geo3x3.fs

namespace Geo3x3

module Geo3x3 =
    let encode lat lng level =
        if level < 1 then
            ""
        else
            let (res, lng1) =
                if lng >= 0.0 then
                    ("E", lng)
                else
                    ("W", lng + 180.0) in
            let lng2 = lng1 in
            let lat2 = lat + 90.0 in (* 180:the North Pole,  0:the South Pole *)
            let rec loop i lat lng unit =
                if i >= level then
                    ""
                else
                    let x = int (lng / unit) in
                    let y = int (lat / unit) in
                    let n = x + y * 3 + 1 in
                    let lng2 = lng - (float x) * unit in
                    let lat2 = lat - (float y) * unit in
                    (string n) + (loop (i + 1) lat2 lng2 (unit / 3.0)) in
            res + (loop 1 lat2 lng2 (180.0 / 3.0))

    let decode code = (* (35., 135., 14, 0.1) *)
        let clen = String.length code in
        if clen < 1 then
            (0.0, 0.0, 0, 180.0) (* err *)
        else
            let c = code.[0] in
            let (flg, beg) =
                if c = '-' || c = 'W' then
                    (true, 1)
                else if c = '+' || c = 'E' then
                    (false, 1)
                else
                    (false, 0) in
            let rec loop i lat lng level unit =
                if i >= clen then
                    (lat, lng, level, unit)
                else
                    let unit2 = unit / 3.0 in
                    let n = (int code.[i]) - 49 in (* ignore err check *)
                    let lat2 = lat + (float (n / 3)) * unit2 in
                    let lng2 = lng + (float (n % 3)) * unit2 in
                    loop (i + 1) lat2 lng2 (level + 1) unit2 in
            let (lat, lng, level, unit) = loop beg 0.0 0.0 1 180.0 in
            let lat2 = lat + (unit / 2.0) in
            let lng2 = lng + (unit / 2.0) in
            let lat3 = lat2 - 90.0 in
            let lng3 = if flg then lng2 - 180.0 else lng2 in
            (lat3, lng3, level, unit)