get east_asian_width in OCaml, using Core_kernel and Camomile. Ported from http://www.shibu.jp/techmemo/zenhan.html

eaw.ml

open Core_kernel

let start_group = [
  161; 164; 167; 170; 173; 176; 182; 188; 198; 208; 215; 222; 230; 232; 236; 240; 242; 247; 252; 254; 257; 273; 275;
  283; 294; 299; 305; 312; 319; 324; 328; 333; 338; 358; 363; 462; 464; 466; 468; 470; 472; 474; 476; 593; 609; 708;
  711; 713; 717; 720; 728; 733; 735; 768; 913; 945; 963; 1025; 1040; 1105; 4352; 4515; 4602; 8208; 8211; 8216; 8220;
  8224; 8228; 8240; 8242; 8245; 8251; 8254; 8308; 8319; 8321; 8364; 8451; 8453; 8457; 8467; 8470; 8481; 8486; 8491;
  8531; 8539; 8544; 8560; 8585; 8632; 8658; 8660; 8679; 8704; 8706; 8711; 8715; 8719; 8721; 8725; 8730; 8733; 8739;
  8741; 8743; 8750; 8756; 8764; 8776; 8780; 8786; 8800; 8804; 8810; 8814; 8834; 8838; 8853; 8857; 8869; 8895; 8978;
  9001; 9312; 9451; 9552; 9600; 9618; 9632; 9635; 9650; 9654; 9660; 9664; 9670; 9675; 9678; 9698; 9711; 9733; 9737;
  9742; 9748; 9756; 9758; 9792; 9794; 9824; 9827; 9831; 9836; 9839; 9886; 9918; 9924; 9935; 9955; 9960; 10045; 10071;
  10102; 11093; 11904; 12353; 13312; 13312; 19894; 19968; 19968; 40908; 40960; 43360; 44032; 55216; 57344; 63744; 63744;
  64046; 64110; 64218; 65024; 65072; 65281; 65504; 65533; 110592; 127232; 127280; 127488; 131072; 131072; 173783; 173824;
  173824; 177973; 177984; 177984; 178206; 194560; 194560; 195102; 196608; 917760; 983040; 1048576]

let end_group = [
  161; 164; 168; 170; 174; 180; 186; 191; 198; 208; 216; 225; 230; 234; 237; 240; 243; 250; 252; 254; 257; 273; 275; 283; 295;
  299; 307; 312; 322; 324; 331; 333; 339; 359; 363; 462; 464; 466; 468; 470; 472; 474; 476; 593; 609; 708; 711; 715; 717; 720;
  731; 733; 735; 879; 937; 961; 969; 1025; 1103; 1105; 4447; 4519; 4607; 8208; 8214; 8217; 8221; 8226; 8231; 8240; 8243; 8245;
  8251; 8254; 8308; 8319; 8324; 8364; 8451; 8453; 8457; 8467; 8470; 8482; 8486; 8491; 8532; 8542; 8555; 8569; 8601; 8633; 8658;
  8660; 8679; 8704; 8707; 8712; 8715; 8719; 8721; 8725; 8730; 8736; 8739; 8741; 8748; 8750; 8759; 8765; 8776; 8780; 8786; 8801;
  8807; 8811; 8815; 8835; 8839; 8853; 8857; 8869; 8895; 8978; 9002; 9449; 9547; 9587; 9615; 9621; 9633; 9641; 9651; 9655; 9661;
  9665; 9672; 9675; 9681; 9701; 9711; 9734; 9737; 9743; 9749; 9756; 9758; 9792; 9794; 9825; 9829; 9834; 9837; 9839; 9887; 9919;
  9933; 9953; 9955; 9983; 10045; 10071; 10111; 11097; 12350; 13311; 19893; 19903; 19903; 40907; 40959; 40959; 42182; 43388; 55203;
  55291; 63743; 64047; 64111; 64217; 64255; 64255; 65049; 65131; 65376; 65510; 65533; 110593; 127277; 127386; 127569; 173782;
  173791; 173823; 177972; 177983; 178205; 178207; 194367; 194559; 195101; 195103; 196605; 262141; 917999; 1048573; 1114109]


let kSurrogateBits = 10

let kHighSurrogateMin = 0xD800
let kHighSurrogateMax = 0xDBFF
let kHighSurrogateMask = (1 lsl kSurrogateBits) - 1

let kLowSurrogateMin = 0xDC00
let kLowSurrogateMax = 0xDFFF
let kLowSurrogateMask = (1 lsl kSurrogateBits) - 1

let kSurrogateMin = kHighSurrogateMin
let kSurrogateMax = kLowSurrogateMax
let kSurrogateMask = (1 lsl (kSurrogateBits + 1)) - 1

let kHighSurrogateOffset = kHighSurrogateMin - (0x10000 lsr 10)

let rec binary_range_search_impl heads tails value ~head ~tail =
  if head > tail then
    if tail < 0 || List.length tails - 1 < tail then
      false
    else
      value <= List.nth_exn tails tail
  else
    let where = ((head + tail) |> float_of_int) /. 2.
                |> Float.round_down |> int_of_float in
    let v = List.nth_exn heads where in
    if value = v then
      true
    else if value < v then
      binary_range_search_impl heads tails value ~head ~tail:(where - 1)
    else
      binary_range_search_impl heads tails value ~head:(where + 1) ~tail

let binary_range_search heads tails value =
  let head = 0
  and tail = List.length heads - 1 in
  binary_range_search_impl heads tails value ~head ~tail

let isHighSurrogate uc = (uc land (lnot kHighSurrogateMask)) = kHighSurrogateMin

let isLowSurrogate uc = (uc land (lnot kLowSurrogateMask)) = kLowSurrogateMin


let isSurrogate uc = (uc land (lnot kSurrogateMask)) = kSurrogateMin

let decodeSurrogatePair high low = ((high land kHighSurrogateMask) lsl kSurrogateBits) + (low land kLowSurrogateMask) + 0x10000

open CamomileLibrary
let (>>) f g x y = g (f x y)

let charCodeAt = UTF8.look >> UChar.code

exception UTF16_DECODE_ERROR

let rec east_asian_width ?(width=0) ?(i=0) text =
  if i >= String.length text then
    width
  else
    let proc_for_next ~code ~i =
      if binary_range_search start_group end_group code then
        east_asian_width ~width:(width + 2) ~i:(UTF8.next text i) text
      else
        east_asian_width ~width:(width + 1) ~i:(UTF8.next text i) text
    in
    let code = charCodeAt text i in
    if isSurrogate code then
      if not (isHighSurrogate code) then
        raise UTF16_DECODE_ERROR
      else
        let i = UTF8.next text i in
        let low_code = charCodeAt text i in
        if not (isLowSurrogate low_code) then
          raise UTF16_DECODE_ERROR
        else
          let code = decodeSurrogatePair code low_code in
          proc_for_next ~code ~i
    else
      proc_for_next ~code ~i

let () =
  Printf.printf "east_asian_width \"A\" : %d\n" (east_asian_width "A");
  Printf.printf "east_asian_width \"ABC\" : %d\n" (east_asian_width "ABC");
  Printf.printf "east_asian_width \"あいうえお\" : %d\n" (east_asian_width "あいうえお");
  Printf.printf "east_asian_width \"あいabc\" : %d\n" (east_asian_width "あいabc")
;;