util/strs: add new package for string utility funcs

Updates #5309

Change-Id: I677cc6e01050b6e10d8d6907d961b11c7a787a05
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/strs/strs.go

@@ -0,0 +1,36 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package strs contains string-related utility funcs.
+package strs
+
+import "strings"
+
+// CutPrefix returns s without the provided leading prefix string
+// and reports whether it found the prefix.
+// If s doesn't start with prefix, CutPrefix returns s, false.
+// If prefix is the empty string, CutPrefix returns s, true.
+//
+// TODO: remove this once Go 1.20 is out with it.
+// See https://github.com/tailscale/tailscale/issues/5309
+func CutPrefix(s, prefix string) (after string, found bool) {
+	if !strings.HasPrefix(s, prefix) {
+		return s, false
+	}
+	return s[len(prefix):], true
+}
+
+// CutSuffix returns s without the provided ending suffix string
+// and reports whether it found the suffix.
+// If s doesn't end with suffix, CutSuffix returns s, false.
+// If suffix is the empty string, CutSuffix returns s, true.
+//
+// See https://github.com/tailscale/tailscale/issues/5309
+// TODO: remove this once Go 1.20 is out with it.
+func CutSuffix(s, suffix string) (before string, found bool) {
+	if !strings.HasSuffix(s, suffix) {
+		return s, false
+	}
+	return s[:len(s)-len(suffix)], true
+}

util/strs/strs_test.go

@@ -0,0 +1,30 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strs
+
+import "testing"
+
+func TestCut(t *testing.T) {
+	tests := []struct {
+		fn       func(string, string) (string, bool)
+		in1, in2 string
+		want     string
+		wantOK   bool
+	}{
+		{CutPrefix, "foo", "fo", "o", true},
+		{CutPrefix, "bar", "fo", "bar", false},
+		{CutSuffix, "foo", "o", "fo", true},
+		{CutSuffix, "bar", "fo", "bar", false},
+	}
+	for i, tt := range tests {
+		got, gotOK := tt.fn(tt.in1, tt.in2)
+		if got != tt.want {
+			t.Errorf("%d. got %q; want %q", i, got, tt.want)
+		}
+		if gotOK != tt.wantOK {
+			t.Errorf("%d. got %v; want %v", i, gotOK, tt.wantOK)
+		}
+	}
+}