//////////////////////////////////////////////////////////////////////
// LibFile: arrays.scad
//   List and Array manipulation functions.
//   To use, add the following lines to the beginning of your file:
//   ```
//   use <BOSL2/std.scad>
//   ```
//////////////////////////////////////////////////////////////////////


// Section: Terminology
//   - **List**: An ordered collection of zero or more items.  ie: `["a", "b", "c"]`
//   - **Vector**: A list of numbers. ie: `[4, 5, 6]`
//   - **Array**: A nested list of lists, or list of lists of lists, or deeper.  ie: `[[2,3], [4,5], [6,7]]`
//   - **Dimension**: The depth of nesting of lists in an array.  A List is 1D.  A list of lists is 2D.  etc.
//   - **Set**: A list of unique items.


// Section: List Query Operations


// Function: select()
// Description:
//   Returns a portion of a list, wrapping around past the beginning, if end<start. 
//   The first item is index 0. Negative indexes are counted back from the end.
//   The last item is -1.  If only the `start` index is given, returns just the value
//   at that position.
// Usage:
//   select(list,start)
//   select(list,start,end)
// Arguments:
//   list = The list to get the portion of.
//   start = The index of the first item.
//   end = The index of the last item.
// Example:
//   l = [3,4,5,6,7,8,9];
//   select(l, 5, 6);   // Returns [8,9]
//   select(l, 5, 8);   // Returns [8,9,3,4]
//   select(l, 5, 2);   // Returns [8,9,3,4,5]
//   select(l, -3, -1); // Returns [7,8,9]
//   select(l, 3, 3);   // Returns [6]
//   select(l, 4);      // Returns 7
//   select(l, -2);     // Returns 8
//   select(l, [1:3]);  // Returns [4,5,6]
//   select(l, [1,3]);  // Returns [4,6]
function select(list, start, end=undef) =
    assert( is_list(list) || is_string(list), "Invalid list.")
    let(l=len(list))
    l==0 ? []
    :   end==undef? 
            is_num(start)?
                list[ (start%l+l)%l ]
            :   assert( is_list(start) || is_range(start), "Invalid start parameter")
                [for (i=start) list[ (i%l+l)%l ] ]
        :   assert(is_finite(start), "Invalid start parameter.")
            assert(is_finite(end), "Invalid end parameter.")
            let( s = (start%l+l)%l, e = (end%l+l)%l )
            (s <= e)? [for (i = [s:1:e]) list[i]]
            :   concat([for (i = [s:1:l-1]) list[i]], [for (i = [0:1:e]) list[i]]) ;


// Function: slice()
// Description:
//   Returns a slice of a list.  The first item is index 0.
//   Negative indexes are counted back from the end.  The last item is -1.
// Arguments:
//   list = The array/list to get the slice of.
//   start = The index of the first item to return.
//   end = The index after the last item to return, unless negative, in which case the last item to return.
// Example:
//   slice([3,4,5,6,7,8,9], 3, 5);   // Returns [6,7]
//   slice([3,4,5,6,7,8,9], 2, -1);  // Returns [5,6,7,8,9]
//   slice([3,4,5,6,7,8,9], 1, 1);   // Returns []
//   slice([3,4,5,6,7,8,9], 6, -1);  // Returns [9]
//   slice([3,4,5,6,7,8,9], 2, -2);  // Returns [5,6,7,8]
function slice(list,start,end) =
    assert( is_list(list), "Invalid list" )
    assert( is_finite(start) && is_finite(end), "Invalid number(s)" )
    let( l = len(list) )
    l==0 ? []
    :   let(
            s = start<0? (l+start): start,
            e = end<0? (l+end+1): end
        ) [for (i=[s:1:e-1]) if (e>s) list[i]];


// Function: in_list()
// Description: Returns true if value `val` is in list `list`. When `val==NAN` the answer will be false for any list.
// Arguments:
//   val = The simple value to search for.
//   list = The list to search.
//   idx = If given, searches the given subindex for matches for `val`.
// Example:
//   in_list("bar", ["foo", "bar", "baz"]);  // Returns true.
//   in_list("bee", ["foo", "bar", "baz"]);  // Returns false.
//   in_list("bar", [[2,"foo"], [4,"bar"], [3,"baz"]], idx=1);  // Returns true.
function in_list(val,list,idx=undef) = 
    assert( is_list(list) && (is_undef(idx) || is_finite(idx)),
            "Invalid input." )
    let( s = search([val], list, num_returns_per_match=1, index_col_num=idx)[0] )
    s==[] || s==[[]] ? false
    : is_undef(idx) ? val==list[s] 
    : val==list[s][idx];


// Function: min_index()
// Usage:
//   min_index(vals,[all]);
// Description:
//   Returns the index of the first occurrence of the minimum value in the given list. 
//   If `all` is true then returns a list of all indices where the minimum value occurs.
// Arguments:
//   vals = vector of values
//   all = set to true to return indices of all occurences of the minimum.  Default: false
// Example:
//   min_index([5,3,9,6,2,7,8,2,1]); // Returns: 8
//   min_index([5,3,9,6,2,7,8,2,7],all=true); // Returns: [4,7]
function min_index(vals, all=false) =
    assert( is_vector(vals) && len(vals)>0 , "Invalid or empty list of numbers.")
    all ? search(min(vals),vals,0) : search(min(vals), vals)[0];


// Function: max_index()
// Usage:
//   max_index(vals,[all]);
// Description:
//   Returns the index of the first occurrence of the maximum value in the given list. 
//   If `all` is true then returns a list of all indices where the maximum value occurs.
// Arguments:
//   vals = vector of values
//   all = set to true to return indices of all occurences of the maximum.  Default: false
// Example:
//   max_index([5,3,9,6,2,7,8,9,1]); // Returns: 2
//   max_index([5,3,9,6,2,7,8,9,1],all=true); // Returns: [2,7]
function max_index(vals, all=false) =
    assert( is_vector(vals) && len(vals)>0 , "Invalid or empty list of numbers.")
    all ? search(max(vals),vals,0) : search(max(vals), vals)[0];


// Function: list_increasing()
// Usage:
//    list_increasing(list)
// Description:
//   Returns true if the list is (non-strictly) increasing
// Example:
//   list_increasing([1,2,3,4]);  // Returns: true
//   list_increasing([1,3,2,4]);  // Returns: false
//   list_increasing([4,3,2,1]);  // Returns: false
function list_increasing(list) =
    assert(is_list(list)||is_string(list))
    len([for (p=pair(list)) if(p.x>p.y) true])==0;


// Function: list_decreasing()
// Usage:
//    list_decreasing(list)
// Description:
//   Returns true if the list is (non-strictly) decreasing
// Example:
//   list_decreasing([1,2,3,4]);  // Returns: false
//   list_decreasing([4,2,3,1]);  // Returns: false
//   list_decreasing([4,3,2,1]);  // Returns: true
function list_decreasing(list) =
    assert(is_list(list)||is_string(list))
    len([for (p=pair(list)) if(p.x<p.y) true])==0;



// Section: Basic List Generation


// Function: repeat()
// Usage:
//   repeat(val, n)
// Description:
//   Generates a list or array of `n` copies of the given `list`.
//   If the count `n` is given as a list of counts, then this creates a
//   multi-dimensional array, filled with `val`.
// Arguments:
//   val = The value to repeat to make the list or array.
//   n = The number of copies to make of `val`.
// Example:
//   repeat(1, 4);        // Returns [1,1,1,1]
//   repeat(8, [2,3]);    // Returns [[8,8,8], [8,8,8]]
//   repeat(0, [2,2,3]);  // Returns [[[0,0,0],[0,0,0]], [[0,0,0],[0,0,0]]]
//   repeat([1,2,3],3);   // Returns [[1,2,3], [1,2,3], [1,2,3]]
function repeat(val, n, i=0) =
    is_num(n)? [for(j=[1:1:n]) val] :
    assert( is_list(n), "Invalid count number.")
    (i>=len(n))? val :
    [for (j=[1:1:n[i]]) repeat(val, n, i+1)];


