use dict for 'visit' var in day 16

not sure if this is an improvement? it's more or less the same
as the array version

.gitignore

@@ -1 +1,29 @@
 /cookie
+/day09/sample2.in
+/day09/sample3.in
+/day09/sample4.in
+/day09/sample5.in
+/day09/sample6.in
+/day09/sample7.in
+/day09/sample8.in
+/day10/sample1.in
+/day10/sample2.in
+/day10/sample3.in
+/day10/sample4.in
+/day10/sample5.in
+/day10/sample7.in
+/day10/sample9.in
+/day11/sample2.in
+/day11/sample3.in
+/day11/sample4.in
+/day11/sample5.in
+/day12/sample3.in
+/day13/sample2.in
+/day13/sample3.in
+/day14/sample2.in
+/day14/sample3.in
+/day16/sample2.in
+/day16/sample3.in
+/day17/sample2.in
+/day18/sample2.in
+/day18/sample3.in

day16/sol.tcl

@@ -20,16 +20,16 @@ proc read-input {input} {
 
 proc reset-visited {} {
     global visit
-    array unset visit
+    set visit [dict create]
 }
 
 proc mark-visited {x y d} {
     global visit
     set v [switch -exact -- $d n {expr 1} s {expr 2} w {expr 4} e {expr 8} default {error "invalid dir '$d'"}]
-    incr visit($y,$x) 0
-    set result [expr {($visit($y,$x) & $v) != 0}]
-    set visit($y,$x) [expr {$visit($y,$x) | $v}]
-    return $result
+    dict incr visit $y,$x 0
+    set old [dict get $visit $y,$x]
+    dict set visit $y,$x [expr {$old | $v}]
+    return [expr {($old & $v) != 0}]
 }
 
 proc print-visited {} {
@@ -39,7 +39,7 @@ proc print-visited {} {
     for {set i 0} {$i < $height} {incr i} {
         set row {}
         for {set j 0} {$j < $width} {incr j} {
-            if {[info exist visit($i,$j)] && $visit($i,$j)} {
+            if {[dict exists $visit $i,$j] && [dict get $visit $i,$j]} {
                 lappend row "#"
             } else {
                 lappend row "."
@@ -93,7 +93,7 @@ proc solve {} {
         set beams [follow $beams]
     }
     print-visited
-    puts [array size visit]
+    puts [dict size $visit]
 }
 
 proc lmax {list} {
@@ -125,8 +125,8 @@ proc solve2 {} {
         while {[llen $beams] > 0} {
             set beams [follow $beams]
         }
-        puts [array size visit]
-        lappend l [array size visit]
+        puts [dict size $visit]
+        lappend l [dict size $visit]
     }
 
     puts [lmax $l]
@@ -135,7 +135,7 @@ proc solve2 {} {
 read-input stdin
 puts [array get grid]
 puts [follow {0 0 e}]
-puts [array get visit]
+puts [dict get $visit]
 
 solve
 solve2

day17/astar.py

@@ -0,0 +1,105 @@
+from heapq import heappush, heappop
+
+def search(start, is_goal, neighbors, heuristic=None, worst=None):
+    if heuristic == None:
+        def heuristic(x):
+            return 0
+    if not callable(is_goal):
+        goal = is_goal
+        def is_goal(this):
+            return this == goal
+    if not isinstance(start, list):
+        start = [start]
+    i = 0 # tiebreaker
+    q = []
+    for s in start:
+        i += 1
+        heappush(q, (heuristic(s), i, s, None))
+    done = {}
+    if worst:
+        best_worst = min(worst(s) for s in start)
+    while q:
+        z, _, this, prev = heappop(q)
+        if this in done:
+            continue
+        cost_so_far = z - heuristic(this)
+        #print(this,z, cost_so_far)
+
+        done[this] = (cost_so_far, prev)
+
+        if is_goal(this):
+            print("astar: visited", len(done), "nodes")
+            print("astar: pending", len(q), "nodes")
+            # reconsruct the path
+            n = this
+            path = []
+            while n is not None:
+                path.append(n)
+                _, n = done[n]
+            path.reverse()
+            return cost_so_far, this, path
+
+        for c, n in neighbors(this):
+            c = cost_so_far + c
+            if n not in done or c < done[n][0]:
+                h = heuristic(n)
+                if worst:
+                    if c+h > best_worst:
+                        # if the best possible cost for this node
+                        # is worse than the lowest worst-case cost we've seen
+                        # then don't even bother exploring it
+                        continue
+                    w = worst(n)
+                    if c+w < best_worst:
+                        best_worst = c+w
+                i += 1
+                heappush(q, (c+h, i, n, this))
+
+    return float('inf'), None, []
+
+def test():
+    data = [
+        "aabqponm",
+        "abcryxxl",
+        "accszzxk",
+        "acctuvwj",
+        "abdefghi",
+    ]
+    #data = [
+    #    "aabbaaba",
+    #    "abcbaaba",
+    #    "accabcbb",
+    #    "accbbbba",
+    #    "abbbabab",
+    #]
+    start = (0,0)
+    goal = (5,2)
+
+    def get(x,y):
+        return ord(data[y][x])
+
+    def neighbors(src):
+        x,y = src
+        here = get(x,y)
+        n = []
+        def push(x,y):
+            if 0 <= y < len(data):
+                if 0 <= x < len(data[y]):
+                    if get(x,y) <= here+1:
+                        n.append((1, (x,y)))
+        push(x-1, y)
+        push(x+1,y)
+        push(x, y-1)
+        push(x, y+1)
+        return n
+
+    def heuristic(n):
+        return abs(goal[0] - n[0]) + abs(goal[1] - n[1])
+
+    c, _, path = search(start, goal, neighbors, heuristic)
+    print(*data, sep="\n")
+    print(*path, sep="\n")
+    assert c == 31, c
+
+if __name__ == '__main__':
+    test()

