src/net/http/pattern.go - go - Git at Google

 // Copyright 2023 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // Patterns for ServeMux routing.

 package http

 import (
 	"errors"
 	"fmt"
 	"net/url"
 	"strings"
 	"unicode"
 )

 // A pattern is something that can be matched against an HTTP request.
 // It has an optional method, an optional host, and a path.
 type pattern struct {
 	str    string // original string
 	method string
 	host   string
 	// The representation of a path differs from the surface syntax, which
 	// simplifies most algorithms.
 	//
 	// Paths ending in '/' are represented with an anonymous "..." wildcard.
 	// For example, the path "a/" is represented as a literal segment "a" followed
 	// by a segment with multi==true.
 	//
 	// Paths ending in "{$}" are represented with the literal segment "/".
 	// For example, the path "a/{$}" is represented as a literal segment "a" followed
 	// by a literal segment "/".
 	segments []segment
 	loc      string // source location of registering call, for helpful messages
 }

 func (p *pattern) String() string { return p.str }

 func (p *pattern) lastSegment() segment {
 	return p.segments[len(p.segments)-1]
 }

 // A segment is a pattern piece that matches one or more path segments, or
 // a trailing slash.
 //
 // If wild is false, it matches a literal segment, or, if s == "/", a trailing slash.
 // Examples:
 //
 //	"a" => segment{s: "a"}
 //	"/{$}" => segment{s: "/"}
 //
 // If wild is true and multi is false, it matches a single path segment.
 // Example:
 //
 //	"{x}" => segment{s: "x", wild: true}
 //
 // If both wild and multi are true, it matches all remaining path segments.
 // Example:
 //
 //	"{rest...}" => segment{s: "rest", wild: true, multi: true}
 type segment struct {
 	s     string // literal or wildcard name or "/" for "/{$}".
 	wild  bool
 	multi bool // "..." wildcard
 }

 // parsePattern parses a string into a Pattern.
 // The string's syntax is
 //
 //	[METHOD] [HOST]/[PATH]
 //
 // where:
 //   - METHOD is an HTTP method
 //   - HOST is a hostname
 //   - PATH consists of slash-separated segments, where each segment is either
 //     a literal or a wildcard of the form "{name}", "{name...}", or "{$}".
 //
 // METHOD, HOST and PATH are all optional; that is, the string can be "/".
 // If METHOD is present, it must be followed by at least one space or tab.
 // Wildcard names must be valid Go identifiers.
 // The "{$}" and "{name...}" wildcard must occur at the end of PATH.
 // PATH may end with a '/'.
 // Wildcard names in a path must be distinct.
 func parsePattern(s string) (_ *pattern, err error) {
 	if len(s) == 0 {
 		return nil, errors.New("empty pattern")
 	}
 	off := 0 // offset into string
 	defer func() {
 		if err != nil {
 			err = fmt.Errorf("at offset %d: %w", off, err)
 		}
 	}()

 	method, rest, found := s, "", false
 	if i := strings.IndexAny(s, " \t"); i >= 0 {
 		method, rest, found = s[:i], strings.TrimLeft(s[i+1:], " \t"), true
 	}
 	if !found {
 		rest = method
 		method = ""
 	}
 	if method != "" && !validMethod(method) {
 		return nil, fmt.Errorf("invalid method %q", method)
 	}
 	p := &pattern{str: s, method: method}

 	if found {
 		off = len(method) + 1
 	}
 	i := strings.IndexByte(rest, '/')
 	if i < 0 {
 		return nil, errors.New("host/path missing /")
 	}
 	p.host = rest[:i]
 	rest = rest[i:]
 	if j := strings.IndexByte(p.host, '{'); j >= 0 {
 		off += j
 		return nil, errors.New("host contains '{' (missing initial '/'?)")
 	}
 	// At this point, rest is the path.
 	off += i