// Function: list_range()
// Usage:
//   list_range(n, [s], [e])
//   list_range(n, [s], [step])
//   list_range(e, [step])
//   list_range(s, e, [step])
// Description:
//   Returns a list, counting up from starting value `s`, by `step` increments,
//   until either `n` values are in the list, or it reaches the end value `e`.
//   If both `n` and `e` are given, returns `n` values evenly spread from `s`
//   to `e`, and `step` is ignored.
// Arguments:
//   n = Desired number of values in returned list, if given.
//   s = Starting value.  Default: 0
//   e = Ending value to stop at, if given.
//   step = Amount to increment each value.  Default: 1
// Example:
//   list_range(4);                  // Returns [0,1,2,3]
//   list_range(n=4, step=2);        // Returns [0,2,4,6]
//   list_range(n=4, s=3, step=3);   // Returns [3,6,9,12]
//   list_range(n=5, s=0, e=10);     // Returns [0, 2.5, 5, 7.5, 10]
//   list_range(e=3);                // Returns [0,1,2,3]
//   list_range(e=7, step=2);        // Returns [0,2,4,6]
//   list_range(s=3, e=5);           // Returns [3,4,5]
//   list_range(s=3, e=8, step=2);   // Returns [3,5,7]
//   list_range(s=4, e=8.3, step=2); // Returns [4,6,8]
//   list_range(n=4, s=[3,4], step=[2,3]);  // Returns [[3,4], [5,7], [7,10], [9,13]]
function list_range(n=undef, s=0, e=undef, step=undef) =
    assert( is_undef(n) || is_finite(n), "Parameter `n` must be a number.")
    assert( is_undef(n) || is_undef(e) || is_undef(step), "At most 2 of n, e, and step can be given.")
    let( step = (n!=undef && e!=undef)? (e-s)/(n-1) : default(step,1) )
    is_undef(e) ? 
        assert( is_consistent([s, step]), "Incompatible data.")
        [for (i=[0:1:n-1]) s+step*i ]
    :   assert( is_vector([s,step,e]), "Start `s`, step `step` and end `e` must be numbers.")
        [for (v=[s:step:e]) v] ;
    


// Section: List Manipulation

// Function: reverse()
// Description: Reverses a list/array.
// Arguments:
//   list = The list to reverse.
// Example:
//   reverse([3,4,5,6]);  // Returns [6,5,4,3]
function reverse(list) =
    assert(is_list(list)||is_string(list))
    [ for (i = [len(list)-1 : -1 : 0]) list[i] ];


// Function: list_rotate()
// Usage:
//   rlist = list_rotate(list,n);
// Description:
//   Rotates the contents of a list by `n` positions left.
//   If `n` is negative, then the rotation is `abs(n)` positions to the right.
// Arguments:
//   list = The list to rotate.
//   n = The number of positions to rotate by.  If negative, rotated to the right.  Positive rotates to the left.  Default: 1
// Example:
//   l1 = list_rotate([1,2,3,4,5],-2); // Returns: [4,5,1,2,3]
//   l2 = list_rotate([1,2,3,4,5],-1); // Returns: [5,1,2,3,4]
//   l3 = list_rotate([1,2,3,4,5],0);  // Returns: [1,2,3,4,5]
//   l4 = list_rotate([1,2,3,4,5],1);  // Returns: [2,3,4,5,1]
//   l5 = list_rotate([1,2,3,4,5],2);  // Returns: [3,4,5,1,2]
//   l6 = list_rotate([1,2,3,4,5],3);  // Returns: [4,5,1,2,3]
//   l7 = list_rotate([1,2,3,4,5],4);  // Returns: [5,1,2,3,4]
//   l8 = list_rotate([1,2,3,4,5],5);  // Returns: [1,2,3,4,5]
//   l9 = list_rotate([1,2,3,4,5],6);  // Returns: [2,3,4,5,1]
function list_rotate(list,n=1) =
    assert(is_list(list)||is_string(list), "Invalid list or string.")
    assert(is_finite(n), "Invalid number")
    select(list,n,n+len(list)-1);


// Function: deduplicate()
// Usage:
//   deduplicate(list,[close],[eps]);
// Description:
//   Removes consecutive duplicate items in a list.
//   When `eps` is zero, the comparison between consecutive items is exact.
//   Otherwise, when all list items and subitems are numbers, the comparison is within the tolerance `eps`.
//   This is different from `unique()` in that the list is *not* sorted.
// Arguments:
//   list = The list to deduplicate.
//   closed = If true, drops trailing items if they match the first list item.
//   eps = The maximum tolerance between items.
// Examples:
//   deduplicate([8,3,4,4,4,8,2,3,3,8,8]);  // Returns: [8,3,4,8,2,3,8]
//   deduplicate(closed=true, [8,3,4,4,4,8,2,3,3,8,8]);  // Returns: [8,3,4,8,2,3]
//   deduplicate("Hello");  // Returns: ["H","e","l","o"]
//   deduplicate([[3,4],[7,2],[7,1.99],[1,4]],eps=0.1);  // Returns: [[3,4],[7,2],[1,4]]
//   deduplicate([[7,undef],[7,undef],[1,4],[1,4+1e-12]],eps=0);    // Returns: [[7,undef],[1,4],[1,4+1e-12]]
function deduplicate(list, closed=false, eps=EPSILON) =
    assert(is_list(list)||is_string(list))
    let( l = len(list),
         end = l-(closed?0:1) )
    is_string(list) || (eps==0)
    ? [for (i=[0:1:l-1]) if (i==end || list[i] != list[(i+1)%l]) list[i]]
    : [for (i=[0:1:l-1]) if (i==end || !approx(list[i], list[(i+1)%l], eps)) list[i]];


// Function: deduplicate_indexed()
// Usage:
//   new_idxs = deduplicate_indexed(list, indices, [closed], [eps]);
// Description:
//   Given a list, and indices into it, removes consecutive indices that
//   index to the same values in the list.
// Arguments:
//   list = The list that the indices index into.
//   indices = The list of indices to deduplicate.
//   closed = If true, drops trailing indices if what they index matches what the first index indexes.
//   eps = The maximum difference to allow between numbers or vectors.
// Examples:
//   deduplicate_indexed([8,6,4,6,3], [1,4,3,1,2,2,0,1]);  // Returns: [1,4,3,2,0,1]
//   deduplicate_indexed([8,6,4,6,3], [1,4,3,1,2,2,0,1], closed=true);  // Returns: [1,4,3,2,0]
//   deduplicate_indexed([[7,undef],[7,undef],[1,4],[1,4],[1,4+1e-12]],eps=0);    // Returns: [0,2,4]
function deduplicate_indexed(list, indices, closed=false, eps=EPSILON) =
    assert(is_list(list)||is_string(list), "Improper list or string.")
    indices==[]? [] :
    assert(is_vector(indices), "Indices must be a list of numbers.")
    let( l = len(indices),
         end = l-(closed?0:1) ) 
    [ for (i = [0:1:l-1]) 
        let(
           a = list[indices[i]],
           b = list[indices[(i+1)%l]],
           eq = (a == b)? true :
                (a*0 != b*0) || (eps==0)? false :
                is_num(a) || is_vector(a) ? approx(a, b, eps=eps) 
                : false
        ) 
        if (i==end || !eq) indices[i]
    ];


