Decoding Polylines from Google Maps Direction API with Clojure
January 17, 2016, 12:56 pm
My first crack at porting some imperative code to Clojure goodness.
I needed some code to turn the polyline points encoding you get back from Google Directions API. This stuff:
"overview_polyline" : { "points" : "_rtwFpgubMtBuGVNFPfAr@TNIVkB|FsBpG_C_BqA}@mGeEwH_FsDcCaGwDkNiJsJmG_SoMyByAzA}EPi@nA{Dp@qBtDdCLh@@VCT_@nAKLMBOAIG}@{Aq@eAWWOMIEWEY?UDYJi@]^w@|AcC|Ew@bBy@hCg@dBQ`AU~A_ChH_D|J_AzCc@rAwXl|@yLt_@k@hCSrBAtABp@LlALp@FPZ~@Tf@Z^X^jAx@PXRTrCxB~@`@lA^`@HVLv@j@n@h@`@Rb@Dl@Gl@WVQTU^w@PaA@gAKcAMa@Q]c@i@}@g@iDoAeDaAeDq@qC]i@Cg@@eAT[L[Vk@h@SZQ^_@bAeJ`XoAbEmB~GqAvDkDzIsFvNcAvCcFrRkBnHcE`Pq@~Bi@rAk@hA}@hA_@f@cAfAoDjDiAv@iB|@a@XoAf@iBj@yA`@aB{Dz@yCl@yAmE`A}Aj@qCjAuC~AkBnA_@ZcBxAoApA{BhCoBtCuJrOkAxAkBzCeH~KkAlBeMrR{I|M}CtEyBjD}EpHmLbQ{BdDiBvCUBMJy@l@g@VeAZs@HiABuKT_BEiBUaEkAcA]oCeAmAi@iCsAyCkBgEaDcCuBa@]kHgGkDyCQQOOG_@Fk@h@YfI_DxAi@vAg@^`@h@l@h@dL@v@Ef@Od@m@fAy@nBmAxCc@dAALAHQf@Qv@G^Et@@n@Fr@Z`BHlABnAp@Cj@Q"
I found Jeffrey Sambells' Java code here that does the job. I am just getting my teeth into writing Clojure for my day job at GoCatch, so I need a Clojure version. This is my first attempt. It's midnight and I haven't had a chance to check the line ends up on a map correctly, but it looks pretty good to me:
;; Port of the Java code to decode google polylines that I found here -> http://jeffreysambells.com/2010/05/27/decoding-polylines-from-google-maps-direction-api-with-java (defn decode-next-result [encoded] ;; keep scanning through encoded till b>=0x20 ;; returns the next latitude/longitude increment (loop [[current & rest] encoded shift 0 result 0] (let [b (- (int current) 63) result (bit-or result (bit-shift-left (bit-and b 0x1f) shift)) shift (+ shift 5)] (if (>= b 0x20) (do ;; if we are encoding the next result then we ;; must have more characters to scan (assert rest) ;; keep looking for our next result (recur rest shift result)) ;; we found our next result (let [return-value (if (not= (bit-and result 1) 0) (bit-not (bit-shift-right result 1)) (bit-shift-right result 1))] [return-value rest]))))) (defn lat-lng-double [lat-lng-int] (/ lat-lng-int 1E5)) (defn path-for-encoded-polyline [encoded] (loop [rest encoded lat 0 lng 0 results []] (if rest ;; if there is anthing in the encoded array ;; we should have two more results at least (let [next-result (decode-next-result rest) new-rest (second next-result) next-lat-result (+ lat (first next-result)) next-result (decode-next-result new-rest ) new-rest (second next-result) next-lng-result (+ lng (first next-result))] ;; add our lat lng result to the results (recur new-rest next-lat-result next-lng-result (conj results {:latitude (lat-lng-double next-lat-result) :longitude (lat-lng-double next-lng-result)}))) ;; we are done, return our results results))) #_ (def example-polyline "pdymEssfy[rJhAlANNeALyBD{@XqFBW^_AZi@NMp@sAPu@Bk@Ce@Qs@uA_Di@{@u@oAi@kAYm@_@mASgBOgBAcC?sBKyBYkFCyD@q@XyDLu@pAsDzDyJ`AaCJa@He@Bq@FoCw@[aBcAMQm@a@eAw@k@]yBaAwBaAqAaA_@S}DmBwD{BuLeHQKu@y@}BiCIOoEaLaJmUSQKSa@sAe@wDK}@@[g@}Am@uAkBkEkCmGg@gAk@{AQk@e@wBo@yAcAyAs@aAgDqEYWSUYa@o@{AoDeKcAwCkBqGg@eCQyAGcBC{AHsCmCf@iD^iAx@}AdAcA|@w@rAmAlBaBt@a@t@Y|Bo@xAs@z@y@n@iAf@wANq@J}@FyAEgAIkBImB?s@DkAHu@eCVeDRiGF}FGiB[sBc@{Am@qBMu@MiBe@gNIwAUmAk@uCMcAU_FKcAe@aCw@uByA}BaAwAMUw@{AoAoCk@mAYi@eAuBmBuD_AuBMICCiB_EeCsFYu@Uu@c@yCo@}CAe@i@sBIc@K_ACaADyAr@iFvBgMbB_Kz@eFj@kD^}A`@uA`CiHPcABYB[Dq@@]Dg@eBZeAr@oAl@w@VUdBsAr@c@~CuBbC_Bn@]d@_@V]NUPa@Pq@Hs@@{@Cm@Ii@Ug@UcA_@kAo@uCqAeHg@kCSi@GK]uAUs@OWEGSYQQk@e@}@a@c@Kc@AiBQiAA[CiC?GAwBAUAu@USI[UQ[GMMk@EqBG_DKsCCcBJcDcCIuBK") #_ (path-for-encoded-polyline (seq (char-array example-polyline)))
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