Module: Arrolio::TextDirection

Defined in:
lib/arrolio/text_direction.rb

Overview

Unicode text direction detection for BIDI-aware layout. Detects whether a string is predominantly left-to-right (Latin, CJK), right-to-left (Arabic, Hebrew), or mixed. Used by the text layout engine to apply correct character ordering within lines.

This module implements the "paragraph level" detection from Unicode UAX #9 (BIDI Algorithm) — a simplified version that suffices for most standards documents. Full BIDI reordering (handling embedded runs, override controls) is future work.

Constant Summary collapse

LTR = 
:ltr
RTL = 
:rtl
MIXED = 
:mixed
RTL_RANGES =

Unicode blocks containing RTL characters.

[
  0x0590..0x05FF,   # Hebrew
  0x0600..0x06FF,   # Arabic
  0x0700..0x074F,   # Syriac
  0x0750..0x077F,   # Arabic Supplement
  0x08A0..0x08FF,   # Arabic Extended-A
  0xFB1D..0xFB4F,   # Hebrew Presentation Forms
  0xFB50..0xFDFF,   # Arabic Presentation Forms-A
  0xFE70..0xFEFF    # Arabic Presentation Forms-B
].freeze

Class Method Summary collapse

Class Method Details

.detect(string) ⇒ Object

Returns the dominant direction of a string. :ltr — all or predominantly LTR characters. :rtl — predominantly RTL characters. :mixed — significant presence of both LTR and RTL.



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# File 'lib/arrolio/text_direction.rb', line 36

def detect(string)
  return LTR if string.nil? || string.empty?

  chars = string.each_char.to_a
  rtl_count = chars.count { |c| rtl?(c) }
  ltr_count = chars.count { |c| ltr?(c) }

  return LTR if rtl_count.zero?
  return RTL if ltr_count.zero?

  ratio = rtl_count.to_f / (rtl_count + ltr_count)
  ratio > 0.6 ? RTL : (ratio < 0.3 ? LTR : MIXED)
end

.ltr?(char) ⇒ Boolean

Is this character an LTR character (Latin, Cyrillic, Greek, CJK, etc.)? Excludes whitespace, punctuation, and digits (which are neutral in BIDI).

Returns:

  • (Boolean) 


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# File 'lib/arrolio/text_direction.rb', line 59

def ltr?(char)
  cp = char.ord
  return false if cp < 0x0041  # below 'A'
  return false if rtl?(char)
  return false if neutral?(char)

  # Latin, Latin Extended, Cyrillic, Greek, CJK, etc.
  true
end

.neutral?(char) ⇒ Boolean

BIDI-neutral characters (whitespace, punctuation, digits).

Returns:

  • (Boolean) 


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# File 'lib/arrolio/text_direction.rb', line 70

def neutral?(char)
  char.match?(/\s/) || char.match?(/[0-9]/) || char.match?(/[^\p{L}]/)
end

.reverse_for_rtl(string) ⇒ Object

Reverses a string character-by-character for RTL rendering. Does NOT do full BIDI reordering — only flips the base direction. Adequate for simple RTL text without embedded LTR.



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# File 'lib/arrolio/text_direction.rb', line 77

def reverse_for_rtl(string)
  string.reverse
end

.rtl?(char) ⇒ Boolean

Is this character an RTL character?

Returns:

  • (Boolean) 


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# File 'lib/arrolio/text_direction.rb', line 51

def rtl?(char)
  cp = char.ord
  RTL_RANGES.any? { |range| range.cover?(cp) }
end