imagesize.lua

local function read_uint16_le(bytes, offset)
    local a, b = bytes:byte(offset + 1, offset + 2)
    return a + b * 0x100
end

local function read_uint16_be(bytes, offset)
    local a, b = bytes:byte(offset + 1, offset + 2)
    return a * 0x100 + b
end

local function read_uint32_le(bytes, offset)
    local a, b, c, d = bytes:byte(offset + 1, offset + 4)
    return a + b * 0x100 + c * 0x10000 + d * 0x1000000
end

local function read_uint32_be(bytes, offset)
    local a, b, c, d = bytes:byte(offset + 1, offset + 4)
    return a * 0x1000000 + b * 0x10000 + c * 0x100 + d
end

local detectors = {}

function detectors.png(header)
    if header:sub(2, 4) ~= "PNG" then
        error("not a png file")
    end
    if header:sub(13, 16) ~= "IHDR" then
        error("IHDR not found")
    end
    local w = read_uint32_be(header, 0x10)
    local h = read_uint32_be(header, 0x14)
    return { w, h }
end

function detectors.gif(header)
    if header:sub(1, 3) ~= "GIF" then
        error("not a gif file")
    end
    local w = read_uint16_le(header, 6)
    local h = read_uint16_le(header, 8)
    return { w, h }
end

function detectors.psd(header)
    if header:sub(1, 4) ~= '8BPS' then
        error("not a psd file")
    end
    local w = read_uint32_be(header, 18)
    local h = read_uint32_be(header, 14)
    return { w, h }
end

local function get_size(path)
    local extension = path:match('%.[a-zA-Z0-9]+$'):sub(2):lower()
    local detector = rawget(detectors, extension)
    if not detector then
        error(string.format("unknown extension '%s'", extension))
    end

    local f = io.open(path, "rb")
    local header = f:read(64)
    f:close()

    return detector(header)
end

return get_size