6 files changed, 167 insertions, 0 deletions
diff --git a/object/tree/mode/append.go b/object/tree/mode/append.go
new file mode 100644
index 00000000..e8f118ee
--- /dev/null
+++ b/object/tree/mode/append.go
@@ -0,0 +1,8 @@
+package mode
+
+import "strconv"
+
+// Append appends the canonical octal encoding of the mode to dst.
+func (mode Mode) Append(dst []byte) []byte {
+	return strconv.AppendUint(dst, uint64(mode), 8)
+}
diff --git a/object/tree/mode/details.go b/object/tree/mode/details.go
new file mode 100644
index 00000000..90ead2ed
--- /dev/null
+++ b/object/tree/mode/details.go
@@ -0,0 +1,48 @@
+package mode
+
+import "lindenii.org/go/furgit/object/typ"
+
+type modeDetails struct {
+	valid         bool
+	isBlobLike    bool
+	isRegularFile bool
+	objectType    typ.Type
+}
+
+func (mode Mode) details() modeDetails {
+	return modeTable[mode]
+}
+
+//nolint:gochecknoglobals
+var modeTable = map[Mode]modeDetails{
+	Directory: {
+		valid:         true,
+		isBlobLike:    false,
+		isRegularFile: false,
+		objectType:    typ.TypeTree,
+	},
+	Regular: {
+		valid:         true,
+		isBlobLike:    true,
+		isRegularFile: true,
+		objectType:    typ.TypeBlob,
+	},
+	Executable: {
+		valid:         true,
+		isBlobLike:    true,
+		isRegularFile: true,
+		objectType:    typ.TypeBlob,
+	},
+	Symlink: {
+		valid:         true,
+		isBlobLike:    true,
+		isRegularFile: false,
+		objectType:    typ.TypeBlob,
+	},
+	Gitlink: {
+		valid:         true,
+		isBlobLike:    false,
+		isRegularFile: false,
+		objectType:    typ.TypeCommit,
+	},
+}
diff --git a/object/tree/mode/doc.go b/object/tree/mode/doc.go
new file mode 100644
index 00000000..8e3f5984
--- /dev/null
+++ b/object/tree/mode/doc.go
@@ -0,0 +1,6 @@
+// Package mode provides canonical Git tree entry modes.
+//
+// A [Mode] is the octal file mode stored in a tree entry,
+// such as a regular file, executable, symlink, directory, or gitlink.
+// It rejects malformed, unsupported, and zero-padded encodings.
+package mode
diff --git a/object/tree/mode/mode.go b/object/tree/mode/mode.go
new file mode 100644
index 00000000..70b6fd38
--- /dev/null
+++ b/object/tree/mode/mode.go
@@ -0,0 +1,24 @@
+package mode
+
+// Mode represents the mode of an entry in a Git tree.
+type Mode uint32
+
+const (
+	// Directory is the mode of a subtree entry.
+	Directory Mode = 0o40000
+
+	// Regular is the mode of a non-executable regular file.
+	Regular Mode = 0o100644
+
+	// Executable is the mode of an executable regular file.
+	Executable Mode = 0o100755
+
+	// Symlink is the mode of a symbolic link.
+	Symlink Mode = 0o120000
+
+	// Gitlink is the mode of a gitlink (submodule commit) entry.
+	Gitlink Mode = 0o160000
+)
+
+// maxModeDigits is the largest number of octal digits in any canonical mode.
+const maxModeDigits = 6
diff --git a/object/tree/mode/ops.go b/object/tree/mode/ops.go
new file mode 100644
index 00000000..149d95db
--- /dev/null
+++ b/object/tree/mode/ops.go
@@ -0,0 +1,38 @@
+package mode
+
+import "lindenii.org/go/furgit/object/typ"
+
+// IsValid reports whether mode is a canonical mode that Git writes.
+func (mode Mode) IsValid() bool {
+	return mode.details().valid
+}
+
+// IsBlobLike reports whether mode names one blob-like tree entry kind.
+//
+// Blob-like entries store blob object IDs as their targets.
+func (mode Mode) IsBlobLike() bool {
+	return mode.details().isBlobLike
+}
+
+// IsRegularFile reports whether mode names one regular-file variant.
+func (mode Mode) IsRegularFile() bool {
+	return mode.details().isRegularFile
+}
+
+// HasSameType reports whether mode and other describe the same tree entry kind.
+//
+// Regular files and executable files have the same type for diff-status purposes.
+func (mode Mode) HasSameType(other Mode) bool {
+	if mode == other {
+		return true
+	}
+
+	return mode.details().isRegularFile && other.details().isRegularFile
+}
+
+// ObjectType returns the type of object that an entry with this mode targets.
+//
+// It returns [typ.TypeUnknown] for invalid modes.
+func (mode Mode) ObjectType() typ.Type {
+	return mode.details().objectType
+}
diff --git a/object/tree/mode/parse.go b/object/tree/mode/parse.go
new file mode 100644
index 00000000..7839a89a
--- /dev/null
+++ b/object/tree/mode/parse.go
@@ -0,0 +1,43 @@
+package mode
+
+import (
+	"errors"
+	"fmt"
+)
+
+// ErrInvalidMode indicates a malformed or unsupported tree entry mode.
+var ErrInvalidMode = errors.New("object/tree/mode: invalid mode")
+
+// Parse decodes a canonical octal tree entry mode.
+//
+// It accepts only the modes Git itself writes
+// and rejects malformed, unsupported, and zero-padded encodings.
+func Parse(raw []byte) (Mode, error) {
+	if len(raw) == 0 {
+		return 0, fmt.Errorf("%w: empty mode", ErrInvalidMode)
+	}
+
+	if raw[0] == '0' {
+		return 0, fmt.Errorf("%w: zero-padded mode %q", ErrInvalidMode, raw)
+	}
+
+	if len(raw) > maxModeDigits {
+		return 0, fmt.Errorf("%w: mode %q too long", ErrInvalidMode, raw)
+	}
+
+	var mode Mode
+
+	for _, c := range raw {
+		if c < '0' || c > '7' {
+			return 0, fmt.Errorf("%w: non-octal byte in mode %q", ErrInvalidMode, raw)
+		}
+
+		mode = mode<<3 | Mode(c-'0')
+	}
+
+	if !mode.IsValid() {
+		return 0, fmt.Errorf("%w: unsupported mode %q", ErrInvalidMode, raw)
+	}
+
+	return mode, nil
+}