Add basic support for parsing configuration files

Now support for switching hash algorithms should be complete!

4 files changed, 926 insertions, 12 deletions

diff --git a/config.go b/config.go
new file mode 100644
index 00000000..b60a5f9d
--- /dev/null
+++ b/config.go
@@ -0,0 +1,485 @@
+package furgit
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"strings"
+	"unicode"
+)
+
+// Config represents a parsed Git configuration.
+type Config struct {
+	entries []ConfigEntry
+}
+
+// ConfigEntry represents a single configuration key-value pair.
+type ConfigEntry struct {
+	Section    string
+	Subsection string
+	Key        string
+	Value      string
+}
+
+// ParseConfig parses a Git configuration from a reader.
+func ParseConfig(r io.Reader) (*Config, error) {
+	parser := &configParser{
+		reader:  bufio.NewReader(r),
+		lineNum: 1,
+	}
+	return parser.parse()
+}
+
+// Get retrieves the first value for a given section, optional subsection, and key.
+// Returns an empty string if not found.
+func (c *Config) Get(section, subsection, key string) string {
+	section = strings.ToLower(section)
+	key = strings.ToLower(key)
+	for _, entry := range c.entries {
+		if strings.EqualFold(entry.Section, section) &&
+			entry.Subsection == subsection &&
+			strings.EqualFold(entry.Key, key) {
+			return entry.Value
+		}
+	}
+	return ""
+}
+
+// GetAll retrieves all values for a given section, optional subsection, and key.
+func (c *Config) GetAll(section, subsection, key string) []string {
+	section = strings.ToLower(section)
+	key = strings.ToLower(key)
+	var values []string
+	for _, entry := range c.entries {
+		if strings.EqualFold(entry.Section, section) &&
+			entry.Subsection == subsection &&
+			strings.EqualFold(entry.Key, key) {
+			values = append(values, entry.Value)
+		}
+	}
+	return values
+}
+
+// Entries returns all configuration entries.
+func (c *Config) Entries() []ConfigEntry {
+	result := make([]ConfigEntry, len(c.entries))
+	copy(result, c.entries)
+	return result
+}
+
+// configParser implements Git config file parsing using character-based reading.
+type configParser struct {
+	reader         *bufio.Reader
+	lineNum        int
+	currentSection string
+	currentSubsec  string
+	peeked         rune
+	hasPeeked      bool
+}
+
+func (p *configParser) parse() (*Config, error) {
+	cfg := &Config{}
+
+	if err := p.skipBOM(); err != nil {
+		return nil, err
+	}
+
+	for {
+		ch, err := p.nextChar()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			return nil, err
+		}
+
+		// Skip whitespace and newlines
+		if ch == '\n' || unicode.IsSpace(ch) {
+			continue
+		}
+
+		// Comments
+		if ch == '#' || ch == ';' {
+			if err := p.skipToEOL(); err != nil && err != io.EOF {
+				return nil, err
+			}
+			continue
+		}
+
+		// Section header
+		if ch == '[' {
+			if err := p.parseSection(); err != nil {
+				return nil, fmt.Errorf("furgit: config: line %d: %w", p.lineNum, err)
+			}
+			continue
+		}
+
+		// Key-value pair
+		if unicode.IsLetter(ch) {
+			p.unreadChar(ch)
+			if err := p.parseKeyValue(cfg); err != nil {
+				return nil, fmt.Errorf("furgit: config: line %d: %w", p.lineNum, err)
+			}
+			continue
+		}
+
+		return nil, fmt.Errorf("furgit: config: line %d: unexpected character %q", p.lineNum, ch)
+	}
+
+	return cfg, nil
+}
+
+func (p *configParser) nextChar() (rune, error) {
+	if p.hasPeeked {
+		p.hasPeeked = false
+		return p.peeked, nil
+	}
+
+	ch, _, err := p.reader.ReadRune()
+	if err != nil {
+		return 0, err
+	}
+
+	if ch == '\r' {
+		next, _, err := p.reader.ReadRune()
+		if err == nil && next == '\n' {
+			ch = '\n'
+		} else if err == nil {
+			// Weird but ok
+			_ = p.reader.UnreadRune()
+		}
+	}
+
+	if ch == '\n' {
+		p.lineNum++
+	}
+
+	return ch, nil
+}
+
+func (p *configParser) unreadChar(ch rune) {