// Function: repeat_entries()
// Usage:
//   newlist = repeat_entries(list, N)
// Description:
//   Takes a list as input and duplicates some of its entries to produce a list
//   with length `N`.  If the requested `N` is not a multiple of the list length then
//   the entries will be duplicated as uniformly as possible.  You can also set `N` to a vector,
//   in which case len(N) must equal len(list) and the output repeats the ith entry N[i] times.
//   In either case, the result will be a list of length `N`.  The `exact` option requires
//   that the final length is exactly as requested.  If you set it to `false` then the
//   algorithm will favor uniformity and the output list may have a different number of
//   entries due to rounding.
//   .
//   When applied to a path the output path is the same geometrical shape but has some vertices
//   repeated.  This can be useful when you need to align paths with a different number of points.
//   (See also subdivide_path for a different way to do that.) 
// Arguments:
//   list = list whose entries will be repeated
//   N = scalar total number of points desired or vector requesting N[i] copies of vertex i.  
//   exact = if true return exactly the requested number of points, possibly sacrificing uniformity.  If false, return uniform points that may not match the number of points requested.  Default: True
// Examples:
//   list = [0,1,2,3];
//   echo(repeat_entries(list, 6));  // Ouputs [0,0,1,2,2,3]
//   echo(repeat_entries(list, 6, exact=false));  // Ouputs [0,0,1,1,2,2,3,3]
//   echo(repeat_entries(list, [1,1,2,1], exact=false));  // Ouputs [0,1,2,2,3]
function repeat_entries(list, N, exact = true) =
    assert(is_list(list) && len(list)>0, "The list cannot be void.")
    assert((is_finite(N) && N>0) || is_vector(N,len(list)),
            "Parameter N must be a number greater than zero or vector with the same length of `list`")
    let(
        length = len(list),
        reps_guess = is_list(N)? N : repeat(N/length,length),
        reps = exact ?
                 _sum_preserving_round(reps_guess) 
               : [for (val=reps_guess) round(val)]
    )
    [for(i=[0:length-1]) each repeat(list[i],reps[i])];


// Function: list_set()
// Usage:
//   list_set(list, indices, values, [dflt], [minlen])
// Description:
//   Takes the input list and returns a new list such that `list[indices[i]] = values[i]` for all of
//   the (index,value) pairs supplied and unchanged for other indices.  If you supply `indices` that are 
//   beyond the length of the list then the list is extended and filled in with the `dflt` value.  
//   If you set `minlen` then the list is lengthed, if necessary, by padding with `dflt` to that length.  
//   Repetitions in `indices` are not allowed. The lists `indices` and `values` must have the same length.  
//   If `indices` is given as a scalar, then that index of the given `list` will be set to the scalar value of `values`.
// Arguments:
//   list = List to set items in.  Default: []
//   indices = List of indices into `list` to set.
//   values = List of values to set.
//   dflt = Default value to store in sparse skipped indices.
//   minlen = Minimum length to expand list to.
// Examples:
//   list_set([2,3,4,5], 2, 21);  // Returns: [2,3,21,5]
//   list_set([2,3,4,5], [1,3], [81,47]);  // Returns: [2,81,4,47]
function list_set(list=[],indices,values,dflt=0,minlen=0) = 
    assert(is_list(list)||is_string(list))
    !is_list(indices)? (
        (is_finite(indices) && indices<len(list))? 
           [for (i=idx(list)) i==indices? values : list[i]]
        :  list_set(list,[indices],[values],dflt) )
    :   assert(is_vector(indices) && is_list(values) && len(values)==len(indices) ,
               "Index list and value list must have the same length")
        let( midx = max(len(list)-1, max(indices)) )
        [ for(i=[0:midx] ) 
            let( j = search(i,indices,0),
                 k = j[0] )
            assert( len(j)<2, "Repeated indices are not acceptable." )
            k!=undef ? values[k] :
            i<len(list) ? list[i]:
                dflt ,
          each repeat(dflt, minlen-max(indices))
        ];


// Function: list_insert()
// Usage:
//   list_insert(list, indices, values);
// Description:
//   Insert `values` into `list` before position `indices`.
// Example:
//   list_insert([3,6,9,12],1,5);  // Returns [3,5,6,9,12]
//   list_insert([3,6,9,12],[1,3],[5,11]);  // Returns [3,5,6,9,11,12]
function list_insert(list, indices, values, _i=0) = 
    assert(is_list(list)||is_string(list))
    ! is_list(indices)? 
        assert( is_finite(indices) && is_finite(values), "Invalid indices/values." ) 
        assert( indices<=len(list), "Indices must be <= len(list) ." )
        [for (i=idx(list)) each ( i==indices?  [ values, list[i] ] : [ list[i] ] ) ]
    :   assert( is_vector(indices) && is_list(values) && len(values)==len(indices) ,
               "Index list and value list must have the same length")
        assert( max(indices)<=len(list), "Indices must be <= len(list) ." )
        let( maxidx = max(indices),
             minidx  = min(indices) )
        [ for(i=[0:1:minidx-1] ) list[i],
          for(i=[minidx: min(maxidx, len(list)-1)] ) 
            let( j = search(i,indices,0),
                 k = j[0],
                 x = assert( len(j)<2, "Repeated indices are not acceptable." )
              )
            each ( k != undef  ? [ values[k], list[i] ] : [ list[i] ] ),
          for(i=[min(maxidx, len(list)-1)+1:1:len(list)-1] ) list[i],
          if(maxidx==len(list)) values[max_index(indices)]
        ];


// Function: list_remove()
// Usage:
//   list_remove(list, indices)
// Description:
//   Remove all items from `list` whose indexes are in `indices`.
// Arguments:
//   list = The list to remove items from.
//   indices = The list of indexes of items to remove.
// Example:
//   list_insert([3,6,9,12],1);      // Returns: [3,9,12]
//   list_insert([3,6,9,12],[1,3]);  // Returns: [3,9]
function list_remove(list, indices) =
    assert(is_list(list)||is_string(list), "Invalid list/string." )
    is_finite(indices) ?
        [
            for (i=[0:1:min(indices, len(list)-1)-1]) list[i],
            for (i=[min(indices, len(list)-1)+1:1:len(list)-1]) list[i]
        ]
    :   indices==[] ? list
    :   assert( is_vector(indices), "Invalid list `indices`." )
        [
            for(i=[0:len(list)-1])
            if ( []==search(i,indices,1) )
            list[i]
        ]; 


// Function: list_remove_values()
// Usage:
//   list_remove_values(list,values,all=false) =
// Description:
//   Removes the first, or all instances of the given `values` from the `list`.
//   Returns the modified list.
// Arguments:
//   list = The list to modify.
//   values = The values to remove from the list.
//   all = If true, remove all instances of the value `value` from the list `list`.  If false, remove only the first.  Default: false
// Example:
//   animals = ["bat", "cat", "rat", "dog", "bat", "rat"];
//   animals2 = list_remove_values(animals, "rat");   // Returns: ["bat","cat","dog","bat","rat"]
//   nonflying = list_remove_values(animals, "bat", all=true);  // Returns: ["cat","rat","dog","rat"]
//   animals3 = list_remove_values(animals, ["bat","rat"]);  // Returns: ["cat","dog","bat","rat"]
//   domestic = list_remove_values(animals, ["bat","rat"], all=true);  // Returns: ["cat","dog"]
//   animals4 = list_remove_values(animals, ["tucan","rat"], all=true);  // Returns: ["bat","cat","dog","bat"]
function list_remove_values(list,values=[],all=false) =
    assert(is_list(list)||is_string(list))
    !is_list(values)? list_remove_values(list, values=[values], all=all) :
    let(
        idxs = all? flatten(search(values,list,0)) : search(values,list,1),
        uidxs = unique(idxs)
    ) list_remove(list,uidxs);