 	// An unclean path with a method that is not CONNECT can never match,
 	// because paths are cleaned before matching.
 	if method != "" && method != "CONNECT" && rest != cleanPath(rest) {
 		return nil, errors.New("non-CONNECT pattern with unclean path can never match")
 	}

 	seenNames := map[string]bool{} // remember wildcard names to catch dups
 	for len(rest) > 0 {
 		// Invariant: rest[0] == '/'.
 		rest = rest[1:]
 		off = len(s) - len(rest)
 		if len(rest) == 0 {
 			// Trailing slash.
 			p.segments = append(p.segments, segment{wild: true, multi: true})
 			break
 		}
 		i := strings.IndexByte(rest, '/')
 		if i < 0 {
 			i = len(rest)
 		}
 		var seg string
 		seg, rest = rest[:i], rest[i:]
 		if i := strings.IndexByte(seg, '{'); i < 0 {
 			// Literal.
 			seg = pathUnescape(seg)
 			p.segments = append(p.segments, segment{s: seg})
 		} else {
 			// Wildcard.
 			if i != 0 {
 				return nil, errors.New("bad wildcard segment (must start with '{')")
 			}
 			if seg[len(seg)-1] != '}' {
 				return nil, errors.New("bad wildcard segment (must end with '}')")
 			}
 			name := seg[1 : len(seg)-1]
 			if name == "$" {
 				if len(rest) != 0 {
 					return nil, errors.New("{$} not at end")
 				}
 				p.segments = append(p.segments, segment{s: "/"})
 				break
 			}
 			name, multi := strings.CutSuffix(name, "...")
 			if multi && len(rest) != 0 {
 				return nil, errors.New("{...} wildcard not at end")
 			}
 			if name == "" {
 				return nil, errors.New("empty wildcard")
 			}
 			if !isValidWildcardName(name) {
 				return nil, fmt.Errorf("bad wildcard name %q", name)
 			}
 			if seenNames[name] {
 				return nil, fmt.Errorf("duplicate wildcard name %q", name)
 			}
 			seenNames[name] = true
 			p.segments = append(p.segments, segment{s: name, wild: true, multi: multi})
 		}
 	}
 	return p, nil
 }

 func isValidWildcardName(s string) bool {
 	if s == "" {
 		return false
 	}
 	// Valid Go identifier.
 	for i, c := range s {
 		if !unicode.IsLetter(c) && c != '_' && (i == 0 || !unicode.IsDigit(c)) {
 			return false
 		}
 	}
 	return true
 }

 func pathUnescape(path string) string {
 	u, err := url.PathUnescape(path)
 	if err != nil {
 		// Invalidly escaped path; use the original
 		return path
 	}
 	return u
 }

 // relationship is a relationship between two patterns, p1 and p2.
 type relationship string

 const (
 	equivalent   relationship = "equivalent"   // both match the same requests
 	moreGeneral  relationship = "moreGeneral"  // p1 matches everything p2 does & more
 	moreSpecific relationship = "moreSpecific" // p2 matches everything p1 does & more
 	disjoint     relationship = "disjoint"     // there is no request that both match
 	overlaps     relationship = "overlaps"     // there is a request that both match, but neither is more specific
 )

 // conflictsWith reports whether p1 conflicts with p2, that is, whether
 // there is a request that both match but where neither is higher precedence
 // than the other.
 //
 //	Precedence is defined by two rules:
 //	1. Patterns with a host win over patterns without a host.
 //	2. Patterns whose method and path is more specific win. One pattern is more
 //	   specific than another if the second matches all the (method, path) pairs
 //	   of the first and more.
 //
 // If rule 1 doesn't apply, then two patterns conflict if their relationship
 // is either equivalence (they match the same set of requests) or overlap
 // (they both match some requests, but neither is more specific than the other).
 func (p1 *pattern) conflictsWith(p2 *pattern) bool {
 	if p1.host != p2.host {
 		// Either one host is empty and the other isn't, in which case the
 		// one with the host wins by rule 1, or neither host is empty
 		// and they differ, so they won't match the same paths.
 		return false
 	}
 	rel := p1.comparePathsAndMethods(p2)
 	return rel == equivalent || rel == overlaps
 }

 func (p1 *pattern) comparePathsAndMethods(p2 *pattern) relationship {
 	mrel := p1.compareMethods(p2)
 	// Optimization: avoid a call to comparePaths.
 	if mrel == disjoint {
 		return disjoint
 	}
 	prel := p1.comparePaths(p2)
 	return combineRelationships(mrel, prel)
 }

