CommunitySolidServer/src/storage/conversion/ChainedConverter.ts

import { Representation } from '../../ldp/representation/Representation';
import { RepresentationMetadata } from '../../ldp/representation/RepresentationMetadata';
import { RepresentationPreferences } from '../../ldp/representation/RepresentationPreferences';
import { CONTENT_TYPE } from '../../util/UriConstants';
import { matchingMediaType } from '../../util/Util';
import { RepresentationConverterArgs } from './RepresentationConverter';
import { TypedRepresentationConverter } from './TypedRepresentationConverter';

/**
 * A meta converter that takes an array of other converters as input.
 * It chains these converters by finding intermediate types that are supported by converters on either side.
 */
export class ChainedConverter extends TypedRepresentationConverter {
  private readonly converters: TypedRepresentationConverter[];

  /**
   * Creates the chain of converters based on the input.
   * The list of `converters` needs to be at least 2 long.
   * @param converters - The chain of converters.
   */
  public constructor(converters: TypedRepresentationConverter[]) {
    super();
    if (converters.length < 2) {
      throw new Error('At least 2 converters are required.');
    }
    this.converters = [ ...converters ];
  }

  protected get first(): TypedRepresentationConverter {
    return this.converters[0];
  }

  protected get last(): TypedRepresentationConverter {
    return this.converters[this.converters.length - 1];
  }

  public async getInputTypes(): Promise<{ [contentType: string]: number }> {
    return this.first.getInputTypes();
  }

  public async getOutputTypes(): Promise<{ [contentType: string]: number }> {
    return this.last.getOutputTypes();
  }

  public async canHandle(input: RepresentationConverterArgs): Promise<void> {
    // We assume a chain can be constructed, otherwise there would be a configuration issue
    // Check if the first converter can handle the input
    const firstChain = await this.getMatchingType(this.converters[0], this.converters[1]);
    const preferences: RepresentationPreferences = { type: [{ value: firstChain, weight: 1 }]};
    await this.first.canHandle({ ...input, preferences });

    // Check if the last converter can produce the output
    const idx = this.converters.length - 1;
    const lastChain = await this.getMatchingType(this.converters[idx - 1], this.converters[idx]);
    const oldMeta = input.representation.metadata;
    const metadata = new RepresentationMetadata(oldMeta, { [CONTENT_TYPE]: lastChain });
    const representation: Representation = { ...input.representation, metadata };
    await this.last.canHandle({ ...input, representation });
  }

  public async handle(input: RepresentationConverterArgs): Promise<Representation> {
    const args = { ...input };
    for (let i = 0; i < this.converters.length - 1; ++i) {
      const value = await this.getMatchingType(this.converters[i], this.converters[i + 1]);
      args.preferences = { type: [{ value, weight: 1 }]};
      args.representation = await this.converters[i].handle(args);
    }
    args.preferences = input.preferences;
    return this.last.handle(args);
  }

  /**
   * Finds the best media type that can be used to chain 2 converters.
   */
  protected async getMatchingType(left: TypedRepresentationConverter, right: TypedRepresentationConverter):
  Promise<string> {
    const leftTypes = await left.getOutputTypes();
    const rightTypes = await right.getInputTypes();
    let bestMatch: { type: string; weight: number } = { type: 'invalid', weight: 0 };

    // Try to find the matching type with the best weight
    const leftKeys = Object.keys(leftTypes);
    const rightKeys = Object.keys(rightTypes);
    for (const leftType of leftKeys) {
      const leftWeight = leftTypes[leftType];
      if (leftWeight <= bestMatch.weight) {
        continue;
      }
      for (const rightType of rightKeys) {
        const rightWeight = rightTypes[rightType];
        const weight = leftWeight * rightWeight;
        if (weight > bestMatch.weight && matchingMediaType(leftType, rightType)) {
          bestMatch = { type: leftType, weight };
          if (weight === 1) {
            return bestMatch.type;
          }
        }
      }
    }

    if (bestMatch.weight === 0) {
      throw new Error(`No match found between ${leftKeys} and ${rightKeys}`);
    }

    return bestMatch.type;
  }
}