// Function: bselect()
// Usage:
//   bselect(array,index);
// Description:
//   Returns the items in `array` whose matching element in `index` is true.
// Arguments:
//   array = Initial list to extract items from.
//   index = List of booleans.
// Example:
//   bselect([3,4,5,6,7], [false,true,true,false,true]);  // Returns: [4,5,7]
function bselect(array,index) =
    assert(is_list(array)||is_string(array), "Improper array." )
    assert(is_list(index) && len(index)>=len(array) , "Improper index list." )
    [for(i=[0:len(array)-1]) if (index[i]) array[i]];


// Function: list_bset()
// Usage:
//   list_bset(indexset, valuelist,[dflt])
// Description:
//   Opposite of `bselect()`.  Returns a list the same length as `indexlist`, where each item will
//   either be 0 if the corresponding item in `indexset` is false, or the next sequential value
//   from `valuelist` if the item is true.  The number of `true` values in `indexset` must be equal 
//   to the length of `valuelist`.
// Arguments:
//   indexset = A list of boolean values.
//   valuelist = The list of values to set into the returned list.
//   dflt = Default value to store when the indexset item is false.
// Example:
//   list_bset([false,true,false,true,false], [3,4]);  // Returns: [0,3,0,4,0]
//   list_bset([false,true,false,true,false], [3,4],dflt=1);  // Returns: [1,3,1,4,1]
function list_bset(indexset, valuelist, dflt=0) =
    assert(is_list(indexset), "The index set is not a list." )
    assert(is_list(valuelist), "The `valuelist` is not a list." )
    let( trueind = search([true], indexset,0)[0] )
    assert( !(len(trueind)>len(valuelist)), str("List `valuelist` too short; its length should be ",len(trueind)) )
    assert( !(len(trueind)<len(valuelist)), str("List `valuelist` too long; its length should be ",len(trueind)) )
    concat(
        list_set([],trueind, valuelist, dflt=dflt),    // Fill in all of the values
        repeat(dflt,len(indexset)-max(trueind)-1)  // Add trailing values so length matches indexset
    );


// Section: List Length Manipulation

// Function: list_shortest()
// Description:
//   Returns the length of the shortest sublist in a list of lists.
// Arguments:
//   array = A list of lists.
function list_shortest(array) =
    assert(is_list(array)||is_string(list), "Invalid input." )
    min([for (v = array) len(v)]);


// Function: list_longest()
// Description:
//   Returns the length of the longest sublist in a list of lists.
// Arguments:
//   array = A list of lists.
function list_longest(array) =
    assert(is_list(array)||is_string(list), "Invalid input." )
    max([for (v = array) len(v)]);


// Function: list_pad()
// Description:
//   If the list `array` is shorter than `minlen` length, pad it to length with the value given in `fill`.
// Arguments:
//   array = A list.
//   minlen = The minimum length to pad the list to.
//   fill = The value to pad the list with.
function list_pad(array, minlen, fill=undef) =
    assert(is_list(array)||is_string(list), "Invalid input." )
    concat(array,repeat(fill,minlen-len(array)));


// Function: list_trim()
// Description:
//   If the list `array` is longer than `maxlen` length, truncates it to be `maxlen` items long.
// Arguments:
//   array = A list.
//   minlen = The minimum length to pad the list to.
function list_trim(array, maxlen) =
    assert(is_list(array)||is_string(list), "Invalid input." )
    [for (i=[0:1:min(len(array),maxlen)-1]) array[i]];


// Function: list_fit()
// Description:
//   If the list `array` is longer than `length` items long, truncates it to be exactly `length` items long.
//   If the list `array` is shorter than `length` items long, pad it to length with the value given in `fill`.
// Arguments:
//   array = A list.
//   minlen = The minimum length to pad the list to.
//   fill = The value to pad the list with.
function list_fit(array, length, fill) =
    assert(is_list(array)||is_string(list), "Invalid input." )
    let(l=len(array)) 
    l==length ? array : 
    l> length ? list_trim(array,length) 
              : list_pad(array,length,fill);


// Section: List Shuffling and Sorting

// Function: shuffle()
// Description:
//   Shuffles the input list into random order.
function shuffle(list) =
    assert(is_list(list)||is_string(list), "Invalid input." )
    len(list)<=1 ? list :
    let (
        rval = rands(0,1,len(list)),
        left  = [for (i=[0:len(list)-1]) if (rval[i]< 0.5) list[i]],
        right = [for (i=[0:len(list)-1]) if (rval[i]>=0.5) list[i]]
    ) 
    concat(shuffle(left), shuffle(right));


// Sort a vector of scalar values
function _sort_scalars(arr) =
    len(arr)<=1 ? arr : 
    let(
        pivot   = arr[floor(len(arr)/2)],
        lesser  = [ for (y = arr) if (y  < pivot) y ],
        equal   = [ for (y = arr) if (y == pivot) y ],
        greater = [ for (y = arr) if (y  > pivot) y ]
    ) 
    concat( _sort_scalars(lesser), equal, _sort_scalars(greater) );


// Sort a vector of vectors based on the first entry only of each vector
function _sort_vectors1(arr) =
    len(arr)<=1 ? arr :
    !(len(arr)>0) ? [] : 
    let(
        pivot   = arr[floor(len(arr)/2)],
        lesser  = [ for (y = arr) if (y[0]  < pivot[0]) y ],
        equal   = [ for (y = arr) if (y[0] == pivot[0]) y ],
        greater = [ for (y = arr) if (y[0]  > pivot[0]) y ]
    ) 
    concat( _sort_vectors1(lesser), equal, _sort_vectors1(greater) );


// Sort a vector of vectors based on the first two entries of each vector
// Lexicographic order, remaining entries of vector ignored
function _sort_vectors2(arr) =
    len(arr)<=1 ? arr :
    !(len(arr)>0) ? [] : 
    let(
        pivot   = arr[floor(len(arr)/2)],
        lesser  = [ for (y = arr) if (y[0] < pivot[0] || (y[0]==pivot[0] && y[1]<pivot[1])) y ],
        equal   = [ for (y = arr) if (y[0] == pivot[0] && y[1]==pivot[1]) y ],
        greater = [ for (y = arr) if (y[0] > pivot[0] || (y[0]==pivot[0] && y[1]>pivot[1])) y ]
    ) 
    concat( _sort_vectors2(lesser), equal, _sort_vectors2(greater) );


// Sort a vector of vectors based on the first three entries of each vector
// Lexicographic order, remaining entries of vector ignored
function _sort_vectors3(arr) =
    len(arr)<=1 ? arr : let(
        pivot   = arr[floor(len(arr)/2)],
        lesser  = [ for (y = arr) 
                      if ( y[0] < pivot[0] 
                           || ( y[0]==pivot[0] 
                                && ( y[1]<pivot[1] 
                                     || ( y[1]==pivot[1] 
                                          && y[2]<pivot[2] ))))
                    y ],
        equal = [ for (y = arr) 
                    if ( y[0] == pivot[0] 
                         && y[1]== pivot[1] 
                         && y[2]==pivot[2] )
                  y ],
        greater = [ for (y = arr) 
                      if ( y[0] > pivot[0] 
                           || ( y[0]==pivot[0] 
                                && ( y[1] > pivot[1] 
                                     || ( y[1]==pivot[1] 
                                          && y[2] > pivot[2] )))) 
                    y ]
    ) concat( _sort_vectors3(lesser), equal, _sort_vectors3(greater) );