+	p.peeked = ch
+	p.hasPeeked = true
+	if ch == '\n' && p.lineNum > 1 {
+		p.lineNum--
+	}
+}
+
+func (p *configParser) skipBOM() error {
+	first, _, err := p.reader.ReadRune()
+	if err == io.EOF {
+		return nil
+	}
+	if err != nil {
+		return err
+	}
+	if first != '\uFEFF' {
+		_ = p.reader.UnreadRune()
+	}
+	return nil
+}
+
+func (p *configParser) skipToEOL() error {
+	for {
+		ch, err := p.nextChar()
+		if err != nil {
+			return err
+		}
+		if ch == '\n' {
+			return nil
+		}
+	}
+}
+
+func (p *configParser) parseSection() error {
+	var name bytes.Buffer
+
+	for {
+		ch, err := p.nextChar()
+		if err != nil {
+			return errors.New("unexpected EOF in section header")
+		}
+
+		if ch == ']' {
+			section := name.String()
+			if !isValidSection(section) {
+				return fmt.Errorf("invalid section name: %q", section)
+			}
+			p.currentSection = strings.ToLower(section)
+			p.currentSubsec = ""
+			return nil
+		}
+
+		if unicode.IsSpace(ch) {
+			return p.parseExtendedSection(&name)
+		}
+
+		if !isKeyChar(ch) && ch != '.' {
+			return fmt.Errorf("invalid character in section name: %q", ch)
+		}
+
+		name.WriteRune(unicode.ToLower(ch))
+	}
+}
+
+func (p *configParser) parseExtendedSection(sectionName *bytes.Buffer) error {
+	for {
+		ch, err := p.nextChar()
+		if err != nil {
+			return errors.New("unexpected EOF in section header")
+		}
+		if !unicode.IsSpace(ch) {
+			if ch != '"' {
+				return errors.New("expected quote after section name")
+			}
+			break
+		}
+	}
+
+	var subsec bytes.Buffer
+	for {
+		ch, err := p.nextChar()
+		if err != nil {
+			return errors.New("unexpected EOF in subsection")
+		}
+
+		if ch == '\n' {
+			return errors.New("newline in subsection")
+		}
+
+		if ch == '"' {
+			break
+		}
+
+		if ch == '\\' {
+			next, err := p.nextChar()
+			if err != nil {
+				return errors.New("unexpected EOF after backslash in subsection")
+			}
+			if next == '\n' {
+				return errors.New("newline after backslash in subsection")
+			}
+			subsec.WriteRune(next)
+		} else {
+			subsec.WriteRune(ch)
+		}
+	}
+
+	ch, err := p.nextChar()
+	if err != nil {
+		return errors.New("unexpected EOF after subsection")
+	}
+	if ch != ']' {
+		return fmt.Errorf("expected ']' after subsection, got %q", ch)
+	}
+
+	section := sectionName.String()
+	if !isValidSection(section) {
+		return fmt.Errorf("invalid section name: %q", section)
+	}
+
+	p.currentSection = strings.ToLower(section)
+	p.currentSubsec = subsec.String()
+	return nil
+}
+
+func (p *configParser) parseKeyValue(cfg *Config) error {
+	if p.currentSection == "" {
+		return errors.New("key-value pair before any section header")
+	}
+
+	var key bytes.Buffer
+	for {
+		ch, err := p.nextChar()
+		if err != nil {
+			return errors.New("unexpected EOF reading key")
+		}
+
+		if ch == '=' || ch == '\n' || unicode.IsSpace(ch) {
+			p.unreadChar(ch)
+			break
+		}
+
+		if !isKeyChar(ch) {
+			return fmt.Errorf("invalid character in key: %q", ch)
+		}
+
+		key.WriteRune(unicode.ToLower(ch))
+	}
+
+	keyStr := key.String()
+	if len(keyStr) == 0 {
+		return errors.New("empty key name")
+	}
+	if !unicode.IsLetter(rune(keyStr[0])) {
+		return errors.New("key must start with a letter")
+	}
+
+	for {
+		ch, err := p.nextChar()
+		if err == io.EOF {
+			cfg.entries = append(cfg.entries, ConfigEntry{
+				Section:    p.currentSection,
+				Subsection: p.currentSubsec,
+				Key:        keyStr,
+				Value:      "true",
+			})
+			return nil
+		}
+		if err != nil {
+			return err
+		}
+
+		if ch == '\n' {
+			cfg.entries = append(cfg.entries, ConfigEntry{
+				Section:    p.currentSection,
+				Subsection: p.currentSubsec,
+				Key:        keyStr,
+				Value:      "true",
+			})
+			return nil
+		}
+
+		if ch == '#' || ch == ';' {
+			if err := p.skipToEOL(); err != nil && err != io.EOF {
