package surf

import (
	"fmt"
	"net/http"
	"net/url"
	"sort"
	"strings"
)

// Overlapping constrained routes.
//
// Laravel matches routes in registration order and checks ->where()
// constraints while matching, so `GET /shelves/items/{id}` (id numeric) and
// `GET /shelves/{shelfId}/follow` live side by side: no request satisfies
// both. ServeMux only looks at the patterns and refuses the pair at
// registration because neither is more specific than the other.
//
// compile therefore groups such routes into overlap families (the transitive
// closure of "ServeMux refuses these two") and registers each family once,
// under a generated method-less pattern with a wildcard wherever the members
// differ. The family handler walks the members in registration order and runs
// the first whose literal segments and constraints match, with its own path
// values and its own fully wrapped handler. Everything outside a family
// registers exactly as before.

// pathSeg is one segment of a simple route path: a literal or a
// single-segment wildcard.
type pathSeg struct {
	raw   string // segment text as written in the route
	lit   string // unescaped literal; empty for a wildcard
	param string // wildcard name; empty for a literal
}

// overlapFamily is one group of routes registered behind a single pattern.
type overlapFamily struct {
	members []int    // indexes into Router.routes, in registration order
	pattern string   // generated method-less ServeMux pattern
	wild    []string // per segment: generated wildcard name, or "" for a shared literal
}

const overlapProbeSegment = "surf~overlap~probe"

var noopHandler = http.NotFoundHandler()

// parseSimplePath splits a route path into literal and single-segment
// wildcard segments. It reports false for every other shape: the root, a
// trailing-slash subtree, an empty segment, a {name...} or {$} wildcard, a
// host, or a malformed escape.
func parseSimplePath(path string) ([]pathSeg, bool) {
	if !strings.HasPrefix(path, "/") || path == "/" || strings.HasSuffix(path, "/") {
		return nil, false
	}
	parts := strings.Split(path[1:], "/")
	segs := make([]pathSeg, len(parts))
	for i, part := range parts {
		if part == "" {
			return nil, false
		}
		if strings.HasPrefix(part, "{") && strings.HasSuffix(part, "}") {
			name := part[1 : len(part)-1]
			if !isWildcardName(name) {
				return nil, false
			}
			segs[i] = pathSeg{raw: part, param: name}
			continue
		}
		if strings.ContainsAny(part, "{}") {
			return nil, false
		}
		lit, err := url.PathUnescape(part)
		if err != nil {
			return nil, false
		}
		segs[i] = pathSeg{raw: part, lit: lit}
	}
	return segs, true
}

func isWildcardName(name string) bool {
	if name == "" {
		return false
	}
	for i, c := range name {
		switch {
		case c == '_', c >= 'a' && c <= 'z', c >= 'A' && c <= 'Z':
		case i > 0 && c >= '0' && c <= '9':
		default:
			return false
		}
	}
	return true
}

// tryHandle registers pattern on mux and reports whether ServeMux accepted it.
func tryHandle(mux *http.ServeMux, pattern string, h http.Handler) (ok bool) {
	defer func() {
		if recover() != nil {
			ok = false
		}
	}()
	mux.Handle(pattern, h)
	return true
}

// muxConflict asks ServeMux itself whether two valid patterns can share a mux.
func muxConflict(a, b string) bool {
	if !tryHandle(http.NewServeMux(), a, noopHandler) || !tryHandle(http.NewServeMux(), b, noopHandler) {
		return false
	}
	mux := http.NewServeMux()
	mux.Handle(a, noopHandler)
	return !tryHandle(mux, b, noopHandler)
}