// Sort a vector of vectors based on the first four entries of each vector
// Lexicographic order, remaining entries of vector ignored
function _sort_vectors4(arr) =
    len(arr)<=1 ? arr : let(
        pivot = arr[floor(len(arr)/2)],
        lesser = [  for (y = arr) 
                      if ( y[0] < pivot[0] 
                           || ( y[0]==pivot[0] 
                                && ( y[1]<pivot[1] 
                                     || ( y[1]==pivot[1] 
                                         && ( y[2]<pivot[2] 
                                              || ( y[2]==pivot[2] 
                                                   && y[3]<pivot[3] ))))))
                      y ],
        equal = [ for (y = arr) 
                    if (  y[0] == pivot[0] 
                          && y[1] == pivot[1] 
                          && y[2] == pivot[2] 
                          && y[3] == pivot[3] ) 
                  y  ],
        greater = [ for (y = arr) 
                      if ( y[0] > pivot[0] 
                           || ( y[0]==pivot[0] 
                                && ( y[1]>pivot[1] 
                                     || ( y[1]==pivot[1] 
                                          && ( y[2]>pivot[2] 
                                               || ( y[2]==pivot[2] 
                                                    && y[3]>pivot[3] )))))) 
                    y ]
    ) concat( _sort_vectors4(lesser), equal, _sort_vectors4(greater) );


// when idx==undef, returns the sorted array
// otherwise, returns the indices of the sorted array
function _sort_general(arr, idx=undef) =
    (len(arr)<=1) ? arr :
    is_undef(idx) 
    ? _sort_scalar(arr)
    : let( arrind=[for(k=[0:len(arr)-1], ark=[arr[k]]) [ k, [for (i=idx) ark[i]] ] ] )
      _indexed_sort(arrind);
      
// given a list of pairs, return the first element of each pair of the list sorted by the second element of the pair
// the sorting is done using compare_vals()
function _indexed_sort(arrind) = 
    arrind==[] ? [] : len(arrind)==1? [arrind[0][0]] :
    let( pivot = arrind[floor(len(arrind)/2)][1] )
    let(
        lesser  = [ for (entry=arrind) if (compare_vals(entry[1], pivot) <0 ) entry ],
        equal   = [ for (entry=arrind) if (compare_vals(entry[1], pivot)==0 ) entry[0] ],
        greater = [ for (entry=arrind) if (compare_vals(entry[1], pivot) >0 ) entry ]
      )
    concat(_indexed_sort(lesser), equal, _indexed_sort(greater));


// returns true for valid index specifications idx in the interval [imin, imax) 
// note that idx can't have any value greater or EQUAL to imax
function _valid_idx(idx,imin,imax) =
    is_undef(idx) 
    || ( is_finite(idx) && idx>=imin && idx< imax )
    || ( is_list(idx) && min(idx)>=imin && max(idx)< imax )
    || ( valid_range(idx) && idx[0]>=imin && idx[2]< imax );
    

// Function: sort()
// Usage:
//   sort(list, [idx])
// Description:
//   Sorts the given list using `compare_vals()`, sorting in lexicographic order, with types ordered according to
//   `undef < boolean < number < string < list`.  Comparison of lists is recursive. 
//   If the list is a list of vectors whose length is from 1 to 4 and the `idx` parameter is not passed, then 
//   `sort` uses a much more efficient method for comparisons and will run much faster.  In this case, all entries
//   in the data are compared using the native comparison operator, so comparisons between types will fail.  
// Arguments:
//   list = The list to sort.
//   idx = If given, do the comparison based just on the specified index, range or list of indices.  
// Example:
//   l = [45,2,16,37,8,3,9,23,89,12,34];
//   sorted = sort(l);  // Returns [2,3,8,9,12,16,23,34,37,45,89]
function sort(list, idx=undef) =
    !is_list(list) || len(list)<=1 ? list :
    is_def(idx) 
    ?   assert( _valid_idx(idx,0,len(list)) , "Invalid indices.")
        let( sarr = _sort_general(list,idx) )
        [for(i=[0:len(sarr)-1]) list[sarr[i]] ] 
    :   let(size = array_dim(list))
        len(size)==1 ? _sort_scalars(list) :
        len(size)==2 && size[1] <=4 
        ? (
            size[1]==0 ? list :
            size[1]==1 ? _sort_vectors1(list) :
            size[1]==2 ? _sort_vectors2(list) :
            size[1]==3 ? _sort_vectors3(list)
       /*size[1]==4*/  : _sort_vectors4(list)
          ) 
        : _sort_general(list);


// Function: sortidx()
// Description:
//   Given a list, calculates the sort order of the list, and returns
//   a list of indexes into the original list in that sorted order.
//   If you iterate the returned list in order, and use the list items
//   to index into the original list, you will be iterating the original
//   values in sorted order.
// Example:
//   lst = ["d","b","e","c"];
//   idxs = sortidx(lst);  // Returns: [1,3,0,2]
//   ordered = select(lst, idxs);   // Returns: ["b", "c", "d", "e"]
// Example:
//   lst = [
//       ["foo", 88, [0,0,1], false],
//       ["bar", 90, [0,1,0], true],
//       ["baz", 89, [1,0,0], false],
//       ["qux", 23, [1,1,1], true]
//   ];
//   idxs1 = sortidx(lst, idx=1); // Returns: [3,0,2,1]
//   idxs2 = sortidx(lst, idx=0); // Returns: [1,2,0,3]
//   idxs3 = sortidx(lst, idx=[1,3]); // Returns: [3,0,2,1]
function sortidx(list, idx=undef) =
    assert( is_list(list) || is_string(list) , "Invalid input to sort." )
    assert( _valid_idx(idx,0,len(list)) , "Invalid indices.")
    list==[] ? [] : 
    let(
        size = array_dim(list),
        aug = is_undef(idx) && (len(size) == 1 || (len(size) == 2 && size[1]<=4))
              ?  zip(list, list_range(len(list)))
              :  0
    )
    is_undef(idx) && len(size) == 1? subindex(_sort_vectors1(aug),1) :
    is_undef(idx) && len(size) == 2 && size[1] <=4
    ? (
        size[1]==0 ? list_range(len(arr)) :
        size[1]==1 ? subindex(_sort_vectors1(aug),1) :
        size[1]==2 ? subindex(_sort_vectors2(aug),2) :
        size[1]==3 ? subindex(_sort_vectors3(aug),3)
    /*size[1]==4*/ : subindex(_sort_vectors4(aug),4)
      ) 
    :   // general case
        _sort_general(list,idx);


// sort() does not accept strings but sortidx does; isn't inconsistent ?


