preserves/implementations/javascript/packages/core/src/decoder.ts

import { Annotated } from "./annotated";
import { DecodeError, ShortPacket } from "./codec";
import { Tag } from "./constants";
import { Set, Dictionary } from "./dictionary";
import { DoubleFloat, SingleFloat } from "./float";
import { Record } from "./record";
import { Bytes, BytesLike, underlying } from "./bytes";
import { Value } from "./values";

export interface DecoderOptions<T extends object> {
    includeAnnotations?: boolean;
    decodePointer?: (v: Value<T>) => T;
}

export class Decoder<T extends object> {
    packet: Uint8Array;
    index: number;
    options: DecoderOptions<T>;

    constructor(packet: BytesLike = new Uint8Array(0), options: DecoderOptions<T> = {}) {
        this.packet = underlying(packet);
        this.index = 0;
        this.options = options;
    }

    get includeAnnotations(): boolean {
        return this.options.includeAnnotations ?? false;
    }

    write(data: BytesLike) {
        if (this.index === this.packet.length) {
            this.packet = underlying(data);
        } else {
            this.packet = Bytes.concat([this.packet.slice(this.index), data])._view;
        }
        this.index = 0;
    }

    nextbyte(): number {
        if (this.index >= this.packet.length) throw new ShortPacket("Short packet");
        // ^ NOTE: greater-than-or-equal-to, not greater-than.
        return this.packet[this.index++];
    }

    nextbytes(n: number): DataView {
        const start = this.index;
        this.index += n;
        if (this.index > this.packet.length) throw new ShortPacket("Short packet");
        // ^ NOTE: greater-than, not greater-than-or-equal-to.
        return new DataView(this.packet.buffer, this.packet.byteOffset + start, n);
    }

    varint(): number {
        // TODO: Bignums :-/
        const v = this.nextbyte();
        if (v < 128) return v;
        return (this.varint() << 7) + (v - 128);
    }

    peekend(): boolean {
        const matched = this.nextbyte() === Tag.End;
        if (!matched) this.index--;
        return matched;
    }

    nextvalues(): Value<T>[] {
        const result = [];
        while (!this.peekend()) result.push(this.next());
        return result;
    }

    nextint(n: number): number {
        // TODO: Bignums :-/
        if (n === 0) return 0;
        let acc = this.nextbyte();
        if (acc & 0x80) acc -= 256;
        for (let i = 1; i < n; i++) acc = (acc * 256) + this.nextbyte();
        return acc;
    }

    wrap(v: Value<T>): Value<T> {
        return this.includeAnnotations ? new Annotated(v) : v;
    }

    static dictionaryFromArray<T extends object>(vs: Value<T>[]): Dictionary<Value<T>, T> {
        const d = new Dictionary<Value<T>, T>();
        if (vs.length % 2) throw new DecodeError("Missing dictionary value");
        for (let i = 0; i < vs.length; i += 2) {
            d.set(vs[i], vs[i+1]);
        }
        return d;
    }

    unshiftAnnotation(a: Value<T>, v: Annotated<T>) {
        if (this.includeAnnotations) {
            v.annotations.unshift(a);
        }
        return v;
    }

    next(): Value<T> {
        const tag = this.nextbyte();
        switch (tag) {
            case Tag.False: return this.wrap(false);
            case Tag.True: return this.wrap(true);
            case Tag.Float: return this.wrap(new SingleFloat(this.nextbytes(4).getFloat32(0, false)));
            case Tag.Double: return this.wrap(new DoubleFloat(this.nextbytes(8).getFloat64(0, false)));
            case Tag.End: throw new DecodeError("Unexpected Compound end marker");
            case Tag.Annotation: {
                const a = this.next();
                const v = this.next() as Annotated<T>;
                return this.unshiftAnnotation(a, v);
            }
            case Tag.Pointer: {
                const d = this.options.decodePointer;
                if (d === void 0) {
                    throw new DecodeError("No decodePointer function supplied");
                }
                return this.wrap(d(this.next()));
            }
            case Tag.SignedInteger: return this.wrap(this.nextint(this.varint()));
            case Tag.String: return this.wrap(Bytes.from(this.nextbytes(this.varint())).fromUtf8());
            case Tag.ByteString: return this.wrap(Bytes.from(this.nextbytes(this.varint())));
            case Tag.Symbol: return this.wrap(Symbol.for(Bytes.from(this.nextbytes(this.varint())).fromUtf8()));
            case Tag.Record: {
                const vs = this.nextvalues();
                if (vs.length === 0) throw new DecodeError("Too few elements in encoded record");
                return this.wrap(Record(vs[0], vs.slice(1)));
            }
            case Tag.Sequence: return this.wrap(this.nextvalues());
            case Tag.Set: return this.wrap(new Set(this.nextvalues()));
            case Tag.Dictionary: return this.wrap(Decoder.dictionaryFromArray(this.nextvalues()));
            default: {
                if (tag >= Tag.SmallInteger_lo && tag <= Tag.SmallInteger_lo + 15) {
                    const v = tag - Tag.SmallInteger_lo;
                    return this.wrap(v > 12 ? v - 16 : v);
                }
                if (tag >= Tag.MediumInteger_lo && tag <= Tag.MediumInteger_lo + 15) {
                    const n = tag - Tag.MediumInteger_lo;
                    return this.wrap(this.nextint(n + 1));
                }
                throw new DecodeError("Unsupported Preserves tag: " + tag);
            }
        }
    }

    try_next(): Value<T> | undefined {
        const start = this.index;

        if (start >= this.packet.length) return void 0;
        // ^ important somewhat-common case optimization - avoid the exception

        try {
            return this.next();
        } catch (e) {
            if (ShortPacket.isShortPacket(e)) {
                this.index = start;
                return void 0;
            }
            throw e;
        }
    }
}

export function decode<T extends object>(bs: BytesLike, options?: DecoderOptions<T>) {
    return new Decoder(bs, options).next();
}

export function decodeWithAnnotations<T extends object>(bs: BytesLike, options: DecoderOptions<T> = {}): Annotated<T> {
    return decode(bs, { ... options, includeAnnotations: true }) as Annotated<T>;
}