add filesys.InsertPathPart function

api/filesys/util.go

@@ -3,7 +3,11 @@
 
 package filesys
 
-import "path/filepath"
+import (
+	"os"
+	"path/filepath"
+	"strings"
+)
 
 // RootedPath returns a rooted path, e.g. "/foo/bar" as
 // opposed to "foo/bar".
@@ -28,3 +32,94 @@ func StripLeadingSeps(s string) string {
 	}
 	return s[k:]
 }
+
+// PathSplit converts a file path to a slice of string.
+// If the path is absolute (if the path has a leading slash),
+// then the first entry in the result is an empty string.
+// Desired:  path == PathJoin(PathSplit(path))
+func PathSplit(incoming string) []string {
+	if incoming == "" {
+		return []string{}
+	}
+	dir, path := filepath.Split(incoming)
+	if dir == string(os.PathSeparator) {
+		if path == "" {
+			return []string{""}
+		}
+		return []string{"", path}
+	}
+	dir = strings.TrimSuffix(dir, string(os.PathSeparator))
+	if dir == "" {
+		return []string{path}
+	}
+	return append(PathSplit(dir), path)
+}
+
+// PathJoin converts a slice of string to a file path.
+// If the first entry is an empty string, then the returned
+// path is absolute (it has a leading slash).
+// Desired:  path == PathJoin(PathSplit(path))
+func PathJoin(incoming []string) string {
+	if len(incoming) == 0 {
+		return ""
+	}
+	if incoming[0] == "" {
+		return string(os.PathSeparator) + filepath.Join(incoming[1:]...)
+	}
+	return filepath.Join(incoming...)
+}
+
+// InsertPathPart inserts 'part' at position 'pos' in the given filepath.
+// The first position is 0.
+//
+// E.g. if part == 'PEACH'
+//
+//                  OLD : NEW                    : POS
+//      --------------------------------------------------------
+//              {empty} : PEACH                  : irrelevant
+//                    / : /PEACH                 : irrelevant
+//                  pie : PEACH/pie              : 0 (or negative)
+//                 /pie : /PEACH/pie             : 0 (or negative)
+//                  raw : raw/PEACH              : 1 (or larger)
+//                 /raw : /raw/PEACH             : 1 (or larger)
+//      a/nice/warm/pie : a/nice/warm/PEACH/pie  : 3
+//     /a/nice/warm/pie : /a/nice/warm/PEACH/pie : 3
+//
+// * An empty part results in no change.
+//
+// * Absolute paths get their leading '/' stripped, treated like
+//   relative paths, and the leading '/' is re-added on output.
+//   The meaning of pos is intentionally the same in either absolute or
+//   relative paths; if it weren't, this function could convert absolute
+//   paths to relative paths, which is not desirable.
+//
+// * For robustness (liberal input, conservative output) Pos values that
+//   that are too small (large) to index the split filepath result in a
+//   prefix (postfix) rather than an error.  Use extreme position values
+//   to assure a prefix or postfix (e.g. 0 will always prefix, and
+//   9999 will presumably always postfix).
+func InsertPathPart(path string, pos int, part string) string {
+	if part == "" {
+		return path
+	}
+	parts := PathSplit(path)
+	if pos < 0 {
+		pos = 0
+	} else if pos > len(parts) {
+		pos = len(parts)
+	}
+	if len(parts) > 0 && parts[0] == "" && pos < len(parts) {
+		// An empty string at 0 indicates an absolute path, and means
+		// we must increment pos.  This change means that a position
+		// specification has the same meaning in relative and absolute paths.
+		// E.g. in either the path 'a/b/c' or the path '/a/b/c',
+		// 'a' is at 0, 'b' is at 1 and 'c' is at 2, and inserting at
+		// zero means a new first field _without_ changing an absolute
+		// path to a relative path.
+		pos++
+	}
+	result := make([]string, len(parts)+1)
+	copy(result, parts[0:pos])
+	result[pos] = part
+	return PathJoin(append(result, parts[pos:]...))
+}

api/filesys/util_test.go

@@ -1,6 +1,7 @@
 package filesys_test
 
 import (
+	"os"
 	"path/filepath"
 	"testing"
 
@@ -93,6 +94,11 @@ func TestFilePathSplit(t *testing.T) {
 			dir:  "",
 			file: "rabbit.jpg",
 		},
+		{
+			full: "/",
+			dir:  "/",
+			file: "",
+		},
 		{
 			full: "/beans",
 			dir:  "/",
@@ -124,6 +130,169 @@ func TestFilePathSplit(t *testing.T) {
 	}
 }
 
