# Phone Letters ## Challenge: Phone Letters Previously, people typed words on a phone by pressing combinations of numbers. Each number mapped to a few different possible letters, as shown below. ``` 1 2 3 ABC DEF 4 5 6 GHI JKL MNO 7 8 9 PQRS TUV WXYZ 0 ``` In this task, you will write a generator that accepts a `@ud` number and returns a `(list tape)` containing all the different strings that it could represent. * Note that `1` and `0` do not map to any letters in the phonepad. Let's crash if the input `@ud` contains any `1` or `0`. * Let's return the list of strings sorted in alphabetical order, all in lowercase. Recall that `@ud` numbers need a dot marking every three digit places if the number is higher than `999`, i.e. `234.567.892` is `234,567,892`. Example usage: ``` > +phone-letters 29 <<"aw" "ax" "ay" "az" "bw" "bx" "by" "bz" "cw" "cx" "cy" "cz">> ``` ## Solutions *These solutions were submitted by the Urbit community as part of a competition in \~2024.8. They are made available under the MIT License and CC0. We ask you to acknowledge authorship should you utilize these elsewhere.* ### Solution #1 *Our style winner, a clean and well-commented solution by \~norweg-rivlex.* ```hoon :: phone-letters.hoon :: convert a positive integer into a list of the possible :: phone number mnemonics :: |= n=@ud ^- (list tape) :: :: mapping from phone number digits to the letters that :: may be used for it, which letters are in alphabetic :: order =/ digit-letters %- my :~ ['2' "abc"] ['3' "def"] ['4' "ghi"] ['5' "jkl"] ['6' "mno"] ['7' "pqrs"] ['8' "tuv"] ['9' "wxyz"] == :: :: get the letters for the passed digit, or die if :: none such exist =/ letters-for-digit |= [digit=@t] ^- tape (need (~(get by digit-letters) digit)) :: :: for a list of incomplete mnemonics, being constructed :: from end to start, for a passed digit, result in a :: new list with the new letters prepended in order =/ prepend-all-for-digit |= [digit=@t values=(list tape)] ^- (list tape) ?: =(digit '.') values =/ letters (flop (letters-for-digit digit)) =| result=(list tape) |- ^- (list tape) ?~ letters result %= $ letters t.letters result (weld (turn values |=(item=tape [i.letters item])) result) == :: :: digits of the passed number in reverse order =/ digits=tape (flop "{}") :: :: building the result =| result=(list tape) :: :: main loop - get each digit in reverse order, and :: build the list of results by adding the possible :: letters in turn |- ?~ digits result =/ next ?~ result (turn (letters-for-digit i.digits) |=(d=@t ~[d])) (prepend-all-for-digit i.digits result) %= $ digits t.digits result next == ``` ### Solution #2 *The speed winner by \~diblud-ricbet.* ```hoon :: phone-letters.hoon :: Convert a @ud entry into an old-school T9 keyboard into a list of possible tapes. :: |= n=@ud ^- (list tape) ?< =(n 0) =/ char-map %- malt :~ [2 `(list tape)`~["a" "b" "c"]] [3 `(list tape)`~["d" "e" "f"]] [4 `(list tape)`~["g" "h" "i"]] [5 `(list tape)`~["j" "k" "l"]] [6 `(list tape)`~["m" "n" "o"]] [7 `(list tape)`~["p" "q" "r" "s"]] [8 `(list tape)`~["t" "u" "v"]] [9 `(list tape)`~["w" "x" "y" "z"]] == =/ output `(list tape)`~ |- ?: =(n 0) (sort output aor) %= $ n (div n 10) output ?