xstrings: Addon string routines from git-backup[1]

Move them here to go123 like in previous patches. Taken from git-backup
0be1f647.

[1] https://lab.nexedi.com/kirr/git-backup

xstrings/xstrings.go

+// Copyright (C) 2015-2016  Nexedi SA and Contributors.
+//                          Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+
+// Package provides addons to standard package strings
+package xstrings
+
+import (
+    "fmt"
+    "strings"
+)
+
+// split string into lines. The last line, if it is empty, is omitted from the result
+// (rationale is: string.Split("hello\nworld\n", "\n") -> ["hello", "world", ""])
+func SplitLines(s, sep string) []string {
+    sv := strings.Split(s, sep)
+    l := len(sv)
+    if l > 0 && sv[l-1] == "" {
+        sv = sv[:l-1]
+    }
+    return sv
+}
+
+// split string by sep and expect exactly 2 parts
+func Split2(s, sep string) (s1, s2 string, err error) {
+    parts := strings.Split(s, sep)
+    if len(parts) != 2 {
+        return "", "", fmt.Errorf("split2: %q has %v parts (expected 2, sep: %q)", s, len(parts), sep)
+    }
+    return parts[0], parts[1], nil
+}
+
+// (head+sep+tail) -> head, tail
+func HeadTail(s, sep string) (head, tail string, err error) {
+    parts := strings.SplitN(s, sep, 2)
+    if len(parts) != 2 {
+        return "", "", fmt.Errorf("headtail: %q has no %q", s, sep)
+    }
+    return parts[0], parts[1], nil
+}

xstrings/xstrings_test.go

+// Copyright (C) 2015-2016  Nexedi SA and Contributors.
+//                          Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+
+package xstrings
+
+import (
+    "reflect"
+    "testing"
+)
+
+func TestSplitLines(t *testing.T) {
+    var tests = []struct { input, sep string; output []string } {
+        {"",                    "\n",   []string{}},
+        {"hello",               "\n",   []string{"hello"}},
+        {"hello\n",             "\n",   []string{"hello"}},
+        {"hello\nworld",        "\n",   []string{"hello", "world"}},
+        {"hello\nworld\n",      "\n",   []string{"hello", "world"}},
+        {"hello\x00world\x00",  "\n",   []string{"hello\x00world\x00"}},
+        {"hello\x00world\x00",  "\x00", []string{"hello", "world"}},
+    }
+
+    for _, tt := range tests {
+        sv := SplitLines(tt.input, tt.sep)
+        if !reflect.DeepEqual(sv, tt.output) {
+            t.Errorf("splitlines(%q, %q) -> %q  ; want %q", tt.input, tt.sep, sv, tt.output)
+        }
+    }
+}
+
+func TestSplit2(t *testing.T) {
+    var tests = []struct { input, s1, s2 string; ok bool } {
+        {"", "", "", false},
+        {" ", "", "", true},
+        {"hello", "", "", false},
+        {"hello world", "hello", "world", true},
+        {"hello world 1", "", "", false},
+    }
+
+    for _, tt := range tests {
+        s1, s2, err := Split2(tt.input, " ")
+        ok := err == nil
+        if s1 != tt.s1 || s2 != tt.s2 || ok != tt.ok {
+            t.Errorf("split2(%q) -> %q %q %v  ; want %q %q %v", tt.input, s1, s2, ok, tt.s1, tt.s2, tt.ok)
+        }
+    }
+}
+
+func TestHeadtail(t *testing.T) {
+    var tests = []struct { input, head, tail string; ok bool } {
+        {"",                "", "", false},
+        {" ",               "", "", true},
+        {"  ",              "", " ", true},
+        {"hello world",     "hello", "world", true},
+        {"hello world 1",   "hello", "world 1", true},
+        {"hello  world 2",  "hello", " world 2", true},
+    }
+
+    for _, tt := range tests {
+        head, tail, err := HeadTail(tt.input, " ")
+        ok := err == nil
+        if head != tt.head || tail != tt.tail || ok != tt.ok {
+            t.Errorf("headtail(%q) -> %q %q %v  ; want %q %q %v", tt.input, head, tail, ok, tt.head, tt.tail, tt.ok)
+        }
+    }
+}