+				return err
+			}
+			cfg.entries = append(cfg.entries, ConfigEntry{
+				Section:    p.currentSection,
+				Subsection: p.currentSubsec,
+				Key:        keyStr,
+				Value:      "true",
+			})
+			return nil
+		}
+
+		if ch == '=' {
+			break
+		}
+
+		if !unicode.IsSpace(ch) {
+			return fmt.Errorf("unexpected character after key: %q", ch)
+		}
+	}
+
+	value, err := p.parseValue()
+	if err != nil {
+		return err
+	}
+
+	cfg.entries = append(cfg.entries, ConfigEntry{
+		Section:    p.currentSection,
+		Subsection: p.currentSubsec,
+		Key:        keyStr,
+		Value:      value,
+	})
+
+	return nil
+}
+
+func (p *configParser) parseValue() (string, error) {
+	var value bytes.Buffer
+	var inQuote bool
+	var inComment bool
+	trimLen := 0
+
+	for {
+		ch, err := p.nextChar()
+		if err == io.EOF {
+			if inQuote {
+				return "", errors.New("unexpected EOF in quoted value")
+			}
+			if trimLen > 0 {
+				return value.String()[:trimLen], nil
+			}
+			return value.String(), nil
+		}
+		if err != nil {
+			return "", err
+		}
+
+		if ch == '\n' {
+			if inQuote {
+				return "", errors.New("newline in quoted value")
+			}
+			if trimLen > 0 {
+				return value.String()[:trimLen], nil
+			}
+			return value.String(), nil
+		}
+
+		if inComment {
+			continue
+		}
+
+		if unicode.IsSpace(ch) && !inQuote {
+			if trimLen == 0 && value.Len() > 0 {
+				trimLen = value.Len()
+			}
+			if value.Len() > 0 {
+				value.WriteRune(ch)
+			}
+			continue
+		}
+
+		if !inQuote && (ch == '#' || ch == ';') {
+			inComment = true
+			continue
+		}
+
+		if trimLen > 0 {
+			trimLen = 0
+		}
+
+		if ch == '\\' {
+			next, err := p.nextChar()
+			if err == io.EOF {
+				return "", errors.New("unexpected EOF after backslash")
+			}
+			if err != nil {
+				return "", err
+			}
+
+			switch next {
+			case '\n':
+				continue
+			case 'n':
+				value.WriteRune('\n')
+			case 't':
+				value.WriteRune('\t')
+			case 'b':
+				value.WriteRune('\b')
+			case '\\', '"':
+				value.WriteRune(next)
+			default:
+				return "", fmt.Errorf("invalid escape sequence: \\%c", next)
+			}
+			continue
+		}
+
+		if ch == '"' {
+			inQuote = !inQuote
+			continue
+		}
+
+		value.WriteRune(ch)
+	}
+}
+
+func isValidSection(s string) bool {
+	if len(s) == 0 {
+		return false
+	}
+	for _, ch := range s {
+		if !unicode.IsLetter(ch) && !unicode.IsDigit(ch) && ch != '-' && ch != '.' {
+			return false
+		}
+	}
+	return true
+}
+
+func isKeyChar(ch rune) bool {
+	return unicode.IsLetter(ch) || unicode.IsDigit(ch) || ch == '-'
+}
diff --git a/config_test.go b/config_test.go
new file mode 100644
index 00000000..65a5c504
--- /dev/null
+++ b/config_test.go
@@ -0,0 +1,365 @@
+package furgit
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestParseConfigSimple(t *testing.T) {
+	input := `
+[core]
+	repositoryformatversion = 0
+	filemode = true
+	bare = false
+
+[user]
+	name = Alice Example
+	email = alice@example.com
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	if got := cfg.Get("core", "", "repositoryformatversion"); got != "0" {
+		t.Errorf("core.repositoryformatversion = %q, want %q", got, "0")
+	}
+	if got := cfg.Get("core", "", "filemode"); got != "true" {
+		t.Errorf("core.filemode = %q, want %q", got, "true")
+	}
+	if got := cfg.Get("user", "", "name"); got != "Alice Example" {
+		t.Errorf("user.name = %q, want %q", got, "Alice Example")
+	}
+	if got := cfg.Get("user", "", "email"); got != "alice@example.com" {
+		t.Errorf("user.email = %q, want %q", got, "alice@example.com")
+	}
+}
+
+func TestParseConfigSubsection(t *testing.T) {
+	input := `
+[remote "origin"]
+	url = https://villosa.example.org/group1/group2//repos/repo
+	fetch = +refs/heads/*:refs/remotes/origin/*
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	if got := cfg.Get("remote", "origin", "url"); got != "https://villosa.example.org/group1/group2//repos/repo" {