// Function: unique()
// Usage:
//   unique(arr);
// Description:
//   Returns a sorted list with all repeated items removed.
// Arguments:
//   arr = The list to uniquify.
function unique(arr) =
    assert(is_list(arr)||is_string(arr), "Invalid input." )
    len(arr)<=1? arr : 
    let( sorted = sort(arr))
    [   for (i=[0:1:len(sorted)-1])
            if (i==0 || (sorted[i] != sorted[i-1]))
                sorted[i]
    ];


// Function: unique_count()
// Usage:
//   unique_count(arr);
// Description:
//   Returns `[sorted,counts]` where `sorted` is a sorted list of the unique items in `arr` and `counts` is a list such 
//   that `count[i]` gives the number of times that `sorted[i]` appears in `arr`.  
// Arguments:
//   arr = The list to analyze. 
function unique_count(arr) =
      assert(is_list(arr) || is_string(arr), "Invalid input." )
      arr == [] ? [[],[]] : 
      let( arr=sort(arr) )
      let( ind = [0, for(i=[1:1:len(arr)-1]) if (arr[i]!=arr[i-1]) i] )
      [ select(arr,ind), deltas( concat(ind,[len(arr)]) ) ];


// Section: List Iteration Helpers

// Function: idx()
// Usage:
//   i = idx(list);
//   for(i=idx(list)) ...
// Description:
//   Returns the range of indexes for the given list.
// Arguments:
//   list = The list to returns the index range of.
//   step = The step size to stride through the list.  Default: 1
//   end = The delta from the end of the list.  Default: -1
//   start = The starting index.  Default: 0
// Example(2D):
//   colors = ["red", "green", "blue"];
//   for (i=idx(colors)) right(20*i) color(colors[i]) circle(d=10);
function idx(list, step=1, end=-1,start=0) =
    assert(is_list(list)||is_string(list), "Invalid input." )
    [start : step : len(list)+end];


// Function: enumerate()
// Description:
//   Returns a list, with each item of the given list `l` numbered in a sublist.
//   Something like: `[[0,l[0]], [1,l[1]], [2,l[2]], ...]`
// Arguments:
//   l = List to enumerate.
//   idx = If given, enumerates just the given subindex items of `l`.
// Example:
//   enumerate(["a","b","c"]);  // Returns: [[0,"a"], [1,"b"], [2,"c"]]
//   enumerate([[88,"a"],[76,"b"],[21,"c"]], idx=1);  // Returns: [[0,"a"], [1,"b"], [2,"c"]]
//   enumerate([["cat","a",12],["dog","b",10],["log","c",14]], idx=[1:2]);  // Returns: [[0,"a",12], [1,"b",10], [2,"c",14]]
// Example(2D):
//   colors = ["red", "green", "blue"];
//   for (p=enumerate(colors)) right(20*p[0]) color(p[1]) circle(d=10);
function enumerate(l,idx=undef) =
    assert(is_list(l)||is_string(list), "Invalid input." )
    assert( _valid_idx(idx,0,len(l)), "Invalid index/indices." )
    (idx==undef)
    ?   [for (i=[0:1:len(l)-1]) [i,l[i]]]
    :   [for (i=[0:1:len(l)-1]) [ i, for (j=idx) l[i][j]] ];


// Function: force_list()
// Usage:
//   list = force_list(value, [n], [fill])
// Description:
//   Coerces non-list values into a list.  Makes it easy to treat a scalar input
//   consistently as a singleton list, as well as list inputs.
//   - If `value` is a list, then that list is returned verbatim.
//   - If `value` is not a list, and `fill` is not given, then a list of `n` copies of `value` will be returned.
//   - If `value` is not a list, and `fill` is given, then a list `n` items long will be returned where `value` will be the first item, and the rest will contain the value of `fill`.
// Arguments:
//   value = The value or list to coerce into a list.
//   n = The number of items in the coerced list.  Default: 1
//   fill = The value to pad the coerced list with, after the firt value.  Default: undef (pad with copies of `value`)
// Examples:
//   x = force_list([3,4,5]);  // Returns: [3,4,5]
//   y = force_list(5);  // Returns: [5]
//   z = force_list(7, n=3);  // Returns: [7,7,7]
//   w = force_list(4, n=3, fill=1);  // Returns: [4,1,1]
function force_list(value, n=1, fill) =
    is_list(value) ? value :
    is_undef(fill)? [for (i=[1:1:n]) value] : [value, for (i=[2:1:n]) fill];


// Function: pair()
// Usage:
//   pair(v)
// Description:
//   Takes a list, and returns a list of adjacent pairs from it.
// Example(2D): Note that the last point and first point do NOT get paired together.
//   for (p = pair(circle(d=20, $fn=12)))
//       move(p[0])
//           rot(from=BACK, to=p[1]-p[0])
//               trapezoid(w1=1, w2=0, h=norm(p[1]-p[0]), anchor=FRONT);
// Example:
//   l = ["A","B","C","D"];
//   echo([for (p=pair(l)) str(p.y,p.x)]);  // Outputs: ["BA", "CB", "DC"]
function pair(v) =
    assert(is_list(v)||is_string(v), "Invalid input." )
    [for (i=[0:1:len(v)-2]) [v[i],v[i+1]]];


// Function: pair_wrap()
// Usage:
//   pair_wrap(v)
// Description:
//   Takes a list, and returns a list of adjacent pairs from it, wrapping around from the end to the start of the list.
// Example(2D): 
//   for (p = pair_wrap(circle(d=20, $fn=12)))
//       move(p[0])
//           rot(from=BACK, to=p[1]-p[0])
//               trapezoid(w1=1, w2=0, h=norm(p[1]-p[0]), anchor=FRONT);
// Example:
//   l = ["A","B","C","D"];
//   echo([for (p=pair_wrap(l)) str(p.y,p.x)]);  // Outputs: ["BA", "CB", "DC", "AD"]
function pair_wrap(v) =
    assert(is_list(v)||is_string(v), "Invalid input." )
    [for (i=[0:1:len(v)-1]) [v[i],v[(i+1)%len(v)]]];


// Function: triplet()
// Usage:
//   triplet(v)
// Description:
//   Takes a list, and returns a list of adjacent triplets from it.
// Example:
//   l = ["A","B","C","D","E"];
//   echo([for (p=triplet(l)) str(p.z,p.y,p.x)]);  // Outputs: ["CBA", "DCB", "EDC"]
function triplet(v) =
    assert(is_list(v)||is_string(v), "Invalid input." )
    [for (i=[0:1:len(v)-3]) [v[i],v[i+1],v[i+2]]];


// Function: triplet_wrap()
// Usage:
//   triplet_wrap(v)
// Description:
//   Takes a list, and returns a list of adjacent triplets from it, wrapping around from the end to the start of the list.
// Example:
//   l = ["A","B","C","D"];
//   echo([for (p=triplet_wrap(l)) str(p.z,p.y,p.x)]);  // Outputs: ["CBA", "DCB", "ADC", "BAD"]
function triplet_wrap(v) =
    assert(is_list(v)||is_string(v), "Invalid input." )
    [for (i=[0:1:len(v)-1]) [v[i],v[(i+1)%len(v)],v[(i+2)%len(v)]]];


// Function: permute()
// Usage:
//   list = permute(l, [n]);
// Description:
//   Returns an ordered list of every unique permutation of `n` items out of the given list `l`.
//   For the list `[1,2,3,4]`, with `n=2`, this will return `[[1,2], [1,3], [1,4], [2,3], [2,4], [3,4]]`.
//   For the list `[1,2,3,4]`, with `n=3`, this will return `[[1,2,3], [1,2,4], [1,3,4], [2,3,4]]`.
// Arguments:
//   l = The list to provide permutations for.
//   n = The number of items in each permutation. Default: 2
// Example:
//   pairs = permute([3,4,5,6]);  // Returns: [[3,4],[3,5],[3,6],[4,5],[4,6],[5,6]]
//   triplets = permute([3,4,5,6],n=3);  // Returns: [[3,4,5],[3,4,6],[3,5,6],[4,5,6]]
// Example(2D):
//   for (p=permute(regular_ngon(n=7,d=100))) stroke(p);
function permute(l,n=2,_s=0) =
    assert(is_list(l), "Invalid list." )
    assert( is_finite(n) && n>=1 && n<=len(l), "Invalid number `n`." )
    n==1
    ?   [for (i=[_s:1:len(l)-1]) [l[i]]] 
    :   [for (i=[_s:1:len(l)-n], p=permute(l,n=n-1,_s=i+1)) concat([l[i]], p)];



