lib/git: implement Gitignore

Gitignore is a type that represent ".gitignore" file.

There are two ways to populate Gitignore, by using [LoadGitignore]
function, or by using [Gitignore.Parse] method.

After the Gitignore created, one can check if a path is ignored by
using [Gitignore.IsIgnored] method, relative to the Gitignore directory.

1 files changed, 106 insertions, 0 deletions

diff --git a/lib/git/gitignore.go b/lib/git/gitignore.go
new file mode 100644
index 00000000..e9313cd0
--- /dev/null
+++ b/lib/git/gitignore.go
@@ -0,0 +1,106 @@
+// SPDX-License-Identifier: BSD-3-Clause
+// SPDX-FileCopyrightText: 2025 M. Shulhan <ms@kilabit.info>
+
+package git
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// Gitignore is a type that represent ".gitignore" file.
+// The content of Gitignore can be populated from [LoadGitignore] function or
+// [Gitignore.Parse] method.
+type Gitignore struct {
+	// dir path to directory that contains ".gitignore" file.
+	dir string
+
+	// path to the ".gitignore" file.
+	path string
+
+	// excludePatterns contains list of excluded pattern from
+	// ".gitignore" file.
+	excludePatterns []ignorePattern
+
+	// includePatterns contains list of include pattern, the one that
+	// start with "!".
+	includePatterns []ignorePattern
+}
+
+// LoadGitignore load the gitignore file inside directory `dir`.
+// It will return nil without error if the ".gitignore" file is not exists.
+//
+// Any invalid pattern will be ignored.
+func LoadGitignore(dir string) (ign *Gitignore, err error) {
+	var logp = `LoadGitignore`
+	var content []byte
+
+	ign = &Gitignore{
+		path: filepath.Join(dir, `.gitignore`),
+	}
+	content, err = os.ReadFile(ign.path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+		return nil, fmt.Errorf(`%s: %w`, logp, err)
+	}
+	ign.Parse(dir, content)
+	return ign, nil
+}
+
+// Parse the raw content of ".gitignore" file that located inside the `dir`
+// directory.
+// This is an alternative to populate Gitignore content beside
+// [LoadGitignore].
+// Any invalid pattern inside the `content` will be ignored.
+func (ign *Gitignore) Parse(dir string, content []byte) {
+	ign.dir = dir
+	var lines = bytes.Split(content, []byte{'\n'})
+	var line []byte
+	for _, line = range lines {
+		var pat ignorePattern
+		pat = parsePattern(line)
+		if pat.pattern == nil {
+			// Skip invalid pattern.
+			continue
+		}
+		if pat.isNegate {
+			ign.includePatterns = append(ign.includePatterns, pat)
+		} else {
+			ign.excludePatterns = append(ign.excludePatterns, pat)
+		}
+	}
+}
+
+// IsIgnored return true if the `path` is ignored by this Gitignore content.
+// The `path` is relative to Gitignore directory.
+func (ign *Gitignore) IsIgnored(path string) bool {
+	path = strings.TrimSpace(path)
+	if path == `` {
+		return true
+	}
+	var fullpath = filepath.Join(ign.dir, path)
+	var fi os.FileInfo
+	fi, _ = os.Stat(fullpath)
+	if fi != nil {
+		if fi.IsDir() {
+			path += "/"
+		}
+	}
+	var pat ignorePattern
+	for _, pat = range ign.includePatterns {
+		if pat.isMatch(path) {
+			return false
+		}
+	}
+	for _, pat = range ign.excludePatterns {
+		if pat.isMatch(path) {
+			return true
+		}
+	}
+	return false
+}