 // compareMethods determines the relationship between the method
 // part of patterns p1 and p2.
 //
 // A method can either be empty, "GET", or something else.
 // The empty string matches any method, so it is the most general.
 // "GET" matches both GET and HEAD.
 // Anything else matches only itself.
 func (p1 *pattern) compareMethods(p2 *pattern) relationship {
 	if p1.method == p2.method {
 		return equivalent
 	}
 	if p1.method == "" {
 		// p1 matches any method, but p2 does not, so p1 is more general.
 		return moreGeneral
 	}
 	if p2.method == "" {
 		return moreSpecific
 	}
 	if p1.method == "GET" && p2.method == "HEAD" {
 		// p1 matches GET and HEAD; p2 matches only HEAD.
 		return moreGeneral
 	}
 	if p2.method == "GET" && p1.method == "HEAD" {
 		return moreSpecific
 	}
 	return disjoint
 }

 // comparePaths determines the relationship between the path
 // part of two patterns.
 func (p1 *pattern) comparePaths(p2 *pattern) relationship {
 	// Optimization: if a path pattern doesn't end in a multi ("...") wildcard, then it
 	// can only match paths with the same number of segments.
 	if len(p1.segments) != len(p2.segments) && !p1.lastSegment().multi && !p2.lastSegment().multi {
 		return disjoint
 	}

 	// Consider corresponding segments in the two path patterns.
 	var segs1, segs2 []segment
 	rel := equivalent
 	for segs1, segs2 = p1.segments, p2.segments; len(segs1) > 0 && len(segs2) > 0; segs1, segs2 = segs1[1:], segs2[1:] {
 		rel = combineRelationships(rel, compareSegments(segs1[0], segs2[0]))
 		if rel == disjoint {
 			return rel
 		}
 	}
 	// We've reached the end of the corresponding segments of the patterns.
 	// If they have the same number of segments, then we've already determined
 	// their relationship.
 	if len(segs1) == 0 && len(segs2) == 0 {
 		return rel
 	}
 	// Otherwise, the only way they could fail to be disjoint is if the shorter
 	// pattern ends in a multi. In that case, that multi is more general
 	// than the remainder of the longer pattern, so combine those two relationships.
 	if len(segs1) < len(segs2) && p1.lastSegment().multi {
 		return combineRelationships(rel, moreGeneral)
 	}
 	if len(segs2) < len(segs1) && p2.lastSegment().multi {
 		return combineRelationships(rel, moreSpecific)
 	}
 	return disjoint
 }

 // compareSegments determines the relationship between two segments.
 func compareSegments(s1, s2 segment) relationship {
 	if s1.multi && s2.multi {
 		return equivalent
 	}
 	if s1.multi {
 		return moreGeneral
 	}
 	if s2.multi {
 		return moreSpecific
 	}
 	if s1.wild && s2.wild {
 		return equivalent
 	}
 	if s1.wild {
 		if s2.s == "/" {
 			// A single wildcard doesn't match a trailing slash.
 			return disjoint
 		}
 		return moreGeneral
 	}
 	if s2.wild {
 		if s1.s == "/" {
 			return disjoint
 		}
 		return moreSpecific
 	}
 	// Both literals.
 	if s1.s == s2.s {
 		return equivalent
 	}
 	return disjoint
 }

 // combineRelationships determines the overall relationship of two patterns
 // given the relationships of a partition of the patterns into two parts.
 //
 // For example, if p1 is more general than p2 in one way but equivalent
 // in the other, then it is more general overall.
 //
 // Or if p1 is more general in one way and more specific in the other, then
 // they overlap.
 func combineRelationships(r1, r2 relationship) relationship {
 	switch r1 {
 	case equivalent:
 		return r2
 	case disjoint:
 		return disjoint
 	case overlaps:
 		if r2 == disjoint {
 			return disjoint
 		}
 		return overlaps
 	case moreGeneral, moreSpecific:
 		switch r2 {
 		case equivalent:
 			return r1
 		case inverseRelationship(r1):
 			return overlaps
 		default:
 			return r2
 		}
 	default:
 		panic(fmt.Sprintf("unknown relationship %q", r1))
 	}
 }

 // If p1 has relationship `r` to p2, then
 // p2 has inverseRelationship(r) to p1.
 func inverseRelationship(r relationship) relationship {
 	switch r {
 	case moreSpecific:
 		return moreGeneral
 	case moreGeneral:
 		return moreSpecific
 	default:
 		return r
 	}
 }

 // isLitOrSingle reports whether the segment is a non-dollar literal or a single wildcard.
 func isLitOrSingle(seg segment) bool {
 	if seg.wild {
 		return !seg.multi
 	}
 	return seg.s != "/"
 }

