// https://codepen.io/P233/pen/AtzIw
// This following mixin is modified from the original Bourbon triangle add-on (https://github.com/thoughtbot/bourbon/blob/master/app/assets/stylesheets/addons/_triangle.scss) in order to customise triangle shape and foreground / background colours, which can also create CSS ribbon incredibly easy.
// @include triangle($width $height, $foreground-color $background-color, $direction)
// $height and $background-color can be omitted, if so, $height will equal to $width and $background-color will be transparent.
// $direction contains the following arguments: up, down, left, right, up-right, up-left, down-right, down-left, inset-up, inset-down, inset-left, inset-right.
@mixin triangle ($size, $color, $direction) {
    height: 0;
    width: 0;
    $width: nth($size, 1);
    $height: nth($size, length($size));
    $foreground-color: nth($color, 1);
    $background-color: transparent !default;
    @if (length($color)==2) {
        $background-color: nth($color, 2);
    }
    @if ($direction==up) or ($direction==down) or ($direction==right) or ($direction==left) {
        $width: $width / 2;
        @if $direction==up {
            border-left: $width solid $background-color;
            border-right: $width solid $background-color;
            border-bottom: $height solid $foreground-color;
        }
        @else if $direction==right {
            border-top: $width solid $background-color;
            border-bottom: $width solid $background-color;
            border-left: $height solid $foreground-color;
        }
        @else if $direction==down {
            border-left: $width solid $background-color;
            border-right: $width solid $background-color;
            border-top: $height solid $foreground-color;
        }
        @else if $direction==left {
            border-top: $width solid $background-color;
            border-bottom: $width solid $background-color;
            border-right: $height solid $foreground-color;
        }
    }
    @else if ($direction==up-right) or ($direction==up-left) {
        border-top: $height solid $foreground-color;
        @if $direction==up-right {
            border-left: $width solid $background-color;
        }
        @else if $direction==up-left {
            border-right: $width solid $background-color;
        }
    }
    @else if ($direction==down-right) or ($direction==down-left) {
        border-bottom: $height solid $foreground-color;
        @if $direction==down-right {
            border-left: $width solid $background-color;
        }
        @else if $direction==down-left {
            border-right: $width solid $background-color;
        }
    }
    @else if ($direction==inset-up) {
        border-width: $height $width;
        border-style: solid;
        border-color: $background-color $background-color $foreground-color;
    }
    @else if ($direction==inset-down) {
        border-width: $height $width;
        border-style: solid;
        border-color: $foreground-color $background-color $background-color;
    }
    @else if ($direction==inset-right) {
        border-width: $width $height;
        border-style: solid;
        border-color: $background-color $background-color $background-color $foreground-color;
    }
    @else if ($direction==inset-left) {
        border-width: $width $height;
        border-style: solid;
        border-color: $background-color $foreground-color $background-color $background-color;
    }
}