// Section: Set Manipulation

// Function: set_union()
// Usage:
//   s = set_union(a, b, [get_indices]);
// Description:
//   Given two sets (lists with unique items), returns the set of unique items that are in either `a` or `b`.
//   If `get_indices` is true, a list of indices into the new union set are returned for each item in `b`,
//   in addition to returning the new union set.  In this case, a 2-item list is returned, `[INDICES, NEWSET]`,
//   where INDICES is the list of indices for items in `b`, and NEWSET is the new union set.
// Arguments:
//   a = One of the two sets to merge.
//   b = The other of the two sets to merge.
//   get_indices = If true, indices into the new union set are also returned for each item in `b`.  Returns `[INDICES, NEWSET]`.  Default: false
// Example:
//   set_a = [2,3,5,7,11];
//   set_b = [1,2,3,5,8];
//   set_u = set_union(set_a, set_b);
//   // set_u now equals [2,3,5,7,11,1,8]
//   set_v = set_union(set_a, set_b, get_indices=true);
//   // set_v now equals [[5,0,1,2,6], [2,3,5,7,11,1,8]]
function set_union(a, b, get_indices=false) =
    assert( is_list(a) && is_list(b), "Invalid sets." )
    let(
        found1 = search(b, a),
        found2 = search(b, b),
        c = [ for (i=idx(b))
                if (found1[i] == [] && found2[i] == i)
                    b[i] 
            ],
        nset = concat(a, c)
    ) 
    ! get_indices ? nset :
    let(
        la = len(a),
        found3 = search(b, c),
        idxs =  [ for (i=idx(b))
                    (found1[i] != [])? found1[i] : la + found3[i]
                ]
    ) [idxs, nset];


// Function: set_difference()
// Usage:
//   s = set_difference(a, b);
// Description:
//   Given two sets (lists with unique items), returns the set of items that are in `a`, but not `b`.
// Arguments:
//   a = The starting set.
//   b = The set of items to remove from set `a`.
// Example:
//   set_a = [2,3,5,7,11];
//   set_b = [1,2,3,5,8];
//   set_d = set_difference(set_a, set_b);
//   // set_d now equals [7,11]
function set_difference(a, b) =
    assert( is_list(a) && is_list(b), "Invalid sets." )
    let( found = search(a, b, num_returns_per_match=1) )
    [ for (i=idx(a)) if(found[i]==[]) a[i] ];


// Function: set_intersection()
// Usage:
//   s = set_intersection(a, b);
// Description:
//   Given two sets (lists with unique items), returns the set of items that are in both sets.
// Arguments:
//   a = The starting set.
//   b = The set of items to intersect with set `a`.
// Example:
//   set_a = [2,3,5,7,11];
//   set_b = [1,2,3,5,8];
//   set_i = set_intersection(set_a, set_b);
//   // set_i now equals [2,3,5]
function set_intersection(a, b) =
    assert( is_list(a) && is_list(b), "Invalid sets." )
    let( found = search(a, b, num_returns_per_match=1) )
    [ for (i=idx(a)) if(found[i]!=[]) a[i] ];



// Section: Array Manipulation

// Function: add_scalar()
// Usage:  
//   add_scalar(v,s);
// Description:
//   Given an array and a scalar, returns the array with the scalar added to each item in it.
//   If given a list of arrays, recursively adds the scalar to the each array.
// Arguments:
//   v = The initial array.
//   s = A scalar value to add to every item in the array.
// Example:
//   add_scalar([1,2,3],3);            // Returns: [4,5,6]
//   add_scalar([[1,2,3],[3,4,5]],3);  // Returns: [[4,5,6],[6,7,8]]
function add_scalar(v,s) = 
    is_finite(s) ? [for (x=v) is_list(x)? add_scalar(x,s) : is_finite(x) ? x+s: x] : v;


// Function: subindex()
// Usage:
//   subindex(M, idx)
// Description:
//   Extracts the entries listed in idx from each entry in M.  For a matrix this means
//   selecting a specified set of columns.  If idx is a number the return is a vector, 
//   otherwise it is a list of lists (the submatrix).  
//   This function will return `undef` at all entry positions indexed by idx not found in the input list M.
// Arguments:
//   M = The given list of lists.
//   idx = The index, list of indices, or range of indices to fetch.
// Example:
//   M = [[1,2,3,4],[5,6,7,8],[9,10,11,12],[13,14,15,16]];
//   subindex(M,2);      // Returns [3, 7, 11, 15]
//   subindex(M,[2]);    // Returns [[3], [7], [11], [15]]
//   subindex(M,[2,1]);  // Returns [[3, 2], [7, 6], [11, 10], [15, 14]]
//   subindex(M,[1:3]);  // Returns [[2, 3, 4], [6, 7, 8], [10, 11, 12], [14, 15, 16]]
//   N = [ [1,2], [3], [4,5], [6,7,8] ];
//   subindex(N,[0,1]);  // Returns [ [1,2], [3,undef], [4,5], [6,7] ]
function subindex(M, idx) =
    assert( is_list(M), "The input is not a list." )
    assert( !is_undef(idx) && _valid_idx(idx,0,1/0), "Invalid index input." ) 
    is_finite(idx)
      ? [for(row=M) row[idx]]
      : [for(row=M) [for(i=idx) row[i]]];


// Function: submatrix()
// Usage: submatrix(M, idx1, idx2)
// Description:
//   The input must be a list of lists (a matrix or 2d array).  Returns a submatrix by selecting the rows listed in idx1 and columsn listed in idx2.
// Arguments:
//   M = Given list of lists
//   idx1 = rows index list or range
//   idx2 = column index list or range
// Example:
//   M = [[ 1, 2, 3, 4, 5],
//        [ 6, 7, 8, 9,10],
//        [11,12,13,14,15],
//        [16,17,18,19,20],
//        [21,22,23,24,25]];
//   submatrix(M,[1:2],[3:4]);  // Returns [[9, 10], [14, 15]]
//   submatrix(M,[1], [3,4]));  // Returns [[9,10]]
//   submatrix(M,1, [3,4]));  // Returns [[9,10]]
//   submatrix(M,1,3));  // Returns [[9]]
//   submatrix(M, [3,4],1); // Returns  [[17],[22]]);
//   submatrix(M, [1,3],[2,4]); // Returns [[8,10],[18,20]]);
//   A = [[true,    17, "test"],
//        [[4,2],   91, false],
//        [6,    [3,4], undef]];
//   submatrix(A,[0,2],[1,2]);   // Returns [[17, "test"], [[3, 4], undef]]

function submatrix(M,idx1,idx2) =
    [for(i=idx1) [for(j=idx2) M[i][j] ] ];


