// Column math
// ===========


// Is Symmetrical
// --------------
// Returns true if a grid is symmetrical.
// - [$columns]   : <number> | <list>
@function is-symmetrical(
  $columns: susy-get(columns)
) {
  $columns: valid-columns($columns);
  @return if(type-of($columns) == number, $columns, null);
}


// Susy Count
// ----------
// Find the number of columns in a given layout
// - [$columns]   : <number> | <list>
@function susy-count(
  $columns: susy-get(columns)
) {
  $columns: valid-columns($columns);
  @return is-symmetrical($columns) or length($columns);
}


// Susy Sum
// --------
// Find the total sum of column-units in a layout
// - [$columns]   : <number> | <list>
// - [$gutters]   : <ratio>
// - [$spread]    : false/narrow | wide | wider
@function susy-sum(
  $columns  : susy-get(columns),
  $gutters  : susy-get(gutters),
  $spread   : false
) {
  $columns: valid-columns($columns);
  $gutters: valid-gutters($gutters);

  $spread: if($spread == wide, 0, if($spread == wider, 1, -1));
  $gutter-sum: (susy-count($columns) + $spread) * $gutters;
  $column-sum: is-symmetrical($columns);

  @if not($column-sum) {
    @each $column in $columns {
      $column-sum: ($column-sum or 0) + $column;
    }
  }

  @return $column-sum + $gutter-sum;
}


// Susy Slice
// ----------
// Return a subset of columns at a given location.
// - $span        : <number>
// - $location    : <number>
// - [$columns]   : <number> | <list>
@function susy-slice(
  $span,
  $location,
  $columns: susy-get(columns)
) {
  $columns: valid-columns($columns);
  $sub-columns: $span;

  @if not(is-symmetrical($columns)) {
    $location: $location or 1;
    $sub-columns: ();
    @for $i from $location to ($location + $span) {
      $sub-columns: append($sub-columns, nth($columns, $i));
    }
  }

  @return $sub-columns;
}


// Susy
// ----
// Find the sum of a column-span.
// - $span        : <number>
// - $location    : <number>
// - [$columns]   : <number> | <list>
// - [$gutters]   : <ratio>
// - [$spread]    : false/narrow | wide | wider
@function susy(
  $span,
  $location     : false,
  $columns      : susy-get(columns),
  $gutters      : susy-get(gutters),
  $spread       : false
) {
  $columns: valid-columns($columns);
  $gutters: valid-gutters($gutters);
  $span: susy-slice($span, $location, $columns);

  @return susy-sum($span, $gutters, $spread);
}