preserves/implementations/javascript/src/record.ts

import { Tag } from "./constants";
import { Encoder } from "./codec";
import { PreserveOn } from "./symbols";
import { DefaultPointer, is, Value } from "./values";

export const IsPreservesRecord = Symbol.for('IsPreservesRecord');

export class Record<T extends object = DefaultPointer> extends Array<Value<T>> {
    readonly label: Value<T>;

    constructor(label: Value<T>, fields: Value<T>[]) {
        if (arguments.length === 1) {
            // Using things like someRecord.map() involves the runtime
            // apparently instantiating instances of this.constructor
            // as if it were just plain old Array, so we have to be
            // somewhat calling-convention-compatible. This is
            // something that isn't part of the user-facing API.
            super(label);
            this.label = label; // needed just to keep the typechecker happy
            return;
        }

        super(fields.length);
        fields.forEach((f, i) => this[i] = f);
        this.label = label;
        Object.freeze(this);
    }

    get(index: number, defaultValue?: Value<T>): Value<T> | undefined {
        return (index < this.length) ? this[index] : defaultValue;
    }

    set(index: number, newValue: Value<T>): Record<T> {
        return new Record(this.label, this.map((f, i) => (i === index) ? newValue : f));
    }

    getConstructorInfo(): RecordConstructorInfo<T> {
        return { label: this.label, arity: this.length };
    }

    equals(other: any): boolean {
        return Record.isRecord(other) &&
            is(this.label, other.label) &&
            this.every((f, i) => is(f, other.get(i)));
    }

    // hashCode(): number {
    //     let h = hash(this.label);
    //     this.forEach((f) => h = ((31 * h) + hash(f)) | 0);
    //     return h;
    // }

    static fallbackToString: (f: Value<any>) => string = (_f) => '<unprintable_preserves_field_value>';

    toString(): string {
        return this.asPreservesText();
    }

    asPreservesText(): string {
        if (!('label' in this)) {
            // A quasi-Array from someRecord.map() or similar. See constructor.
            return super.toString();
        }
        return this.label.asPreservesText() +
            '(' + this.map((f) => {
                try {
                    return f.asPreservesText();
                } catch (e) {
                    return Record.fallbackToString(f);
                }
            }).join(', ') + ')';
    }

    static makeConstructor<T extends object = any>(labelSymbolText: string, fieldNames: string[]): RecordConstructor<T> {
        return Record.makeBasicConstructor<T>(Symbol.for(labelSymbolText), fieldNames);
    }

    static makeBasicConstructor<T extends object = any>(label: Value<T>, fieldNames: string[]): RecordConstructor<T> {
        const arity = fieldNames.length;
        const ctor: RecordConstructor<T> = (...fields: Value<T>[]): Record<T> => {
            if (fields.length !== arity) {
                throw new Error("Record: cannot instantiate " + (label && label.toString()) +
                    " expecting " + arity + " fields with " + fields.length + " fields");
            }
            return new Record<T>(label, fields);
        };
        const constructorInfo = { label, arity };
        ctor.constructorInfo = constructorInfo;
        ctor.isClassOf = (v: any): v is Record<T> => Record.isClassOf(constructorInfo, v);
        ctor._ = {};
        fieldNames.forEach((name, i) => {
            ctor._[name] = function (r: Value<T>): Value<T> | undefined {
                if (!ctor.isClassOf(r)) {
                    throw new Error("Record: attempt to retrieve field "+label.toString()+"."+name+
                        " from non-"+label.toString()+": "+(r && r.toString()));
                }
                return r.get(i);
            };
        });
        return ctor;
    }

    [PreserveOn](encoder: Encoder<T>) {
        encoder.emitbyte(Tag.Record);
        encoder.push(this.label);
        this.forEach((f) => encoder.push(f));
        encoder.emitbyte(Tag.End);
    }

    get [IsPreservesRecord](): boolean {
        return true;
    }

    static isRecord<T extends object = DefaultPointer>(x: any): x is Record<T> {
        return !!x?.[IsPreservesRecord];
    }

    static isClassOf<T extends object = DefaultPointer>(ci: RecordConstructorInfo<T>, v: any): v is Record<T> {
        return (Record.isRecord(v)) && is(ci.label, v.label) && (ci.arity === v.length);
    }
}

export interface RecordConstructor<T extends object = DefaultPointer> {
    (...fields: Value<T>[]): Record<T>;
    constructorInfo: RecordConstructorInfo<T>;
    isClassOf(v: any): v is Record<T>;
    _: { [getter: string]: (r: Value<T>) => Value<T> | undefined };
}

export interface RecordConstructorInfo<T extends object = DefaultPointer> {
    label: Value<T>;
    arity: number;
}