 // describeConflict returns an explanation of why two patterns conflict.
 func describeConflict(p1, p2 *pattern) string {
 	mrel := p1.compareMethods(p2)
 	prel := p1.comparePaths(p2)
 	rel := combineRelationships(mrel, prel)
 	if rel == equivalent {
 		return fmt.Sprintf("%s matches the same requests as %s", p1, p2)
 	}
 	if rel != overlaps {
 		panic("describeConflict called with non-conflicting patterns")
 	}
 	if prel == overlaps {
 		return fmt.Sprintf(`%[1]s and %[2]s both match some paths, like %[3]q.
 But neither is more specific than the other.
 %[1]s matches %[4]q, but %[2]s doesn't.
 %[2]s matches %[5]q, but %[1]s doesn't.`,
 			p1, p2, commonPath(p1, p2), differencePath(p1, p2), differencePath(p2, p1))
 	}
 	if mrel == moreGeneral && prel == moreSpecific {
 		return fmt.Sprintf("%s matches more methods than %s, but has a more specific path pattern", p1, p2)
 	}
 	if mrel == moreSpecific && prel == moreGeneral {
 		return fmt.Sprintf("%s matches fewer methods than %s, but has a more general path pattern", p1, p2)
 	}
 	return fmt.Sprintf("bug: unexpected way for two patterns %s and %s to conflict: methods %s, paths %s", p1, p2, mrel, prel)
 }

 // writeMatchingPath writes to b a path that matches the segments.
 func writeMatchingPath(b *strings.Builder, segs []segment) {
 	for _, s := range segs {
 		writeSegment(b, s)
 	}
 }

 func writeSegment(b *strings.Builder, s segment) {
 	b.WriteByte('/')
 	if !s.multi && s.s != "/" {
 		b.WriteString(s.s)
 	}
 }

 // commonPath returns a path that both p1 and p2 match.
 // It assumes there is such a path.
 func commonPath(p1, p2 *pattern) string {
 	var b strings.Builder
 	var segs1, segs2 []segment
 	for segs1, segs2 = p1.segments, p2.segments; len(segs1) > 0 && len(segs2) > 0; segs1, segs2 = segs1[1:], segs2[1:] {
 		if s1 := segs1[0]; s1.wild {
 			writeSegment(&b, segs2[0])
 		} else {
 			writeSegment(&b, s1)
 		}
 	}
 	if len(segs1) > 0 {
 		writeMatchingPath(&b, segs1)
 	} else if len(segs2) > 0 {
 		writeMatchingPath(&b, segs2)
 	}
 	return b.String()
 }

 // differencePath returns a path that p1 matches and p2 doesn't.
 // It assumes there is such a path.
 func differencePath(p1, p2 *pattern) string {
 	var b strings.Builder

 	var segs1, segs2 []segment
 	for segs1, segs2 = p1.segments, p2.segments; len(segs1) > 0 && len(segs2) > 0; segs1, segs2 = segs1[1:], segs2[1:] {
 		s1 := segs1[0]
 		s2 := segs2[0]
 		if s1.multi && s2.multi {
 			// From here the patterns match the same paths, so we must have found a difference earlier.
 			b.WriteByte('/')
 			return b.String()

 		}
 		if s1.multi && !s2.multi {
 			// s1 ends in a "..." wildcard but s2 does not.
 			// A trailing slash will distinguish them, unless s2 ends in "{$}",
 			// in which case any segment will do; prefer the wildcard name if
 			// it has one.
 			b.WriteByte('/')
 			if s2.s == "/" {
 				if s1.s != "" {
 					b.WriteString(s1.s)
 				} else {
 					b.WriteString("x")
 				}
 			}
 			return b.String()
 		}
 		if !s1.multi && s2.multi {
 			writeSegment(&b, s1)
 		} else if s1.wild && s2.wild {
 			// Both patterns will match whatever we put here; use
 			// the first wildcard name.
 			writeSegment(&b, s1)
 		} else if s1.wild && !s2.wild {
 			// s1 is a wildcard, s2 is a literal.
 			// Any segment other than s2.s will work.
 			// Prefer the wildcard name, but if it's the same as the literal,
 			// tweak the literal.
 			if s1.s != s2.s {
 				writeSegment(&b, s1)
 			} else {
 				b.WriteByte('/')
 				b.WriteString(s2.s + "x")
 			}
 		} else if !s1.wild && s2.wild {
 			writeSegment(&b, s1)
 		} else {
 			// Both are literals. A precondition of this function is that the
 			// patterns overlap, so they must be the same literal. Use it.
 			if s1.s != s2.s {
 				panic(fmt.Sprintf("literals differ: %q and %q", s1.s, s2.s))
 			}
 			writeSegment(&b, s1)
 		}
 	}
 	if len(segs1) > 0 {
 		// p1 is longer than p2, and p2 does not end in a multi.
 		// Anything that matches the rest of p1 will do.
 		writeMatchingPath(&b, segs1)
 	} else if len(segs2) > 0 {
 		writeMatchingPath(&b, segs2)
 	}
 	return b.String()
 }
	// Copyright 2023 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// Patterns for ServeMux routing.

