-- This is released into the public domain.
-- No warranty is provided, implied or otherwise.

-- bmp: Portable OC BMP/DIB library
-- Flexible: Reading can be set to
--  ignore first 14 bytes,
--  allowing reuse of the library on
--  ICO/CUR data. Yes, really.

-- handle(i, valDiv, valMod)
local function bitsCore(i, fieldOfs, fieldWidth, handle)
 local adv = math.floor(fieldOfs / 8)
 i = i + adv
 fieldOfs = fieldOfs - (adv * 8)
 -- above 3 lines are a removable optimization
 while fieldWidth > 0 do
  local bitsHere = math.min(fieldWidth, math.max(0, 8 - fieldOfs))
  if bitsHere > 0 then
   local pow1 = math.floor(2 ^ bitsHere)
   -- offset
   -- pixels are "left to right" in the MostSigBitFirst stream
   local aFieldOfs = 8 - (fieldOfs + bitsHere)
   local pow2 = math.floor(2 ^ aFieldOfs)
   handle(i, pow2, pow1)
   fieldWidth = fieldWidth - bitsHere
   fieldOfs = 0
  else
   -- in case the 'adv' opt. gets removed
   fieldOfs = fieldOfs - 8
  end
  i = i + 1
 end
end

local function encode16(t)
 return string.char(t % 0x100) .. string.char(math.floor(t / 0x100))
end

local function encode32(t)
 return encode16(t % 0x10000) .. encode16(math.floor(t / 0x10000))
end

-- This & the BMP equivalent return a header,
--  a buffer size, and a 1-based data pointer,
--  and are used to initially create the image.
-- Notably, for bpp <= 8, a paletteSize of 0 is illegal.
-- topDown adjusts the order of scanlines.
-- cMode adjusts some values.
-- IT IS RECOMMENDED YOU PROPERLY SET THE MASK UP.
local function prepareDIB(w, h, p, bpp, paletteSize, topDown, cMode)
 if bpp <= 8 then
  if paletteSize == 0 then
   error("A palette size of 0 is invalid for <= 8-bit images. Use 16-bit or 32-bit for no palette, or specify the amount of palette entries.")
  end
 end

 local scanWB = math.ceil((bpp * w) / 32) * 4
 local palSize = paletteSize * 4
 local bufSize = scanWB * h * p

 local aH = h
 if cMode then
  -- O.o why change format? who knows!
  aH = aH * 2
  bufSize = bufSize + ((math.ceil(w / 32) * 4) * h * p)
 end
 if topDown then
  aH = 0x100000000 - aH
 end
 return
  "\x28\x00\x00\x00" .. -- 0x0E
  encode32(w) .. -- 0x12
  encode32(aH) .. -- 0x16
  encode16(p) .. -- 0x1A
  encode16(bpp) .. -- 0x1C
  "\x00\x00\x00\x00" .. -- 0x1E
  encode32(bufSize) .. -- 0x22
  "\x00\x00\x00\x00" .. -- 0x26
  "\x00\x00\x00\x00" .. -- 0x2A
  encode32(paletteSize) .. -- 0x2E
  encode32(paletteSize), -- 0x32 then EOH
  -- -14 here to move back into headless units
  0x36 + palSize + bufSize - 14,
  0x36 + palSize + 1 - 14 -- 1-based data pointer
end

