local syntax = require "core.syntax"

local tokenizer = {}

local function push_token(t, type, text)
  local prev_type = t[#t-1]
  local prev_text = t[#t]
  if prev_type and (prev_type == type or prev_text:find("^%s*$")) then
    t[#t-1] = type
    t[#t] = prev_text .. text
  else
    table.insert(t, type)
    table.insert(t, text)
  end
end


local function push_tokens(t, syn, pattern, full_text, find_results)
  if #find_results > 2 then
    -- We do some manipulation with find_results so that it's arranged
    -- like this:
    -- { start, end, i_1, i_2, i_3, …, i_last }
    -- Each position spans characters from i_n to ((i_n+1) - 1), to form
    -- consecutive spans of text.
    --
    -- If i_1 is not equal to start, start is automatically inserted at
    -- that index.
    if find_results[3] ~= find_results[1] then
      table.insert(find_results, 3, find_results[1])
    end
    -- Copy the ending index to the end of the table, so that an ending index
    -- always follows a starting index after position 3 in the table.
    table.insert(find_results, find_results[2] + 1)
    -- Then, we just iterate over our modified table.
    for i = 3, #find_results - 1 do
      local start = find_results[i]
      local fin = find_results[i + 1] - 1
      local type = pattern.type[i - 2]
        -- ↑ (i - 2) to convert from [3; n] to [1; n]
      local text = full_text:sub(start, fin)
      push_token(t, syn.symbols[text] or type, text)
    end
  else
    local start, fin = find_results[1], find_results[2]
    local text = full_text:sub(start, fin)
    push_token(t, syn.symbols[text] or pattern.type, text)
  end
end


-- State is a 32-bit number that is four separate bytes, illustrating how many
-- differnet delimiters we have open, and which subsyntaxes we have active.
-- At most, there are 3 subsyntaxes active at the same time. Beyond that,
-- does not support further highlighting. 

-- You can think of it as a maximum 4 integer (0-255) stack. It always has
-- 1 integer in it. Calling `push_subsyntax` increases the stack depth. Calling
-- `pop_subsyntax` decreases it. The integers represent the index of a pattern
-- that we're following in the syntax. The top of the stack can be any valid 
-- pattern index, any integer lower in the stack must represent a pattern that
-- specifies a subsyntax.

-- If you do not have subsyntaxes in your syntax, the three most
-- singificant numbers will always be 0, the stack will only ever be length 1
-- and the state variable will only ever range from 0-255.
local function retrieve_syntax_state(incoming_syntax, state)
  local current_syntax, subsyntax_info, current_pattern_idx, current_level =
    incoming_syntax, nil, state, 0
  if state > 0 and (state > 255 or current_syntax.patterns[state].syntax) then
    -- If we have higher bits, then decode them one at a time, and find which
    -- syntax we're using. Rather than walking the bytes, and calling into
    -- `syntax` each time, we could probably cache this in a single table.
    for i = 0, 2 do
      local target = bit32.extract(state, i*8, 8)
      if target ~= 0 then
        if current_syntax.patterns[target].syntax then
          subsyntax_info = current_syntax.patterns[target]
          current_syntax = type(subsyntax_info.syntax) == "table" and
            subsyntax_info.syntax or syntax.get(subsyntax_info.syntax)
          current_pattern_idx = 0
          current_level = i+1
        else
          current_pattern_idx = target
          break
        end
      else
        break
      end
    end
  end
  return current_syntax, subsyntax_info, current_pattern_idx, current_level
end

function tokenizer.tokenize(incoming_syntax, text, state)
  local res = {}
  local i = 1

  if #incoming_syntax.patterns == 0 then
    return { "normal", text }
  end

  state = state or 0
  -- incoming_syntax    : the parent syntax of the file.
  -- state              : a 32-bit number representing syntax state (see above) 
  
  -- current_syntax     : the syntax we're currently in.
  -- subsyntax_info     : info about the delimiters of this subsyntax.
  -- current_pattern_idx: the index of the pattern we're on for this syntax.
  -- current_level      : how many subsyntaxes deep we are.
  local current_syntax, subsyntax_info, current_pattern_idx, current_level =
    retrieve_syntax_state(incoming_syntax, state)
  