	package http

	import (
	"errors"
	"fmt"
	"net/url"
	"strings"
	"unicode"
	)

	// A pattern is something that can be matched against an HTTP request.
	// It has an optional method, an optional host, and a path.
	type pattern struct {
	str string // original string
	method string
	host string
	// The representation of a path differs from the surface syntax, which
	// simplifies most algorithms.
	//
	// Paths ending in '/' are represented with an anonymous "..." wildcard.
	// For example, the path "a/" is represented as a literal segment "a" followed
	// by a segment with multi==true.
	//
	// Paths ending in "{$}" are represented with the literal segment "/".
	// For example, the path "a/{$}" is represented as a literal segment "a" followed
	// by a literal segment "/".
	segments []segment
	loc string // source location of registering call, for helpful messages
	}

	func (p *pattern) String() string { return p.str }

	func (p *pattern) lastSegment() segment {
	return p.segments[len(p.segments)-1]
	}

	// A segment is a pattern piece that matches one or more path segments, or
	// a trailing slash.
	//
	// If wild is false, it matches a literal segment, or, if s == "/", a trailing slash.
	// Examples:
	//
	// "a" => segment{s: "a"}
	// "/{$}" => segment{s: "/"}
	//
	// If wild is true and multi is false, it matches a single path segment.
	// Example:
	//
	// "{x}" => segment{s: "x", wild: true}
	//
	// If both wild and multi are true, it matches all remaining path segments.
	// Example:
	//
	// "{rest...}" => segment{s: "rest", wild: true, multi: true}
	type segment struct {
	s string // literal or wildcard name or "/" for "/{$}".
	wild bool
	multi bool // "..." wildcard
	}

	// parsePattern parses a string into a Pattern.
	// The string's syntax is
	//
	// [METHOD] [HOST]/[PATH]
	//
	// where:
	// - METHOD is an HTTP method
	// - HOST is a hostname
	// - PATH consists of slash-separated segments, where each segment is either
	// a literal or a wildcard of the form "{name}", "{name...}", or "{$}".
	//
	// METHOD, HOST and PATH are all optional; that is, the string can be "/".
	// If METHOD is present, it must be followed by at least one space or tab.
	// Wildcard names must be valid Go identifiers.
	// The "{$}" and "{name...}" wildcard must occur at the end of PATH.
	// PATH may end with a '/'.
	// Wildcard names in a path must be distinct.
	func parsePattern(s string) (_ *pattern, err error) {
	if len(s) == 0 {
	return nil, errors.New("empty pattern")
	}
	off := 0 // offset into string
	defer func() {
	if err != nil {
	err = fmt.Errorf("at offset %d: %w", off, err)
	}
	}()

	method, rest, found := s, "", false
	if i := strings.IndexAny(s, " \t"); i >= 0 {
	method, rest, found = s[:i], strings.TrimLeft(s[i+1:], " \t"), true
	}
	if !found {
	rest = method
	method = ""
	}
	if method != "" && !validMethod(method) {
	return nil, fmt.Errorf("invalid method %q", method)
	}
	p := &pattern{str: s, method: method}

	if found {
	off = len(method) + 1
	}
	i := strings.IndexByte(rest, '/')
	if i < 0 {
	return nil, errors.New("host/path missing /")
	}
	p.host = rest[:i]
	rest = rest[i:]
	if j := strings.IndexByte(p.host, '{'); j >= 0 {
	off += j
	return nil, errors.New("host contains '{' (missing initial '/'?)")
	}
	// At this point, rest is the path.
	off += i