+		t.Errorf("remote.origin.url = %q, want %q", got, "https://villosa.example.org/group1/group2//repos/repo")
+	}
+	if got := cfg.Get("remote", "origin", "fetch"); got != "+refs/heads/*:refs/remotes/origin/*" {
+		t.Errorf("remote.origin.fetch = %q, want %q", got, "+refs/heads/*:refs/remotes/origin/*")
+	}
+}
+
+func TestParseConfigCaseInsensitive(t *testing.T) {
+	input := `
+[Core]
+	FileMode = true
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	if got := cfg.Get("core", "", "filemode"); got != "true" {
+		t.Errorf("core.filemode = %q, want %q", got, "true")
+	}
+	if got := cfg.Get("CORE", "", "FILEMODE"); got != "true" {
+		t.Errorf("CORE.FILEMODE = %q, want %q", got, "true")
+	}
+}
+
+func TestParseConfigBooleanKeys(t *testing.T) {
+	input := `
+[core]
+	bare
+	ignorecase
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	if got := cfg.Get("core", "", "bare"); got != "true" {
+		t.Errorf("core.bare = %q, want %q", got, "true")
+	}
+	if got := cfg.Get("core", "", "ignorecase"); got != "true" {
+		t.Errorf("core.ignorecase = %q, want %q", got, "true")
+	}
+}
+
+func TestParseConfigQuotedValues(t *testing.T) {
+	input := `
+[user]
+	name = "Bob Smith"
+	comment = "Has a \"quoted\" word"
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	if got := cfg.Get("user", "", "name"); got != "Bob Smith" {
+		t.Errorf("user.name = %q, want %q", got, "Bob Smith")
+	}
+	if got := cfg.Get("user", "", "comment"); got != `Has a "quoted" word` {
+		t.Errorf("user.comment = %q, want %q", got, `Has a "quoted" word`)
+	}
+}
+
+func TestParseConfigEscapeSequences(t *testing.T) {
+	input := `
+[test]
+	newline = "line1\nline2"
+	tab = "col1\tcol2"
+	backslash = "path\\to\\file"
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	if got := cfg.Get("test", "", "newline"); got != "line1\nline2" {
+		t.Errorf("test.newline = %q, want %q", got, "line1\nline2")
+	}
+	if got := cfg.Get("test", "", "tab"); got != "col1\tcol2" {
+		t.Errorf("test.tab = %q, want %q", got, "col1\tcol2")
+	}
+	if got := cfg.Get("test", "", "backslash"); got != "path\\to\\file" {
+		t.Errorf("test.backslash = %q, want %q", got, "path\\to\\file")
+	}
+}
+
+func TestParseConfigComments(t *testing.T) {
+	input := `
+# This is a comment
+; This is also a comment
+[core]
+	# Comment in section
+	bare = false  # inline comment
+	filemode = true ; another inline comment
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	if got := cfg.Get("core", "", "bare"); got != "false" {
+		t.Errorf("core.bare = %q, want %q", got, "false")
+	}
+	if got := cfg.Get("core", "", "filemode"); got != "true" {
+		t.Errorf("core.filemode = %q, want %q", got, "true")
+	}
+}
+
+func TestParseConfigMultipleValues(t *testing.T) {
+	input := `
+[remote "origin"]
+	fetch = +refs/heads/main:refs/remotes/origin/main
+	fetch = +refs/heads/dev:refs/remotes/origin/dev
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	values := cfg.GetAll("remote", "origin", "fetch")
+	if len(values) != 2 {
+		t.Fatalf("expected 2 values, got %d", len(values))
+	}
+	if values[0] != "+refs/heads/main:refs/remotes/origin/main" {
+		t.Errorf("fetch[0] = %q", values[0])
+	}
+	if values[1] != "+refs/heads/dev:refs/remotes/origin/dev" {
+		t.Errorf("fetch[1] = %q", values[1])
+	}
+}
+
+func TestParseConfigSubsectionWithEscapes(t *testing.T) {
+	input := `
+[branch "feature/my-branch"]
+	remote = origin
+	merge = refs/heads/feature/my-branch
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	if got := cfg.Get("branch", "feature/my-branch", "remote"); got != "origin" {
+		t.Errorf("branch.feature/my-branch.remote = %q, want %q", got, "origin")
+	}
+}
+
+func TestParseConfigEmptyValue(t *testing.T) {
+	input := `
+[core]
+	empty =
+	whitespace =    
