package main

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

type reverse struct {
	sort.Interface
}

func (r reverse) Less(i, j int) bool {
	return r.Interface.Less(j, i)
}

// ParseByQuality parses a field into the HTTP message to return the parameters
// sorted by importance and others with a quality value of 0 (if any).
// Returns nil if the field does not exist in the HTTP message.
//
// [HTTP spec](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html)
// [Quality Values](http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.9)
func ParseByQuality(r *http.Request, field string) (params, zero []string) {
	fieldValue := r.Header.Get(field)
	if fieldValue == "" {
		return
	}

	qParams := make(map[float64][]string)
	qSorted := make([]float64, 0)

	for _, f := range strings.Split(fieldValue, ",") {
		name := ""
		q := 1.0 // by default

		if strings.ContainsRune(f, ';') {
			name = strings.Split(f, ";")[0]
			name = strings.Trim(name, " ")

			q_ := strings.Split(f, "=")[1]
			q_ = strings.Trim(q_, " ")
			q, _ = strconv.ParseFloat(q_, 64)

			if q == 0 { // content not acceptable for the client
				zero = append(zero, name)
				continue
			}
		} else {
			name = strings.Trim(f, " ")
		}

		if _, ok := qParams[q]; !ok {
			qParams[q] = make([]string, 0)
		}
		qParams[q] = append(qParams[q], name)
	}

	// Sort the quality values.
	for k, _ := range qParams {
		qSorted = append(qSorted, k)
	}
	sort.Sort(reverse{sort.Float64Slice(qSorted)})

	for _, v := range qSorted {
		params = append(params, qParams[v]...)
	}
	return
}