~ output (~(got by char-map) (mod n 10)) %- zing %+ turn output |= existing=tape %+ turn (~(got by char-map) (mod n 10)) |= letter=tape %+ weld letter existing == ``` ## Unit Tests Following a principle of test-driven development, the unit tests below allow us to check for expected behavior. To run the tests yourself, follow the instructions in the [Unit Tests](/build-on-urbit/userspace/unit-tests.md) section. ```hoon /+ *test /= phone-letters /gen/phone-letters |% :: tests for success :: ++ test-01 %+ expect-eq !> `(list tape)`~["a" "b" "c"] !> %- phone-letters 2 ++ test-02 %+ expect-eq !> `(list tape)`~["d" "e" "f"] !> %- phone-letters 3 ++ test-03 %+ expect-eq !> `(list tape)`~["g" "h" "i"] !> %- phone-letters 4 ++ test-04 %+ expect-eq !> `(list tape)`~["j" "k" "l"] !> %- phone-letters 5 ++ test-05 %+ expect-eq !> `(list tape)`~["m" "n" "o"] !> %- phone-letters 6 ++ test-06 %+ expect-eq !> `(list tape)`~["p" "q" "r" "s"] !> %- phone-letters 7 ++ test-07 %+ expect-eq !> `(list tape)`~["t" "u" "v"] !> %- phone-letters 8 ++ test-08 %+ expect-eq !> `(list tape)`~["w" "x" "y" "z"] !> %- phone-letters 9 ++ test-09 %+ expect-eq !> `(list tape)`~["aw" "ax" "ay" "az" "bw" "bx" "by" "bz" "cw" "cx" "cy" "cz"] !> %- phone-letters 29 ++ test-10 %+ expect-eq !> `(list tape)`~["dj" "dk" "dl" "ej" "ek" "el" "fj" "fk" "fl"] !> %- phone-letters 35 ++ test-11 %+ expect-eq !> `(list tape)`~["pg" "ph" "pi" "qg" "qh" "qi" "rg" "rh" "ri" "sg" "sh" "si"] !> %- phone-letters 74 ++ test-12 %+ expect-eq !> `(list tape)`~["aga" "agb" "agc" "aha" "ahb" "ahc" "aia" "aib" "aic" "bga" "bgb" "bgc" "bha" "bhb" "bhc" "bia" "bib" "bic" "cga" "cgb" "cgc" "cha" "chb" "chc" "cia" "cib" "cic"] !> %- phone-letters 242 ++ test-13 %+ expect-eq !> `(list tape)`~["amw" "amx" "amy" "amz" "anw" "anx" "any" "anz" "aow" "aox" "aoy" "aoz" "bmw" "bmx" "bmy" "bmz" "bnw" "bnx" "bny" "bnz" "bow" "box" "boy" "boz" "cmw" "cmx" "cmy" "cmz" "cnw" "cnx" "cny" "cnz" "cow" "cox" "coy" "coz"] !> %- phone-letters 269 ++ test-14 %+ expect-eq !> `(list tape)`~["tta" "ttb" "ttc" "tua" "tub" "tuc" "tva" "tvb" "tvc" "uta" "utb" "utc" "uua" "uub" "uuc" "uva" "uvb" "uvc" "vta" "vtb" "vtc" "vua" "vub" "vuc" "vva" "vvb" "vvc"] !> %- phone-letters 882 ++ test-15 %+ expect-eq !> `(list tape)`~["pdm" "pdn" "pdo" "pem" "pen" "peo" "pfm" "pfn" "pfo" "qdm" "qdn" "qdo" "qem" "qen" "qeo" "qfm" "qfn" "qfo" "rdm" "rdn" "rdo" "rem" "ren" "reo" "rfm" "rfn" "rfo" "sdm" "sdn" "sdo" "sem" "sen" "seo" "sfm" "sfn" "sfo"] !> %- phone-letters 736 ++ test-16 %+ expect-eq !> `(list tape)`~["jtdg" "jtdh" "jtdi" "jteg" "jteh" "jtei" "jtfg" "jtfh" "jtfi" "judg" "judh" "judi" "jueg" "jueh" "juei" "jufg" "jufh" "jufi" "jvdg" "jvdh" "jvdi" "jveg" "jveh" "jvei" "jvfg" "jvfh" "jvfi" "ktdg" "ktdh" "ktdi" "kteg" "kteh" "ktei" "ktfg" "ktfh" "ktfi" "kudg" "kudh" "kudi" "kueg" "kueh" "kuei" "kufg" "kufh" "kufi" "kvdg" "kvdh" "kvdi" "kveg" "kveh" "kvei" "kvfg" "kvfh" "kvfi" "ltdg" "ltdh" "ltdi" "lteg" "lteh" "ltei" "ltfg" "ltfh" "ltfi" "ludg" "ludh" "ludi" "lueg" "lueh" "luei" "lufg" "lufh" "lufi" "lvdg" "lvdh" "lvdi" "lveg" "lveh" "lvei" "lvfg" "lvfh" "lvfi"] !> %- phone-letters 5.834 ++ test-17 %+ expect-eq !