/**
 * Expression evaluation of the structured css.
 *
 * The evaluator understands numbers, strings, variables, arithmetic and a
 * small set of colour functions. It is used by the interpolations
 * (`#{$gap * 2}`) and by the functions of the stylesheets.
 */

export type Scope = Record<string, string>;

export interface EvaluateOptions {
	/** Variables of the current scope. */
	scope?: Scope;
	/** Names of the mixins that can be called. */
	functions?: Record<string, (...args: string[]) => string>;
}

type Token =
	| { type: "number"; value: number }
	| { type: "string"; value: string }
	| { type: "variable"; value: string }
	| { type: "operator"; value: string }
	| { type: "function"; value: string }
	| { type: "punctuation"; value: string };

const OPERATORS = ["+", "-", "*", "/", "(", ")", ","];

function tokenize(source: string): Token[] {
	const tokens: Token[] = [];
	let index = 0;

	while (index < source.length) {
		const char = source[index] as string;

		if (/\s/.test(char)) {
			index += 1;
			continue;
		}

		if (char === "$") {
			const match = /^\$[\w-]+/.exec(source.slice(index));
			if (!match) throw new Error(`Invalid variable in "${source}"`);
			tokens.push({ type: "variable", value: match[0] });
			index += match[0].length;
			continue;
		}

		if (char === "#") {
			const match = /^#[\da-fA-F]{3,8}/.exec(source.slice(index));
			if (match) {
				tokens.push({ type: "string", value: match[0] });
				index += match[0].length;
				continue;
			}
		}

		if (/[\d.]/.test(char)) {
			const match = /^[\d.]+/.exec(source.slice(index));
			if (!match) throw new Error(`Invalid number in "${source}"`);
			tokens.push({ type: "number", value: Number(match[0]) });
			index += match[0].length;
			continue;
		}

		if (/[a-zA-Z_]/.test(char)) {
			const match = /^[a-zA-Z_][\w-]*/.exec(source.slice(index));
			if (!match) throw new Error(`Invalid identifier in "${source}"`);
			const word = match[0];
			let cursor = index + word.length;
			while (cursor < source.length && /\s/.test(source[cursor] as string)) {
				cursor += 1;
			}
			if (source[cursor] === "(") {
				tokens.push({ type: "function", value: word });
				index = cursor;
				continue;
			}
			tokens.push({ type: "string", value: word });
			index += word.length;
			continue;
		}

		if (char === '"' || char === "'") {
			const end = source.indexOf(char, index + 1);
			const value =
				end === -1 ? source.slice(index + 1) : source.slice(index + 1, end);
			tokens.push({ type: "string", value });
			index = end === -1 ? source.length : end + 1;
			continue;
		}

		const operator = OPERATORS.find((candidate) =>
			source.startsWith(candidate, index),
		);
		if (operator) {
			tokens.push({
				type:
					operator === "(" || operator === ")" || operator === ","
						? "punctuation"
						: "operator",
				value: operator,
			});
			index += operator.length;
			continue;
		}

		throw new Error(`Unexpected character "${char}" in "${source}"`);
	}

	return tokens;
}

class Parser {
	private index = 0;

	constructor(
		private readonly tokens: Token[],
		private readonly options: EvaluateOptions,
	) {}

	parse(): string {
		const value = this.expression();
		if (this.index < this.tokens.length) {
			throw new Error("Unexpected token in expression");
		}
		return value;
	}

	private peek(): Token | undefined {
		return this.tokens[this.index];
	}

	private expression(): string {
		let left = this.term();
		while (this.peek()?.type === "operator") {
			const operator = (this.peek() as Token).value;
			if (operator !== "+" && operator !== "-") break;
			this.index += 1;
			const right = this.term();
			left = operator === "+" ? add(left, right) : subtract(left, right);
		}
		return left;
	}

	private term(): string {
		let left = this.factor();
		while (this.peek()?.type === "operator") {
			const operator = (this.peek() as Token).value;
			if (operator !== "*" && operator !== "/") break;
			this.index += 1;
			const right = this.factor();
			left = operator === "*" ? multiply(left, right) : divide(left, right);
		}
		return left;
	}

	private factor(): string {
		const token = this.peek();
		if (!token) throw new Error("Unexpected end of expression");
		this.index += 1;