  -- Should be used to set the state variable. Don't modify it directly.
  local function set_subsyntax_pattern_idx(pattern_idx)
    current_pattern_idx = pattern_idx
    state = bit32.replace(state, pattern_idx, current_level*8, 8)
  end
  
  
  local function push_subsyntax(entering_syntax, pattern_idx)
    set_subsyntax_pattern_idx(pattern_idx)
    current_level = current_level + 1
    subsyntax_info = entering_syntax
    current_syntax = type(entering_syntax.syntax) == "table" and
      entering_syntax.syntax or syntax.get(entering_syntax.syntax)
    current_pattern_idx = 0
  end
  
  local function pop_subsyntax()
    set_subsyntax_pattern_idx(0)
    current_level = current_level - 1
    set_subsyntax_pattern_idx(0)
    current_syntax, subsyntax_info, current_pattern_idx, current_level = 
      retrieve_syntax_state(incoming_syntax, state)
  end
  
  local function find_text(text, p, offset, at_start, close)
    local target, res = p.pattern or p.regex, { 1, offset - 1 }, p.regex
    local code = type(target) == "table" and target[close and 2 or 1] or target
    if p.regex and type(p.regex) ~= "table" then
      p._regex = p._regex or regex.compile(p.regex)
      code = p._regex
    end    
    repeat
      res = p.pattern and { text:find(at_start and "^" .. code or code, res[2]+1) } 
        or { regex.match(code, text, res[2]+1, at_start and regex.ANCHORED or 0) }
      if res[1] and close and target[3] then
        local count = 0
        for i = res[1] - 1, 1, -1 do
          if text:byte(i) ~= target[3]:byte() then break end
          count = count + 1
        end
        -- Check to see if the escaped character is there,
        -- and if it is not itself escaped.
        if count % 2 == 0 then break end
      end
    until not res[1] or not close or not target[3]
    return table.unpack(res)
  end
  
  while i <= #text do
    -- continue trying to match the end pattern of a pair if we have a state set
    if current_pattern_idx > 0 then
      local p = current_syntax.patterns[current_pattern_idx]
      local s, e = find_text(text, p, i, false, true)

      local cont = true
      -- If we're in subsyntax mode, always check to see if we end our syntax
      -- first, before the found delimeter, as ending the subsyntax takes
      -- precedence over ending the delimiter in the subsyntax.
      if subsyntax_info then
        local ss, se = find_text(text, subsyntax_info, i, false, true)
        -- If we find that we end the subsyntax before the 
        -- delimiter, push the token, and signal we shouldn't
        -- treat the bit after as a token to be normally parsed
        -- (as it's the syntax delimiter).
        if ss and (s == nil or ss < s) then
          push_token(res, p.type, text:sub(i, ss - 1))
          i = ss
          cont = false
        end
      end
      -- If we don't have any concerns about syntax delimiters,
      -- continue on as normal.
      if cont then
        if s then
          push_token(res, p.type, text:sub(i, e))
          set_subsyntax_pattern_idx(0)
          i = e + 1
        else
          push_token(res, p.type, text:sub(i))
          break
        end
      end
    end
    -- General end of syntax check. Applies in the case where
    -- we're ending early in the middle of a delimiter, or
    -- just normally, upon finding a token.
    if subsyntax_info then
      local s, e = find_text(text, subsyntax_info, i, true, true)
      if s then
        push_token(res, subsyntax_info.type, text:sub(i, e))
        -- On finding unescaped delimiter, pop it.
        pop_subsyntax()
        i = e + 1
      end
    end

    -- find matching pattern
    local matched = false
    for n, p in ipairs(current_syntax.patterns) do
      local find_results = { find_text(text, p, i, true, false) }
      if find_results[1] then
        -- matched pattern; make and add tokens
        push_tokens(res, current_syntax, p, text, find_results)
        -- update state if this was a start|end pattern pair
        if type(p.pattern or p.regex) == "table" then
          -- If we have a subsyntax, push that onto the subsyntax stack.
          if p.syntax then
            push_subsyntax(p, n)
          else          
            set_subsyntax_pattern_idx(n)
          end
        end
        -- move cursor past this token
        i = find_results[2] + 1
        matched = true
        break
      end
    end

    -- consume character if we didn't match
    if not matched then
      push_token(res, "normal", text:sub(i, i))
      i = i + 1
    end
  end

  return res, state
end


local function iter(t, i)
  i = i + 2
  local type, text = t[i], t[i+1]
  if type then
    return i, type, text
  end
end

function tokenizer.each_token(t)
  return iter, t, -1
end


return tokenizer