day17/input

@@ -0,0 +1,141 @@
+541325532413154112543324624344224545523462433352343463247775356367644347443563677657343744452636444345256642465244525244265544431245144444221
+331414241322155325566365653323365536535625534662334663573476555335357734444677365546643757652664642422365232553435632422462652133234221512215
+112554244333323423245642454645423242233656354535646574735346555646776767536545363675664344376545324446564445556565525526354621135153235435355
+542134154422331162256546652564242435642355365533543774673455365777557755536655735643465454333776472665655234623522255366242524413124115451453
+255251321341123224246525356462642224343225675747637556637674454344574363677445356433635464757763654622224224643632533433452565562111314332341
+323422512325314235242263655345522326644277643546736735556435374637543437655333567455673737467355557747324624235426622233354356446132133215154
+541534415132425356522655256624346542356354755664376557445744476334744743465536755754336555544636554557737333443224244456235552453253423313145
+515553552411263445633352624623665533344334337643473435634547645473665346447473446437553466757655455367463472264425533355633442452453213442124
+544214521555643342656232623463353546563633737773445375774334667743475334577363467775435576654463636667756476334523533536656245462245431325552
+444435344234254322226534364244465643743553676564764753654766336746655546757334456666436757744355475475363753752322654652363332225633645155434
+212254314564226533533534434524557665735744735556577636573746346635443434567673456637573775336347467573445644354564643546236422336444661324543
+512413516564664633325622256245356466543343753467665546337365773767564653766657446456356444363564457774376334434756443242324324342343553415431
+123353233256452626246523564247633554636547453756573433544433634767734774746646334333454765563334375553346673354374344625323266355556655225412
+332255633633636625263535525635667435356665377666744736446636545565764577677455774735573646553437444674337675673667576235366552324234533442215
+223515534224455443532343536547353366775533575556634444434737867846787446578668886484577736735663344633473457576544343265455656255646522424132
+533546255455524366545365473433534564467543663577337455744574545784658875787488555445786876364433545775774533573546546554333656323556264426531
+243536232465655424464653354376357773364566775433766547487787585756455675665875447777646868877576375577734473634775475667446545452346443626462
+155623534353226323442675576457657475474357735635687785478546678646666448578775875848665587878733645577643473333467575545543563266636464343253
+236332623432542664353547736564656564374753553734575675767575568884556566475855685848485654864454663766645444574436774645376563234666326346443
+566563433543322264347477663533766545634464344785486845767768576676857744758456485667758884677656583377736363347534654543534433463566366343363
+555223422622655243643633767456657367445676386557685464545777868586864564484676684746564574488864774674453365474765677673543553322344356325443
+666422265262655424456466666473554475447334585887857754757574584787868465865767476668456448656887677466674546647447665677555645354264545422234
+563226422242366336753544557367656637776577466488685868775888744478784748675465587674767868556748745676644554534653566733733676636233445642336
+322564234435646554674567377365335337747666488746766875574658858558556446886654658478845478476676888748467647663554665733455654534522662534653
+465555623454633354445333653446736364557747546574447766445585886557886678584858674585768866455555764486685853766473443674575654742435232244663
+343444333345462646773576447564676464457648776785566755578878787756544684757567485468846844788457564488844564553777663743437536532223424654325
+262222464566654745464554475655357345664565788445885684557675677648778887486484857545754565467575775554645447446774764536337467454235536662643
+232225445454333756373336674365336584675774545658567674845565785577665785455566754657474747566857554848688844854757347546577375635422363642242
+426462234255434436537565533757765666754465558644788765657454865698895866888566895465865676454875485688586685856333457773757443455725433334645
+536564545645475634464356564636775454456854468854545858586758597989866959966875956568758855675865676846478855774847754365447455345662662465465
+635444422225455545664356536555546766677768765875554767747678579757857766887776587776987878886577846778548485657454356354566563375337625625244
+354436534567735673766437774474565877786884768576467564566555758968956556889765687986686987865756455865744655777886676375663557576465533642236
+564665643553557647746457345554877775644875885554756589687679868898765598699897996787695896548855684484468767648688664437743364557436753642366
+562364622554366374664353374858757467476747844755657868879675897768996785676596568757988866759674578456455784488886586755755657734764436232336
+264332654365666637556563657457756576848475886566996976778877778996886659858776775557879588599578845855748764454447585737365747667555744553465
+534465655563445763544656646887457545478877655476988979997999978759788955558656988897978799577686578664666466758445658354357534445674363656354
+442265464553344643664345447584444584678685788996976769556796788869989699698556766786866576777679974865845548854444757574344344733474574455225
+462336673754537443367573555745685445476767587556988776979999959589587557999558699996957667867556799866545875787566646454665453464764454365364
+335263637555343654737775467464784587484485585556778965787785896585755977777555689698897596999577598986548777644884557658454754554636344736262
+234335766637333434546376845678668845775685795875967556579589797876866976986978887685859995675859857698744557566865874477576747466454533772422
+255434574536534753637354455547567667765778595757897575599857866877978577859965585577889676598875996979764867485875854674453536534453655577222
+434566643345555356464888454746767544687558976776679678677955866868767765969586776966879879685887568689876475854674445665645643347335464344356
+534646336647757665335846455746868845678596765786877885787975659799696678879959758659766657987769879796977674644648784577553743545437435374322
+526365737663437746666855544855754657787857895779598797959957869888886986987996999677785876678665985658777858668448457478565773676435774655566
+265667734463653544354785567557576787658656969686597997765589868689979668997699966665695687879755957566977888847446885775864346753353374573535
+253564543633573537474886666446566577786885575979777887666977886787987986687989689796888859686995995669797794674644768846685566646675743776375
+244376776367766633545876687588456786855996899689566986669889969797878866997896967996696665557978668567558875577556657546684545554563757455776
+554776573466745737474576745674745567776787959776557766566997867897986769676979869889676685769677969699775595547566885876544885443756344465663
+257575556467766775454686547865888598556866678569556978678967688996698696799696998798787699896756867598989557556768454745464684763565434737545
+537357564546643674458785745866754665595996755986665596688768796966787669977896868776776989968579766986785786678484777564864885657473634734343
+565344664665535477747684777576746876656866585967989886866888699998877899979767668998996876979767669859877688885887574546786886546537664664373
+565357774673666664574665685457558689795889587659668986987787779769889769786786677968688676789758795789778799968874554888745884477656333347734
+245364636567733378747546688568467886556575995665667899776697867979969896777697699969798666967778955575878757679874445586566685865366343455347
+575656654334567568656486467468659899798876975555697877868779698669996796698678666769878689967795566959879555869767654758764456537655557557767
+346477444766645357588884555657666698776697587778967998678897767668969786678666998777877898879695965756995895677554566657887857845647373757563
+664555364364376544585645485887676665565675956998886886898667766666879668669876889667796967869898765797966889859764865547768786566655366764647
+663335573643645757784454578444798978988975897996797676789878998969987867899789769986867999888866857798899566596886878577678866667673443666553
+665646477444467687777558754567958897787855565999966889777667798796689979878967869689697689969797678558687999957567777748468868754345565456346
+566543636475566856656775656587756577667967799766766879688687889678977997888877768899789968667679679859556898886955486585464677474675534575777
+645577757777546568576475846559988557689978768999899878797888686788977998778999799668866998997767768595597566666695577675758886657334537474335
+647443537674646775664646644485579879756769756799977696899668878789789997979999998676969697669787776678878565876897478757886554676544436373733
+735435337737668684557486784787968675575789587868997868887678797878989898977877897779967796786998868557559568875998648447577777687677333766745
+543365534657558846755485555769955989956966898986769788686688788779788887978788778979999776788888866879568986868576657577867745756657476634333
+575536335353367864458746645779595756876859568788987698977689998898877788798987888787886766966799886679868555575596986655665884467344356374634
+473743773666654456848656885888576959596756697769979988779698977879787779899997998997896679869976797956886957756797747546478877674557735477576
+645375747353758646565577776469995878658675789787886696789697888878888779887978789889769798679989886788698567658597588456647457688467657554557
+345467535536675474848846568657895799758598699988889898977697988798798789797789877898877968986996666757886787886969865748864884847444663434663
+656456334766548844754566855865579785857878878769986899686879798779888888997888797898876768676868879999656856958868968558486468757444637647463
+464343374457778676587755854879865876988558978768686966967978999997987897797897799788898988987869996895799965888998677645657757588743463334546
+347476457754374656475466676555567667657587886679767677688789978887987778988799997779879669999977768767865586875857875547687554665463564573454
