gistfile1.erl

% Exercise 2.4
% Database Handling Using Lists
%
% skers@kth.se 2013
%
-module(db).

-export([ new/0
        , destroy/1
        , write/3
        , read/2
        , match/2
        , delete/2
        ]).

% New
new() ->
    [].

% Destroy
destroy(_) ->
    [].

% Write
write(Key, City, Db) ->
    write(Key, City, Db, []).

write(Key, City, [], NewDb) ->
    NewDb ++ [{Key, City}];
write(Key, City, [{OldKey, OldCity}|T], NewDb) when Key /= OldKey ->
    write(Key, City, T, [{OldKey, OldCity}|NewDb]);
write(Key, City, [{OldKey, _}|T], NewDb)       when Key == OldKey ->
    write(Key, City, T, NewDb).

% Read
read(_, []) ->
    {error, instance};
read(Key, [{Name, City}|_]) when Key == Name ->
    {ok, City};
read(Key, [_|T]) ->
    read(Key, T).

% Match
match(Key, Db) ->
    match(Key, Db, []).

match(_, [], Results) ->
    Results;
match(Key, [{Name, City}|T], Result) when Key == City ->
    match(Key, T, [Name | Result]);
match(Key, [_|T], Result) ->
    match(Key, T, Result).

% Delete
delete(Key, Db) ->
    delete(Key, Db, []).

delete(_, [], NewDb) ->
    NewDb;
delete(Key, [{Name, _}|T], NewDb) when Key == Name ->
    delete(Key, T, NewDb);
delete(Key, [H|T], NewDb) ->
    delete(Key, T, [H|NewDb]).