// Function: zip()
// Usage:
//   zip(v1, v2, v3, [fit], [fill]);
//   zip(vecs, [fit], [fill]);
// Description:
//   Zips together corresponding items from two or more lists.
//   Returns a list of lists, where each sublist contains corresponding
//   items from each of the input lists.  `[[A1, B1, C1], [A2, B2, C2], ...]`
// Arguments:
//   vecs = A list of two or more lists to zipper together.
//   fit = If `fit=="short"`, the zips together up to the length of the shortest list in vecs.  If `fit=="long"`, then pads all lists to the length of the longest, using the value in `fill`.  If `fit==false`, then requires all lists to be the same length.  Default: false.
//   fill = The default value to fill in with if one or more lists if short.  Default: undef
// Example:
//   v1 = [1,2,3,4];
//   v2 = [5,6,7];
//   v3 = [8,9,10,11];
//   zip(v1,v3);                       // returns [[1,8], [2,9], [3,10], [4,11]]
//   zip([v1,v3]);                     // returns [[1,8], [2,9], [3,10], [4,11]]
//   zip([v1,v2], fit="short");        // returns [[1,5], [2,6], [3,7]]
//   zip([v1,v2], fit="long");         // returns [[1,5], [2,6], [3,7], [4,undef]]
//   zip([v1,v2], fit="long, fill=0);  // returns [[1,5], [2,6], [3,7], [4,0]]
//   zip([v1,v2,v3], fit="long");      // returns [[1,5,8], [2,6,9], [3,7,10], [4,undef,11]]
// Example:
//   v1 = [[1,2,3], [4,5,6], [7,8,9]];
//   v2 = [[20,19,18], [17,16,15], [14,13,12]];
//   zip(v1,v2);    // Returns [[1,2,3,20,19,18], [4,5,6,17,16,15], [7,8,9,14,13,12]]
function zip(vecs, v2, v3, fit=false, fill=undef) =
    (v3!=undef)? zip([vecs,v2,v3], fit=fit, fill=fill) :
    (v2!=undef)? zip([vecs,v2], fit=fit, fill=fill) :
    assert(in_list(fit, [false, "short", "long"]), "Invalid fit value." )
    assert(all([for(v=vecs) is_list(v)]), "One of the inputs to zip is not a list")
    let(
        minlen = list_shortest(vecs),
        maxlen = list_longest(vecs)
    )
    assert(fit!=false || minlen==maxlen, "Input vectors to zip must have the same length")
    (fit == "long")
    ?   [for(i=[0:1:maxlen-1]) [for(v=vecs) for(x=(i<len(v)? v[i] : (fill==undef)? [fill] : fill)) x] ] 
    :   [for(i=[0:1:minlen-1]) [for(v=vecs) for(x=v[i]) x] ];


// Function: array_group()
// Description:
//   Takes a flat array of values, and groups items in sets of `cnt` length.
//   The opposite of this is `flatten()`.
// Arguments:
//   v = The list of items to group.
//   cnt = The number of items to put in each grouping.
//   dflt = The default value to fill in with is the list is not a multiple of `cnt` items long.
// Example:
//   v = [1,2,3,4,5,6];
//   array_group(v,2) returns [[1,2], [3,4], [5,6]]
//   array_group(v,3) returns [[1,2,3], [4,5,6]]
//   array_group(v,4,0) returns [[1,2,3,4], [5,6,0,0]]
function array_group(v, cnt=2, dflt=0) = [for (i = [0:cnt:len(v)-1]) [for (j = [0:1:cnt-1]) default(v[i+j], dflt)]];


// Function: flatten()
// Description: Takes a list of lists and flattens it by one level.
// Arguments:
//   l = List to flatten.
// Example:
//   flatten([[1,2,3], [4,5,[6,7,8]]]) returns [1,2,3,4,5,[6,7,8]]
function flatten(l) = [for (a = l) each a];


// Function: full_flatten()
// Description: 
//   Collects in a list all elements recursively found in any level of the given list.
//   The output list is ordered in depth first order.
// Arguments:
//   l = List to flatten.
// Example:
//   full_flatten([[1,2,3], [4,5,[6,7,8]]]) returns [1,2,3,4,5,6,7,8]
function full_flatten(l) = [for(a=l) if(is_list(a)) (each full_flatten(a)) else a ];


// Internal.  Not exposed.
function _array_dim_recurse(v) =
    !is_list(v[0])
    ?   sum( [for(entry=v) is_list(entry) ? 1 : 0] ) == 0 ? [] : [undef]
    :   let(
          firstlen = len(v[0]),
          first = sum( [for(entry = v) len(entry) == firstlen  ? 0 : 1]   ) == 0 ? firstlen : undef,
          leveldown = flatten(v)
        ) 
        is_list(leveldown[0])
        ?  concat([first],_array_dim_recurse(leveldown))
        : [first];


// Function: array_dim()
// Usage:
//   array_dim(v, [depth])
// Description:
//   Returns the size of a multi-dimensional array.  Returns a list of
//   dimension lengths.  The length of `v` is the dimension `0`.  The
//   length of the items in `v` is dimension `1`.  The length of the
//   items in the items in `v` is dimension `2`, etc.  For each dimension,
//   if the length of items at that depth is inconsistent, `undef` will
//   be returned.  If no items of that dimension depth exist, `0` is
//   returned.  Otherwise, the consistent length of items in that
//   dimensional depth is returned.
// Arguments:
//   v = Array to get dimensions of.
//   depth = Dimension to get size of.  If not given, returns a list of dimension lengths.
// Examples:
//   array_dim([[[1,2,3],[4,5,6]],[[7,8,9],[10,11,12]]]);     // Returns [2,2,3]
//   array_dim([[[1,2,3],[4,5,6]],[[7,8,9],[10,11,12]]], 0);  // Returns 2
//   array_dim([[[1,2,3],[4,5,6]],[[7,8,9],[10,11,12]]], 2);  // Returns 3
//   array_dim([[[1,2,3],[4,5,6]],[[7,8,9]]]);                // Returns [2,undef,3]
function array_dim(v, depth=undef) =
    assert( is_undef(depth) || ( is_finite(depth) && depth>=0 ), "Invalid depth.")
    ! is_list(v) ? 0 :
    (depth == undef)
    ?   concat([len(v)], _array_dim_recurse(v))
    :   (depth == 0)
        ?  len(v)
        :  let( dimlist = _array_dim_recurse(v))
           (depth > len(dimlist))? 0 : dimlist[depth-1] ;

// This function may return undef!


// Function: transpose()
// Usage:
//    transpose(arr, [reverse])
// Description:
//    Returns the transpose of the given input array.  The input should be a list of lists that are
//    all the same length.  If you give a vector then transpose returns it unchanged.  
//    When reverse=true, the transpose is done across to the secondary diagonal.  (See example below.)
//    By default, reverse=false.
// Example:
//   arr = [
//       ["a", "b", "c"],
//       ["d", "e", "f"],
//       ["g", "h", "i"]
//   ];
//   t = transpose(arr);
//   // Returns:
//   // [
//   //     ["a", "d", "g"],
//   //     ["b", "e", "h"],
//   //     ["c", "f", "i"],
//   // ]
// Example:
//   arr = [
//       ["a", "b", "c"],
//       ["d", "e", "f"]
//   ];
//   t = transpose(arr);
//   // Returns:
//   // [
//   //     ["a", "d"],
//   //     ["b", "e"],
//   //     ["c", "f"],
//   // ]
// Example:
//   arr = [
//       ["a", "b", "c"],
//       ["d", "e", "f"],
//       ["g", "h", "i"]
//   ];
//   t = transpose(arr, reverse=true);
//   // Returns:
//   // [
//   //  ["i", "f", "c"],
//   //  ["h", "e", "b"],
//   //  ["g", "d", "a"]
//   // ]
// Example: Transpose on a list of numbers returns the list unchanged
//   transpose([3,4,5]);  // Returns: [3,4,5]
function transpose(arr, reverse=false) =
    assert( is_list(arr) && len(arr)>0, "Input to transpose must be a nonempty list.")
    is_list(arr[0])
    ?   let( len0 = len(arr[0]) )
        assert([for(a=arr) if(!is_list(a) || len(a)!=len0) 1 ]==[], "Input to transpose has inconsistent row lengths." )
        reverse
        ? [for (i=[0:1:len0-1]) 
              [ for (j=[0:1:len(arr)-1]) arr[len(arr)-1-j][len0-1-i] ] ] 
        : [for (i=[0:1:len0-1]) 
              [ for (j=[0:1:len(arr)-1]) arr[j][i] ] ] 
    :  assert( is_vector(arr), "Input to transpose must be a vector or list of lists.")
           arr;


// vim: expandtab tabstop=4 shiftwidth=4 softtabstop=4 nowrap