local GPS = { }

function GPS.locate(timeout, debug)
  local pt = { }
  timeout = timeout or 10
  pt.x, pt.y, pt.z = gps.locate(timeout, debug)
  if pt.x then
    return pt
  end
end

function GPS.isAvailable()
  return device.wireless_modem and GPS.locate()
end

function GPS.getPoint(timeout, debug)
 
  local pt = GPS.locate(timeout, debug)
  if not pt then
    return
  end

  pt.x = math.floor(pt.x)
  pt.y = math.floor(pt.y)
  pt.z = math.floor(pt.z)

  if pocket then
    pt.y = pt.y - 1
  end

  return pt
end

function GPS.getHeading()

  if not turtle then
    return
  end

  local apt = GPS.locate()
  if not apt then
    return
  end

  local heading = turtle.point.heading

  while not turtle.forward() do
    turtle.turnRight()
    if turtle.getHeading() == heading then
      printError('GPS.getPoint: Unable to move forward')
      return
    end
  end

  local bpt = GPS.locate()
  if not bpt then
    return
  end

  if apt.x < bpt.x then
    return 0
  elseif apt.z < bpt.z then
    return 1
  elseif apt.x > bpt.x then
    return 2
  end
  return 3
end

function GPS.getPointAndHeading()
  local heading = GPS.getHeading()
  if heading then
    local pt = GPS.getPoint()
    if pt then
      pt.heading = heading
    end
    return pt
  end
end

-- from stock gps API
local function trilaterate( A, B, C ) 
  local a2b = B.position - A.position
  local a2c = C.position - A.position

  if math.abs( a2b:normalize():dot( a2c:normalize() ) ) > 0.999 then
    return nil 
  end 

  local d = a2b:length()
  local ex = a2b:normalize( )
  local i = ex:dot( a2c )
  local ey = (a2c - (ex * i)):normalize()
  local j = ey:dot( a2c )
  local ez = ex:cross( ey )

  local r1 = A.distance
  local r2 = B.distance
  local r3 = C.distance

  local x = (r1*r1 - r2*r2 + d*d) / (2*d)
  local y = (r1*r1 - r3*r3 - x*x + (x-i)*(x-i) + j*j) / (2*j)
 
  local result = A.position + (ex * x) + (ey * y)

  local zSquared = r1*r1 - x*x - y*y 
  if zSquared > 0 then
    local z = math.sqrt( zSquared )
    local result1 = result + (ez * z)
    local result2 = result - (ez * z)
    
    local rounded1, rounded2 = result1:round(), result2:round()
    if rounded1.x ~= rounded2.x or rounded1.y ~= rounded2.y or rounded1.z ~= rounded2.z then
      return rounded1, rounded2
    else
      return rounded1
    end 
  end 
  return result:round()
end

local function narrow( p1, p2, fix )
  local dist1 = math.abs( (p1 - fix.position):length() - fix.distance )
  local dist2 = math.abs( (p2 - fix.position):length() - fix.distance )

  if math.abs(dist1 - dist2) < 0.05 then
    return p1, p2
  elseif dist1 < dist2 then
    return p1:round()
  else
    return p2:round()
  end 
end
-- end stock gps api

function GPS.trilaterate(tFixes)
  local pos1, pos2 = trilaterate(tFixes[1], tFixes[2], tFixes[3])

  if pos2 then
    pos1, pos2 = narrow(pos1, pos2, tFixes[4])
  end

  if pos1 and pos2 then
    print("Ambiguous position")
    print("Could be "..pos1.x..","..pos1.y..","..pos1.z.." or "..pos2.x..","..pos2.y..","..pos2.z )
    return
  end

  return pos1
end

return GPS