+453766454355684874884885874686876989695967697979669789668987897877898978887777778979796779986768878976856765759976868744885877575746735565357
+736636647435668778784654646498787999669755687687679986876999779979898978899977999898789768998797877699878975986897764765448684554875364476735
+664436647443567755668485684766596755555769978796767678869978778798988789998777777787997666686699786798677785879868965874785748455564663637575
+433553345755375866788885748475965768586787978989798989696789999888799989878797978798798968789786967779856877995576767745856455666836337554377
+344765347543374446674554858577596665697799687787679778869887879977999987999887979878796886977978987769865666868568956585866764566535774537475
+546576646364778657445448847759585995895697878889778996669898789879989987799998979898797868677699766858578859977755677446755745878665767345653
+533546474574554677555477646465886687856557896666778797998699899977899878877779778779867866897987667857887599895995588754757778486775455634365
+364776533553388544476648645496597697688656897776688987669777788789989798888777777798988866877996776958558977857698778487778775567443437764654
+767575643445774646865587678859868695688785899996688688877668879779779878879789879897668976796797988686797559759759646754568666464766356634777
+663564774636658444884475474759797966777788768776999686687989799887988789889998777878788897969767689958777798865786678765766587576735546445447
+765545346777444457678486575588879885678987596687678987878666897997977897979987978988767978769779979865598988867978478475444464744564356435353
+366746667447578457875445867488677995859667698887779896696998788978887788989797887876769887968897796987657789656777858786758778587445346364346
+734757577657577844678557878567659668878757786878767979787666669879779997977898778999886989869968789676788565887969468567446757675547746434465
+467334563557566456758584574878865795766868658996987868887697669888997898899788998889989668879666986669776669687989566454585878855556675374743
+645575457653677884784585588855655968856577875887769876987669779888778799897898777698889967796888787578585855865768848684455877667364474457775
+767557477466374868477886488774898767588796985976996797898896789868898899977876979969877766999996989576756859996757575867468868557576573774434
+365367356554353486664888444486576978789969667598668798779679967888788999796986688989987679989688677766569979655954458755548884553647454343365
+376336736735457876455766664576868899958567899859897696666876898967968667669679868898898888678777677965768667886556674567766854753575675577476
+655377576336353777555487674664459995998755596688788868897878878786779897766898666796798699986769596797666758675744757446578744875557637667343
+346446435544365588467868786546676559596578965787878788879688679686797787696768968886787989768785998568785579588888765488767877567565467755576
+354754743465663748676677684474685586888867755998899878766698966778769666867868877666879989869975667775996755968584457547448868665676574646644
+375476567446766648657685558868684876569896777896588689767787787777779797666866966896997697886777658855887666756556865488575868637476356664546
+445443376634364754578848556856444996665555889556557986866898987898998767968769789997987768997597899598677768594578757748767547673755546757466
+665475644545575348685574648545765657959867595696876978696669997879986966979668799877788966668889557878656677776786455675445767467546444535474
+536536364733663343757854644865786455596579777665886777767776796878787689898768677668688795585588869858867558758654576484588863577736365556466
+545677765673477647458587675877857747699998588967985579966698996796669968868886777876896876566779758656867579564454754855855877653657775474465
+526634533344674357784857678766684666995887697768597868868688669877886679676996996687797775768787576675996687784778756557686657567343466435376
+555367765673445476467455475554586466585989789887787769676867966767676796687889896986877586778976986876898895566876577547785676354466367535475
+566676343464545754336756566678644764665555658957956858897698698967768969799997687799675666769956667655559976654874668868864344365374536646744
+642375475753557633436874464648878466547566967655869789865959766986877898877867796965686987965766886865865675747548555887646747776445567347646
+656326775677733743553476855846684858777567795889655999758996557887666699887977775858955696758766587856867888675575847875564373546374366533364
+356523464767556365754554458685647747687488976778688855697796599685596579789897557585667697756877865679768565567847486476653374377755437773466
+543364557655446374776484667446844587777758978869866755687997557969558968989668567678676658666557568857756646588867747655633365453645345346563
+236562656367457666473574756767787484864787678758988675958855996858589889785858687986999988859888998997488777755668648846466573577345536445525
+435425454743354543343534475784886747857444675996758965666578678996855698756785979795587987959775967867864775685466445885737645534737473665362
+656365663566663445676743748865778774886455467789977566999699575957678987978899885895585598567758556556745674586867685465366373764453753632226
+554364645654577774575347585474486665445484788787958557777995767555969688786699755588897576959596976845578586878455558754775543573553676466226
+566365526354555656773564765857585674685586764446576595576665977697577778959689988577969857867777544786674878657577858777336353333336763243544
+225645463477536636737547746855854577665455586777855595668788968669666569588785557968878679887868765668784484567586587654537447376767635645624
+326343324673543334344333453846654874888454446758467889966666595596659955785659698886979596579556454785744657484756587664635335666734746333362
+532662633633547457656367653444766558747845867568768765978675586896665555887897795567797775544675848675458648484474634746576546564554542654353
+355663354646374655677455353646574544555776878645666884896787558788857879795677987878598875568878457856645555765646757774773335733663526234223
+353422562247634437354366757547848484486845777858677644677756767685987785766977575755599856685775568757588558668567757543337544345564535234266
+625553254333746337646674767563388458857564687664846777856669755559789686587569959585684667544648664446655584855854567365533744356742364465444
+256524322524363455666676457677636858464476685768488658555885585999765659786578666675884764747557458554788587577463354747475464475463564363232
+344555253446556735453434354367637586487674456777465645884457744886765674748655466874854454886874745687874444873743475747434554764722562353544
+436336633646457553373364653633435565546884675847685764745576446566586448474646588477788544776856755887755886845455353675573457356665354253225
+353553554345353455733547735663756547845588454848874587864766784664557578755786484855447586484587747576577446434565477633534576642362554466224
+432525456624322674674455473444735465585878467468565785857768745885767848488547466474785564885557558774875737646763773465674537654223644552654
+525335354433465274655555537746447754355487677744768868776677444557756865777755446886557446678584767648468645477543475677473764432555343662443
+536234524632522465555647777335435334437785748846755474465446555657555454476786768444746644457754678455575643465746465774734443646245244645242
+543442426643426534477636563376653536756355858585748486864457846887545584748846488758855855645876464887457465374457355477665635626433523524452
+223322223546552553575645377567767764633457777574576586688766475584868555884876446564687754664544674746647356433575667577763565225654256544622
+125432653256262246645676433475543465775457456787477566566648476474677745847886586875848787748885486346376633653653363657636242326523563666434
+154555463335463666563736355477665434373773447735676677764476767476757886646475666486685555778786577733434376677574533676446342454463463634432
+141552262546462536364674453376344346655766473635575565475486864645767844585465884467586667565773377733634663557773445746422526356353452324542
+312423654364225622226637376343347535467656737454465678885885746745857776864757467665478865575446666474647437757675757473645242266443443325554
+333352266326435462625366644556555677753333564555775666584874665848744466545484587654854464745467733645434647766364433755226524223666554626514
+435132545534652226544462427766367457556547533646553733455368768864748656858757658764667566573447736343373755745347737256566566356246552554344
+251251242554456622643453332655755467667663776573476475365556464548586464744786567334573766376654353443634574576453465245553624544653336543111
+212442344546526645642263244654774376666364765664337477476566755735634734545547537453654634467644564673444576465347666352444336246443534225131
+112553156446434342262332632662776733455533536546447357345533465336456677743746473675453643333635675467467446533763524242322555366262662253531
+211154334644242354452264624333677734447567476373463347474347444453644577737565635654763766744377535655635747756455462332222243526345542432111
+452531315135444354222633542256663373756457456776563646334754543544575364335767774564575736767736775545744343363363624653264666332236225512123
+554211544546426525425562633634342635553437466647567764434744664546375647473767773675456535466347457734537637645252643554463622523335443333224
+334533452455542655362226365236322545337345466373753756677574663767643737437554464546345477754476745457537446262223644636236336656625515513231
+452555231215326224366256565643522266556465346746347363633645746436473667767556763464673664753345776465737364364536446645553352366221223144345
+514335112142516342564642545262662364262353574573475464554553456636477477446756645474374646364357353537766264336222226532264353454144251545325
+152522253244513266563264433552644226246665674554677745755364563737344773646764375565657457543353753423534335226422232656264234264153315553535
+324514541241141455232546254624542356544332452737546335476443477344545643434547765335573737455674436465526432346264244552654644651313224214233
+213141543243524321434324633656252333252323653563475667336736466576733354556565346365373663374333256546644423653255346636444624514124522425212

