export enum AssertionMode {
    SKIP = "SKIP",
    LOG = "LOG",
    THROW = "THROW",
}

/** Configurations for the different assertion modes. */
export namespace AssertionConfig {
    /** The types of the assertion configurations. */
    export type Type = typeof Skip|typeof Throw|ReturnType<typeof Log>
    /** An assertion configuration suitable for production, that skips assertions without evaluating them. */
    export const Skip = {mode: AssertionMode.SKIP} as const
    /** An assertion configuration suitable for testing, that throws an exception when an assertion fails. */
    export const Throw = {mode: AssertionMode.THROW} as const
    /** An assertion configuration suitable for debugging, that logs an exception when an assertion fails but allows the program to continue. */
    export function Log(logger: (message?: unknown, ...params: unknown[]) => void) {
        return {mode: AssertionMode.LOG, logger} as const
    }
}

/** Assertion class for test-only checks on preconditions and postconditions. */
export class Assertions {
    config: AssertionConfig.Type = AssertionConfig.Skip

    /** General assert method. Checks that the condition is true. */
    check<T>(value: T, condition: (value: T) => boolean, message: (value: T) => string): T {
        if (this.config.mode === AssertionMode.SKIP) {
            return value
        }
        const result = condition(value)
        if (result) {
            return value
        }
        const err = Error(message(value))
        if (this.config.mode === AssertionMode.THROW) {
            throw err
        } else {
            // All we can do is stand by and watch in horror...
            this.config.logger(err)
            return value
        }
    }

    /** Checks that the value is an integer. */
    checkInteger(value: number, message: (value: number) => string): number {
        return this.check(value, Number.isSafeInteger, message)
    }

    /** Checks that the value is an integer and either positive or zero. */
    checkPositiveIntegerOrZero(value: number, message: (value: number) => string): number {
        return this.check(value, (value) => {
            return Number.isSafeInteger(value) && value >= 0
        }, message)
    }
}

/** Global assertion object for use by other modules. */
export const assertion = new Assertions();