	// An unclean path with a method that is not CONNECT can never match,
	// because paths are cleaned before matching.
	if method != "" && method != "CONNECT" && rest != cleanPath(rest) {
	return nil, errors.New("non-CONNECT pattern with unclean path can never match")
	}

	seenNames := map[string]bool{} // remember wildcard names to catch dups
	for len(rest) > 0 {
	// Invariant: rest[0] == '/'.
	rest = rest[1:]
	off = len(s) - len(rest)
	if len(rest) == 0 {
	// Trailing slash.
	p.segments = append(p.segments, segment{wild: true, multi: true})
	break
	}
	i := strings.IndexByte(rest, '/')
	if i < 0 {
	i = len(rest)
	}
	var seg string
	seg, rest = rest[:i], rest[i:]
	if i := strings.IndexByte(seg, '{'); i < 0 {
	// Literal.
	seg = pathUnescape(seg)
	p.segments = append(p.segments, segment{s: seg})
	} else {
	// Wildcard.
	if i != 0 {
	return nil, errors.New("bad wildcard segment (must start with '{')")
	}
	if seg[len(seg)-1] != '}' {
	return nil, errors.New("bad wildcard segment (must end with '}')")
	}
	name := seg[1 : len(seg)-1]
	if name == "$" {
	if len(rest) != 0 {
	return nil, errors.New("{$} not at end")
	}
	p.segments = append(p.segments, segment{s: "/"})
	break
	}
	name, multi := strings.CutSuffix(name, "...")
	if multi && len(rest) != 0 {
	return nil, errors.New("{...} wildcard not at end")
	}
	if name == "" {
	return nil, errors.New("empty wildcard")
	}
	if !isValidWildcardName(name) {
	return nil, fmt.Errorf("bad wildcard name %q", name)
	}
	if seenNames[name] {
	return nil, fmt.Errorf("duplicate wildcard name %q", name)
	}
	seenNames[name] = true
	p.segments = append(p.segments, segment{s: name, wild: true, multi: multi})
	}
	}
	return p, nil
	}

	func isValidWildcardName(s string) bool {
	if s == "" {
	return false
	}
	// Valid Go identifier.
	for i, c := range s {
	if !unicode.IsLetter(c) && c != '_' && (i == 0 \|\| !unicode.IsDigit(c)) {
	return false
	}
	}
	return true
	}

	func pathUnescape(path string) string {
	u, err := url.PathUnescape(path)
	if err != nil {
	// Invalidly escaped path; use the original
	return path
	}
	return u
	}

	// relationship is a relationship between two patterns, p1 and p2.
	type relationship string

	const (
	equivalent relationship = "equivalent" // both match the same requests
	moreGeneral relationship = "moreGeneral" // p1 matches everything p2 does & more
	moreSpecific relationship = "moreSpecific" // p2 matches everything p1 does & more
	disjoint relationship = "disjoint" // there is no request that both match
	overlaps relationship = "overlaps" // there is a request that both match, but neither is more specific
	)

	// conflictsWith reports whether p1 conflicts with p2, that is, whether
	// there is a request that both match but where neither is higher precedence
	// than the other.
	//
	// Precedence is defined by two rules:
	// 1. Patterns with a host win over patterns without a host.
	// 2. Patterns whose method and path is more specific win. One pattern is more
	// specific than another if the second matches all the (method, path) pairs
	// of the first and more.
	//
	// If rule 1 doesn't apply, then two patterns conflict if their relationship
	// is either equivalence (they match the same set of requests) or overlap
	// (they both match some requests, but neither is more specific than the other).
	func (p1 pattern) conflictsWith(p2 pattern) bool {
	if p1.host != p2.host {
	// Either one host is empty and the other isn't, in which case the
	// one with the host wins by rule 1, or neither host is empty
	// and they differ, so they won't match the same paths.
	return false
	}
	rel := p1.comparePathsAndMethods(p2)
	return rel == equivalent \|\| rel == overlaps
	}

	func (p1 pattern) comparePathsAndMethods(p2 pattern) relationship {
	mrel := p1.compareMethods(p2)
	// Optimization: avoid a call to comparePaths.
	if mrel == disjoint {
	return disjoint
	}
	prel := p1.comparePaths(p2)
	return combineRelationships(mrel, prel)
	}