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	if got := cfg.Get("core", "", "empty"); got != "" {
+		t.Errorf("core.empty = %q, want empty string", got)
+	}
+	if got := cfg.Get("core", "", "whitespace"); got != "" {
+		t.Errorf("core.whitespace = %q, want empty string", got)
+	}
+}
+
+func TestParseConfigInvalidInputs(t *testing.T) {
+	cases := []struct {
+		name  string
+		input string
+	}{
+		{
+			name:  "key before section",
+			input: "key = value\n",
+		},
+		{
+			name:  "invalid section no closing bracket",
+			input: "[section\n",
+		},
+		{
+			name:  "invalid escape in value",
+			input: "[test]\nkey = \"invalid\\x\"\n",
+		},
+		{
+			name:  "unclosed quote in value",
+			input: "[test]\nkey = \"unclosed\n",
+		},
+		{
+			name:  "unclosed quote in subsection",
+			input: "[section \"unclosed]\n",
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := ParseConfig(strings.NewReader(tc.input))
+			if err == nil {
+				t.Errorf("expected error for %q, got nil", tc.name)
+			}
+		})
+	}
+}
+
+func TestParseConfigEntries(t *testing.T) {
+	input := `
+[core]
+	bare = false
+[user]
+	name = Alice
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	entries := cfg.Entries()
+	if len(entries) != 2 {
+		t.Fatalf("expected 2 entries, got %d", len(entries))
+	}
+
+	if entries[0].Section != "core" || entries[0].Key != "bare" || entries[0].Value != "false" {
+		t.Errorf("entry[0] = %+v", entries[0])
+	}
+	if entries[1].Section != "user" || entries[1].Key != "name" || entries[1].Value != "Alice" {
+		t.Errorf("entry[1] = %+v", entries[1])
+	}
+}
+
+func TestParseConfigGetNotFound(t *testing.T) {
+	input := `
+[core]
+	bare = false
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	if got := cfg.Get("nonexistent", "", "key"); got != "" {
+		t.Errorf("expected empty string for nonexistent key, got %q", got)
+	}
+}
+
+func TestParseConfigComplexSubsection(t *testing.T) {
+	input := `
+[url "https://villosa.example.org/"]
+	insteadOf = gh:
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	if got := cfg.Get("url", "https://villosa.example.org/", "insteadof"); got != "gh:" {
+		t.Errorf("url.https://villosa.example.org/.insteadof = %q, want %q", got, "gh:")
+	}
+}
+
+func TestParseConfigBooleanKeyWithInlineComment(t *testing.T) {
+	input := `
+[core]
+	bare ; this is a comment
+	filemode # another comment
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	if got := cfg.Get("core", "", "bare"); got != "true" {
+		t.Errorf("core.bare = %q, want %q", got, "true")
+	}
+	if got := cfg.Get("core", "", "filemode"); got != "true" {
+		t.Errorf("core.filemode = %q, want %q", got, "true")
+	}
+}
+
+func TestParseConfigLineContinuation(t *testing.T) {
+	input := `[section]
+	# Quoted value with line continuation
+	quoted = "line1\
+line2\
+line3"
+	
+	# Unquoted value with line continuation
+	unquoted = one\
+two\
+three
+`
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	if got := cfg.Get("section", "", "quoted"); got != "line1line2line3" {
+		t.Errorf("section.quoted = %q, want %q", got, "line1line2line3")
+	}
+	if got := cfg.Get("section", "", "unquoted"); got != "onetwothree" {
+		t.Errorf("section.unquoted = %q, want %q", got, "onetwothree")
+	}
+}
+
+func TestParseConfigDOSLineEndings(t *testing.T) {
+	input := "[core]\r\n\tbare = true\r\n\tfilemode = false\r\n"
+	cfg, err := ParseConfig(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseConfig error: %v", err)
+	}
+
+	if got := cfg.Get("core", "", "bare"); got != "true" {
+		t.Errorf("core.bare = %q, want %q", got, "true")
+	}
+	if got := cfg.Get("core", "", "filemode"); got != "false" {
+		t.Errorf("core.filemode = %q, want %q", got, "false")
+	}
+}
diff --git a/repo.go b/repo.go
index 250b989d..858e2037 100644
--- a/repo.go
+++ b/repo.go
@@ -1,6 +1,8 @@
 package furgit
 