> `(list tape)`~["jjdd" "jjde" "jjdf" "jjed" "jjee" "jjef" "jjfd" "jjfe" "jjff" "jkdd" "jkde" "jkdf" "jked" "jkee" "jkef" "jkfd" "jkfe" "jkff" "jldd" "jlde" "jldf" "jled" "jlee" "jlef" "jlfd" "jlfe" "jlff" "kjdd" "kjde" "kjdf" "kjed" "kjee" "kjef" "kjfd" "kjfe" "kjff" "kkdd" "kkde" "kkdf" "kked" "kkee" "kkef" "kkfd" "kkfe" "kkff" "kldd" "klde" "kldf" "kled" "klee" "klef" "klfd" "klfe" "klff" "ljdd" "ljde" "ljdf" "ljed" "ljee" "ljef" "ljfd" "ljfe" "ljff" "lkdd" "lkde" "lkdf" "lked" "lkee" "lkef" "lkfd" "lkfe" "lkff" "lldd" "llde" "lldf" "lled" "llee" "llef" "llfd" "llfe" "llff"] !> %- phone-letters 5.533 ++ test-18 %+ expect-eq !> `(list tape)`~["jggp" "jggq" "jggr" "jggs" "jghp" "jghq" "jghr" "jghs" "jgip" "jgiq" "jgir" "jgis" "jhgp" "jhgq" "jhgr" "jhgs" "jhhp" "jhhq" "jhhr" "jhhs" "jhip" "jhiq" "jhir" "jhis" "jigp" "jigq" "jigr" "jigs" "jihp" "jihq" "jihr" "jihs" "jiip" "jiiq" "jiir" "jiis" "kggp" "kggq" "kggr" "kggs" "kghp" "kghq" "kghr" "kghs" "kgip" "kgiq" "kgir" "kgis" "khgp" "khgq" "khgr" "khgs" "khhp" "khhq" "khhr" "khhs" "khip" "khiq" "khir" "khis" "kigp" "kigq" "kigr" "kigs" "kihp" "kihq" "kihr" "kihs" "kiip" "kiiq" "kiir" "kiis" "lggp" "lggq" "lggr" "lggs" "lghp" "lghq" "lghr" "lghs" "lgip" "lgiq" "lgir" "lgis" "lhgp" "lhgq" "lhgr" "lhgs" "lhhp" "lhhq" "lhhr" "lhhs" "lhip" "lhiq" "lhir" "lhis" "ligp" "ligq" "ligr" "ligs" "lihp" "lihq" "lihr" "lihs" "liip" "liiq" "liir" "liis"] !> %- phone-letters 5.447 ++ test-19 %+ expect-eq !> `(list tape)`~["gwad" "gwae" "gwaf" "gwbd" "gwbe" "gwbf" "gwcd" "gwce" "gwcf" "gxad" "gxae" "gxaf" "gxbd" "gxbe" "gxbf" "gxcd" "gxce" "gxcf" "gyad" "gyae" "gyaf" "gybd" "gybe" "gybf" "gycd" "gyce" "gycf" "gzad" "gzae" "gzaf" "gzbd" "gzbe" "gzbf" "gzcd" "gzce" "gzcf" "hwad" "hwae" "hwaf" "hwbd" "hwbe" "hwbf" "hwcd" "hwce" "hwcf" "hxad" "hxae" "hxaf" "hxbd" "hxbe" "hxbf" "hxcd" "hxce" "hxcf" "hyad" "hyae" "hyaf" "hybd" "hybe" "hybf" "hycd" "hyce" "hycf" "hzad" "hzae" "hzaf" "hzbd" "hzbe" "hzbf" "hzcd" "hzce" "hzcf" "iwad" "iwae" "iwaf" "iwbd" "iwbe" "iwbf" "iwcd" "iwce" "iwcf" "ixad" "ixae" "ixaf" "ixbd" "ixbe" "ixbf" "ixcd" "ixce" "ixcf" "iyad" "iyae" "iyaf" "iybd" "iybe" "iybf" "iycd" "iyce" "iycf" "izad" "izae" "izaf" "izbd" "izbe" "izbf" "izcd" "izce" "izcf"] !> %- phone-letters 4.923 :: tests for failure :: ++ test-20 %- expect-fail |. (phone-letters 0) ++ test-21 %- expect-fail |. (phone-letters 1) ++ test-22 %- expect-fail |. (phone-letters 12) ++ test-23 %- expect-fail |. (phone-letters 13) ++ test-24 %- expect-fail |. (phone-letters 14) ++ test-25 %- expect-fail |. (phone-letters 15) ++ test-26 %- expect-fail |. (phone-letters 16) ++ test-27 %- expect-fail |. (phone-letters 17) ++ test-28 %- expect-fail |. (phone-letters 18) ++ test-29 %- expect-fail |. (phone-letters 19) ++ test-30 %- expect-fail |. (phone-letters 20) ++ test-31 %- expect-fail |. (phone-letters 21) ++ test-32 %- expect-fail |. (phone-letters 5.814) ++ test-33 %- expect-fail |. (phone-letters 5.804) ++ test-34 %- expect-fail |. (phone-letters 53.204) ++ test-35 %- expect-fail |. (phone-letters 12.345) ++ test-36 %- expect-fail |. (phone-letters 59.491) ++ test-37 %- expect-fail |. (phone-letters 87.650) ++ test-38 %- expect-fail |. (phone-letters 87.651) ++ test-39 %- expect-fail |. (phone-letters 81.123) ++ test-40 %- expect-fail |. (phone-letters 10.000) -- ``` --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.urbit.org/hoon/examples/phone-letters.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.