Bit operations in Lua - lua

Question

I try to parse gpx files and to output encoded polylines (Google algorithm)
test.gpx
<trkseg>
<trkpt lon="-120.2" lat="38.5"/>
<trkpt lon="-120.95" lat="40.7"/>
<trkpt lon="-126.453" lat="43.252"/>
</trkseg>
I managed most of it, but have trouble with encoding the numbers
gpx2epl:
file = io.open(arg[1], "r")
io.input(file)
--
function round(number, precision)
return math.floor(number*math.pow(10,precision)+0.5) / math.pow(10,precision)
end
function encodeNumber(number)
return number
end
--
local Olatitude = 0
local Olongitude = 0
--
while true do
local line = io.read()
if line == nil
then
break
end
if string.match(line, "trkpt") then
local latitude
local longitude
local encnum
latitude = string.match(line, 'lat="(.-)"')
longitude = string.match(line, 'lon="(.-)"')
latitude = round(latitude,5)*100000
longitude = round(longitude,5)*100000
encnum = encodeNumber(latitude-Olatitude)
print(encnum)
encnum = encodeNumber(longitude-Olongitude)
print(encnum)
Olatitude = latitude
Olongitude = longitude
end
end
This script produces the expected output (see: Google Link), with the exception of encoded latitude and longitude.
3850000
-12020000
220000
-75000
255200
-550300
Mapquest provides an implementation in Javascript:
function encodeNumber(num) {
var num = num << 1;
if (num < 0) {
num = ~(num);
}
var encoded = '';
while (num >= 0x20) {
encoded += String.fromCharCode((0x20 | (num & 0x1f)) + 63);
num >>= 5;
}
encoded += String.fromCharCode(num + 63);
return encoded;
}
Can this be done in Lua? Can somebody please help me out. I have no idea how to implement this in Lua.
Edit:
Based on Doug's advice, I did:
function encodeNumber(number)
local num = number
num = num * 2
if num < 0
then
num = (num * -1) - 1
end
while num >= 32
do
local num2 = 32 + (num % 32) + 63
print(string.char(num2))
num = num / 32
end
print(string.char(num + 63) .. "\n-----")
end
encodeNumber(3850000) -- _p~iF
encodeNumber(-12020000) -- ~ps|U
encodeNumber(220000) -- _ulL
encodeNumber(-75000) -- nnqC
encodeNumber(255200) -- _mqN
encodeNumber(-550300) -- vxq`#
It's near expected output, but only near ... Any hint?

Answer 1

Taking encodeNumber piecemeal...
var num = num << 1;
This is just num = num * 2
num = ~(num);
This is num = (- num) - 1
0x20 | (num & 0x1f)
Is equivalent to 32 + (num % 32)
num >>= 5
Is equivalent to num = math.floor(num / 32)
ADDENDUM
To concatenate the characters, use a table to collect them:
function encodeNumber(number)
local num = number
num = num * 2
if num < 0
then
num = (num * -1) - 1
end
local t = {}
while num >= 32
do
local num2 = 32 + (num % 32) + 63
table.insert(t,string.char(num2))
num = math.floor(num / 32) -- use floor to keep integer portion only
end
table.insert(t,string.char(num + 63))
return table.concat(t)
end