rewrite: remove condition file (#3003)

Nothing it declares is used by CoreDNS and it does not appear to be used
by any third-party packages (according to godoc.org).

plugin/rewrite/condition.go

-package rewrite
-
-import (
-	"context"
-	"fmt"
-	"regexp"
-	"strings"
-
-	"github.com/coredns/coredns/plugin/pkg/replacer"
-	"github.com/coredns/coredns/request"
-
-	"github.com/miekg/dns"
-)
-
-// Operators that are defined.
-const (
-	Is         = "is"
-	Not        = "not"
-	Has        = "has"
-	NotHas     = "not_has"
-	StartsWith = "starts_with"
-	EndsWith   = "ends_with"
-	Match      = "match"
-	NotMatch   = "not_match"
-)
-
-var repl = replacer.New()
-
-// condition is a rewrite condition.
-type condition func(string, string) bool
-
-var conditions = map[string]condition{
-	Is:         isFunc,
-	Not:        notFunc,
-	Has:        hasFunc,
-	NotHas:     notHasFunc,
-	StartsWith: startsWithFunc,
-	EndsWith:   endsWithFunc,
-	Match:      matchFunc,
-	NotMatch:   notMatchFunc,
-}
-
-// isFunc is condition for Is operator. It checks for equality.
-func isFunc(a, b string) bool { return a == b }
-
-// notFunc is condition for Not operator. It checks for inequality.
-func notFunc(a, b string) bool { return a != b }
-
-// hasFunc is condition for Has operator. It checks if b is a substring of a.
-func hasFunc(a, b string) bool { return strings.Contains(a, b) }
-
-// notHasFunc is condition for NotHas operator. It checks if b is not a substring of a.
-func notHasFunc(a, b string) bool { return !strings.Contains(a, b) }
-
-// startsWithFunc is condition for StartsWith operator. It checks if b is a prefix of a.
-func startsWithFunc(a, b string) bool { return strings.HasPrefix(a, b) }
-
-// endsWithFunc is condition for EndsWith operator. It checks if b is a suffix of a.
-func endsWithFunc(a, b string) bool {
-	// TODO(miek): IsSubDomain
-	return strings.HasSuffix(a, b)
-}
-
-// matchFunc is condition for Match operator. It does regexp matching of a against pattern in b
-// and returns if they match.
-func matchFunc(a, b string) bool {
-	matched, _ := regexp.MatchString(b, a)
-	return matched
-}
-
-// notMatchFunc is condition for NotMatch operator. It does regexp matching of a against pattern in b
-// and returns if they do not match.
-func notMatchFunc(a, b string) bool {
-	matched, _ := regexp.MatchString(b, a)
-	return !matched
-}
-
-// If is statement for a rewrite condition.
-type If struct {
-	A        string
-	Operator string
-	B        string
-}
-
-// True returns true if the condition is true and false otherwise.
-// If r is not nil, it replaces placeholders before comparison.
-func (i If) True(r *dns.Msg) bool {
-	if c, ok := conditions[i.Operator]; ok {
-		a, b := i.A, i.B
-		if r != nil {
-			ctx := context.TODO()
-			state := request.Request{Req: r, W: nil} // hmm W nil?
-			a = repl.Replace(ctx, state, nil, i.A)
-			b = repl.Replace(ctx, state, nil, i.B)
-		}
-		return c(a, b)
-	}
-	return false
-}
-
-// NewIf creates a new If condition.
-func NewIf(a, operator, b string) (If, error) {
-	if _, ok := conditions[operator]; !ok {
-		return If{}, fmt.Errorf("invalid operator %v", operator)
-	}
-	return If{
-		A:        a,
-		Operator: operator,
-		B:        b,
-	}, nil
-}