reduce allocation in funcPatsubst

benchmark                 old ns/op     new ns/op     delta
BenchmarkFuncPatsubst     2030          1102          -45.71%

benchmark                 old allocs     new allocs     delta
BenchmarkFuncPatsubst     9              1              -88.89%

benchmark                 old bytes     new bytes     delta
BenchmarkFuncPatsubst     297           32            -89.23%

1 files changed, 11 insertions, 3 deletions

diff --git a/func.go b/func.go
index 62e9af4..f6c3b41 100644
--- a/func.go
+++ b/func.go
@@ -182,10 +182,18 @@ func (f *funcPatsubst) Eval(w io.Writer, ev *Evaluator) {
 	pat := fargs[0]
 	repl := fargs[1]
 	ws := newWordScanner(fargs[2])
-	sw := ssvWriter{w: w}
+	space := false
 	for ws.Scan() {
-		t := substPatternBytes(pat, repl, ws.Bytes())
-		sw.Write(t)
+		if space {
+			writeByte(w, ' ')
+		}
+		pre, subst, post := substPatternBytes(pat, repl, ws.Bytes())
+		w.Write(pre)
+		if subst != nil {
+			w.Write(subst)
+			w.Write(post)
+		}
+		space = true
 	}
 	freeBuf(abuf)
 	addStats("funcbody", "patsubst", t)