/*
 * Copyright © 2019 Atomist, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// tslint:disable-next-line:no-null-keyword
export type NeverMatch = null;

/**
 * Result of a Mapper. A result of the desired type if
 * we match. undefined if we don't match.
 * Return DoNotSetAnyGoals (null) to shortcut evaluation of the present set of rules,
 * terminating evaluation and guarantee the return of undefined if we've reached this point.
 * Only do so if you are sure
 */
export type MappingResult<V> = Promise<V | undefined | NeverMatch>;

/**
 * Mapping function from F to V
 */
export type Mapper<F, V> = (from: F) => MappingResult<V>;

/**
 * Mapper from source to value, if it can be resolved.
 * This is a central interface used throughout the SDM.
 */
export interface Mapping<F, V> {

    /**
     * Name of the Mapping. Must be unique
     */
    readonly name: string;

    /**
     * Compute a value for the given event. Return undefined
     * if we don't find a mapped value.
     * Return DoNotSetAnyGoals (null) to shortcut evaluation of the present set of rules,
     * terminating evaluation and guarantee the return of undefined if we've reached this point.
     * Only do so if you are sure
     * that this evaluation must be short circuited if it has reached this point.
     * If a previous rule has matched, it will still be used.
     * The value may be static
     * or computed on demand, depending on the implementation.
     */
    mapping: Mapper<F, V>;
}

export function isMapping(a: any): a is Mapping<any, any> {
    const maybe = a as Mapping<any, any>;
    return !!maybe.name && !!maybe.mapping;
}

export enum MappingCompositionStyle {
    ApplyFunctionToOutput = "like Array.map",
}

/**
 * This will be a union type when we add another one
 */
export interface ExplicableMappingStructure<F, V, V1 = V> {
    component: Mapping<F, V1>;
    applyFunction: (v: V1) => V;
    compositionStyle: MappingCompositionStyle.ApplyFunctionToOutput;
}

/**
 * Possible inner structure of Mapping, based on generic Mapping composition.
 * F = input
 * V = output
 * V1 = output type of the structural components
 */
export interface ExplicableMapping<F, V, V1 = V> extends Mapping<F, V> {
    structure: ExplicableMappingStructure<F, V, V1>;
}

export function isExplicableMapping<F, V>(input: Mapping<F, V>): input is ExplicableMapping<F, V> {
    const maybe = input as ExplicableMapping<F, V>;
    return !!maybe.structure;
}

export function mapMapping<F, V1, V2>(inputMapping: Mapping<F, V1>, f: (v1: V1) => V2): Mapping<F, V2> & ExplicableMapping<F, V2, V1> {
    return {
        name: inputMapping.name,
        mapping: (input: F) => {
            return inputMapping.mapping(input).then(f);
        },
        structure: {
            component: inputMapping,
            applyFunction: f,
            compositionStyle: MappingCompositionStyle.ApplyFunctionToOutput,
        },
    };
}