implement thumbnail support for txt files

thumbnails/pkg/preprocessor/preprocessor.go

@@ -0,0 +1,106 @@
+package preprocessor
+
+import (
+	"bufio"
+	"github.com/golang/freetype"
+	"github.com/golang/freetype/truetype"
+	"github.com/pkg/errors"
+	"golang.org/x/image/font"
+	"golang.org/x/image/font/gofont/goregular"
+	"image"
+	"image/draw"
+	"io"
+	"mime"
+	"strings"
+)
+
+const (
+	fontSize         = 12
+	spacing  float64 = 1.5
+)
+
+type FileConverter interface {
+	Convert(r io.Reader) (image.Image, error)
+}
+
+type ImageDecoder struct{}
+
+func (i ImageDecoder) Convert(r io.Reader) (image.Image, error) {
+	img, _, err := image.Decode(r)
+	if err != nil {
+		return nil, errors.Wrap(err, `could not decode the image`)
+	}
+	return img, nil
+}
+
+type TxtToImageConverter struct{}
+
+func (t TxtToImageConverter) Convert(r io.Reader) (image.Image, error) {
+	img := image.NewRGBA(image.Rect(0, 0, 640, 480))
+	draw.Draw(img, img.Bounds(), image.White, image.Point{}, draw.Src)
+
+	c := freetype.NewContext()
+	// Ignoring the error since we are using the embedded Golang font.
+	// This shouldn't return an error.
+	f, _ := truetype.Parse(goregular.TTF)
+	c.SetFont(f)
+	c.SetFontSize(fontSize)
+	c.SetClip(img.Bounds())
+	c.SetDst(img)
+	c.SetSrc(image.Black)
+	c.SetHinting(font.HintingFull)
+	pt := freetype.Pt(10, 10+int(c.PointToFixed(fontSize)>>6))
+
+	scanner := bufio.NewScanner(r)
+	for scanner.Scan() {
+		txt := scanner.Text()
+		cs := chunks(txt, 80)
+		for _, s := range cs {
+			_, err := c.DrawString(strings.TrimSpace(s), pt)
+			if err != nil {
+				return nil, err
+			}
+			pt.Y += c.PointToFixed(fontSize * spacing)
+			if pt.Y.Round() >= img.Bounds().Dy() {
+				return img, scanner.Err()
+			}
+		}
+
+	}
+	return img, scanner.Err()
+}
+
+// Code from https://stackoverflow.com/a/61469854
+// Written By Igor Mikushkin
+func chunks(s string, chunkSize int) []string {
+	if chunkSize >= len(s) {
+		return []string{s}
+	}
+	var chunks []string
+	chunk := make([]rune, chunkSize)
+	length := 0
+	for _, r := range s {
+		chunk[length] = r
+		length++
+		if length == chunkSize {
+			chunks = append(chunks, string(chunk))
+			length = 0
+		}
+	}
+	if length > 0 {
+		chunks = append(chunks, string(chunk[:length]))
+	}
+	return chunks
+}
+
+func ForType(mimeType string) FileConverter {
+	// We can ignore the error here because we parse it in IsMimeTypeSupported before and if it fails
+	// return the service call. So we should only get here when the mimeType parses fine.
+	mimeType, _, _ = mime.ParseMediaType(mimeType)
+	switch mimeType {
+	case "text/plain":
+		return TxtToImageConverter{}
+	default:
+		return ImageDecoder{}
+	}
+}