 import (
+	"crypto/sha1"
+	"crypto/sha256"
 	"encoding/hex"
 	"fmt"
 	"os"
@@ -25,10 +27,10 @@ type Repository struct {
 	closeOnce sync.Once
 }
 
-// OpenRepository opens the repository at the provided path with the specified hash size.
-// This will be replaced later with a function that auto-detects the hash size based
-// on the git configuration.
-func OpenRepository(path string, hashSize int) (*Repository, error) {
+// OpenRepository opens the repository at the provided path. The path is expected to be
+// the actual repository directory, i.e., the repository itself for bare repositories,
+// or the .git subdirectory for non-bare repositories.
+func OpenRepository(path string) (*Repository, error) {
 	fi, err := os.Stat(path)
 	if err != nil {
 		return nil, err
@@ -36,9 +38,38 @@ func OpenRepository(path string, hashSize int) (*Repository, error) {
 	if !fi.IsDir() {
 		return nil, ErrInvalidObject
 	}
+
+	cfgPath := filepath.Join(path, "config")
+	f, err := os.Open(cfgPath)
+	if err != nil {
+		return nil, fmt.Errorf("furgit: unable to open config: %w", err)
+	}
+	defer f.Close()
+
+	cfg, err := ParseConfig(f)
+	if err != nil {
+		return nil, fmt.Errorf("furgit: failed to parse config: %w", err)
+	}
+
+	algo := cfg.Get("extensions", "", "objectformat")
+	if algo == "" {
+		algo = "sha1"
+	}
+
+	var hashSize int
+	switch algo {
+	case "sha1":
+		hashSize = sha1.Size
+	case "sha256":
+		hashSize = sha256.Size
+	default:
+		return nil, fmt.Errorf("furgit: unsupported hash algorithm %q", algo)
+	}
+
 	if _, ok := hashFuncs[hashSize]; !ok {
-		return nil, fmt.Errorf("furgit: unsupported hash size %d", hashSize)
+		return nil, fmt.Errorf("furgit: hash algorithm %q is not supported by the hash functions provided by this build", algo)
 	}
+
 	return &Repository{rootPath: path, HashSize: hashSize}, nil
 }
 
