Day 7

% The root of the filesystem
root(root).

% A directory can have a sub-directory
sub_directory(root, a).
sub_directory(root, d).
sub_directory(a, e).

% We don't need to know the file name, so we record only the file sizes
file(root, 14848514).
file(root, 8504156).
file(a, 29116).
file(a, 2557).
file(a, 62596).
file(e, 584).
file(d, 4060174).
file(d, 8033020).
file(d, 5626152).
file(d, 7214296).

% The root is a directory. An subdirectory of a directory is a directory itself.
directory(X) :- root(X).
directory(X) :- sub_directory(Y, X), directory(Y).

% The size of a directory is the size of all files in the directory plus the sizes of all subdirectories
directory_size(Dir, Size) :- findall(FileSize, file(Dir, FileSize), SizeList), sum_list(SizeList, DirectFiles), findall(SubDirSize, (sub_directory(Dir, SubDir), directory_size(SubDir, SubDirSize)), SubDirSizeList), sum_list(SubDirSizeList, SubDirs), Size is DirectFiles + SubDirs.

% Find all directories with a size at most 100000 and sum their sizes
solve(SizeSum) :- findall(DirSize, (directory(Dir), directory_size(Dir, DirSize), DirSize =< 100000), DirSizeList), sum_list(DirSizeList, SizeSum).