import { SdtfError } from '@shapediver/sdk.sdtf-core';
import { ISdtfBufferCache } from './ISdtfBufferCache';

export class SdtfBinaryBufferCache implements ISdtfBufferCache {
    readonly cacheIdFullBuffer = 'full';

    /**
     * Caching object used to store already fetched/loaded buffers.
     * The key is a hash of the buffer's URI.
     * The id encapsulates the offset and length of the buffer; or {@link cacheIdFullBuffer} for the entire buffer.
     * @private
     */
    readonly cache: { [key: string]: { [offset: string]: DataView } };

    constructor() {
        this.cache = {};
    }

    /**
     * Calculates the cache key by hashing the given buffer URI.
     * Simple, __insecure__ hashing function that's short, fast, and has no dependencies.
     * @protected
     */
    calcCacheKey(uri: string = ''): string {
        let hash = 0;
        for (let i = 0; i < uri.length; i++) {
            const char = uri.charCodeAt(i);
            hash = (hash << 5) - hash + char;
            hash &= hash; // Convert to 32bit integer
        }
        return new Uint32Array([hash])[0].toString(36);
    }

    /**
     * Calculates the cache ID based on the given buffer range.
     * @protected
     */
    calcCacheId(offset: number, length: number): string {
        return `${offset};${length}`;
    }

    /**
     * Returns the buffer with the given key and range from cache.
     * @protected
     */
    loadFromCache(key: string, offset: number, length: number): DataView | undefined {
        const item = this.cache[key];
        if (!item) {
            // Buffer not found in cache
            return undefined;
        }

        const id = this.calcCacheId(offset, length);
        if (item[id]) {
            // Buffer found in cache for this range
            return item[id];
        }

        if (item[this.cacheIdFullBuffer]) {
            // Entire buffer object stored for this key - return range.
            const buffer = item[this.cacheIdFullBuffer];

            // However, the internally stored buffer might already contain a byte offset - take this into account!
            return new DataView(buffer.buffer, buffer.byteOffset + offset, length);
        }
    }

    /**
     * Stores the buffer with the given key and range in cache.
     * @protected
     */
    storeInCache(key: string, id: string, buffer: DataView): void {
        const item = this.cache[key] ?? {};
        item[id] = buffer;
        this.cache[key] = item;
    }

    setBinaryBody(binaryBody?: DataView): void {
        if (!binaryBody) return;

        this.cache[this.calcCacheKey()] = {
            [this.cacheIdFullBuffer]: binaryBody,
        };
    }

    async getBuffer(uri: string | undefined, offset: number, length: number): Promise<DataView> {
        const cacheKey = this.calcCacheKey(uri);

        return (
            this.loadFromCache(cacheKey, offset, length) ??
            (await this.acquireBuffer(uri, length, offset))
        );
    }

    /**
     * Resolves and loads external buffer files - this is not supported in this mode!
     * However, this can be overwritten by other buffer cache implementations.
     * @protected
     */
    async acquireBuffer(
        uri: string | undefined,
        offset: number,
        length: number
    ): Promise<DataView> {
        throw new SdtfError(
            'Resolution of external buffers is not supported in this mode. ' +
                "Please use 'ISdtfParser.readFromFile()' or 'ISdtfParser.readFromUrl()' to instantiate the sdTF asset and to enable this functionality."
        );
    }
}