// mayOverlap is a cheap pre-filter: two simple paths with a different segment
// count or two different literals at one position can never match the same
// request. Anything else goes to muxConflict.
func mayOverlap(a, b string) bool {
	sa, okA := parseSimplePath(a)
	sb, okB := parseSimplePath(b)
	if !okA || !okB {
		return true
	}
	if len(sa) != len(sb) {
		return false
	}
	for i := range sa {
		if sa[i].param == "" && sb[i].param == "" && sa[i].lit != sb[i].lit {
			return false
		}
	}
	return true
}

func routePattern(rt route) string { return rt.method + " " + rt.path }

func routeConflict(a, b route) bool {
	return a.method == b.method && mayOverlap(a.path, b.path) && muxConflict(routePattern(a), routePattern(b))
}

type unionFind []int

func newUnionFind(n int) unionFind {
	u := make(unionFind, n)
	for i := range u {
		u[i] = i
	}
	return u
}

func (u unionFind) find(i int) int {
	for u[i] != i {
		u[i] = u[u[i]]
		i = u[i]
	}
	return i
}

// union joins two sets and reports whether they were separate.
func (u unionFind) union(a, b int) bool {
	ra, rb := u.find(a), u.find(b)
	if ra == rb {
		return false
	}
	if ra < rb {
		u[rb] = ra
	} else {
		u[ra] = rb
	}
	return true
}

func (u unionFind) groups() [][]int {
	byRoot := map[int][]int{}
	var roots []int
	for i := range u {
		root := u.find(i)
		if _, ok := byRoot[root]; !ok {
			roots = append(roots, root)
		}
		byRoot[root] = append(byRoot[root], i)
	}
	sort.Ints(roots)
	var out [][]int
	for _, root := range roots {
		if members := byRoot[root]; len(members) > 1 {
			out = append(out, members)
		}
	}
	return out
}

// overlapFamilies finds every overlap family in routes. A route whose
// pattern ServeMux rejects for another reason than a conflict is left alone
// so handleRoute reports it as before.
func overlapFamilies(routes []route) ([]overlapFamily, error) {
	uf := newUnionFind(len(routes))
	probe := http.NewServeMux()
	var outside []int // routes the probe mux refused
	for j, rt := range routes {
		pat := routePattern(rt)
		if !tryHandle(http.NewServeMux(), pat, noopHandler) {
			continue
		}
		if tryHandle(probe, pat, noopHandler) {
			for _, i := range outside {
				if routeConflict(routes[i], rt) {
					uf.union(i, j)
				}
			}
			continue
		}
		for i := 0; i < j; i++ {
			if routeConflict(routes[i], rt) {
				uf.union(i, j)
			}
		}
		outside = append(outside, j)
	}
	if len(uf.groups()) == 0 {
		return nil, nil
	}
	// A family's generated pattern may itself overlap another route or
	// another family's pattern; fold those in until nothing changes.
	for {
		groups := uf.groups()
		fams := make([]overlapFamily, len(groups))
		for fi, members := range groups {
			fam, err := generaliseFamily(routes, members)
			if err != nil {
				return nil, err
			}
			fams[fi] = fam
		}
		changed := false
		for _, fam := range fams {
			root := uf.find(fam.members[0])
			for k, rt := range routes {
				if uf.find(k) == root || inAnyFamily(uf, fams, k) {
					continue
				}
				if mayOverlap(fam.pattern, rt.path) && muxConflict(fam.pattern, routePattern(rt)) {
					changed = uf.union(fam.members[0], k) || changed
				}
			}
		}
		for a := 0; a < len(fams); a++ {
			for b := a + 1; b < len(fams); b++ {
				if mayOverlap(fams[a].pattern, fams[b].pattern) && muxConflict(fams[a].pattern, fams[b].pattern) {
					changed = uf.union(fams[a].members[0], fams[b].members[0]) || changed
				}
			}
		}
		if !changed {
			return fams, nil
		}
	}
}

func inAnyFamily(uf unionFind, fams []overlapFamily, k int) bool {
	root := uf.find(k)
	for _, fam := range fams {
		if uf.find(fam.members[0]) == root {
			return true
		}
	}
	return false
}

func describeRoute(rt route) string {
	return fmt.Sprintf("%s %s (plugin %q)", rt.method, rt.path, rt.pluginID)
}

