guile-uniseg/uniseg.scm

(define-module (uniseg)
  #:use-module (srfi srfi-1)
  #:use-module (ice-9 match)
  #:use-module (srfi srfi-41)
  #:use-module (uniseg graphemes)
  #:use-module (uniseg graphemes stream)
  #:use-module (uniseg charsets emoji)
  #:use-module (uniseg charsets eastasian)
  #:use-module (uniseg charsets graphemes)
  #:export (emoji?
            char->grapheme-property
            char->eastasian-property
            char-width
            string-width))

(define (emoji? char)
  (char-set-contains? char-set:emoji-extended-pictographic char))

(define (char->prop sets char)
  (let ((pair
         (find
          (λ (p) (char-set-contains? (second p) char))
          sets)))
    (and pair
         (first pair))))

(define (char->grapheme-property char)
  "Find the Unicode grapheme cluster property, as defined by https://www.unicode.org/reports/tr44/"
  (or (char->prop grapheme-charsets char)
      'other))

(define (char->eastasian-property char)
  "Find the Unicode East Asian width property, as defined by https://www.unicode.org/reports/tr11/"
  (define (<==> lo hi)
    "Create a range predicate, inclusive between two numbers"
    (λ (num)
      (and (>= num lo)
           (<= num hi))))

  ;; The spec requires that any values not in the table be double-width, if
  ;; they are in certain ranges. So let's take care of that here.
  ;; These characters are not in the char-sets as they are undesignated.
  ;; See comments in unicode's EastAsianWidth.txt
  (define range-unified-ideographs-ext-a?
    (<==> #x3400 #x4DBF))
  (define range-unified-ideographs?
    (<==> #x4E00 #x9FFF))
  (define range-compatibility-ideographs?
    (<==> #xF900 #xFAFF))
  (define range-plane-2?
    (<==> #x20000 #x2FFFD))
  (define range-plane-3?
    (<==> #x30000 #x3FFFD))

  (or (char->prop eastasian-charsets char)
      (let ((int (char->integer char)))
        (match int
          ((or (? range-unified-ideographs-ext-a?)
               (? range-unified-ideographs?)
               (? range-compatibility-ideographs?)
               (? range-plane-2?)
               (? range-plane-3?))
           'doublewidth)
          (else
           'neutral)))))

(define (char-width rune)
  "Convert from a character to its visible width. Returns the width as well as the grapheme property to reduce calculations"
  (define grapheme-prop (char->grapheme-property rune))
  (values
   (or
    (match grapheme-prop
      ((or 'control
            'carriage-return
            'line-feed
            'extend
            'zero-width-joiner)
       0)
      ('regional-indicator
        2)
      ('extended-pictographic
        (if
         (char-set-contains? char-set:emoji-presentation rune)
         2
         1))
      (else #f)
      )
    (match (char->integer rune)
      (#x2E3A 3)
      (#x2E3B 4)
      (else #f))
    (match (char->eastasian-property rune)
      ('doublewidth 2)
      ;; Default for monospace is to render ambiguous as 1
      ('ambiguous 1)
      (else 1)))
   grapheme-prop))

(define (string-width str)
  "Get the width of a string by adding up the widths of each grapheme"
  (stream-fold
   (λ (val grapheme)
     (+ val (grapheme-delta-width grapheme)))
   0
   (string->grapheme-stream str)))