day17/sample1.in

@@ -0,0 +1,13 @@
+2413432311323
+3215453535623
+3255245654254
+3446585845452
+4546657867536
+1438598798454
+4457876987766
+3637877979653
+4654967986887
+4564679986453
+1224686865563
+2546548887735
+4322674655533

day17/sol.py

@@ -0,0 +1,58 @@
+import astar
+
+def read_input(f):
+    map = []
+    for line in f:
+        map.append([int(x) for x in line.strip()])
+    return map
+
+def solve(map,part=1):
+    start = (0,0,(0,0),0)
+    goal = (len(map)-1,len(map[-1])-1)
+    def is_goal(x):
+        return x[:2] == goal
+    def neighbors(state):
+        y,x,dir,count = state
+        next = []
+        def go(dx,dy):
+            if (-dx,-dy) == dir:
+                # can't turn around
+                return
+            newx = x + dx
+            newy = y + dy
+            if (dx,dy) == dir:
+                newcount = count+1
+            else:
+                newcount = 1
+            if part == 1:
+                if newcount > 3:
+                    return
+            if part == 2:
+                if newcount > 10:
+                    return
+                if newcount <= count < 4:
+                    return
+            if (0 <= newy < len(map)) and (0 <= newx < len(map[newy])):
+                next.append((map[newy][newx], (newy, newx, (dx,dy), newcount)))
+        go(-1,0)
+        go(+1,0)
+        go(0,-1)
+        go(0,+1)
+        return next
+    
+    cost, node, path = astar.search(start, is_goal, neighbors)
+    print(cost)
+    print([(x,y) for (y,x,_,_) in path])
+    print(''.join(dir2s(d) for (_,_,d,_) in path))
+
+def dir2s(d):
+    if d == (0,1): return "v"
+    if d == (0,-1): return "^"
+    if d == (1,0): return ">"
+    if d == (-1,0): return "<"
+    return "?"
+
+import sys
+map = read_input(sys.stdin)
+solve(map, part=1)
+solve(map, part=2)

