"use strict";

const splitFilter = require("split-filter");
const assert = require("assert");

const NoChange = require("../astformer/actions/no-change");
const nixTypes = require("./util/nix-types");

module.exports = {
	name: "desugar-inherits",
	visitors: {
		NixAttributeSet: (node) => {
			let [ inherits, regularBindings ] = splitFilter(node.bind, (binding) => binding.type === "NixInherit");

			if (inherits.length === 0) {
				return NoChange;
			} else {
				let tempCounter = 0;
				let inherits_ = inherits.map((inherit) => {
					return {
						tempName: `$temp$${tempCounter++}`,
						sourceExpression: inherit.expression,
						names: inherit.attrs.attr.map((attribute) => {
							assert(attribute.type === "NixAttributeIdentifier");
							return attribute.name;
						})
					};
				});

				let letBindings = inherits_.map((inherit) => {
					return nixTypes.NixBinding(
						[ nixTypes.NixAttributeIdentifier(inherit.tempName) ],
						inherit.sourceExpression
					);
				});

				let inheritedAttributeBindings = inherits_.flatMap((inherit) => {
					return inherit.names.map((name) => {
						return nixTypes.NixBinding(
							[ nixTypes.NixAttributeIdentifier(name) ],
							nixTypes.NixAttributeSelection(
								nixTypes.NixIdentifier(inherit.tempName),
								[ nixTypes.NixAttributeIdentifier(name) ]
							)
						);
					});
				});

				if (node.recursive === false) {
					// When not recursive, we need to ensure that the temporary variables are defined a scope *higher* than the attribute set that actually makes use of their properties, because we cannot access those variables from the attribute bindings otherwise.
					
					return nixTypes.NixLetIn(
						letBindings,
						{
							... node,
							bind: [
								... regularBindings,
								... inheritedAttributeBindings
							]
						}
					);
				} else {
					// ... but when recursive, we *must* specify the temporary variables as part of the recursive attribute set itself, because otherwise we don't have the ability to bidirectionally reference between fixed attributes and inherited ones; see, for example, eval-okay-scope-7.nix.
					// FIXME: Mark the temporary variables as 'internal' so that they don't get returned in the final object; this would make the generated code more readable.

					return {
						... node,
						bind: [
							... regularBindings,
							... letBindings,
							... inheritedAttributeBindings
						]
					};
				}

			}
		}
	}
};