return {
 headerMinSzBMP = 0x36,
 headerMinSzDIB = 0x36 - 14,
 -- get/set are (index) and (index, value) respectively
 -- they are 1-based
 -- If "packed" is used, two things happen:
 -- 1. The +1 offeset for 1-based is replaced
 --     with packed (so -13 is pure packed-DIB)
 -- 2. We don't try to use the BMP header
 connect = function (get, set, cMode, packed)
  -- NOTE: Internally, BMP addresses are used,
  --        so that the Wikipedia page can be used
  --        as a valid reference for header fields.
  -- verify cMode
  if cMode ~= nil and cMode ~= "mask" and cMode ~= "colour" then
   error("Unknown cMode " .. cMode)
  end
  -- NOTE: 0-base is used
  local function get8(i)
   return get(i + (packed or 1))
  end
  local function get16(i)
   return get8(i) + (256 * get8(i + 1))
  end
  local function get32(i)
   return get16(i) + (65536 * get16(i + 2))
  end
  local function set8(i, v)
   set(i + (packed or 1), v)
  end
  local function set32(i, v)
   local st = encode32(v)
   set8(i, st:byte(1))
   set8(i + 1, st:byte(2))
   set8(i + 2, st:byte(3))
   set8(i + 3, st:byte(4))
  end
  local function getBits(i, fieldOfs, fieldWidth)
   local v = 0
   local vp = 1
   bitsCore(i, fieldOfs, fieldWidth, function (i, valDiv, valMod)
    local data = math.floor(get8(i) / valDiv) % valMod
    v = v + (data * vp)
    vp = vp * valMod
   end)
   return v
  end
  local function setBits(i, fieldOfs, fieldWidth, v)
   return bitsCore(i, fieldOfs, fieldWidth, function (i, valDiv, valMod)
    local data = get8(i)
    -- Firstly need to eliminate the old data
    data = data - ((math.floor(data / valDiv) % valMod) * valDiv)
    -- Now to insert the new data
    data = data + ((v % valMod) * valDiv)
    set8(i, data)
    -- Advance
    v = math.floor(v / valMod)
   end)
  end
  -- direct header reads (all of them)
  local hdrSize = get32(0x0E)
  if hdrSize < 0x28 then
   error("OS/2 Bitmaps Incompatible")
  end
  local width = get32(0x12)
  local height = get32(0x16)
  local planes = get16(0x1A)
  local bpp = get16(0x1C)
  local compression = get32(0x1E)
  local paletteCol = get32(0x2E)

  -- negative height means sane coords
  local upDown = true
  if height >= 0x80000000 then
   height = height - 0x100000000
   height = -height
   upDown = false
  end

  -- postprocess

  -- The actual values used for addressing, for cMode to mess with
  local basePtr = 14 + hdrSize + (paletteCol * 4)
  local scanWB = math.ceil((bpp * width) / 32) * 4
  local monoWB = math.ceil(width / 32) * 4
  local planeWB = scanWB * height

  if not packed then
   basePtr = get32(0x0A) -- 'BM' header
  end

  -- Cursor/Icon
  if cMode then
   height = math.floor(height / 2)
   assert(planes == 1, "planes ~= 1 for cursor")
   planeWB = planeWB + (monoWB * height)
  end
  if cMode == "mask" then
   if upDown then
    basePtr = basePtr + (scanWB * height)
   end
   bpp = 1
   scanWB = monoWB
   paletteCol = 0
   compression = 3
  elseif cMode == "colour" then
   if not upDown then
    basePtr = basePtr + (monoWB * height)
   end
  end
  -- Check compression
  if (compression ~= 0) and (compression ~= 3) and (compression ~= 6) then
   error("compression " .. compression .. " unavailable")
  end
  -- paletteSize correction for comp == 0
  if (bpp <= 8) and (paletteCol == 0) and (compression == 0) then
   paletteCol = math.floor(2 ^ bpp)
  end
  return {
   width = width,
   height = height,
   planes = planes,
   bpp = bpp,
   ignoresPalette = (compression ~= 0) or (paletteCol == 0) or (cMode == "mask"),
   paletteCol = paletteCol,
   paletteAddress = 14 + hdrSize + (packed or 1),
   dataAddress = basePtr + (packed or 1),
   dsFull = get32(0x22),
   dsSpan = scanWB,
   dsPlane = planeWB,
   getPalette = function (i)
    return get32(14 + hdrSize + (i * 4))
   end,
   setPalette = function (i, xrgb)
    set32(14 + hdrSize + (i * 4), xrgb)
   end,
   -- Coordinates are 0-based for sanity. Returns raw colour value.
   getPixel = function (x, y, p)
    if upDown then
     y = height - (1 + y)
    end
    local i = basePtr + (y * scanWB) + (p * planeWB)
    return getBits(i, x * bpp, bpp)
   end,
   -- Uses raw colour value.
   setPixel = function (x, y, p, v)
    if upDown then
     y = height - (1 + y)
    end
    local i = basePtr + (y * scanWB) + (p * planeWB)
    setBits(i, x * bpp, bpp, v)
   end
  }
 end,
 -- See prepareDIB above for format.
 prepareBMP = function (...)
  local head, tLen, dtPtr = prepareDIB(...)
  tLen = tLen + 14 -- add BM header
  dtPtr = dtPtr + 14
  head = "BM" .. encode32(tLen) .. "mRWH" .. encode32(dtPtr - 1) .. head
  return head, tLen, dtPtr
 end,
 prepareDIB = prepareDIB
}