Add streaming renderer tests

This checks that the streaming approach produces the same output as the classical buffer-everything approach, for all existing cases.

4streaming.txt

@@ -0,0 +1,7 @@
+goos: darwin
+goarch: arm64
+pkg: github.com/buildkite/terminal-to-html/v3
+BenchmarkRendererNpm-10     	      99	  11804480 ns/op	12303395 B/op	  163908 allocs/op
+BenchmarkStreamingNpm-10    	      93	  12547077 ns/op	10829185 B/op	  163919 allocs/op
+PASS
+ok  	github.com/buildkite/terminal-to-html/v3	3.674s

screen.go

@@ -116,7 +116,10 @@ func (s *Screen) getCurrentLineForWriting() *screenLine {
 		// If MaxLines is not in use, or adding a new line would not make it
 		// larger than MaxLines, then just allocate a new line.
 		if s.MaxLines <= 0 || len(s.screen)+1 <= s.MaxLines {
-			s.screen = append(s.screen, screenLine{nodes: make([]node, 0, 80)})
+			// nodes is preallocated with space for 80 columns, which is
+			// arbitrary, but also the traditional terminal width.
+			newLine := screenLine{nodes: make([]node, 0, 80)}
+			s.screen = append(s.screen, newLine)
 			continue
 		}
 

terminal_test.go

@@ -341,7 +341,7 @@ func TestRendererAgainstCases(t *testing.T) {
 			want := c.expected
 
 			if diff := cmp.Diff(got, want); diff != "" {
-				t.Errorf("%s Render(%q) diff (-got +want):\n%s", c.name, c.input, diff)
+				t.Errorf("Render(%q) diff (-got +want):\n%s", c.input, diff)
 			}
 		})
 	}
@@ -356,7 +356,48 @@ func TestRendererAgainstFixtures(t *testing.T) {
 			got := Render(raw)
 
 			if diff := cmp.Diff(got, want); diff != "" {
-				t.Errorf("%s diff (-got +want):\n%s", base, diff)
+				t.Errorf("Render diff (-got +want):\n%s", diff)
+			}
+		})
+	}
+}
+
+func streamingRender(raw []byte) string {
+	var buf strings.Builder
+	s := &Screen{
+		MaxLines: 300,
+		ScrollOutFunc: func(line string) {
+			fmt.Fprintln(&buf, line)
+		},
+	}
+	s.Write(raw)
+	buf.WriteString(s.AsHTML())
+	return buf.String()
+}
+
+func TestStreamingRendererAgainstCases(t *testing.T) {
+	for _, c := range rendererTestCases {
+		t.Run(c.name, func(t *testing.T) {
+			got := streamingRender([]byte(c.input))
+			want := c.expected
+
+			if diff := cmp.Diff(got, want); diff != "" {
+				t.Errorf("streamingRender(%q) diff (-got +want):\n%s", c.input, diff)
+			}
+		})
+	}
+}
+
+func TestStreamingRendererAgainstFixtures(t *testing.T) {
+	for _, base := range TestFiles {
+		t.Run(fmt.Sprintf("for fixture %q", base), func(t *testing.T) {
+			raw := loadFixture(t, base, "raw")
+			want := string(loadFixture(t, base, "rendered"))
+
+			got := streamingRender(raw)
+
+			if diff := cmp.Diff(got, want); diff != "" {
+				t.Errorf("streamingRender diff (-got +want):\n%s", diff)
 			}
 		})
 	}