Feat/streaming optimization by mysamimi · Pull Request #138 · asticode/go-astisub

mysamimi · 2026-02-06T20:39:43Z

Description

This PR refactors the WriteToSRT WriteToWebVTT and WriteToSSA methods to use io.Writer streaming (via bufio.Writer) instead of accumulating the entire file content in an in-memory byte slice.

Motivation

Previously, methods like
WriteToSRT
built the entire output in a []byte buffer before writing it. For large subtitle files or high-throughput services, this caused excessive memory allocation (O(N) with file size) and GC pressure. This change ensures memory usage remains constant (O(1)) regardless of the subtitle duration/line count, as data is flushed to the writer in chunks.

Changes

SRT: Replaced internal srtBytes() building with writeSRT(w io.Writer)
WebVTT: Replaced internal webVTTBytes() (which used append/copy) with writeWebVTT(w io.Writer)
SSA: Replaced internal bytes() building with write(w io.Writer)
Go Version: Upgraded project to Go 1.21+ (1.24) to support modern standard library features and improved GC.
Tests: Updated internal tests to verify streaming behavior. Added Benchmarks.

Benchmarks

While there is a negligible overhead for tiny files due to bufio initialization, the memory scalability is significantly improved.

Results (Apple M3 Max):


BenchmarkWriteToSRT-16	301935	3902 ns/op	6136 B/op	135 allocs/op
BenchmarkWriteToWebVTT-16	237235	4967 ns/op	7030 B/op	160 allocs/op

note: The static B/op in benchmarks reflects the bufio buffer allocation (default 4KB). In real-world large file scenarios, the previous implementation's allocation would grow linearly, whereas this remains constant.

Supersedes

Supersedes #61 (if applicable)

…asticode-master

asticode

Thanks for the PR and sorry for the slow review! ❤️

I've quite a few remarks 👍

go.mod

srt.go

asticode · 2026-03-13T09:54:50Z

webvtt.go

 // WriteToWebVTT writes subtitles in .vtt format
-func (s Subtitles) WriteToWebVTT(o io.Writer) (err error) {
+// if set true in second args write index as item index
+func (s Subtitles) WriteToWebVTT(args ...interface{}) (err error) {


Can you remove replacing the function signature from this PR?

asticode · 2026-03-13T09:55:15Z

webvtt.go

 	}

+	// Init writer
+	w := bufio.NewWriter(o)


Can you avoid using a bufio.Writer here?

asticode · 2026-03-13T09:56:07Z

webvtt.go

 }

-func (l Line) webVTTBytes() (c []byte) {
+func (l Line) writeWebVTT(w io.Writer) (err error) {


Can you provide the *astikit.WriteChainer instead ?

asticode · 2026-03-13T09:56:14Z

webvtt.go

 }

-func (li LineItem) webVTTBytes(previous, next *LineItem) (c []byte) {
+func (li LineItem) writeWebVTT(w io.Writer, previous, next *LineItem) (err error) {


Can you provide the *astikit.WriteChainer instead ?

asticode · 2026-03-13T09:56:47Z

webvtt_internal_test.go

 		},
-	}}.webVTTBytes()))
+	}}.writeWebVTT(w)
+	assert.NoError(t, err)


Can you use require instead of assert?

Can you do this too?

…title formats and add astikit dependency.

…timization

…T formats

asticode · 2026-03-16T08:14:36Z

go.mod

 module github.com/asticode/go-astisub

-go 1.13
+go 1.24.0


It seems your "revert to go 1.13" change was reverted by one of the last commit. Could you revert it back?

asticode · 2026-03-16T08:16:07Z

ssa.go

-		o = appendStringToBytesWithNewLine(o, "; "+c)
+// write writes the block to the writer
+func (b *ssaScriptInfo) write(c *astikit.WriteChainer) (err error) {
+	if _, err = c.Write(astikit.WriteWithLabel("script info header", []byte("[Script Info]"))); err != nil {


Can you group/chain the 2 writes here?

asticode · 2026-03-16T08:25:38Z

ssa.go

+	}
+	for _, comment := range b.comments {
+		if _, err = c.Write(
+			astikit.WriteWithLabel("comment start", []byte("; ")),


Can you simplify with

if _, err = c.Write( astikit.WriteWithLabel("comment", append([]byte("; "+comment), bytesLineSeparator...)), ); err != nil { return }

asticode · 2026-03-16T08:28:05Z

ssa.go

 	}
 	if len(b.collisions) > 0 {
-		o = appendStringToBytesWithNewLine(o, ssaScriptInfoNameCollisions+": "+b.collisions)
+		if _, err = c.Write(