// generaliseFamily validates the member shapes and builds the shared pattern.
func generaliseFamily(routes []route, members []int) (overlapFamily, error) {
	parsed := make([][]pathSeg, len(members))
	for mi, idx := range members {
		segs, ok := parseSimplePath(routes[idx].path)
		if !ok {
			other := members[0]
			if other == idx {
				other = members[1]
			}
			return overlapFamily{}, fmt.Errorf("surf: route conflict for %s and %s: overlapping routes must use literal and single-segment {name} wildcards only",
				describeRoute(routes[idx]), describeRoute(routes[other]))
		}
		parsed[mi] = segs
	}
	n := len(parsed[0])
	for mi := 1; mi < len(parsed); mi++ {
		if len(parsed[mi]) != n {
			return overlapFamily{}, fmt.Errorf("surf: route conflict for %s and %s: overlapping routes differ in segment count",
				describeRoute(routes[members[mi]]), describeRoute(routes[members[0]]))
		}
	}
	// Two routes of one method with the same shape differ only in parameter
	// names: the later one could never be reached by registration order.
	for a := 0; a < len(members); a++ {
		for b := a + 1; b < len(members); b++ {
			if routes[members[a]].method == routes[members[b]].method && sameShape(parsed[a], parsed[b]) {
				return overlapFamily{}, fmt.Errorf("surf: route conflict for %s and %s: the routes have the same shape",
					describeRoute(routes[members[b]]), describeRoute(routes[members[a]]))
			}
		}
	}
	wild := make([]string, n)
	parts := make([]string, n)
	for i := 0; i < n; i++ {
		shared := true
		for mi := range parsed {
			s := parsed[mi][i]
			if s.param != "" || s.lit != parsed[0][i].lit {
				shared = false
				break
			}
		}
		if shared {
			parts[i] = parsed[0][i].raw
			continue
		}
		wild[i] = fmt.Sprintf("surfOverlap%d", i)
		parts[i] = "{" + wild[i] + "}"
	}
	return overlapFamily{
		members: append([]int(nil), members...),
		pattern: "/" + strings.Join(parts, "/"),
		wild:    wild,
	}, nil
}

func sameShape(a, b []pathSeg) bool {
	if len(a) != len(b) {
		return false
	}
	for i := range a {
		if (a[i].param == "") != (b[i].param == "") {
			return false
		}
		if a[i].param == "" && a[i].lit != b[i].lit {
			return false
		}
	}
	return true
}

// familyMember is one route behind a family pattern.
type familyMember struct {
	method      string
	pattern     string
	segs        []pathSeg
	constraints []Constraint
	handler     http.Handler
}

func (m familyMember) pathMatches(vals []string) bool {
	for i, s := range m.segs {
		if s.param == "" && s.lit != vals[i] {
			return false
		}
	}
	return true
}

func (m familyMember) constraintsMatch(vals []string) bool {
	for _, c := range m.constraints {
		value := ""
		for i, s := range m.segs {
			if s.param == c.param {
				value = vals[i]
				break
			}
		}
		if !c.match(value) {
			return false
		}
	}
	return true
}

// methodMatches mirrors ServeMux: a GET route also answers HEAD.
func methodMatches(routeMethod, reqMethod string) bool {
	return routeMethod == reqMethod || (routeMethod == http.MethodGet && reqMethod == http.MethodHead)
}

