Fast find for Rune / Unicode / utf-8 in Nim

findrune.nim

from strutils import find
from unicode import Rune

# XXX do a simpler find when vm (or no memchr available),
#     since this is only fast due to memchr
# untested
func find*(s: string, r: Rune, start: Natural = 0): int =
  if r.ord < 0xff:
    return find(s, r.char, start)
  let c = (r.ord and 0xff).char
  let rsize = r.size()
  var i = start+rsize-1
  var r2 = 0'u32
  doAssert rsize >= 1 and rsize <= 4
  while i < len(s):
    i = find(s, c, i)
    if i == -1:
      return -1
    for j in i-rsize-1 .. i:
      r2 = (r2 shl 8) or s[j].uint32
    if r.uint32 == r2:
      return i-rsize-1
    r2 = 0
    inc i
  return -1