	// compareMethods determines the relationship between the method
	// part of patterns p1 and p2.
	//
	// A method can either be empty, "GET", or something else.
	// The empty string matches any method, so it is the most general.
	// "GET" matches both GET and HEAD.
	// Anything else matches only itself.
	func (p1 pattern) compareMethods(p2 pattern) relationship {
	if p1.method == p2.method {
	return equivalent
	}
	if p1.method == "" {
	// p1 matches any method, but p2 does not, so p1 is more general.
	return moreGeneral
	}
	if p2.method == "" {
	return moreSpecific
	}
	if p1.method == "GET" && p2.method == "HEAD" {
	// p1 matches GET and HEAD; p2 matches only HEAD.
	return moreGeneral
	}
	if p2.method == "GET" && p1.method == "HEAD" {
	return moreSpecific
	}
	return disjoint
	}

	// comparePaths determines the relationship between the path
	// part of two patterns.
	func (p1 pattern) comparePaths(p2 pattern) relationship {
	// Optimization: if a path pattern doesn't end in a multi ("...") wildcard, then it
	// can only match paths with the same number of segments.
	if len(p1.segments) != len(p2.segments) && !p1.lastSegment().multi && !p2.lastSegment().multi {
	return disjoint
	}

	// Consider corresponding segments in the two path patterns.
	var segs1, segs2 []segment
	rel := equivalent
	for segs1, segs2 = p1.segments, p2.segments; len(segs1) > 0 && len(segs2) > 0; segs1, segs2 = segs1[1:], segs2[1:] {
	rel = combineRelationships(rel, compareSegments(segs1[0], segs2[0]))
	if rel == disjoint {
	return rel
	}
	}
	// We've reached the end of the corresponding segments of the patterns.
	// If they have the same number of segments, then we've already determined
	// their relationship.
	if len(segs1) == 0 && len(segs2) == 0 {
	return rel
	}
	// Otherwise, the only way they could fail to be disjoint is if the shorter
	// pattern ends in a multi. In that case, that multi is more general
	// than the remainder of the longer pattern, so combine those two relationships.
	if len(segs1) < len(segs2) && p1.lastSegment().multi {
	return combineRelationships(rel, moreGeneral)
	}
	if len(segs2) < len(segs1) && p2.lastSegment().multi {
	return combineRelationships(rel, moreSpecific)
	}
	return disjoint
	}

	// compareSegments determines the relationship between two segments.
	func compareSegments(s1, s2 segment) relationship {
	if s1.multi && s2.multi {
	return equivalent
	}
	if s1.multi {
	return moreGeneral
	}
	if s2.multi {
	return moreSpecific
	}
	if s1.wild && s2.wild {
	return equivalent
	}
	if s1.wild {
	if s2.s == "/" {
	// A single wildcard doesn't match a trailing slash.
	return disjoint
	}
	return moreGeneral
	}
	if s2.wild {
	if s1.s == "/" {
	return disjoint
	}
	return moreSpecific
	}
	// Both literals.
	if s1.s == s2.s {
	return equivalent
	}
	return disjoint
	}

	// combineRelationships determines the overall relationship of two patterns
	// given the relationships of a partition of the patterns into two parts.
	//
	// For example, if p1 is more general than p2 in one way but equivalent
	// in the other, then it is more general overall.
	//
	// Or if p1 is more general in one way and more specific in the other, then
	// they overlap.
	func combineRelationships(r1, r2 relationship) relationship {
	switch r1 {
	case equivalent:
	return r2
	case disjoint:
	return disjoint
	case overlaps:
	if r2 == disjoint {
	return disjoint
	}
	return overlaps
	case moreGeneral, moreSpecific:
	switch r2 {
	case equivalent:
	return r1
	case inverseRelationship(r1):
	return overlaps
	default:
	return r2
	}
	default:
	panic(fmt.Sprintf("unknown relationship %q", r1))
	}
	}

	// If p1 has relationship `r` to p2, then
	// p2 has inverseRelationship(r) to p1.
	func inverseRelationship(r relationship) relationship {
	switch r {
	case moreSpecific:
	return moreGeneral
	case moreGeneral:
	return moreSpecific
	default:
	return r
	}
	}

	// isLitOrSingle reports whether the segment is a non-dollar literal or a single wildcard.
	func isLitOrSingle(seg segment) bool {
	if seg.wild {
	return !seg.multi
	}
	return seg.s != "/"
	}