// familyDispatch is the handler registered under a family pattern.
type familyDispatch struct {
	pattern string
	wild    []string
	lits    []string
	members []familyMember
	methods []string // every method in the router's table, for Allow
	mux     *http.ServeMux
}

func (f *familyDispatch) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	vals := make([]string, len(f.wild))
	for i, name := range f.wild {
		if name == "" {
			vals[i] = f.lits[i]
		} else {
			vals[i] = r.PathValue(name)
		}
	}
	methodSeen := false
	for _, m := range f.members {
		if !methodMatches(m.method, r.Method) || !m.pathMatches(vals) {
			continue
		}
		methodSeen = true
		if !m.constraintsMatch(vals) {
			continue
		}
		for i, s := range m.segs {
			if s.param != "" {
				r.SetPathValue(s.param, vals[i])
			}
		}
		r.Pattern = m.pattern
		m.handler.ServeHTTP(w, r)
		return
	}
	if methodSeen {
		http.NotFound(w, r)
		return
	}
	allow := f.allowed(r, vals)
	if len(allow) == 0 {
		http.NotFound(w, r)
		return
	}
	w.Header().Set("Allow", strings.Join(allow, ", "))
	http.Error(w, http.StatusText(http.StatusMethodNotAllowed), http.StatusMethodNotAllowed)
}

// allowed lists the methods ServeMux would put in Allow for this path if the
// table held the members on their own patterns: any other route the mux
// matches for the method, plus members whose literal segments match.
func (f *familyDispatch) allowed(r *http.Request, vals []string) []string {
	set := map[string]bool{}
	for _, method := range f.methods {
		_, pat := f.mux.Handler(&http.Request{Method: method, URL: r.URL, Host: r.Host})
		switch pat {
		case "":
		case f.pattern:
			for _, m := range f.members {
				if m.method == method && m.pathMatches(vals) {
					set[method] = true
					break
				}
			}
		default:
			set[method] = true
		}
	}
	if set[http.MethodGet] {
		set[http.MethodHead] = true
	}
	out := make([]string, 0, len(set))
	for method := range set {
		out = append(out, method)
	}
	sort.Strings(out)
	return out
}

// registerOverlapFamilies registers each family pattern on mux, then checks
// that no other route takes a member's requests away from the family.
func registerOverlapFamilies(mux *http.ServeMux, routes []route, handlers []http.Handler, fams []overlapFamily) error {
	if len(fams) == 0 {
		return nil
	}
	methodSet := map[string]bool{}
	for _, rt := range routes {
		methodSet[rt.method] = true
	}
	methods := make([]string, 0, len(methodSet))
	for method := range methodSet {
		methods = append(methods, method)
	}
	sort.Strings(methods)
	for _, fam := range fams {
		d := &familyDispatch{pattern: fam.pattern, wild: fam.wild, methods: methods, mux: mux}
		d.lits = make([]string, len(fam.wild))
		for _, idx := range fam.members {
			rt := routes[idx]
			segs, _ := parseSimplePath(rt.path)
			for i, s := range segs {
				if fam.wild[i] == "" {
					d.lits[i] = s.lit
				}
			}
			d.members = append(d.members, familyMember{
				method:      rt.method,
				pattern:     routePattern(rt),
				segs:        segs,
				constraints: rt.constraints,
				handler:     handlers[idx],
			})
		}
		if !tryHandle(mux, fam.pattern, d) {
			first := routes[fam.members[0]]
			return fmt.Errorf("surf: route conflict for %s: the overlap pattern %s cannot be registered", describeRoute(first), fam.pattern)
		}
	}
	for _, fam := range fams {
		for _, idx := range fam.members {
			rt := routes[idx]
			segs, _ := parseSimplePath(rt.path)
			parts := make([]string, len(segs))
			for i, s := range segs {
				if s.param != "" {
					parts[i] = overlapProbeSegment
				} else {
					parts[i] = s.raw
				}
			}
			probe := &http.Request{Method: rt.method, URL: &url.URL{Path: "/" + strings.Join(parts, "/")}}
			if _, pat := mux.Handler(probe); pat != fam.pattern {
				return fmt.Errorf("surf: route conflict for %s: route %s takes its requests before the overlapping routes are tried", describeRoute(rt), pat)
			}
		}
	}
	return nil
}