Abstract hash function usage (#28138)

Refactor Hash interfaces and centralize hash function. This will allow
easier introduction of different hash function later on.

This forms the "no-op" part of the SHA256 enablement patch.

modules/git/object_format.go

@@ -0,0 +1,103 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+import (
+	"crypto/sha1"
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+type ObjectFormatID int
+
+const (
+	Sha1 ObjectFormatID = iota
+)
+
+// sha1Pattern can be used to determine if a string is an valid sha
+var sha1Pattern = regexp.MustCompile(`^[0-9a-f]{4,40}$`)
+
+type ObjectFormat interface {
+	ID() ObjectFormatID
+	String() string
+
+	// Empty is the hash of empty git
+	Empty() ObjectID
+	// EmptyTree is the hash of an empty tree
+	EmptyTree() ObjectID
+	// FullLength is the length of the hash's hex string
+	FullLength() int
+
+	IsValid(input string) bool
+	MustID(b []byte) ObjectID
+	MustIDFromString(s string) ObjectID
+	NewID(b []byte) (ObjectID, error)
+	NewIDFromString(s string) (ObjectID, error)
+	NewEmptyID() ObjectID
+
+	NewHasher() HasherInterface
+}
+
+/* SHA1 Type */
+type Sha1ObjectFormat struct{}
+
+func (*Sha1ObjectFormat) ID() ObjectFormatID { return Sha1 }
+func (*Sha1ObjectFormat) String() string     { return "sha1" }
+func (*Sha1ObjectFormat) Empty() ObjectID    { return &Sha1Hash{} }
+func (*Sha1ObjectFormat) EmptyTree() ObjectID {
+	return &Sha1Hash{
+		0x4b, 0x82, 0x5d, 0xc6, 0x42, 0xcb, 0x6e, 0xb9, 0xa0, 0x60,
+		0xe5, 0x4b, 0xf8, 0xd6, 0x92, 0x88, 0xfb, 0xee, 0x49, 0x04,
+	}
+}
+func (*Sha1ObjectFormat) FullLength() int { return 40 }
+func (*Sha1ObjectFormat) IsValid(input string) bool {
+	return sha1Pattern.MatchString(input)
+}
+
+func (*Sha1ObjectFormat) MustID(b []byte) ObjectID {
+	var id Sha1Hash
+	copy(id[0:20], b)
+	return &id
+}
+
+func (h *Sha1ObjectFormat) MustIDFromString(s string) ObjectID {
+	return MustIDFromString(h, s)
+}
+
+func (h *Sha1ObjectFormat) NewID(b []byte) (ObjectID, error) {
+	return IDFromRaw(h, b)
+}
+
+func (h *Sha1ObjectFormat) NewIDFromString(s string) (ObjectID, error) {
+	return genericIDFromString(h, s)
+}
+
+func (*Sha1ObjectFormat) NewEmptyID() ObjectID {
+	return NewSha1()
+}
+
+func (h *Sha1ObjectFormat) NewHasher() HasherInterface {
+	return &Sha1Hasher{sha1.New()}
+}
+
+// utils
+func ObjectFormatFromID(id ObjectFormatID) ObjectFormat {
+	switch id {
+	case Sha1:
+		return &Sha1ObjectFormat{}
+	}
+
+	return nil
+}
+
+func ObjectFormatFromString(hash string) (ObjectFormat, error) {
+	switch strings.ToLower(hash) {
+	case "sha1":
+		return &Sha1ObjectFormat{}, nil
+	}
+
+	return nil, fmt.Errorf("unknown hash type: %s", hash)
+}