feat(core): add basic transform function that apply @apply directive

core/mod.ts

@@ -11,3 +11,4 @@ export * from "./extract.ts";
 export * from "./resolve.ts";
 export * from "./types.ts";
 export * from "./ast.ts";
+export * from "./transform.ts";

core/transform.ts

@@ -0,0 +1,49 @@
+// This module is browser compatible.
+
+import { postcss, Root } from "./deps.ts";
+import { generate, Output } from "./generate.ts";
+import type { StaticConfig, StaticContext } from "./types.ts";
+
+// TODO(miyauci): define generic config types
+type Config = Partial<StaticConfig & StaticContext>;
+
+export function transform(input: string, config: Readonly<Config>) {
+  const ast = postcss().process(input).root;
+
+  return applyDirective(ast.clone(), (input) => {
+    return generate(input.split(" "), config);
+  });
+}
+
+export function applyDirective(
+  ast: Root,
+  map: (input: string) => Output,
+): Output {
+  let matched = new Set<string>();
+  let unmatched = new Set<string>();
+  ast.walkAtRules("apply", (root) => {
+    const { ast, matched: _matched, unmatched: _unmatched } = map(root.params);
+    matched = _matched;
+    unmatched = _unmatched;
+
+    if (root.parent?.type === "rule") {
+      const visit = new Root();
+      ast.walkDecls((decl) => {
+        visit.append(decl);
+      });
+
+      root.parent?.append(visit);
+    }
+
+    root.remove();
+  });
+
+  return {
+    ast,
+    get css(): string {
+      return ast.toString();
+    },
+    matched,
+    unmatched,
+  };
+}