path: root/src/thing/cacheable-object.js

// Generally extendable class for caching properties and handling dependencies,
// with a few key properties:
//
// 1) The behavior of every property is defined by its descriptor, which is a
//    static value stored on the subclass (all instances share the same property
//    descriptors).
//
//  1a) Additional properties may not be added past the time of object
//      construction, and attempts to do so (including externally setting a
//      property name which has no corresponding descriptor) will throw a
//      TypeError. (This is done via an Object.seal(this) call after a newly
//      created instance defines its own properties according to the descriptor
//      on its constructor class.)
//
// 2) Properties may have two flags set: update and expose. Properties which
//    update are provided values from the external. Properties which expose
//    provide values to the external, generally dependent on other update
//    properties (within the same object).
//
//  2a) Properties may be flagged as both updating and exposing. This is so
//      that the same name may be used for both "output" and "input".
//
// 3) Exposed properties have values which are computations dependent on other
//    properties, as described by a `compute` function on the descriptor.
//    Depended-upon properties are explicitly listed on the descriptor next to
//    this function, and are only provided as arguments to the function once
//    listed.
//
//  3a) An exposed property may depend only upon updating properties, not other
//      exposed properties (within the same object). This is to force the
//      general complexity of a single object to be fairly simple: inputs
//      directly determine outputs, with the only in-between step being the
//      `compute` function, no multiple-layer dependencies. Note that this is
//      only true within a given object - externally, values provided to one
//      object's `update` may be (and regularly are) the exposed values of
//      another object.
//
//  3b) If a property both updates and exposes, it is automatically regarded as
//      a dependancy. (That is, its exposed value will depend on the value it is
//      updated with.) Rather than a required `compute` function, these have an
//      optional `transform` function, which takes the update value as its first
//      argument and then the usual key-value dependencies as its second. If no
//      `transform` function is provided, the expose value is the same as the
//      update value.
//
// 4) Exposed properties are cached; that is, if no depended-upon properties are
//    updated, the value of an exposed property is not recomputed.
//
//  4a) The cache for an exposed property is invalidated as soon as any of its
//      dependencies are updated, but the cache itself is lazy: the exposed
//      value will not be recomputed until it is again accessed. (Likewise, an
//      exposed value won't be computed for the first time until it is first
//      accessed.)
//
// 5) Updating a property may optionally apply validation checks before passing,
//    declared by a `validate` function on the `update` block. This function
//    should either throw an error (e.g. TypeError) or return false if the value
//    is invalid.
//
// 6) Objects do not expect all updating properties to be provided at once.
//    Incomplete objects are deliberately supported and enabled.
//
//  6a) The default value for every updating property is null; undefined is not
//      accepted as a property value under any circumstances (it always errors).
//      However, this default may be overridden by specifying a `default` value
//      on a property's `update` block. (This value will be checked against
//      the property's validate function.) Note that a property may always be
//      updated to null, even if the default is non-null. (Null always bypasses
//      the validate check.)
//
//  6b) It's required by the external consumer of an object to determine whether
//      or not the object is ready for use (within the larger program). This is
//      convenienced by the static CacheableObject.listAccessibleProperties()
//      function, which provides a mapping of exposed property names to whether
//      or not their dependencies are yet met.

import { color, ENABLE_COLOR } from '../util/cli.js';

import { inspect as nodeInspect } from 'util';

function inspect(value) {
    return nodeInspect(value, {colors: ENABLE_COLOR});
}

export default class CacheableObject {
    #propertyUpdateValues = Object.create(null);
    #propertyUpdateCacheInvalidators = Object.create(null);

    /*
    // Note the constructor doesn't take an initial data source. Due to a quirk
    // of JavaScript, private members can't be accessed before the superclass's
    // constructor is finished processing - so if we call the overridden
    // update() function from inside this constructor, it will error when
    // writing to private members. Pretty bad!
    //
    // That means initial data must be provided by following up with update()
    // after constructing the new instance of the Thing (sub)class.
    */

    constructor() {
        this.#defineProperties();
        this.#initializeUpdatingPropertyValues();
    }

    #initializeUpdatingPropertyValues() {
        for (const [ property, descriptor ] of Object.entries(this.constructor.propertyDescriptors)) {
            const { flags, update } = descriptor;

            if (!flags.update) {
                continue;
            }

            if (update?.default) {
                this[property] = update?.default;
            } else {
                this[property] = null;
            }
        }
    }