Can you simplify all writes below with one big write with conditions:

if _, err = c.Write( astikit.WriteWithCondition("collisions", append([]byte(ssaScriptInfoNameCollisions+": "b.collisions), bytesLineSeparator...), len(b.collisions) > 0), astikit.WriteWithCondition("original editing", append([]byte(ssaScriptInfoNameOriginalEditing+": "b.originalEditing), bytesLineSeparator...), len(b.originalEditing) > 0), [...] ); err != nil { return }

asticode · 2026-03-16T08:28:34Z

ssa.go

 	if len(b.collisions) > 0 {
-		o = appendStringToBytesWithNewLine(o, ssaScriptInfoNameCollisions+": "+b.collisions)
+		if _, err = c.Write(
+			astikit.WriteWithLabel("collisions label", []byte(ssaScriptInfoNameCollisions+": ")),


Also could you delete the appendStringToBytesWithNewLine() function since it's not used anymore?

asticode · 2026-03-16T08:32:54Z

webvtt.go

 }

 // WriteToWebVTT writes subtitles in .vtt format
+// if set true in second args write index as item index


Can you remove this comment too?

asticode · 2026-03-16T08:36:34Z

webvtt.go

+		}
+
+		// Lines
+		lines := r.InlineStyle.WebVTTLines


Can you "compute" all variables first (lines, ra, scroll, etc.) and then add one big write with condition writes? Something like

lines := r.InlineStyle.WebVTTLines [...] ra := r.InlineStyle.WebVTTRegionAnchor [...] if _, err = c.Write( astikit.WriteWithLabel("region id", []byte("Region: id="+r.ID)), astikit.WriteWithCondition("lines", append(bytesSpace, []byte("lines="+strconv.Itoa(lines))...)), [...] ); err != nil { return }

asticode · 2026-03-16T08:37:18Z

webvtt.go

-			} else if item.Style != nil && item.Style.InlineStyle != nil && item.Style.InlineStyle.WebVTTPosition != nil {
-				c = append(c, bytesSpace...)
-				c = append(c, []byte("position:"+item.Style.InlineStyle.WebVTTPosition.String())...)
+			// Align


Same as above, could you compute all variables first and make one big Write?

asticode · 2026-03-16T08:38:03Z

webvtt.go

+	// Voice name
 	if l.VoiceName != "" {
-		c = append(c, []byte("<v "+l.VoiceName+">")...)
+		if _, err = c.Write(


Can you use WriteWithCondition to remove the if instead?

asticode · 2026-03-16T08:38:32Z

webvtt_internal_test.go

 		},
-	}}.webVTTBytes()))
+	}}.writeWebVTT(w)
+	assert.NoError(t, err)


Can you do this too?

Mohammad Samimi and others added 20 commits November 30, 2021 17:33

use item index in WriteToWebVTT

1e7c16b

test repo

969140e

rev test repo

102b36b

fix index in Order method

d2f7670

remove index plus one!

abc0985

add FixIndex for Items

820c2cb

fix unnessery change

9abe444

fix

4ae3e61

fix bad build test & add ClipFrom, ClipTo method

82e090d

overload WriteToWebVTT, 2 arg force use item index

aa08cba

remove change in Order method

d6b6e46

add new tests, fix bug :)

f8731f9

fix clipFrom

b0e80b6

fix clipFrom

44a67b7

add clone method

06b2226

Merge branch 'master' of https://github.com/asticode/go-astisub into …

2d08a66

…asticode-master

Merge branch 'asticode-master'

b0feb66

Merge remote-tracking branch 'upstream/master'

4776701

feat: optimize writers with streaming and upgrade to Go 1.24

764fc08

chore: Update Go version to 1.24 in the test workflow.

8fd230f

asticode requested changes Mar 13, 2026

View reviewed changes

mysamimi added 5 commits March 13, 2026 22:34

refactor: use astikit.WriteChainer for robust write operations in sub…

fa8dbf1

…title formats and add astikit dependency.

Merge remote-tracking branch 'upstream/master' into feat/streaming-op…

914aeaf

…timization

Revert Go version to 1.13 and cleanup webvtt comments

a2d8524

Align WriteToWebVTT signature and logic with upstream master

266762f

refactor: wrap write errors with contextual information in SSA and SR…

932f2c7

…T formats

asticode reviewed Mar 16, 2026

View reviewed changes

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mysamimi commented Feb 6, 2026

Description

Motivation

Changes

Benchmarks

Supersedes

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