-- set-table-col-widths.lua
--
-- Pandoc filter which sets the widths of columns to explicit
-- percentages of the available width according to values given
-- in an attribute `table-column-widths` or `table-col-widths` of
-- an enclosing div.
-- for example
--
-- ````markdown
-- ::: {table-col-widths="20 20 40"}
--
-- |This|That|Description
-- |----|----|-----------
-- |Foo |Bar |Foo and bar
--
-- :::
-- ````
--
-- will make the first two colums 20% wide and the third
-- column 40% wide, taking up a total of 80% of the available
-- width.
--
-- If the attribute contains more widths than there are
-- columns in an enclosed table the list of new widths will be
-- truncated to fit, ignoring excess items. If on the other
-- hand the attribute contains fewer widths than there are
-- columns in an enclosed table the list of new widths will be
-- extended to fit **by appending copies of the last item!**
--
-- Note that any substrings in the attribute value which are not
-- sequences of digits will be ignored except as separators.
-- In particular something like "20.5" will count as two column 
-- widths of 20% and 5% respectively!
-- If you want to make your attribute values more human
-- friendly you can include commas, spaces and per cent signs
-- like `20%, 20%, 40%`. The filter still only "sees" the
-- three substrings `20`, `20` and `40` and something "crazy"
-- like `-20:20 -- !40` would be
-- entirely equivalent!
--
-- Note that a percentage of zero has a special meaning:
-- it makes the width of the corresponding column "automatic"
-- so that it will fit the content of the row with the
-- widest content for that column.
--
-- If no sequences of digits are found in the attribute value
-- all column widths are set to zero — i.e. automatic —, so that
-- a human-friendly way of making all column widths
-- automatic is to set the attribute value to the string
-- `auto`, although `foo` or `Ni` or `""` will have the same
-- effect!
--
-- By default the enclosing div will be "removed", i.e. replaced
-- with its possibly altered content.  To avoid this set a class `.keep-div` on the div!

local function copy_table (t)
  local c = {}
  for k,v in pairs(t) do
    c[k] = v
  end
  return c
end

local function gen_set_widths (wanted)
  local function set_widths (table)
    local old = table.widths
    local new = copy_table(wanted)
    if #new > #old then
      -- If there are more wanted widths than columns
      -- then truncate the list of new widths
      local start = #old+1
      for i=#new, start, -1 do
        new[i] = nil
      end
    elseif #old > #new then
      -- If there are fewer wanted widths than columns then
      -- extend the list of new widths with copies of the last
      -- new width specified
      local last = new[#new]
      local start = #new+1
      for i=start, #old do
        new[i] = last
      end
    end
    table.widths = new
    return table
  end
  return { Table = set_widths }
end

-- "Parse" a string of wanted widths by extracting all
-- sequences of digits with string.gmatch, meaning that
-- any sequences of non-digits are ignored except as separators.
-- These are taken to be percentages of the total width
-- available, so are divided by 100 to get the floats Pandoc wants
local function extract_widths (str)
  wd = {}
  for w in str:gmatch('%d+') do
    wd[#wd+1] = tonumber(w, 10) / 100
  end
  -- If no widths were found default to automatic width for all columns!
  if #wd == 0 then
    wd[1] = 0.0
  end
  return wd
end

function Div (div)
  local wd_string = div.attributes['table-column-widths']
    or div.attributes['table-col-widths']
  if not wd_string then return nil end
  new_widths = extract_widths(wd_string)
  filter = gen_set_widths(new_widths)
  new_div = pandoc.walk_block(div, filter)
  if div.classes:includes('keep-div') then
    return new_div
  else
    return new_div.content
  end
end