From 7cb2284e2b60e03ef7ced2df25d6055bf8472bcb Mon Sep 17 00:00:00 2001
From: Jonas Knobloch <jonas.knobloch@t-online.de>
Date: Fri, 3 Apr 2026 20:16:18 +0200
Subject: Add file reader

---
 dataset/file.go | 110 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 110 insertions(+)
 create mode 100644 dataset/file.go

diff --git a/dataset/file.go b/dataset/file.go
new file mode 100644
index 0000000..081fbdf
--- /dev/null
+++ b/dataset/file.go
@@ -0,0 +1,110 @@
+package dataset
+
+import (
+	"bufio"
+	"fmt"
+	"iter"
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+)
+
+type FileReader struct {
+	shards []string
+	err    error
+}
+
+func NewFileReader(name, pattern string) (*FileReader, error) {
+	var shards []string
+
+	if matches, err := filepath.Glob(filepath.Join(name, pattern)); err != nil {
+		return nil, err
+	} else {
+		shards = matches
+	}
+
+	if len(shards) == 0 {
+		return nil, fmt.Errorf("no matching files in %q", name)
+	}
+
+	slices.Sort(shards)
+
+	f := &FileReader{
+		shards: shards,
+	}
+
+	return f, nil
+}
+
+func (f *FileReader) Err() error {
+	return f.err
+}
+
+func (f *FileReader) Texts() iter.Seq2[int, string] {
+	return func(yield func(int, string) bool) {
+		for n, name := range f.shards {
+			for text := range f.read(name) {
+				if f.err != nil {
+					return
+				}
+
+				if !yield(n, text) {
+					return
+				}
+			}
+		}
+	}
+}
+
+func (f *FileReader) read(name string) iter.Seq[string] {
+	return func(yield func(string) bool) {
+		var file *os.File
+
+		if r, err := os.Open(name); err != nil {
+			f.err = err
+
+			return
+		} else {
+			file = r
+		}
+
+		defer file.Close()
+
+		scanner := bufio.NewScanner(file)
+
+		buf := make([]byte, 0, 1024*1024)
+
+		scanner.Buffer(buf, 1024*1024)
+
+		scanner.Split(func(data []byte, atEOF bool) (advance int, token []byte, err error) {
+			if atEOF && len(data) == 0 {
+				return 0, nil, nil
+			}
+
+			if i := strings.IndexAny(string(data), "\r\n"); i >= 0 {
+				if i+1 < len(data) && data[i] == '\r' && data[i+1] == '\n' {
+					return i + 2, data[0 : i+2], nil
+				}
+
+				return i + 1, data[0 : i+1], nil
+			}
+
+			if atEOF {
+				return len(data), data, nil
+			}
+
+			return 0, nil, nil
+		})
+
+		for scanner.Scan() {
+			if !yield(scanner.Text()) {
+				return
+			}
+		}
+
+		if err := scanner.Err(); err != nil {
+			f.err = err
+		}
+	}
+}
-- 
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