diff --git a/repo_test.go b/repo_test.go
index 409919cf..43cc9919 100644
--- a/repo_test.go
+++ b/repo_test.go
@@ -2,6 +2,8 @@ package furgit
 
 import (
 	"bytes"
+	"crypto/sha1"
+	"crypto/sha256"
 	"encoding/binary"
 	"errors"
 	"fmt"
@@ -23,7 +25,8 @@ func writeLooseBlob(t *testing.T, repo *Repository, data []byte) Hash {
 
 func TestOpenRepositoryAndLooseRead(t *testing.T) {
 	root := t.TempDir()
-	repo, err := OpenRepository(root, testHashSize)
+	setupRepoConfig(t, root)
+	repo, err := OpenRepository(root)
 	if err != nil {
 		t.Fatalf("OpenRepository error: %v", err)
 	}
@@ -45,7 +48,8 @@ func TestOpenRepositoryAndLooseRead(t *testing.T) {
 
 func TestResolveRefLooseAndPacked(t *testing.T) {
 	root := t.TempDir()
-	repo, err := OpenRepository(root, testHashSize)
+	setupRepoConfig(t, root)
+	repo, err := OpenRepository(root)
 	if err != nil {
 		t.Fatalf("OpenRepository error: %v", err)
 	}
@@ -85,7 +89,8 @@ func TestResolveRefLooseAndPacked(t *testing.T) {
 
 func TestResolveHEAD(t *testing.T) {
 	root := t.TempDir()
-	repo, err := OpenRepository(root, testHashSize)
+	setupRepoConfig(t, root)
+	repo, err := OpenRepository(root)
 	if err != nil {
 		t.Fatalf("OpenRepository error: %v", err)
 	}
@@ -110,7 +115,8 @@ func TestResolveHEAD(t *testing.T) {
 func TestReadObjectTypeSizeLoose(t *testing.T) {
 	t.Parallel()
 	root := t.TempDir()
-	repo, err := OpenRepository(root, testHashSize)
+	setupRepoConfig(t, root)
+	repo, err := OpenRepository(root)
 	if err != nil {
 		t.Fatalf("OpenRepository error: %v", err)
 	}
@@ -130,6 +136,7 @@ func TestReadObjectTypeSizeLoose(t *testing.T) {
 func TestReadObjectTypeSizePackedObjects(t *testing.T) {
 	t.Parallel()
 	root := t.TempDir()
+	setupRepoConfig(t, root)
 
 	objs := []testPackObject{
 		{finalType: ObjBlob, body: []byte("packed base payload")},
@@ -142,7 +149,7 @@ func TestReadObjectTypeSizePackedObjects(t *testing.T) {
 	}
 	ids := writeTestPack(t, root, "pack-basic", objs)
 
-	repo, err := OpenRepository(root, testHashSize)
+	repo, err := OpenRepository(root)
 	if err != nil {
 		t.Fatalf("OpenRepository error: %v", err)
 	}
@@ -168,8 +175,9 @@ func TestReadObjectTypeSizePackedObjects(t *testing.T) {
 func TestReadObjectTypeSizePackRefDeltaLooseBase(t *testing.T) {
 	t.Parallel()
 	root := t.TempDir()
+	setupRepoConfig(t, root)
 
-	repo, err := OpenRepository(root, testHashSize)
+	repo, err := OpenRepository(root)
 	if err != nil {
 		t.Fatalf("OpenRepository error: %v", err)
 	}
@@ -200,7 +208,8 @@ func TestReadObjectTypeSizePackRefDeltaLooseBase(t *testing.T) {
 
 func TestWriteLooseObjectAllTypes(t *testing.T) {
 	root := t.TempDir()
-	repo, err := OpenRepository(root, testHashSize)
+	setupRepoConfig(t, root)
+	repo, err := OpenRepository(root)
 	if err != nil {
 		t.Fatalf("OpenRepository error: %v", err)
 	}
@@ -504,3 +513,27 @@ func encodeOfsDistance(dist uint64) []byte {
 	}
 	return out
 }
+
+func setupRepoConfig(t *testing.T, root string) {
+	var algo string
+	switch testHashSize {
+	case sha1.Size:
+		algo = "sha1"
+	case sha256.Size:
+		algo = "sha256"
+	default:
+		t.Fatalf("unsupported testHashSize: %d", testHashSize)
+	}
+
+	cfg := fmt.Sprintf(`
+[core]
+	repositoryformatversion = 0
+[extensions]
+	objectformat = %s
+`, algo)
+
+	err := os.WriteFile(filepath.Join(root, "config"), []byte(cfg), 0o644)
+	if err != nil {
+		t.Fatalf("write config: %v", err)
+	}
+}