// Parsing combinators
const ok = <T>(r: T, e: string): Result<T> => ({ status: "ok", value: r, remain: e });
const err = <T, E>(m: string, e?: E): Result<T> => ({ status: "err", msg: m, data: e });
const wrapErr = <T>(e: string, r: Result<T>): Result<T> => r.status === "ok" ? r : { ...r, msg: `${e}: ${r.msg}`}

type Result<T> = {status:"ok",value:T, remain:string} | {status:"err", msg: string, data?: unknown}
type Parser<T = unknown> = (i: string) => Result<T>;

const re = (r: RegExp): Parser<string> => (i: string) => {
    const r2 = r.source.startsWith("^") ? r : new RegExp(`^${r.source}`)
    const m = r2.exec(i);
    if (!m) return err("failed to match " + r.source)
    return ok(m[0], i.slice(m[0].length))
};
const map = <T, U>(p: Parser<T>, f: (_: T) => U): Parser<U> => (i: string) => {
    const res = p(i);
    if (res.status === "ok") return {...res, value: f(res.value)}
    else return res;
};
const opt = <T>(p: Parser<T>): Parser<T|null> => (i: string) => p(i).status === "ok" ? p(i) : ok(null,i);
const seq = (...parsers) => (i: string) => parsers.reduce((acc: Result<unknown[]>, next: Parser, idx) => {
    if (acc.status === "err") return acc;
    const res = next(acc.remain);
    if (res.status === "err") return wrapErr(`failed seq #${idx}`, res);
    return ok([...acc.value, res.value], res.remain)
}, ok([], i));
const alt = (...parsers) => (i: string) => {
    const res = parsers.reduce((acc: Result<unknown>, next: Parser) => {
        if (acc.status === "ok") return acc;
        return next(i)
    }, err("nothing matched"));
    if (res.status === "err") return err("failed alt")
    return res;
};

const _: Parser = re(/\s*/) // whitespace

// Grammar
const ident: Parser = re(/(\_[A-Za-z0-9_]+)|([A-Za-z][A-Za-z0-9_]*)/);
const ty: Parser = alt(
    re(/uint/), re(/int/), re(/ptr/),
    re(/str/), ident,
)

const exprL: Parser = alt(
    seq(re(/\(/), _, (i) => expr(i), _, re(/\)/)),
    map(re(/(\+|\-)?[0-9]+/), (n) => ({expr:"intLit", value:parseInt(n)})),
    map(ident, (name) => ({expr:"ident", name})),
)
const expr2: Parser = alt(
    map(seq(exprL, _, re(/</), _, exprL), ([left,,,,right]) => ({expr:"<", left, right})),
    map(seq(exprL, _, re(/>/), _, exprL), ([left,,,,right]) => ({expr:">", left, right})),
    exprL)
const expr1: Parser = alt(
    map(seq(expr2, _, re(/==/), _, expr2), ([left,,,,right]) => ({expr: "==", left, right})),
    expr2)
const expr: Parser = expr1

const top: Parser = alt(
    map(seq(re(/extern/), _, ident), (([, name]) => ({type: "extern", name}))),
    seq(re(/struct/), _, ident, _, re(/:/)),
    map(seq(re(/fn/), _, ident, _, re(/:/)), (([,, name]) => ({type: "func", name}))),
)
const stmt: Parser = alt(
    map(seq(re(/let/), _, ident, _, re(/=/), _, (i) => expr(i)),
        ([,, name,,,, value]) => ({stmt: "let",name, value})),
    map(seq(re(/if/), _, (i) => expr(i), _, re(/:/)), ([,, cond]) => ({stmt: "if", cond})),
    map((i) => expr(i), (expr) => ({stmt:"expr", expr})),
)


// Parsing driver
interface Program {
    structs: object[]
    functions: object[]
}

function parseProgram(input: string): Program {
    let currentFunc: string | null = null;
    let indentStack = [0];
    let expectIndent = false;

    for (const line of input.split(/\r?\n/)) {
        const leadingWhitespace = /^(?<space>\s*)(?<rest>.*)$/.exec(line)
        const numSpaces = leadingWhitespace?.groups?.space?.length ?? 0;
        const rest = leadingWhitespace?.groups?.rest ?? "";

        console.log("stack", indentStack)

        const lastIndent = indentStack[indentStack.length - 1];
        if (numSpaces > lastIndent) {
            if (expectIndent) {
                indentStack.push(numSpaces)
                expectIndent = false;
            } else {
                // Possible error?
            }
        } else if (numSpaces === lastIndent) {
            if (expectIndent) {
                console.log("empty block")
            }
        } else {
            indentStack.pop();
            console.log("dedented")
        }

        if (numSpaces === 0) {
            // Parse top level
            const result = opt(top)(rest)
            console.log("top", JSON.stringify(result))
            if (result.status === "ok") {
                if (result.value === null) continue;
                switch(result.value.type) {
                    case "func":
                        currentFunc = result.value.name;
                        expectIndent = true;
                        break;
                }
            }
        } else if (currentFunc) {
            const result = stmt(rest);
            console.log("stmt", JSON.stringify(rest), JSON.stringify(result))
        }
    }
}

// Codegen

// Main
async function main() {
    const filename = Bun.argv[2];
    const contents = await Bun.file(filename).text()

    const programAst = parseProgram(contents);
}

main();