day18/input

@@ -0,0 +1,618 @@
+L 7 (#357ea0)
+U 2 (#931393)
+L 4 (#3a7350)
+U 10 (#02c563)
+L 4 (#81f110)
+U 5 (#74d543)
+L 9 (#256002)
+U 6 (#708cf3)
+L 4 (#4c90e0)
+U 8 (#0ac253)
+L 8 (#4c90e2)
+U 5 (#693483)
+R 6 (#256000)
+U 6 (#27cc33)
+R 8 (#3bf170)
+U 3 (#376663)
+L 4 (#035da0)
+U 6 (#045753)
+L 10 (#826390)
+U 4 (#7a8c43)
+L 10 (#271b30)
+U 10 (#6d21f3)
+L 10 (#4a97c2)
+D 2 (#641f33)
+L 6 (#8468c2)
+D 3 (#4316e3)
+R 10 (#2dc6b2)
+D 11 (#5776a3)
+R 7 (#6319d0)
+D 7 (#6573e3)
+L 11 (#7e0de0)
+D 6 (#7d1f63)
+L 6 (#4c51e2)
+D 5 (#5de033)
+L 5 (#59cb30)
+U 11 (#2bb943)
+L 3 (#434650)
+U 6 (#2bb941)
+L 8 (#20d440)
+U 6 (#3c4843)
+L 4 (#535852)
+D 6 (#818223)
+L 7 (#144d02)
+U 4 (#818221)
+L 3 (#564072)
+U 5 (#2e7f43)
+L 12 (#4c51e0)
+U 9 (#3b61d3)
+L 9 (#14d352)
+U 4 (#727ac3)
+L 7 (#2f39c2)
+U 7 (#0516d3)
+L 4 (#9d1aa2)
+U 4 (#22d7e3)
+R 9 (#321810)
+U 6 (#740ad3)
+R 5 (#4cfb40)
+D 7 (#740ad1)
+R 8 (#38f8c0)
+D 11 (#1f22e3)
+R 4 (#0e5630)
+U 18 (#62bcb3)
+R 3 (#292920)
+D 6 (#7b6263)
+R 3 (#0d3bd0)
+U 9 (#82c2b3)
+L 8 (#426530)
+U 13 (#40cf11)
+L 2 (#2ca6c0)
+U 4 (#577ac1)
+L 6 (#1ec6a0)
+U 3 (#247691)
+L 10 (#69b330)
+U 6 (#3db1f3)
+L 5 (#357750)
+U 5 (#adbae3)
+L 13 (#508510)
+D 8 (#0ebc21)
+L 2 (#11ae92)
+D 4 (#8e3c61)
+L 12 (#11ae90)
+U 9 (#4e7451)
+L 8 (#11ac20)
+U 3 (#633511)
+L 3 (#1091b2)
+U 16 (#5b17e1)
+R 3 (#25f402)
+U 4 (#50ed21)
+R 13 (#50e172)
+U 8 (#ac0503)
+R 13 (#5ca752)
+U 9 (#119051)
+R 4 (#012c32)
+U 8 (#aabdf1)
+R 2 (#678e72)
+U 5 (#46f071)
+R 7 (#890120)
+U 4 (#0c2131)
+R 7 (#62ffe0)
+U 10 (#08cdc1)
+R 7 (#0927c0)
+D 14 (#880c31)
+R 7 (#0927c2)
+U 9 (#4043a1)
+R 3 (#533660)
+U 4 (#314eb3)
+R 5 (#89f670)
+D 7 (#59a0f3)
+R 11 (#41cfc0)
+D 9 (#20c953)
+L 8 (#83fc42)
+D 9 (#798a53)
+L 2 (#83fc40)
+D 11 (#2f7c73)
+R 10 (#5a7880)
+D 4 (#72f003)
+R 4 (#184650)
+D 3 (#1146d1)
+R 5 (#5ce420)
+D 4 (#915c31)
+R 11 (#257a20)
+U 5 (#78faf3)
+R 13 (#595510)
+U 8 (#29a813)
+R 3 (#050b70)
+U 8 (#1bcac3)
+R 12 (#53c3a0)
+U 3 (#752613)
+R 10 (#282602)
+U 7 (#6d5a53)
+R 6 (#6266d2)
+U 6 (#0ff141)
+R 8 (#40f1e2)
+U 6 (#0ff143)
+R 7 (#814a02)
+U 9 (#22a8b3)
+R 2 (#469720)
+U 2 (#358e33)
+R 12 (#9d5ce0)
+D 5 (#358e31)
+R 3 (#2cb960)
+D 3 (#17cf63)
+L 4 (#a20580)
+D 4 (#29c2e1)
+L 10 (#9c87f0)
+D 5 (#3cd241)
+R 8 (#4c6400)
+D 2 (#3aca21)
+R 2 (#661a10)
+D 9 (#997e51)
+L 3 (#661a12)
+D 4 (#4c65e1)
+L 7 (#82e570)
+D 4 (#72e2a1)
+R 14 (#369ab0)
+D 3 (#256c01)
+R 7 (#7449e0)
+D 7 (#582f41)
+R 8 (#820b42)
+U 5 (#212c51)
+R 7 (#30a812)
+U 6 (#8bd2f1)
+L 5 (#8e4622)
+U 12 (#016441)
+L 6 (#030532)
+U 9 (#7ffcc1)
+L 4 (#362f62)
+U 4 (#41b291)
+R 10 (#77ccb0)
+U 7 (#3e3811)
+R 5 (#52abd0)
+U 3 (#3e3813)
+R 13 (#30e6c0)
+U 6 (#2763d3)
+R 13 (#468d70)
+U 9 (#2763d1)
+R 4 (#490740)
+U 5 (#16c7b1)
+R 13 (#0698c0)
+U 3 (#3092a1)
+L 6 (#34dc72)
+U 5 (#6eedb1)
+L 4 (#34dc70)
+U 5 (#3bb131)
+L 13 (#60d040)
+U 3 (#502ad1)
+L 3 (#42e190)
+U 16 (#1550a1)
+L 5 (#9144e0)
+U 5 (#0e1761)
+L 9 (#0740d2)
+U 7 (#4ff951)
+L 8 (#2ca952)
+U 7 (#309051)
+L 3 (#a03c52)
+U 12 (#05c2d1)
+L 3 (#800432)
+U 8 (#3dce71)
+L 5 (#192882)
+U 9 (#4c2d43)
+L 5 (#419c22)
+U 5 (#b1aaf3)
+L 4 (#66bec2)
+U 6 (#24f6a3)
+L 7 (#2d7eb2)
