# slug.ludic - a frame's tags as a file name: lower case letters and digits, words joined by dashes,
# at most 24 characters, so a folder of photographs browses well
const PHT_ALPHA: string = "0123456789abcdefghijklmnopqrstuvwxyz"

@alloc_ok("on a photograph taken: its file name")
export function photo_slug(tags: string) -> string {
  var out = ""
  var n = 0
  var i = 0
  var last_dash = true
  while i < len(tags) and n < 24 {
    var c = tags[i]
    if c >= 65 and c <= 90 { c = c + 32 }
    var k = -1
    if c >= 48 and c <= 57 { k = c - 48 }
    if c >= 97 and c <= 122 { k = c - 97 + 10 }
    if k >= 0 {
      let ch: string = PHT_ALPHA[k..k + 1]
      out = out + ch
      last_dash = false
      n += 1
    } else if (c == 32 or c == 44) and not last_dash and n > 0 {
      out = out + "-"
      last_dash = true
      n += 1
    }
    i += 1
  }
  while Text.ends_with(out, "-") { out = Text.slice(out, 0, len(out) - 1) }
  if len(out) == 0 { return "photo" }
  return out
}