Automate breakpoints

Breakpoints and container widths are now defined as maps.

scss/mixins/_breakpoints.scss

@@ -0,0 +1,85 @@
+// Breakpoint viewport sizes and media queries.
+//
+// Breakpoints are defined as a map of (name: minimum width), order from small to large:
+//
+//    (xs: 0, sm: 34rem, md: 45rem)
+//
+// The map defined in the `$grid-breakpoints` global variable is used as the `$breakpoints` argument by default.
+
+// Name of the next breakpoint, or null for the last breakpoint.
+//
+//    >> breakpoint-next(sm)
+//    md
+//    >> breakpoint-next(sm, $breakpoints: (xs: 0, sm: 34rem, md: 45rem))
+//    md
+//    >> breakpoint-next(sm, $breakpoint-names: (xs sm md))
+//    md
+@function breakpoint-next($name, $breakpoints: $grid-breakpoints, $breakpoint-names: map-keys($breakpoints)) {
+  $n: index($breakpoint-names, $name);
+  @return if($n < length($breakpoint-names), nth($breakpoint-names, $n + 1), null);
+}
+
+// Minimum breakpoint width. Null for the smallest (first) breakpoint.
+//
+//    >> breakpoint-min(sm, (xs: 0, sm: 34rem, md: 45rem))
+//    34rem
+@function breakpoint-min($name, $breakpoints: $grid-breakpoints) {
+  $min: map-get($breakpoints, $name);
+  @return if($min != 0, $min, null);
+}
+
+// Maximum breakpoint width. Null for the largest (last) breakpoint.
+// The maximum value is calculated as the minimum of the next one less 0.1.
+//
+//    >> breakpoint-max(sm, (xs: 0, sm: 34rem, md: 45rem))
+//    44.9rem
+@function breakpoint-max($name, $breakpoints: $grid-breakpoints) {
+  $next: breakpoint-next($name, $breakpoints);
+  @return if($next, breakpoint-min($next, $breakpoints) - 0.1, null);
+}
+
+// Media of at least the minimum breakpoint width. No query for the smallest breakpoint.
+@mixin media-breakpoint-min($name, $breakpoints: $grid-breakpoints) {
+  $min: breakpoint-min($name, $breakpoints);
+  @if $min {
+    @media (min-width: $min) {
+      @content;
+    }
+  } @else {
+    @content;
+  }
+}
+
+// Media of at most the maximum breakpoint width. No query for the largest breakpoint.
+@mixin media-breakpoint-max($name, $breakpoints: $grid-breakpoints) {
+  $max: breakpoint-max($name, $breakpoints);
+  @if $max {
+    @media (max-width: $max) {
+      @content;
+    }
+  } @else {
+    @content;
+  }
+}
+
+// Media between the breakpoint's minimum and maximum widths.
+// No minimum for the smallest breakpoint, and no maximum for the largest one.
+@mixin media-breakpoint-only($name, $breakpoints: $grid-breakpoints) {
+  // Nested media query combination does not work in libsass yet
+  // https://github.com/sass/libsass/issues/185
+  // Work around until the issue is resolved:
+  $min: breakpoint-min($name, $breakpoints);
+  $max: breakpoint-max($name, $breakpoints);
+  @if $min and $max {
+    @media (min-width: $min) and (max-width: $max) {
+      @content;
+    }
+  } @else {
+    // One of min or max is a no-op, so this branch is not affected by libsass#185
+    @include media-breakpoint-min($name, $breakpoints) {
+      @include media-breakpoint-max($name, $breakpoints) {
+        @content;
+      }
+    }
+  }
+}