+U 10 (#5c6651)
+L 6 (#167982)
+D 6 (#5e9201)
+L 7 (#167980)
+D 10 (#1ba941)
+L 6 (#1887a2)
+D 4 (#4c2d41)
+R 12 (#465332)
+D 7 (#4f4261)
+L 5 (#18f3b2)
+D 14 (#3a0001)
+R 5 (#18f3b0)
+D 5 (#477441)
+R 11 (#03f1b0)
+D 7 (#66e581)
+L 6 (#09d890)
+D 8 (#3db093)
+L 9 (#252580)
+D 7 (#1b8853)
+L 3 (#963ea0)
+D 6 (#1b8851)
+L 5 (#132d70)
+D 8 (#3db091)
+L 8 (#0f6be0)
+D 6 (#66e583)
+L 5 (#5e59d0)
+U 10 (#1fe881)
+R 4 (#4b8912)
+U 10 (#7eb3c1)
+L 7 (#4b8910)
+D 8 (#166151)
+L 7 (#49db70)
+U 8 (#02a761)
+L 10 (#39e420)
+U 3 (#492dd1)
+R 6 (#48e850)
+U 5 (#703fb1)
+R 16 (#4c6b72)
+U 3 (#10fdd1)
+R 2 (#359bc2)
+U 3 (#a2b691)
+L 4 (#3c6712)
+U 3 (#0613b1)
+L 11 (#53dde2)
+U 11 (#662e11)
+L 10 (#7164d2)
+U 3 (#3af111)
+R 4 (#3a2be2)
+U 3 (#040021)
+R 3 (#0da842)
+U 3 (#b32a51)
+R 4 (#52af42)
+U 15 (#035bb1)
+R 6 (#0d6b52)
+U 5 (#261bd1)
+R 5 (#7314e0)
+U 3 (#50ed33)
+L 13 (#526630)
+U 5 (#704043)
+L 9 (#3f1350)
+U 7 (#0c7273)
+R 5 (#493b90)
+U 2 (#30c193)
+R 9 (#33ba10)
+U 7 (#75b9c3)
+R 7 (#0c2ef0)
+U 8 (#042593)
+R 10 (#5b3cc0)
+U 10 (#66b891)
+R 8 (#33d8e0)
+U 5 (#43e851)
+R 3 (#30ca70)
+U 2 (#099de1)
+R 8 (#54ef50)
+U 5 (#4adfa1)
+R 5 (#87a9b2)
+U 6 (#765601)
+R 2 (#87a9b0)
+U 8 (#02cc61)
+L 14 (#063ef0)
+U 6 (#242451)
+R 5 (#04c3f0)
+U 3 (#2622d1)
+R 11 (#770830)
+U 9 (#64cd61)
+R 8 (#3f7700)
+U 3 (#97bc41)
+L 14 (#4328a0)
+U 6 (#1d3ed1)
+L 10 (#3f5090)
+U 10 (#549f11)
+R 5 (#401c12)
+U 9 (#1db141)
+R 9 (#3ab0c2)
+U 6 (#027201)
+R 12 (#8398b2)
+U 2 (#5e76d1)
+R 6 (#6b9210)
+D 6 (#59c901)
+R 9 (#6b9212)
+D 5 (#23be31)
+R 5 (#170352)
+D 6 (#6404d1)
+R 12 (#4577e0)
+D 7 (#a84041)
+L 14 (#4a3910)
+D 4 (#528a21)
+L 4 (#459bd0)
+D 8 (#5c3d21)
+L 4 (#6efdb2)
+D 6 (#20a931)
+R 3 (#15cd22)
+D 7 (#7c9c13)
+R 6 (#7e8af2)
+D 6 (#2b0663)
+R 7 (#304942)
+D 3 (#277bb3)
+R 6 (#2ade02)
+D 3 (#b2ff33)
+R 13 (#2524b2)
+D 3 (#1cc843)
+R 4 (#6c16c2)
+D 3 (#876163)
+R 3 (#436032)
+D 8 (#6520e1)
+R 2 (#1c69d2)
+D 15 (#a123d1)
+R 5 (#1c69d0)
+U 6 (#785fd1)
+R 3 (#18e722)
+D 11 (#a7a271)
+R 7 (#04d402)
+D 3 (#20a933)
+R 10 (#83d842)
+D 5 (#89d7b1)
+R 4 (#672c02)
+D 10 (#394b21)
+R 5 (#520a32)
+D 4 (#5b1951)
+R 8 (#187f52)
+D 3 (#471901)
+R 7 (#2d9542)
+D 13 (#b049b1)
+R 3 (#526ba2)
+D 9 (#6445d3)
+R 5 (#7ddda2)
+D 2 (#6445d1)
+R 17 (#8f7f92)
+D 3 (#824f13)
+R 9 (#29ec62)
+D 4 (#31cb73)
+R 8 (#53c0a2)
+D 7 (#1b1321)
+L 6 (#6183d2)
+D 3 (#4e9f01)
+L 3 (#6582e2)
+D 13 (#484c11)
+R 9 (#0683e0)
+D 8 (#8186e1)
+L 7 (#7c7340)
+D 7 (#8186e3)
+L 4 (#440f90)
+D 8 (#1ba571)
+L 7 (#267130)
+D 6 (#288601)
+L 3 (#6c3dc0)
+U 8 (#36ceb1)
+L 2 (#03b370)
+U 10 (#27e7d1)
+L 3 (#4ecfd0)
+U 6 (#840633)
+L 6 (#281d50)
+U 4 (#840631)
+L 8 (#70c000)
+D 11 (#27e7d3)
+L 8 (#506fe0)
+D 2 (#36ceb3)
+L 7 (#549e70)
+D 4 (#3855e1)
+R 7 (#826de2)
+D 4 (#459461)
+R 8 (#1b5db2)
+D 7 (#588f91)
+L 8 (#97c742)
+D 4 (#72fbf1)
+L 5 (#5ea652)
+D 5 (#565203)
+L 13 (#4d0d22)
+D 9 (#753983)
+R 13 (#40f5b2)
+D 5 (#459463)
+L 4 (#00e1e2)
+D 3 (#054db1)
+R 7 (#1109b2)
+D 3 (#51ac81)
+R 6 (#786302)
+D 5 (#1d4111)
+R 11 (#384d62)
+D 8 (#0abf31)
+R 3 (#9e4960)
+D 2 (#56d781)
+R 5 (#2370b0)
+U 15 (#1e2401)
+R 5 (#015242)
+D 2 (#09c081)
+R 10 (#068442)
+D 6 (#8b50f1)
