str/src/vm/commands/rsub.ts

import {Command, ParseContext, StrTerm} from "./command.js";
import {LexInput} from "../lexer.js";
import {StrVM} from "../vm.js";
import {Awaitable} from "../../util/types.js";
import {LSubData} from "./lsub.js";

export type RSubData = {
    offset: StrTerm,
    length?: StrTerm,
}

export class RSub extends Command<RSubData> {
    attemptParse(context: ParseContext): RSubData {
        return {
            offset: context.popTerm(),
            length: context.popOptionalTerm(),
        }
    }

    getDisplayName(): string {
        return 'rsub'
    }

    isParseCandidate(token: LexInput): boolean {
        return this.isKeyword(token, 'rsub')
    }

    execute(vm: StrVM, data: LSubData): Awaitable<StrVM> {
        return vm.inPlace(ctx =>
            ctx.replaceSubject(sub =>
                sub.modify(s => {
                    const offset = ctx.resolveInt(data.offset)
                    const length = data.length ? ctx.resolveInt(data.length) : s.length
                    return s.split('')  // fixme: do the math so we don't have to do this bs
                        .reverse()
                        .slice(offset, offset + length)
                        .reverse()
                        .join('')
                })))
    }
}