		if (token.type === "number") return String(token.value);
		if (token.type === "string") {
			if (token.value.startsWith("#")) return token.value;
			return token.value;
		}
		if (token.type === "variable") {
			const value = this.options.scope?.[token.value];
			if (value === undefined) {
				throw new Error(`Unknown variable ${token.value}`);
			}
			return value;
		}
		if (token.type === "punctuation" && token.value === "(") {
			const value = this.expression();
			const closing = this.peek();
			if (closing?.type === "punctuation" && closing.value === ")") {
				this.index += 1;
			}
			return value;
		}
		if (token.type === "function") {
			const args: string[] = [];
			const opening = this.peek();
			if (opening?.type === "punctuation" && opening.value === "(") {
				this.index += 1;
				while (this.index < this.tokens.length) {
					const current = this.peek() as Token;
					if (current.type === "punctuation" && current.value === ")") {
						this.index += 1;
						break;
					}
					if (current.type === "punctuation" && current.value === ",") {
						this.index += 1;
						continue;
					}
					args.push(this.expression());
				}
			}
			const fn = this.options.functions?.[token.value];
			if (!fn) throw new Error(`Unknown function ${token.value}`);
			return fn(...args);
		}

		throw new Error(`Unexpected token ${token.value}`);
	}
}

function toNumber(value: string): number {
	const numeric = Number(value);
	return Number.isNaN(numeric) ? 0 : numeric;
}

function add(left: string, right: string): string {
	if (left.startsWith("#") || right.startsWith("#")) {
		return `${left}${right}`;
	}
	const result = toNumber(left) + toNumber(right);
	return Number.isInteger(result) ? String(result) : String(result);
}

function subtract(left: string, right: string): string {
	const result = toNumber(left) - toNumber(right);
	return Number.isInteger(result) ? String(result) : String(result);
}

function multiply(left: string, right: string): string {
	const result = toNumber(left) * toNumber(right);
	return Number.isInteger(result) ? String(result) : String(result);
}

function divide(left: string, right: string): string {
	const result = toNumber(left) / toNumber(right);
	return Number.isInteger(result) ? String(result) : String(result);
}

/** Parses a colour into its channels. */
export function parseColor(value: string):
	| {
			r: number;
			g: number;
			b: number;
	  }
	| undefined {
	const hex = value.replace("#", "");
	if (hex.length !== 3 && hex.length !== 6) return undefined;
	const expanded =
		hex.length === 3
			? hex
					.split("")
					.map((char) => char + char)
					.join("")
			: hex;
	return {
		r: Number.parseInt(expanded.slice(0, 2), 16),
		g: Number.parseInt(expanded.slice(2, 4), 16),
		b: Number.parseInt(expanded.slice(4, 6), 16),
	};
}

function toHex(value: number): string {
	return Math.max(0, Math.min(255, Math.round(value)))
		.toString(16)
		.padStart(2, "0");
}

/** Darkens a colour by the given percentage. */
export function darken(color: string, amount: number): string {
	const parsed = parseColor(color);
	if (!parsed) return color;
	const factor = 1 - amount / 100;
	return `#${toHex(parsed.r * factor)}${toHex(parsed.g * factor)}${toHex(parsed.b * factor)}`;
}

/** Lightens a colour by the given percentage. */
export function lighten(color: string, amount: number): string {
	const parsed = parseColor(color);
	if (!parsed) return color;
	const factor = 1 + amount / 100;
	return `#${toHex(parsed.r * factor)}${toHex(parsed.g * factor)}${toHex(parsed.b * factor)}`;
}

/** Builds a rgba colour. */
export function rgba(color: string, alpha: number): string {
	const parsed = parseColor(color);
	if (!parsed) return color;
	return `rgba(${parsed.r}, ${parsed.g}, ${parsed.b}, ${alpha})`;
}

/** The functions available inside a stylesheet. */
export const builtinFunctions: Record<string, (...args: string[]) => string> = {
	darken: (...args) =>
		String(args[0]).startsWith("#")
			? darken(String(args[0]), Number(args[1]))
			: `darken(${args.join(", ")})`,
	lighten: (...args) =>
		String(args[0]).startsWith("#")
			? lighten(String(args[0]), Number(args[1]))
			: `lighten(${args.join(", ")})`,
	rgba: (...args) =>
		String(args[0]).startsWith("#")
			? rgba(String(args[0]), Number(args[1]))
			: `rgba(${args.join(", ")})`,
};

/** Evaluates an expression of a stylesheet. */
export function evaluate(
	source: string,
	options: EvaluateOptions = {},
): string {
	const trimmed = source.trim();
	if (trimmed.length === 0) return "";
	const parser = new Parser(tokenize(trimmed), {
		scope: options.scope ?? {},
		functions: { ...builtinFunctions, ...(options.functions ?? {}) },
	});
	return parser.parse();
}