	// describeConflict returns an explanation of why two patterns conflict.
	func describeConflict(p1, p2 *pattern) string {
	mrel := p1.compareMethods(p2)
	prel := p1.comparePaths(p2)
	rel := combineRelationships(mrel, prel)
	if rel == equivalent {
	return fmt.Sprintf("%s matches the same requests as %s", p1, p2)
	}
	if rel != overlaps {
	panic("describeConflict called with non-conflicting patterns")
	}
	if prel == overlaps {
	return fmt.Sprintf(`%[1]s and %[2]s both match some paths, like %[3]q.
	But neither is more specific than the other.
	%[1]s matches %[4]q, but %[2]s doesn't.
	%[2]s matches %[5]q, but %[1]s doesn't.`,
	p1, p2, commonPath(p1, p2), differencePath(p1, p2), differencePath(p2, p1))
	}
	if mrel == moreGeneral && prel == moreSpecific {
	return fmt.Sprintf("%s matches more methods than %s, but has a more specific path pattern", p1, p2)
	}
	if mrel == moreSpecific && prel == moreGeneral {
	return fmt.Sprintf("%s matches fewer methods than %s, but has a more general path pattern", p1, p2)
	}
	return fmt.Sprintf("bug: unexpected way for two patterns %s and %s to conflict: methods %s, paths %s", p1, p2, mrel, prel)
	}

	// writeMatchingPath writes to b a path that matches the segments.
	func writeMatchingPath(b *strings.Builder, segs []segment) {
	for _, s := range segs {
	writeSegment(b, s)
	}
	}

	func writeSegment(b *strings.Builder, s segment) {
	b.WriteByte('/')
	if !s.multi && s.s != "/" {
	b.WriteString(s.s)
	}
	}

	// commonPath returns a path that both p1 and p2 match.
	// It assumes there is such a path.
	func commonPath(p1, p2 *pattern) string {
	var b strings.Builder
	var segs1, segs2 []segment
	for segs1, segs2 = p1.segments, p2.segments; len(segs1) > 0 && len(segs2) > 0; segs1, segs2 = segs1[1:], segs2[1:] {
	if s1 := segs1[0]; s1.wild {
	writeSegment(&b, segs2[0])
	} else {
	writeSegment(&b, s1)
	}
	}
	if len(segs1) > 0 {
	writeMatchingPath(&b, segs1)
	} else if len(segs2) > 0 {
	writeMatchingPath(&b, segs2)
	}
	return b.String()
	}

	// differencePath returns a path that p1 matches and p2 doesn't.
	// It assumes there is such a path.
	func differencePath(p1, p2 *pattern) string {
	var b strings.Builder

	var segs1, segs2 []segment
	for segs1, segs2 = p1.segments, p2.segments; len(segs1) > 0 && len(segs2) > 0; segs1, segs2 = segs1[1:], segs2[1:] {
	s1 := segs1[0]
	s2 := segs2[0]
	if s1.multi && s2.multi {
	// From here the patterns match the same paths, so we must have found a difference earlier.
	b.WriteByte('/')
	return b.String()

	}
	if s1.multi && !s2.multi {
	// s1 ends in a "..." wildcard but s2 does not.
	// A trailing slash will distinguish them, unless s2 ends in "{$}",
	// in which case any segment will do; prefer the wildcard name if
	// it has one.
	b.WriteByte('/')
	if s2.s == "/" {
	if s1.s != "" {
	b.WriteString(s1.s)
	} else {
	b.WriteString("x")
	}
	}
	return b.String()
	}
	if !s1.multi && s2.multi {
	writeSegment(&b, s1)
	} else if s1.wild && s2.wild {
	// Both patterns will match whatever we put here; use
	// the first wildcard name.
	writeSegment(&b, s1)
	} else if s1.wild && !s2.wild {
	// s1 is a wildcard, s2 is a literal.
	// Any segment other than s2.s will work.
	// Prefer the wildcard name, but if it's the same as the literal,
	// tweak the literal.
	if s1.s != s2.s {
	writeSegment(&b, s1)
	} else {
	b.WriteByte('/')
	b.WriteString(s2.s + "x")
	}
	} else if !s1.wild && s2.wild {
	writeSegment(&b, s1)
	} else {
	// Both are literals. A precondition of this function is that the
	// patterns overlap, so they must be the same literal. Use it.
	if s1.s != s2.s {
	panic(fmt.Sprintf("literals differ: %q and %q", s1.s, s2.s))
	}
	writeSegment(&b, s1)
	}
	}
	if len(segs1) > 0 {
	// p1 is longer than p2, and p2 does not end in a multi.
	// Anything that matches the rest of p1 will do.
	writeMatchingPath(&b, segs1)
	} else if len(segs2) > 0 {
	writeMatchingPath(&b, segs2)
	}
	return b.String()
	}