‑1)")
			return mediaRange{}, false // skip invalid entry
		}
		r.q = v
	}

	switch {
	case mt == "*/*":
		r.spec = 0
	case strings.HasSuffix(mt, "/*"):
		r.spec = 1
	}
	return r, true
}

func hasExplicitQ(r mediaRange) bool {
	return strings.Contains(strings.ToLower(r.raw), ";q=")
}

// ParseAccept returns media ranges sorted by q (desc) then specificity.
func ParseAccept(header string) []mediaRange {
	if header == "" {
		return []mediaRange{{mt: "*/*", q: 1, spec: 0, raw: "*/*"}}
	}

	var out []mediaRange
	for field := range strings.SplitSeq(header, ",") {
		if r, ok := parseMediaRange(field); ok {
			out = append(out, r)
		}
	}

	sort.SliceStable(out, func(i, j int) bool {
		ei, ej := hasExplicitQ(out[i]), hasExplicitQ(out[j])
		if ei != ej {
			return ei // explicit beats implicit
		}
		if out[i].q != out[j].q {
			return out[i].q > out[j].q
		}
		return out[i].spec > out[j].spec
	})
	return out
}

// Matches reports whether content type is acceptable per the header.
func Matches(header, ct string) bool {
	for _, r := range ParseAccept(header) {
		switch {
		case r.q == 0:
			continue
		case r.mt == "*/*":
			return true
		case strings.HasSuffix(r.mt, "/*"):
			if strings.HasPrefix(ct, r.mt[:len(r.mt)-1]) {
				return true
			}
		case r.mt == ct:
			return true
		}
	}
	return false
}

// Negotiate selects the best server type according to the header.
// Returns the chosen type and true, or "", false when nothing matches.
func Negotiate(header string, serverTypes []string) (string, bool) {
	for _, r := range ParseAccept(header) {
		if r.q == 0 {
			continue
		}
		for _, s := range serverTypes {
			if Matches(r.mt, s) {
				return s, true
			}
		}
	}
	return "", false
}

// PrimaryAcceptMatches only checks if the first accept matches
func PrimaryAcceptMatches(header, produced string) bool {
	for _, r := range ParseAccept(header) {
		if r.q == 0 {
			continue // explicitly unacceptable – skip
		}
		return Matches(r.mt, produced)
	}
	return false
}
