bidi — the Unicode Bidirectional Algorithm

github.com/go-opentype/bidi is a pure-Go, CGO_ENABLED=0, standard-library-only implementation of the Unicode Bidirectional Algorithm (UAX #9) for laying out mixed left-to-right / right-to-left text.

Unlike most Go bidi implementations, it does not depend on golang.org/x/text. The Bidi_Class and paired-bracket Unicode properties are compiled into small generated lookup tables (see cmd/genbidi), so the package builds anywhere the standard library does.

Usage

package main

import (
    "fmt"

    "github.com/go-opentype/bidi"
)

func main() {
    // A logical-order string mixing English and Hebrew.
    s := "abc אבג def"

    // Visual (left-to-right) order for display.
    fmt.Println(bidi.VisualOrder(s, bidi.Auto))

    // Or work with levels directly.
    runes := []rune(s)
    levels := bidi.ResolveLevels(runes, bidi.LeftToRight)
    order := bidi.Reorder(runes, levels) // visual-order permutation of indices
    fmt.Println(levels, order)

    // Inspect a single rune's Bidi_Class.
    fmt.Println(bidi.ClassOf('א')) // R
}

API

Symbol	Purpose
ClassOf(r rune) Class	Bidi_Class of a rune
Class enum	L R AL EN ES ET AN CS NSM BN B S WS ON LRE RLE LRO RLO PDF LRI RLI FSI PDI
ResolveLevels(text []rune, base Direction) []Level	resolved embedding level per rune
BaseLevel(text []rune, base Direction) Level	paragraph level (rules P2/P3)
Reorder(text []rune, levels []Level) []int	rule L2 visual-order permutation
VisualOrder(text string, base Direction) string	resolve + reorder convenience
Direction	LeftToRight, RightToLeft, Auto
Level	embedding level (even = LTR, odd = RTL)

Implemented vs. deferred

Implemented — the algorithm runs through rule L2, the full extent covered by the Unicode conformance file BidiCharacterTest.txt:

• P2, P3 base paragraph level from the first strong character.
• X1–X8 explicit embeddings and isolates (with overflow handling and the directional status stack); X9 removal of the deprecated formatting characters and BN; X10 isolating run sequences with sos/eos.
• W1–W7 weak types.
• N0 paired-bracket resolution (BD16, incl. the U+2329/U+232A canonical equivalence), N1–N2 neutral types.
• I1, I2 implicit levels.
• L1 separator / trailing-whitespace reset, L2 reordering.

Deferred (out of scope for a bidi engine — the job of a shaper):

• L3 (combining marks) and L4 (glyph mirroring of paired brackets and other mirrored characters) — VisualOrder does not substitute mirrored glyphs; that belongs to the rendering/shaping stage.
• Arabic cursive shaping / joining — see shape.
• P1 paragraph splitting — the caller drives it; the API operates per paragraph (an inline Paragraph_Separator is still handled by X8/L1).

Conformance

The package is validated against the entire BidiCharacterTest.txt (all cases pass: paragraph level, per-character levels and visual order). A curated representative subset is embedded under testdata and run by TestConformance. CI enforces exactly 100% statement coverage, go vet, gofmt, and cross-compilation for the six 64-bit architectures plus js/wasm, darwin/arm64 and windows/amd64.

Regenerating the tables

go run ./cmd/genbidi .

This fetches the latest DerivedBidiClass.txt and BidiBrackets.txt from the Unicode Character Database and rewrites bidiclass_table.go and bidibrackets_table.go.

Source: github.com/go-opentype/bidi · pkg.go.dev