refactor(svg): use interface for better testing

cmd/termsvg/export/export.go

@@ -25,21 +25,28 @@ func (cmd *Cmd) Run() error {
 }
 
 func export(input, output string) error {
-	file, err := os.ReadFile(input)
+	inputFile, err := os.ReadFile(input)
 	if err != nil {
 		return err
 	}
 
 	if output == "" {
-		output = input
+		output = input + ".svg"
 	}
 
-	cast, err := asciicast.Unmarshal(file)
+	cast, err := asciicast.Unmarshal(inputFile)
 	if err != nil {
 		return err
 	}
 
-	svg.Export(*cast, output)
+	outputFile, err := os.Create(output)
+	if err != nil {
+		return err
+	}
+
+	defer outputFile.Close()
+
+	svg.Export(*cast, outputFile)
 
 	return nil
 }

internal/svg/svg.go

@@ -2,7 +2,7 @@ package svg
 
 import (
 	"fmt"
-	"os"
+	"io"
 	"strings"
 
 	svg "github.com/ajstarks/svgo"
@@ -27,6 +27,10 @@ type Canvas struct {
 	bgColors map[string]byte
 }
 
+type Output interface {
+	io.Writer
+}
+
 func createSVG(svg *svg.SVG, cast asciicast.Cast) {
 	canvas := &Canvas{SVG: svg, Cast: cast}
 	canvas.width = cast.Header.Width * colWidth
@@ -125,16 +129,10 @@ func (c *Canvas) createFrames() {
 	}
 }
 
-func Export(input asciicast.Cast, output string) {
-	file, err := os.Create(fmt.Sprintf("%s.svg", output))
-	if err != nil {
-		panic(err)
-	}
-
-	defer file.Close()
-	input.Compress()
+func Export(input asciicast.Cast, output Output) {
+	input.Compress() // to reduce the number of frames
 
-	createSVG(svg.New(file), input)
+	createSVG(svg.New(output), input)
 }
 
 func (c *Canvas) addBG(bg vt10x.Color) {