justenoughicu incorrect length calculation

[mhosken]

In the function icu_bidi_runs there is a calculation to reduce the length of a run based on 2-part UTF-16 codes. There is a bug in this calculation. The loop walks forward reducing the length each time a trailing character is found. But this means that if there is more than one trailing character in a string, then the final character(s) will not be tested and the length may come out too long.

I suggest running the loop backwards.

[simoncozens]

Confirmed. Here's a spec test. Length is currently coming out as 4:

SILE = require("core/sile")
local icu = require("justenoughicu")

describe("SILE.linebreak", function()
  local chars = { 0x10000, 0x10001, 0x10002 }
  local utf8string = ""
  for i = 1,#chars do
    utf8string = utf8string .. SU.utf8char(chars[i])
  end

  it("should be the right length in UTF8", function()
    assert.is.equal(#utf8string, 12)
  end)

  it("should be the right length from ICU", function()
    local res = icu.bidi_runs(utf8string, "LTR")
    assert.is.equal(res.length, 3)
  end)
end)