+func TestPathSplitAndJoin(t *testing.T) {
+	cases := map[string]struct {
+		original string
+		expected []string
+	}{
+		"Empty": {
+			original: "",
+			expected: []string{},
+		},
+		"One": {
+			original: "hello",
+			expected: []string{"hello"},
+		},
+		"Two": {
+			original: "hello/there",
+			expected: []string{"hello", "there"},
+		},
+		"Three": {
+			original: "hello/my/friend",
+			expected: []string{"hello", "my", "friend"},
+		},
+	}
+	for n, c := range cases {
+		f := func(t *testing.T, original string, expected []string) {
+			actual := PathSplit(original)
+			if len(actual) != len(expected) {
+				t.Fatalf(
+					"expected len %d, got len %d",
+					len(expected), len(actual))
+			}
+			for i := range expected {
+				if expected[i] != actual[i] {
+					t.Fatalf(
+						"at i=%d, expected '%s', got '%s'",
+						i, expected[i], actual[i])
+				}
+			}
+			joined := PathJoin(actual)
+			if joined != original {
+				t.Fatalf(
+					"when rejoining, expected '%s', got '%s'",
+					original, joined)
+			}
+		}
+		t.Run("relative"+n, func(t *testing.T) {
+			f(t, c.original, c.expected)
+		})
+		t.Run("absolute"+n, func(t *testing.T) {
+			f(t,
+				string(os.PathSeparator)+c.original,
+				append([]string{""}, c.expected...))
+		})
+	}
+}
+
+func TestInsertPathPart(t *testing.T) {
+	cases := map[string]struct {
+		original string
+		pos      int
+		part     string
+		expected string
+	}{
+		"rootOne": {
+			original: "/",
+			pos:      0,
+			part:     "___",
+			expected: "/___",
+		},
+		"rootTwo": {
+			original: "/",
+			pos:      444,
+			part:     "___",
+			expected: "/___",
+		},
+		"rootedFirst": {
+			original: "/apple",
+			pos:      0,
+			part:     "___",
+			expected: "/___/apple",
+		},
+		"rootedSecond": {
+			original: "/apple",
+			pos:      444,
+			part:     "___",
+			expected: "/apple/___",
+		},
+		"rootedThird": {
+			original: "/apple/banana",
+			pos:      444,
+			part:     "___",
+			expected: "/apple/banana/___",
+		},
+		"emptyLow": {
+			original: "",
+			pos:      -3,
+			part:     "___",
+			expected: "___",
+		},
+		"emptyHigh": {
+			original: "",
+			pos:      444,
+			part:     "___",
+			expected: "___",
+		},
+		"peachPie": {
+			original: "a/nice/warm/pie",
+			pos:      3,
+			part:     "PEACH",
+			expected: "a/nice/warm/PEACH/pie",
+		},
+		"rootedPeachPie": {
+			original: "/a/nice/warm/pie",
+			pos:      3,
+			part:     "PEACH",
+			expected: "/a/nice/warm/PEACH/pie",
+		},
+		"longStart": {
+			original: "a/b/c/d/e/f",
+			pos:      0,
+			part:     "___",
+			expected: "___/a/b/c/d/e/f",
+		},
+		"rootedLongStart": {
+			original: "/a/b/c/d/e/f",
+			pos:      0,
+			part:     "___",
+			expected: "/___/a/b/c/d/e/f",
+		},
+		"longMiddle": {
+			original: "a/b/c/d/e/f",
+			pos:      3,
+			part:     "___",
+			expected: "a/b/c/___/d/e/f",
+		},
+		"rootedLongMiddle": {
+			original: "/a/b/c/d/e/f",
+			pos:      3,
+			part:     "___",
+			expected: "/a/b/c/___/d/e/f",
+		},
+		"longEnd": {
+			original: "a/b/c/d/e/f",
+			pos:      444,
+			part:     "___",
+			expected: "a/b/c/d/e/f/___",
+		},
+		"rootedLongEnd": {
+			original: "/a/b/c/d/e/f",
+			pos:      444,
+			part:     "___",
+			expected: "/a/b/c/d/e/f/___",
+		},
+	}
+	for n, c := range cases {
+		t.Run(n, func(t *testing.T) {
+			actual := InsertPathPart(c.original, c.pos, c.part)
+			if actual != c.expected {
+				t.Fatalf("expected '%s', got '%s'", c.expected, actual)
+			}
+		})
+	}
+}
+
 func TestStripTrailingSeps(t *testing.T) {
 	cases := []struct {
 		full string