    #defineProperties() {
        for (const [ property, descriptor ] of Object.entries(this.constructor.propertyDescriptors)) {
            const { flags } = descriptor;

            const definition = {
                configurable: false,
                enumerable: true
            };

            if (flags.update) {
                definition.set = this.#getUpdateObjectDefinitionSetterFunction(property);
            }

            if (flags.expose) {
                definition.get = this.#getExposeObjectDefinitionGetterFunction(property);
            }

            Object.defineProperty(this, property, definition);
        }

        Object.seal(this);
    }

    #getUpdateObjectDefinitionSetterFunction(property) {
        const { update } = this.#getPropertyDescriptor(property);
        const validate = update?.validate;
        const allowNull = update?.allowNull;

        return (newValue) => {
            const oldValue = this.#propertyUpdateValues[property];

            if (newValue === undefined) {
                throw new ValueError(`Properties cannot be set to undefined`);
            }

            if (newValue === oldValue) {
                return;
            }

            if (newValue !== null && validate) {
                try {
                    const result = validate(newValue);
                    if (result === undefined) {
                        throw new TypeError(`Validate function returned undefined`);
                    } else if (result !== true) {
                        throw new TypeError(`Validation failed for value ${newValue}`);
                    }
                } catch (error) {
                    error.message = `Property ${color.green(property)} (${inspect(this[property])} -> ${inspect(newValue)}): ${error.message}`;
                    throw error;
                }
            }

            this.#propertyUpdateValues[property] = newValue;
            this.#invalidateCachesDependentUpon(property);
        };
    }

    #getUpdatePropertyValidateFunction(property) {
        const descriptor = this.#getPropertyDescriptor(property);
    }

    #getPropertyDescriptor(property) {
        return this.constructor.propertyDescriptors[property];
    }

    #invalidateCachesDependentUpon(property) {
        for (const invalidate of this.#propertyUpdateCacheInvalidators[property] || []) {
            invalidate();
        }
    }

    #getExposeObjectDefinitionGetterFunction(property) {
        const { flags } = this.#getPropertyDescriptor(property);
        const compute = this.#getExposeComputeFunction(property);

        if (compute) {
            let cachedValue;
            const checkCacheValid = this.#getExposeCheckCacheValidFunction(property);
            return () => {
                if (checkCacheValid()) {
                    return cachedValue;
                } else {
                    return (cachedValue = compute());
                }
            };
        } else if (!flags.update && !compute) {
            throw new Error(`Exposed property ${property} does not update and is missing compute function`);
        } else {
            return () => this.#propertyUpdateValues[property];
        }
    }

    #getExposeComputeFunction(property) {
        const { flags, expose } = this.#getPropertyDescriptor(property);

        const compute = expose?.compute;
        const transform = expose?.transform;

        if (flags.update && !transform) {
            return null;
        } else if (flags.update && compute) {
            throw new Error(`Updating property ${property} has compute function, should be formatted as transform`);
        } else if (!flags.update && !compute) {
            throw new Error(`Exposed property ${property} does not update and is missing compute function`);
        }

        const dependencyKeys = expose.dependencies || [];
        const dependencyGetters = dependencyKeys.map(key => () => [key, this.#propertyUpdateValues[key]]);
        const getAllDependencies = () => Object.fromEntries(dependencyGetters.map(f => f()));

        if (flags.update) {
            return () => transform(this.#propertyUpdateValues[property], getAllDependencies());
        } else {
            return () => compute(getAllDependencies());
        }
    }

    #getExposeCheckCacheValidFunction(property) {
        const { flags, expose } = this.#getPropertyDescriptor(property);

        let valid = false;

        const invalidate = () => {
            valid = false;
        };

        const dependencyKeys = new Set(expose?.dependencies);

        if (flags.update) {
            dependencyKeys.add(property);
        }

        for (const key of dependencyKeys) {
            if (this.#propertyUpdateCacheInvalidators[key]) {
                this.#propertyUpdateCacheInvalidators[key].push(invalidate);
            } else {
                this.#propertyUpdateCacheInvalidators[key] = [invalidate];
            }
        }

        return () => {
            if (!valid) {
                valid = true;
                return false;
            } else {
                return true;
            }
        };
    }
}