+R 2 (#74fa32)
+D 6 (#8b50f3)
+R 3 (#5c62e2)
+D 9 (#4e15b1)
+R 9 (#882932)
+D 11 (#5207a3)
+R 6 (#858b62)
+D 6 (#87cd73)
+R 4 (#389ac2)
+D 8 (#0093b3)
+R 6 (#2c2582)
+D 6 (#483b71)
+R 4 (#967a82)
+D 11 (#483b73)
+R 8 (#139412)
+D 3 (#43d833)
+R 3 (#39ce82)
+D 12 (#1b3e53)
+R 10 (#70ef10)
+D 4 (#2cb373)
+L 13 (#32fb60)
+D 3 (#5239e3)
+L 13 (#98c7d2)
+D 4 (#323a43)
+L 13 (#0b22a2)
+D 8 (#2ce053)
+L 7 (#7696e2)
+D 7 (#505353)
+L 8 (#b06560)
+D 11 (#127133)
+R 3 (#0a2af2)
+D 10 (#7fcd23)
+R 11 (#646af2)
+D 9 (#19db13)
+R 3 (#1facf2)
+D 4 (#278353)
+R 9 (#7da710)
+D 12 (#97eec3)
+R 3 (#324190)
+D 7 (#0d7793)
+R 4 (#afe8a2)
+D 10 (#5c84d3)
+R 5 (#812562)
+D 4 (#2d50f3)
+L 16 (#b1f652)
+D 3 (#051f51)
+L 6 (#47f1c2)
+D 10 (#9968d1)
+L 2 (#64a0a2)
+D 6 (#9968d3)
+L 6 (#6b9ae2)
+D 2 (#6972b3)
+L 8 (#319422)
+D 6 (#a5c243)
+L 6 (#1572e0)
+D 5 (#a2e233)
+R 13 (#1572e2)
+D 8 (#0f0be3)
+R 7 (#5c79c2)
+D 3 (#2ebab1)
+R 3 (#431082)
+D 10 (#090461)
+R 5 (#222c82)
+D 9 (#4e4681)
+L 4 (#732d30)
+U 6 (#678f01)
+L 5 (#387c20)
+U 9 (#71c291)
+L 7 (#aba952)
+D 11 (#059f41)
+L 4 (#222c80)
+D 4 (#3944a1)
+L 8 (#431080)
+D 7 (#22e801)
+R 6 (#03da22)
+D 4 (#264601)
+L 6 (#5eb852)
+D 7 (#2cb323)
+L 13 (#1d2bd2)
+D 2 (#2cb321)
+L 3 (#5b2ef2)
+U 9 (#264603)
+L 2 (#16f302)
+U 6 (#051f53)
+L 9 (#4bd602)
+U 4 (#385d93)
+L 9 (#736600)
+U 6 (#38ebf3)
+L 13 (#41dc90)
+U 5 (#073e13)
+L 6 (#031ff0)
+U 11 (#784b53)
+L 5 (#50b290)
+U 7 (#3e8d53)
+L 8 (#50b292)
+U 5 (#752f43)
+L 15 (#4583e0)
+U 5 (#7dbe13)
+L 3 (#8db280)
+U 4 (#4212a1)
+L 12 (#a4bbf0)
+U 3 (#66d011)
+L 5 (#336120)
+U 4 (#479a81)
+R 6 (#78af90)
+U 4 (#352181)
+R 5 (#78af92)
+U 10 (#421b01)
+R 5 (#4143e0)
+U 4 (#494a51)
+R 13 (#57dea0)
+U 9 (#312a23)
+R 12 (#894440)
+U 6 (#312a21)
+R 5 (#4e2a70)
+U 4 (#61a223)
+R 3 (#1c5820)
+U 7 (#146ba1)
+R 5 (#4da250)
+U 3 (#146ba3)
+R 10 (#555d30)
+U 2 (#52e6c3)
+R 3 (#078620)
+U 12 (#23ce33)
+L 10 (#15b752)
+U 5 (#3447b1)
+L 8 (#656962)
+U 9 (#3447b3)
+L 3 (#65f2b2)
+D 14 (#05c0d3)
+L 4 (#20fb32)
+U 10 (#23fea3)
+L 11 (#6fb332)
+U 9 (#3eac33)
+L 7 (#00ea52)
+D 13 (#8b56d3)
+L 2 (#115f30)
+D 7 (#365993)
+L 2 (#167ef0)
+D 3 (#3a33e3)
+L 2 (#1e9ef0)
+D 7 (#755883)
+L 5 (#6e64b0)
+D 5 (#6b9661)
+L 6 (#76e2b0)
+U 3 (#43f601)
+L 5 (#46e7a0)
+U 7 (#5c9be3)
+L 3 (#328892)
+U 9 (#8c7073)
+R 5 (#3c6020)
+U 6 (#943bd3)
+R 3 (#498750)
+U 10 (#0f8f33)
+L 11 (#36f0f0)
+D 7 (#2af483)
+L 12 (#207430)
+D 3 (#0c9011)
+L 10 (#0ed572)
+D 5 (#426c31)
+L 9 (#7ddd92)
+D 5 (#319fb1)
+R 11 (#7ddd90)
+D 4 (#44de41)
+R 6 (#0ed570)
+D 3 (#094551)
+R 10 (#004c10)
+D 4 (#6b53f3)
+L 11 (#506b90)
+D 4 (#2f2a23)
+L 5 (#39b5f0)
+D 3 (#8ca543)
+L 6 (#39b5f2)
+D 10 (#20ea63)
+L 2 (#66cec0)
+D 2 (#22cf93)
+L 11 (#3ee352)
+D 6 (#591893)
+R 12 (#14d660)
+D 4 (#5c95f3)
+R 3 (#14d662)
+D 4 (#37d7b3)
+R 5 (#025f32)
+D 13 (#7ddd23)
+L 8 (#3a93c2)
+D 4 (#063123)
+L 12 (#9a7522)
+D 3 (#61ae11)
+R 9 (#572b22)
+D 4 (#4fcde1)
+R 9 (#130512)
+U 4 (#a1ee51)
+R 6 (#130510)
+D 4 (#1e2a31)
+R 11 (#275c72)
+D 3 (#129543)
+L 10 (#585302)
+D 11 (#608d93)
+L 8 (#62bf02)
+U 11 (#27f113)
+L 9 (#322732)
+D 3 (#4a7f63)
+L 6 (#664bb2)
+U 6 (#5a4ea3)
+L 9 (#507222)
+U 3 (#2e1ca3)

day18/sample1.in

@@ -0,0 +1,14 @@
+R 6 (#70c710)
+D 5 (#0dc571)
+L 2 (#5713f0)
+D 2 (#d2c081)
+R 2 (#59c680)
+D 2 (#411b91)
+L 5 (#8ceee2)
+U 2 (#caa173)
+L 1 (#1b58a2)
+U 2 (#caa171)
+R 2 (#7807d2)
+U 3 (#a77fa3)
+L 2 (#015232)
+U 2 (#7a21e3)

day18/sol.tcl

@@ -0,0 +1,57 @@
+#!/usr/bin/env tclsh
+
+proc solve {input} {
+    set data {}
+    while {[gets $input line] >= 0} {
+        lappend data $line
+    }
+    set data [join $data]
+    set x 0
+    set y 0
+    set y0 0
+    set y1 1
+    set x0 0
+    set x1 1
+    set grid [dict create 0,0 1]
+    foreach {dir steps color} $data {
+        puts "$x $y $dir $steps $color"
+        for {set i 0} {$i < $steps} {incr i} {
+            switch $dir {
+                {U} {incr y -1}
+                {D} {incr y}
+                {L} {incr x -1}
+                {R} {incr x}
+            }
+            dict set grid $x,$y 1
+            set x0 [expr {min($x0,$x)}]
+            set y0 [expr {min($y0,$y)}]
+            set x1 [expr {max($x1,$x+1)}]
+            set y1 [expr {max($y1,$y+1)}]
+        }
+    }
+    set t 0
+    for {set y $y0} {$y < $y1} {incr y} {
+        set row ""
+        set in 0
+        for {set x $x0} {$x < $x1} {incr x} {
+            if {[dict exists $grid $x,$y]} {
+                if {[dict exists $grid $x,[expr {$y+1}]]} {
+                    set in [expr {!$in}]
+                }
+                lappend row "#"
+                incr t
+            } else {
+                if {$in} {
+                    lappend row "o"
+                    incr t
+                } else {
+                    lappend row "."
+                }
+            }
+        }
+        puts [join $row ""]
+    }
+    puts "$t"
+}
+
+solve stdin

day18/sol2.tcl

@@ -0,0 +1,81 @@
+#!/usr/bin/env tclsh
+
+proc solve {input} {
+    set data {}
+    while {[gets $input line] >= 0} {
+        set color [string trim [lindex $line 2] "()"]
+        set dir [lindex {R D L U} [string index $color end]]
+        set steps [scan [string range $color 1 end-1] "%x"]
+        lappend data $dir $steps $color
+    }
+    puts $data
+    set x 0
+    set y 0
+    set extra 0
+    set yrays {}
+    foreach {dir steps color} $data {
+        puts "$x $y $dir $steps $color"
+        set oldy $y
+        set oldx $x
+        switch $dir {
+            {U} {incr y -$steps}
+            {D} {incr y +$steps}
+            {L} {incr x -$steps}
+            {R} {incr x +$steps}
+        }
+        if {$oldy < $y} {
+            lappend yrays [list $oldy $y $x]
+        } elseif {$y < $oldy} {
+            lappend yrays [list $y $oldy $x]
+        }
+        if {$oldy != $y} {
+            incr extra [expr {abs($oldy - $y)}]
+        }
+        if {$oldx != $x} {
+            incr extra [expr {abs($oldx - $x)}]
+        }
+    }
+    #puts $yrays
+    set ypos {}
+    foreach {a b x} [join $yrays] {
+        lappend ypos $a $b
+    }
+    set ypos [lsort -integer -unique $ypos]
+    set t 0
+    puts $ypos
+    foreach y $ypos yn [difference $ypos] {
+        if {$yn eq ""} {
+            continue
+        }
+        set xpos {}
+        foreach {a b x} [join $yrays] {
+            if {$a <= $y && $y < $b} {
+                lappend xpos $x
+            }
+        }
+        set xpos [lsort -integer $xpos]
+        puts "$y +$yn : $xpos"
+        if {[llength $xpos] == 0} { error "no xpos" }
+        set in 0
+        foreach x $xpos xn [difference $xpos] {
+            set in [expr {!$in}]
+            if {$in && $xn ne {}} {
+                incr t [expr {($xn) * ($yn)}]
+            }
+        }
+    }
+    #puts "$t"
+    # +1 for the outer perimeter, -1 for the inner perimeter
+    incr t [expr {$extra/2 + 1}]
+    puts "$t"
+}
+
+proc difference lst {
+    set out {}
+    for {set i 1} {$i < [llength $lst]} {incr i} {
+        lappend out [expr {[lindex $lst $i] - [lindex $lst $i-1]}]
+    }
+    return $out
+}
+
+solve stdin

prelude.tcl

@@ -4,11 +4,15 @@ namespace import tcl::mathfunc::max
 
 proc {#} args {}
 
+# short aliases for common things
+
 proc llen {lst} { return [llength $lst] }
 proc slen {str} { return [string length $str] }
 proc trim args { return [uplevel [concat string trim $args]] }
 proc replace {str a b} { return [string map [list $a $b] $str] }
 
+# sum and product
+
 proc ladd {list} {
     set t 0
     foreach x $list { incr t $x }
@@ -33,7 +37,13 @@ proc must_regexp args {
     }
 }
 
-# transpose an array of strings
+# transpose a list of strings
+#
+#     % transpose {abc 123}
+#     a1 b2 c3
+#     % transpose a1 b2 c3
+#     abc 123
+#
 proc transpose strings {
     set out {}
     set C [slen [lindex $strings 0]]
@@ -46,3 +56,45 @@ proc transpose strings {
     }
     return $out
 }
+
+# transpose a list of lists
+#
+#     % ltranspose {{a b c} {1 2 3}}
+#     {a 1} {b 2} {c 3}
+#     % ltranspose {{a 1} {b 2} {c 3}}
+#     {a b c} {1 2 3}
+#
+proc ltranspose lists {
+    set out {}
+    set C [llen [lindex $lists 0]]
+    for {set i 0} {$i < $C} {incr i} {
+        set column {}
+        foreach row $lists {
+            lappend column [lindex $row $i]
+        }
+        lappend out $column
+    }
+    return $out
+}
+
+# extracts one or more indexes from a strided list
+# e.g.
+#       % set l {0 1 2   a b c   x y z}
+#       % lextract $l 3 0
+#       0 a x
+#       % lextract $l 3 {1 2}
+#       1 2 b c y z
+#
+# equivalent to [lmap {a b c} $lst {list $b $c}] except that
+# you don't have to name the list elements
+proc lextract {lst stride index} {
+    set i 0
+    set out {}
+    if {$stride <= 0} { error "stride must be positive: $stride" }
+    for {set i 0} {$i < [llength $lst]} {incr i $stride} {
+        foreach j $index {
+            lappend out [lindex $lst $i+$j]
+        }
+    }
+    return $out
+}